bluuebunny/crossref_metadata_2025 huggingface dataset:
Parameterising the impact of shelter on crop microclimates and evaporation fluxes
Authors: Cleugh H. A.
Published: March 2003

This paper presents the modelling framework and parameterisations adopted to enable the effects of wind shelter on crop microclimates and growth to be simulated. Two modelling components were required: the first, SCAM (Soil Canopy Atmosphere Model), is a biophysical land surface model used to simulate the effects of shelter on the plant canopy and soil energy and water exchanges. The second is the Agricultural Production Systems Simulator (APSIM) configured for crop modelling (Carberry et al. 2002). This paper describes the link between the two and the parameterisations developed to simulate the effects of shelter and provide potential evaporation estimates for input to APSIM.

A series of sensitivity analyses using SCAM demonstrate, as expected, that over the course of the growing season and during meteorological conditions typical of agricultural regions in south-eastern Australia, plant evaporation is relatively insensitive to wind speed. This is consistent with theory and illustrates that reducing wind speeds will only reduce evaporation fluxes when the (unsheltered) canopy evaporation rate exceeds the equilibrium evaporation rate. In other words, reductions in water loss as a result of shelter will arise from one or a combination of the following: (i) reduced soil evaporation following rainfall; (ii) reduced evaporation of rainfall intercepted by the plant canopy; and (iii) reduced plant transpiration during wet periods and/or periods of very high demand when transpiration exceeds the equilibrium evaporation rate. Increased storage of soil moisture as a result of shelter early in the season may sustain crop transpiration later in the growing season. While this may not modify seasonal water use, increased soil water availability for the growing crop may confer an advantage in terms of final yield.

The paper then describes the development and validation of a bulk scalar transfer coefficient, or resistance, based on wind tunnel experiments investigating scalar (heat and water vapour) transport in windbreak flows. This coefficient is used in SCAM to parameterise the effect of shelter on the exchanges of heat and water between the atmosphere and the soil and/or plant canopy. A simple parameterisation for this coefficient is developed and implemented. The potential effects of shelter on evaporative demand and hence plant growth are then quantified by using the predictions of plant potential evaporation, calculated from a modified version of SCAM as input to APSIM. The simulations show that events where reduced evaporative demand benefits plant growth can occur intermittently in the latter part of the growing season (i.e. post-anthesis for cereals) for some climatic regimes, as illustrated by the simulations for the Roseworthy, Esperance and Atherton sites used in the National Windbreaks Program.
Characterisation of a windbreak system on the south coast of Western Australia. 1. Microclimate and wind erosion
Authors: Sudmeyer R. A. , Scott P. R.
Published: March 2003

The lack of data relating changes in microclimate and wind erosion to crop growth in shelter is a constraint to the adoption of windbreak systems in Australia. In this experiment microclimate and soil movement were measured in a 450 m wide bay between 2 Pinus pinaster windbreaks in south-western Australia over 4 years. Changes in wind speed and microclimate as a result of wind shelter varied spatially and temporally. When the wind direction was perpendicular to the windbreaks, wind-run reductions greater than 20% extended 18 times the height of the windbreak (H) downwind. However, over the whole growing season wind-run reductions greater than 20% only extended 3–6 H from the windbreaks, and were confined to within 4 H over the whole year. Over the growing season, atmospheric vapour pressure and average daily temperature and potential evaporation in the most sheltered part of the windbreak bay were generally within ± 5–10% of unsheltered values. While growing conditions were generally improved, there were periods at the end of the growing season when sheltered crops experienced increased air temperatures and vapour pressure deficit. The principal benefit of the windbreaks appeared to be reducing wind speed during periods with short duration erosive winds. More than 1 H from the windbreaks, wind erosion was reduced for 36 H downwind of the windbreak that provided most shelter during the period of maximum soil movement. Browsing stock increased the porosity of the lower 1.5 m of the windbreaks, which allowed wind to funnel under the windbreaks. This study highlights the difficulty of maintaining constant shelter in an environment where the prevailing wind direction changes throughout the year and the need to orient windbreaks to provide shelter during those times when strong winds are most damaging to soils or crops.
Characterisation of a windbreak system on the south coast of Western Australia. 2. Crop growth
Authors: Sudmeyer R. A. , Scott P. R.
Published: March 2003

This paper, which is the second in a series of three, describes dryland crop growth and yields in a windbreak bay in south-western Australia and relates changes to microclimate modification by the windbreaks. Over the 4 years of this trial, above ground biomass and the development rate of crops 3–20 times the tree height from the windbreak (H) were similar to crops growing in unsheltered conditions (more than 20 H from the windbreaks). Grain yield was 16–30% higher between 3 H and 20 H than at more than 20 H in 1994, the driest year on record for the district, in other years yield was largely unchanged. In contrast, above ground biomass growth was consistently less within 3 H than further from the windbreaks and grain yield within 3 H was 19–27% less than unsheltered yield. Water use by the trees is the most likely cause of reduced yield within 3 H. Over the 4 years, mean grain yield between 0.5 H and 20 H was 3.8% greater than yield at more than 20 H. This increase was largely due to the yield increase in 1994. As 5.4% of the paddock was directly occupied by, or uncropped next to, the windbreaks, there was a net yield decrease of 2.8% over 4 years compared to estimated production from a similar area with no windbreaks. The principle benefits of the windbreaks were reducing evaporative demand in extremely dry years and protection against extreme wind events. These benefits must be weighed against the costs of establishing and maintaining windbreak systems.
Characterisation of a windbreak system on the south coast of Western Australia. 3. Soil water and hydrology
Authors: Hall D. J. M. , Sudmeyer R. A. , McLernon C. K. , Short R. J.
Published: March 2003

This paper describes changes in soil water and ground water at various distances from a Pinus pinaster windbreak in south-western Australia. Soil water contents were measured by neutron moisture meter and time domain reflectometry at distances from a windbreak ranging from 1 to 20 tree heights (H). Within 3 H of the windbreak, soil water storage was reduced by 100–153 mm/1.8 m when compared to unsheltered conditions (20 H) over the 4 years of the experiment. Beyond 3 H, no significant differences in soil water storage were found which could be related to microclimate modification. Relationships between surface soil water storage (mm/0.4 m) at <6�H and 12–24 H were 1 : 1 regardless of the technique used. Similarly, soil water depletion within the crop rootzone (mm/0.6 m) was similar at distances >3 H. Reductions in the depth and duration of perched water levels occurred within 4 H of the windbreak. Despite this, the windbreaks had no effect on the regional ground-water levels.
Economic assessment of windbreaks on the south-eastern coast of Western Australia
Authors: Jones H. R. , Sudmeyer R. A.
Published: March 2003

Windbreaks in the south-eastern coastal region of Western Australia are generally established to protect agricultural land against damage from wind erosion. However, to date there has been no detailed economic analysis of establishing windbreaks on farms in the region. Accordingly, a decision model was developed to estimate benefits and costs of windbreaks as determined by production inputs and outputs, windbreak system design, frequency and severity of wind damage, and commercial tree products from the windbreak. The model used crop yield responses to shelter at sites across the medium–low rainfall agricultural areas of southern Western Australia. The model lends itself to further development as a risk analysis tool incorporating probabilities and empirical measurements of wind damage.

A benefit:cost analysis, using a partial budget approach, compared situations with and without windbreaks. The model showed that windbreaks improved profitability when they reduced the severity of wind damage to crops. Without wind erosion events, windbreaks had a negative impact on farm profit. Investment in windbreaks is therefore a form of insurance. The relative level of benefits is determined by the frequency of wind damage, severity of damage, distance between windbreaks, pruning of lateral tree roots and the possibility of using trees that can produce commercial timber products.
Broadacre crop yield in the lee of windbreaks in the medium and low rainfall areas of south-western Australia
Authors: Sudmeyer R. A. , Adams M. A. , Eastham J. , Scott P. R. , Hawkins W. , Rowland I. C.
Published: March 2003

In Western Australia, the paucity of documented information detailing crop yield in the lee of windbreaks is a constraint to the adoption of tree windbreaks in dryland farming systems. This paper presents grain yield data for crops growing in the lee of windbreaks in the medium to low rainfall areas of the south-west of Western Australia for 64 field years between 1994 and 1997. Two distinct areas were identified in the lee of windbreaks: a zone of reduced crop yield extending 3–5 times the windbreak height (H) from the trees (competition zone), and a zone of unchanged or improved yield extending 15–20 H (sheltered zone). Yield between 1 and 20 H was less than unsheltered yield in years with average rainfall, similar to unsheltered yield in years, or areas, with low rainfall and higher than unsheltered yield if the unsheltered crop was subjected to sandblasting. Changes in microclimate in shelter appeared to be of benefit in increasing crop yields in drier years or areas. Lupin yield was generally increased in the sheltered zone while cereal yield was generally unchanged. The rate of canopy development may be critical to crop response. In dry years, reduced wind speed in shelter reduced evaporation of soil moisture, increasing the amount of soil water available to establishing crops and reducing sandblasting damage. The principle benefit of windbreaks was their ability to reduce wind erosion and subsequent crop damage. As such, windbreaks are best regarded as a form of insurance.
The tree - crop interface: the effects of root pruning in south-western Australia
Authors: Sudmeyer R. A. , Hall D. J. M. , Eastham J. , Adams M. A.
Published: March 2003

This paper examines the effect severing lateral tree roots (root pruning) has on crop and tree growth and soil water content at 2 sites in the south-west of Western Australia. Crop and tree growth and soil water content were assessed in a Pinus pinaster windbreak system growing on 0.45–1.00 m of sand over clay, and crop growth was assessed adjacent to Eucalyptus globulus windbreaks growing on 4–5 m of sand. Crop yield was depressed by 23–52% within 2.5 times the tree height (H) of unpruned pines and by 44% within 2.5 H of pruned eucalypts. Depressed yields made cropping uneconomical within 1.5 H of the eucalypts and 1 H of the pines. Root pruning most improved crop yields where lateral tree roots were confined close to the soil surface and decreased in effectiveness as the depth to confining layer (clay) increased. Crop losses within 2.5 H of the pines were reduced from 39 to 14% in the year the trees were root pruned and were 25% 1 year after root pruning. Subsequent root pruning of the eucalypts did not improve crop yield. While root pruning severed lateral pine roots, tree growth was not significantly reduced. The principal cause of reduced crop yield near the trees appeared to be reduced soil moisture in the area occupied by tree roots. Competition for nutrients and light appeared to have little effect on crop yield. Root pruning can spatially separate tree and crop roots where the tree roots are confined close to the surface, and significantly improve crop yields without reducing tree growth.
Effect of shelter on the yield and water use of wheat
Authors: Nuberg I. K. , Mylius S. J.
Published: March 2003

Wheat was grown in the field and lysimeters under 3 experimental regimes — full exposure to wind, full shelter within an enclosure, and partial shelter behind an artificial windbreak — to test the hypothesis that a crop in a sheltered environment will be more conservative in its water use and more efficient in using that water to grow biomass. The fully sheltered wheat crop in the field produced 11% more above-ground biomass than the exposed crop and most of this difference was attributed to leaf (20%) and stem (21%) material. However, the sheltered crop had lower 1000-grain weights (35.6 g cf. 40.1 g) and higher protein (14.3% cf. 11.5%). No significant difference between sheltered and exposed yields could be confidently detected.Plants grown under non-water limiting conditions of lysimeters produced 14% more biomass under shelter and were also likely to be more efficient (7%, P = 0.06) in their use of water to produce that biomass than wind-exposed plants. Shelter did not change the total soil water use of the lysimeter- or field-grown wheat. However, the sheltered field crop was more conservative than the exposed crop in its use of soil water up to anthesis and less conservative during grain filling. In the partial shelter regime wheat was grown in the ground and in lysimeters at distances of 3�H (i.e. 3 × windbreak height), 6 H, 12 H, 15 H, 18 H and 24 H from an artificial windbreak. Significant relationships with distance from this windbreak were only observed in total biomass, stem weight and head weight of field-grown wheat at anthesis. In summary, the sheltered wheat was more efficient in production of biomass and did conserve water early in the growing season but the conserved soil water was expended to maintain that biomass at the expense of grain size.
Windbreak research in a South Australian cropping system
Authors: Nuberg I. K. , Mylius S. J. , Edwards J. M. , Davey C.
Published: March 2003

This paper presents the results of 3 years (1994–96) work on the effect of shelter created by a 9 m high Aleppo pine (Pinus halepensis) windbreak in the Mediterranean environment of South Australia on the productivity of wheat, canola, faba beans and oats. Elements of the classic windbreak signature could be detected in the yield responses of 6 of 8 crop years. For example, in the dry year of 1994, yields in the sheltered zone (3–6 H) were 44, 49 and 81% greater than in the exposed zone (18–24 H) for wheat, faba beans and canola, respectively. In 1995, a wet year, only faba beans showed a response (18% increase). In 1996, oats showed an apparent 25% yield increase in the sheltered zone compared with the unsheltered zone. However, these yield differences were not due to shelter alone. For example, covariate analysis of 1995 wheat and bean yield data with soil texture indicated that yield variation across the paddock could be explained by variation in depth to calcareous clay loam. Cumulative water use of wheat was greater in the 3–12 H zone in 1994 (dry year) but less in 1995 (wet year); however, these differences are likely to be also due to soil variation in the 2 plots used. Nevertheless, the windbreak effectively altered the microclimate. Cumulative temperature was measured at 4 distances on the leeward side of the windbreak and used to predict the time taken for wheat to reach anthesis at each location. The correlation between normalised values of observed and predicted times to anthesis was found for 3 wheat crops with different times of sowing in 1995 (r2 = 0.62). It was concluded that (i) shelter will affect the physiology of crops grown in this environment, (ii)�the shelter effect on enhancing crop yield is relatively subtle compared with other sources of yield variation and (iii)�the evidence for a water-saving mechanism in enhancing crop yield is inconclusive.
Effect of windbreaks on potato production for the Atherton Tablelands of North Queensland
Authors: Wright A. J. , Brooks S. J.
Published: March 2003

The effect of windbreaks on the growth and yield of potatoes was measured over a 4-year period. Growth measures included the amount and severity of wind damage to leaves, plant height and number of leaves. Plots were located at various distances from the windbreak in both sheltered and unsheltered positions. The results of this project, while variable both within and between seasons, suggest that windbreaks increase the yield of potatoes between 4.8 and 9.3% for the sheltered portion of the paddock in seasons with higher than average wind speeds. A significant increase in yield has been observed between 3 H and 18 H (where H is the height of the windbreak) in seasons where wind speed was above average, although this result varied within seasons. Increased yield was attributed to a reduction in wind damage to leaves on plants growing in sheltered positions, where it was recorded.
Effect of two tree windbreaks on adjacent pastures in south-western Victoria, Australia
Authors: Bird P. R. , Jackson T. T. , Kearney G. A. , Williams K. W.
Published: March 2003

The effects of 2 tree windbreaks on pasture production in adjoining paddocks were assessed over 4 years in a cool-temperate climate, perennial pasture area in south-western Victoria, Australia. The Willandra windbreak was 2 rows of direct-sown black wattle (Acacia mearnsii) aligned south-east–north-west. The Helm View windbreak was 4 rows of mixed Eucalyptus, Casuarina and Acacia spp., aligned east–west. Pastures at both sites had been sown to perennial ryegrass and subterranean clover. Wind direction data indicated that the north-eastern and south-western paddocks at Willandra were sheltered 39 and 28% of the time, respectively. The northern and southern paddocks at Helm View were sheltered 34 and 42% of the time, respectively. The relationship of rainfall with wind speed and direction was examined to indicate the potential of windbreaks of various orientations to protect livestock.

The windbreaks had clear effects on pasture production in the competition zone along the margins of the windbreak. Average pasture production at Willandra in the zone 0.5–0.75 H (where H is the windbreak height) was 69% of open yield; production in the 0.9–1.5 H zone was 96%. At Helm View, production in the zone 0.7–1.0 H was 74% of open yield, with 81% in the zone 1.1–1.5 H. Averaged over all years, both windbreaks had no effect on pasture growth in the 2–10 H sheltered zone, although in some years there were small significant growth increases at parts of that zone in certain paddocks. Periodic soil moisture measurements at Willandra showed that water use was greater in the competition zone near the windbreak, with a difference in summer water content of the 1.2 m profile of at least 20 mm between soil 4.6 m from the windbreak and further away. The difference was due to greater water use from the deepest part of the profile. No other effects of the windbreak on soil water content were observed.
Effect of synthetic windbreaks on pasture growth in south-western Victoria, Australia
Authors: Bird P. R. , Jackson T. T. , Williams K. W.
Published: March 2003

The impact of a synthetic windbreak on the growth of subterranean clover and perennial ryegrass pasture in the cool-temperate zone of south-western Victoria was investigated over 2 years. Four square plots (10 m sides) at each of 2 sites were fenced with wire mesh 1.2 m tall in 1996 and 1997. Two plots at each site were sheltered with synthetic mesh of 50% porosity attached to the wire mesh. The open wind speed averaged 3.6 m/s in 1996 and 3.1�m/s in 1997. Winds exceeded 6 m/s for 4–23% and 2–8% of the time in 1996 and 1997, respectively.

There were small but significant differences in temperature between the sheltered and open plots. The mean daily temperature was 0–0.4°C warmer with shelter. Temperatures in the shelters were always higher from 0900 to 1800�hours, the differences ranging from 0.1 to 0.9°C. Conversely, lower temperatures (a maximum difference of 0.4°C) usually occurred in the shelters from 1800 to 0600 hours. The mean daily relative humidity was 1.4–3.1% greater in shelter than in the open and the maximum difference was 3.8%.

There was a significant (P< 0.01) effect of shelter on pasture growth in both years. In terms of total production over both years, the results indicate a small but consistent increase in pasture growth of about 9% for sheltered v. open plots at both sites. There was a clear seasonal effect of shelter, with greater production in autumn–winter, and a trend towards greater production in open plots during very wet periods. The results indicate that responses of pasture plants to shelter in the cool-temperate zone of Australia may be modest, and difficult to determine in field experiments, but shelter should contribute significantly to animal production because of improved plant growth in times of scarcity and reduced expenditure of energy for maintenance. Artificial shelters may provide the best means of testing the likely response to shelter of other pasture species or crops.
Effect of artificial wind shelters on the growth and yield of rainfed crops
Authors: Sudmeyer R. A. , Crawford M. C. , Meinke H. , Poulton P. L. , Robertson M. J.
Published: March 2003

There is great interest in quantifying and understanding how shelter modifies crop growth and development under Australian conditions. Small constructed enclosures (shelters) can consistently reduce wind speed, allowing experiments to be run with replicated sheltered and unsheltered treatments in close proximity. The aim of this study was to quantify the effect on microclimate of consistently reducing wind speed by 70% and explain the consequences for dryland wheat (Triticum aestivum), lupin (Lupinus angustifolius) and mungbean (Vigna radiata) growth and development, at sites in Queensland, Victoria and Western Australia. Crops were grown inside and outside of artificial shelters, 10 by 10 m and extending 1 m above the crop canopy throughout the growing season.

Mean daily air and soil temperatures and atmospheric vapour pressure inside the shelters were largely similar to unsheltered conditions. However, clear diurnal trends were evident; daily maximum temperature and vapour pressure deficit (VPD) were increased in shelter when crops were establishing or senescing. When leaf area index (LAI) was reduced in the shelters, soil temperature was greater than in the open, however when LAI was increased in the shelters, soil temperature was less than in the open.

Grain yield in shelters ranged between 78 and 120% of unsheltered yield, depending on seasonal conditions and crop species; the mean yield for all sites, crops and years was 99% of unsheltered yield. In the absence of waterlogging, sheltered crops tended to develop more leaf area than unsheltered crops, with an increase in the ratio of leaf area to above-ground biomass. This greater leaf area did not increase soil water use. While LAI was increased by shelter, only 2 of the 6 sheltered crops that were not waterlogged yielded significantly more grain than the unsheltered crops. This may be because the sheltered crops experienced greater maximum temperatures and VPD during anthesis and grain filling than unsheltered crops. Also, net photosynthesis may not have increased in the shelters after canopy closure (LAI>3–4). Lupins, which developed more leaf area inside shelters, may have experienced strong competition for assimilates between developing branches, flowers and fruit. When rainfall was above average and the soil became waterlogged for part of the growing season, grain yield was reduced inside the shelters. Reduced evaporation inside the shelters may have extended the duration and severity of waterlogging and increased stresses on sheltered plants when potential yield was being set.

The reductions in wind speed achieved inside the artificial shelters were greater than those likely in conventional tree windbreak systems. Analysis of crop growth illustrated that microclimate modification at this high level of shelter can be both beneficial and harmful, depending on the crop species and climatic conditions during the growing season.
Ecology and biogeography of Cassia brewsteri: assessment of potential sites for cultivation
Authors: Cunningham D. C. , Anderson E. R. , Walsh K. B.
Published: March 2003

Cassia brewsteri (F. Muell.) F. Muell. ex Benth. has been identified as a potential multipurpose agroforestry species, and also as a potential source of seed galactomannans (industrial gums). In this study, the natural and cultivated distributions of C. brewsteri were used as a basis for predicting potential sites for cultivation of the tree, using the climate modelling software ANUCLIM. The natural distribution was determined by compilation of data from herbaria and from field studies. The ecological parameters of natural occurrences were characterised in terms of soil and vegetation types by sampling 113 sites throughout the natural distribution in Queensland (18.583–26.150°S, 144.750–152.750°N). In addition to the natural occurrences, a further 11 sites were identified where the tree has been grown successfully in cultivation. Failures of the tree in agroforestry trials in Thailand may be explained by high temperature as a limiting factor in the distribution of the species. Failures of the species in trials in Australia, Zimbabwe and Malawi could not be attributed to climate or soil conditions at the trial sites, indicating that further research on culture of the tree is required to achieve high survival and growth rates.
Fruit viability on grubbed Nassella trichotoma (Nees) tussocks
Authors: Lamoureaux S. , Bourdôt G.
Published: March 2003

An experiment was conducted to determine the influence of time of grubbing in the late spring–early summer, panicle developmental stage at grubbing, and the presence or absence of roots on grubbed plants, on the initial and post-senescent viability of fruits of Nassella trichotoma in North Canterbury, New Zealand. The percentage of fruits viable, following desiccation of panicles removed from plants grubbed in the field, increased from 1% for panicles just beginning to emerge from the leaf sheath, to 49% for panicles fully extended at the time of grubbing. Similarly, as grubbing date was delayed from late November (late spring) until late December (early summer), fruit viability increased from 0.3 to 47%. Overall, 36% of the fruits were viable on panicles at the time of grubbing, increasing slightly to 47 and 44%, respectively, after a period of desiccation with and without the panicles being attached to the grubbed plant's root system. It was concluded that if recruitment of fruits of N. trichotoma to the soil seed bank is to be prevented in North Canterbury, plants must be uprooted before panicle extension. Delaying grubbing to when panicles are fully extended, while possibly enabling a higher percentage of plants to be detected and therefore destroyed, carries with it a high risk of permitting the recruitment of viable fruits to the soil seed bank.
Effects of preharvest drip-irrigation scheduling on strawberry yield, quality and growth
Authors: Kirnak H. , Kaya C. , Higgs D. , Bolat I. , Simsek M. , Ikinci A.
Published: March 2003

Strawberry (Fragaria × ananassa Duch) cultivars, Oso Grande and Camarosa were grown in the field from July 1999 to May 2000 in order to investigate the effectiveness of preharvest drip-irrigation management on fruit yield, quality (i.e. soluble dry matter, fruit size), leaf macro-nutrient composition and normal growth parameters. All plots were irrigated uniformly until 2 weeks before harvest. Differential treatments were then imposed ranging from a complete cut-off of irrigation to full irrigation through the harvest period. Preharvest drip-irrigation management treatments were (i) complete irrigation cut-off, dry (D), (ii) normal irrigation based on class A pan and percentage cover (C), (iii) 75% of normal irrigation, C (IR1), (iv) 50% of normal irrigation, C (IR2), and (v) 25% of normal irrigation, C (IR3). Normal irrigation (control, C) was created by irrigating plants once every 2 days at 100% A pan (Epan) evaporation. No irrigation (D) and IR3 treatments caused reductions in most parameters measured, except water-soluble dry matter concentrations (SDM) in fruit compared with other treatments. There were no significant differences between C, IR1, and IR2 treatments in normal growth parameters or leaf nutrient composition. Fruit size and SDM were both significantly affected by late-season irrigation management; individual fruit weight was significantly reduced and SDM increased even in the IR2 and IR3 treatments compared with control values. Fruit yield was not affected significantly by reduced water application except in the D treatment. These results clearly indicate that reduced preharvest irrigation was partially detrimental; a small reduction in irrigation (IR1) had little or no effect but 50% or less of normal irrigation, while not reducing overall fruit yield, resulted in smaller fruits.
Assessment of Stylosanthes seed yield reduction caused by phytoplasma-associated diseases
Authors: Rue S. J. De La , Hopkinson R. , Gibb K. S.
Published: March 2003

The effect of Stylosanthes little leaf disease on Stylosanthes scabra cv. Seca seed yield was monitored over 2 consecutive seasons at Southedge Research Station in northern Queensland. The time of initial symptom expression was recorded during weekly assessments to determine what effect time of infection after planting had on seed yield. First flowering date was recorded to establish whether the developmental stage of the plant, as indicated by the onset of reproductive activity, at the time of symptom expression influenced the effect of Stylosanthes little leaf disease on yield. At the end of each season, both diseased and asymptomatic plants were harvested and seed yield determined. Seed yield data from the 1999 season showed that there was no significant difference between the mean yield of symptomatic plants, regardless of when they first showed symptoms, and that of asymptomatic plants. However, during the 2000 season plants that showed symptoms early in the season had a significantly lower seed yield than both asymptomatic plants and plants that became diseased later in the season. This decrease in productivity amounted to a yield loss of 98.8 and 56.5% when the plants showed symptoms at 79�and 110 days, respectively, after planting. If plants became diseased within 30 days of first flowering, they did not produce significant amounts of seed. Yield remained low even when the time between first flowering and initial symptom expression increased up to 60 days, after which yield was extremely variable but within the range of that observed for individual asymptomatic plants. It is concluded that Stylosanthes little leaf disease has little or no effect on seed yield if plants have been flowering for about 8 weeks before symptom expression.
Differential accumulation of nutrient elements in some New Zealand mistletoes and their hosts
Authors: Bannister Peter , Strong Graham L. , Andrew Inge
Published: March 2003

It has been generally assumed that differential accumulation of mineral nutrients, leading to greater accumulation of elements in mistletoe tissues, is associated with greater transpiration in the mistletoe than in the host. Only a few investigations have measured both tissue element concentrations and transpiration, or transpiration-related parameters such as carbon isotope ratios (δ13C). Seasonal means for foliar concentrations of Ca, Mg, K, Na, P, N, transpiration and δ13C were obtained from ten mistletoe–host pairs, nine involving the mistletoe Ileostylus micranthus and one with Tupeia antarctica. Annual means of transpiration and δ13C were similar in mistletoes and hosts, but hosts showed greater variation in both transpiration and δ13C than mistletoes. Foliar concentrations of Ca and Mg in mistletoes were similar to those of their hosts, N concentrations were less, and foliar concentrations of K, Na and P were greater in mistletoes than in their hosts. Ratios of foliar concentrations of Ca, Mg and P in mistletoes to those in their hosts (M/H) were greatest when host transpiration was low, when host δ13C was least negative, and when the difference between mistletoe and host δ13C was most negative. The lack of a phloem connection between host and mistletoe, combined with circulation of elements in the host phloem and their transfer into the host xylem, provides a mechanism that explains the accumulation of phloem-mobile elements in the mistletoe.
Comparison of high-light effects with and without methyl viologen indicate barley chlorina mutants exhibit contrasting sensitivities depending on the specific nature of the chlorina mutation: comparison of wild type, chlorophyll-b-less clo f2 and light-sensitive chlorophyll-b-deficient clo
f104 mutants
Authors: Peng Chang-Lian , Gilmore Adam M.
Published: March 2003

This study compared the response to methyl viologen (MV)-induced photooxidation in wild-type barley (wt), and both its chlorina f104-nuclear gene mutant (that restricts Chl a and b synthesis) and its f2-nuclear gene mutant (that inhibits all Chl b synthesis). Without MV, the f2 mutant showed the highest sensitivity to high light, with Fv/Fm being reduced by 80% after 80 min of irradiation. There was little difference in response to high light without MV between f104 and wt. After vacuum infiltration with 100 μM MV and exposure to high light, f104 exhibited the highest sensitivity while f2 was the most tolerant to the photooxidation effects. 77K fluorescence spectral analysis indicated that PSII of f104 was especially damaged, as evidenced by the appearance of a new Chl a emission band around 700 nm at the expense of the F685 and F695 bands from the PSII core-inner antenna. With MV, chlorophyll degraded more rapidly in f104 than in either f2 or wt. During MV treatment, zeaxanthin content increased significantly during the initial period of exposure (0–20 min) in all strains, but decreased sharply in f104 after longer exposure time (20–80 min). β-Carotene, on a chlorophyll basis, was not much changed under high light without MV, but with MV it decreased significantly, mostly in f104, intermediately in f2 and least in wt. We conclude that the light-sensitive chlorosis phenotype of f104 is exacerbated by MV-induced photooxidation.
Enhancement of nodulation in bean (Phaseolus vulgaris) by UV-B irradiation
Authors: Pinto Manuel E. , Edwards Gerald E. , Riquelme Alejandro A. , Ku Maurice S. B.
Published: March 2003

Exposure of bean plants grown in the greenhouse, where UV-B is low, to ambient levels of UV-B light stimulated nodulation more than 2.5-fold. Reduction of UV-B radiation to 3% of ambient levels for outdoor-grown plants through use of Mylar filters consistently reduced nodulation by 45%. The increase in nodulation caused by UV-B was mainly due to an increase in number and size of nodules. The amounts of UV-B-absorbing compounds in roots of UV-B-exposed plants increased almost 5-fold. However, the composition of UV-B-absorbing compounds remained very similar. Exposing leaves to UV-B also significantly increased release of these compounds from roots to the medium. These results suggest that UV-B radiation enhances nodulation, and that a signal may be transported from shoot to roots to play a role in nodulation.
Determination of apoplastic Na+ in intact leaves of cotton by in vivo fluorescence ratio-imaging
Authors: Mühling Karl H. , Läuchli André
Published: March 2003

Salinity may reduce plant growth via Na+-toxicity symptoms in mature leaves after long-term exposure. It has been suggested by other authors that Na+ accumulates in the leaf apoplast and leads to dehydration of leaves, wilting, and finally to death of these leaves. Two methods were employed to determine the Na+ concentration in the leaf apoplast of salt-tolerant cotton plants under salinity. The ratio imaging of sodium-binding benzofuran isophthalate (SBFI) fluorescence was used to detect in vivo concentration changes and gradients of Na+ within the leaf apoplast under salinity stress, and results were compared with the infiltration–centrifugation method. A�significant increase in Na+ concentration was found in the leaf apoplast under salinity (75 mM NaCl), but no further significant increase was determined when NaCl supply was increased from 75 to 150 mM. Both methods revealed that Na+ concentrations remained relatively low, and thus could not be responsible for the decline in yield under salinity. The ratio images showed changes in Na+ concentration and gradients within the leaf apoplast under salt stress, and demonstrated the validity of the method. However, SBFI fluorescence was also influenced by pH, proteins and salt-induced compatible osmolytes.
Sustained downregulation of photosystem II in mistletoes during winter depression of photosynthesis
Authors: Matsubara Shizue , Gilmore Adam M. , Ball Marilyn C. , Anderson Jan M. , Osmond C. Barry
Published: March 2003

Cold acclimation by sustained downregulation of PSII was studied in intact leaves of an Australian mistletoe Amyema miquelii (Lehm. ex Miq.) Tiegh. and its host Eucalyptus. The trends were followed from autumn to spring on leaves that had developed in summer and were exposed to different microclimates with progress of the seasons. In sun leaves of mistletoe, efficiency of excitation energy transfer from light-harvesting pigments to Chl a molecules in PSII core complexes was markedly reduced in winter. Concomitantly, a band in 77K fluorescence emission spectra emerged at 715 nm, in the same way as the cold-hard band found in overwintering snow gum seedlings (Gilmore and Ball 2000, Proceedings of the National Academy of Sciences USA 97, 11 098–11 101). Further, a distinct band, which presumably involves Chl-b-containing antennae complexes, appeared at 705 nm in –2°C fluorescence emission spectra with decreasing intensity of the PSII band. Much shorter PSII fluorescence lifetimes measured in sun leaves of mistletoe that were exhibiting sustained downregulation of PSII indicated enhanced thermal dissipation of excitation energy. Winter acclimation symptoms of the photosynthetic apparatus were more striking in mistletoe sun leaves compared with eucalypt sun leaves. Because the light and temperature environments of sun leaves are similar for the parasite and host, we primarily attribute the enhanced light-acclimation symptoms to the limited photosynthetic capacity of A. miquelii in winter.
Energy dissipation in C3 plants under drought
Authors: Flexas Jaume , Medrano Hipólito
Published: March 2003

A general quantification of the relative contribution of different light energy dissipation processes to total dissipation under different drought conditions is lacking. Here we compare six studies, including enough data for such a general quantification, to build up a general pattern of the relative importance of several energy dissipation mechanisms in response to drought in C3 plants. Such a general pattern apparently emerges independently of specific acclimation to drought, but largely dependent on CO2 availability in the chloroplasts, which may be regulated under drought by adjustments in stomatal and mesophyll conductances. Under irrigation and saturating light, more than 50% of absorbed light is thermally dissipated, while photosynthesis dissipates 20–30% and photorespiration 10–20%. Under mild drought, the contribution of photosynthesis decreases, and that of photorespiration increases in a compensatory manner. During moderate to severe drought, the contribution of both photosynthesis and photorespiration decreases, and thermal dissipation increases up to 70–90% of the total light absorbed. The contribution of other processes, like the Mehler reaction, is shown to be very low under both irrigation and drought. Therefore, in C3 plants subjected to different degrees of drought, more than 90% of the total energy absorbed by leaves is dissipated by the sum of thermal dissipation, photorespiration and photosynthesis.
Effects of drought on light-energy dissipation mechanisms in high-light-acclimated, field-grown grapevines
Authors: Medrano Hipólito , Bota Josefina , Abadía Anunciacion , Sampol Bartolomé , Escalona José M. , Flexas Jaume
Published: March 2003

The response of several light-energy dissipation mechanisms to water shortage was analysed in a 10-year study in field-grown, high-light-acclimated grapevines, and compared with those of greenhouse-grown, low-light-acclimated grapevines. Dissipation mechanisms, except leaf photochemistry, differ among cultivars and acclimate to the prevailing light conditions during growth. However, no additional acclimation to drought was observed. The dependence of the dissipation responses on stomatal conductance suggests that low CO2 availability in the chloroplasts during drought triggers variations in the energy dissipation pattern. In irrigated grapevines under high light, more than 50% of total absorbed energy is thermally dissipated. There is evidence that implicates the xanthophyll cycle as the main thermal dissipation processes. CO2 assimilation is the most important photochemical pathway of dissipation in irrigated plants, but is replaced by photorespiration when CO2 assimilation declines under mild drought. Under moderate to severe drought, both photosynthesis and photorespiration decline, and thermal dissipation increases to account for up to 90% of total dissipation. Involvement of other processes in light dissipation is minimal in grapevines. Even in severely-stressed leaves, the incidence of photoinhibition is very low, indicating that safe dissipation of absorbed energy is very effective in grapevines.
Structural and functional role of the PsbH protein in resistance to light stress in Synechocystis PCC 6803
Authors: Bergantino Elisabetta , Brunetta Alessia , Segalla Anna , Szabò Ildikò , Carbonera Donatella , Bordignon Enrica , Rigoni Fernanda , Giacometti Giorgio M.
Published: March 2003

Four mutants of the cyanobacterium Synechocystis sp. PCC 6803, carrying a modified PsbH subunit on a PSI-less background, were characterized by optically-detected magnetic resonance (ODMR), electron transport kinetics, and oxygen-evolving activity. Their relative tolerance to light stress was measured. Results indicate that: (i) the PsbH protein is deeply involved in determining structural and functional properties of the QB site on the D1 protein, whereas the environment of the primary donor P680 and its acceptors pheophytin and QA are not significantly affected by modifications of this subunit or its deletion; (ii) the charge recombination rate, in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), is reduced by a factor of 2, independently of the particular modification. The same result is found with the strain in which the subunit has been deleted. This result is taken as an indication that PsbH is important in regulating protein dynamics of the entire PSII core complex; (iii)�all investigated mutants display reduced tolerance to light stress, the extent of which depends on the particular modification. In this respect, mutations introduced in the transmembrane portion of the polypeptide are more effective than those involving the extramembrane N-terminal extension.
Cyanogenic polymorphism as an indicator of genetic diversity in the rarespecies Eucalyptus yarraensis (Myrtaceae)
Authors: Goodger Jason Q. D. , Woodrow Ian E.
Published: March 2003

The rare Australian tree Eucalyptus yarraensis Maiden & Cambage is cyanogenic, a quantitative trait potentially indicative of genetic diversity. Cyanogenic plants are capable of releasing cyanide from endogenous cyanide-containing compounds. Cyanide is toxic or deterrent to generalist or non-adapted specialist herbivores. Consequently, cyanogenic plants are afforded an effective means of chemical defense. In this paper we characterize quantitative variation in cyanogenic capability, known as cyanogenic polymorphism, in E. yarraensis for the first time. We show that the cyanogenic glucoside prunasin (R-mandelonitrile-β-D-glucoside) is the only cyanogenic compound in E. yarraensis foliage. We also show that two natural populations of E. yarraensis display extensive intra- and inter-population variation in foliar prunasin concentration. The high prunasin concentrations reported in this paper represent the highest yet recorded for mature eucalypt leaves. The cyanogenic variation could not be attributed to measured physical and chemical parameters, supporting the hypothesis that the variation is genetically based. A preliminary progeny trial also supports this hypothesis, with narrow sense heritability estimated at 1.17 from three half-sibling families. The variation in cyanogenic capability may be a useful tool in the development of a conservation strategy for the species.
Fructan formation in transgenic white clover expressing a fructosyltransferase from Streptococcus salivarius
Authors: Jenkins Colin L. D. , Snow Annette J. , Simpson Richard J. , Higgins Thomas J. , Jacques Nicholas A. , Pritchard Jenifer , Gibson Jenny , Larkin Philip J.
Published: March 2003

White clover (Trifolium repens L.) is an important pasture legume that does not normally accumulate fructan as a storage carbohydrate. We have generated transgenic white clover plants that accumulate fructan, by expressing the fructosyltransferase (Ftf) enzyme from the bacterium Streptococcus salivarius under the control of the cauliflower mosaic virus (CaMV) 35S promoter. Fructan accumulated in leaves, petioles, stolons, flowers, and roots of transgenic plants. Levels of fructan up to approximately 2% dry weight were measured in leaves. The fructan was of high molecular mass ( > 5000 kDa), typical of bacterial fructans. Ftf enzyme activity up to 120 nmol min–1 g–1 fresh weight was determined in leaf extracts of the transformed plants, and appeared to be stable throughout leaf development. Most transformed lines appeared normal, flowered and produced seed, but the growth rate of some transformed lines decreased. Photosynthetic carbon assimilation and levels of endogenous carbohydrates (hexoses, sucrose and starch) were not substantially changed in a clonal line with relatively low fructan. However, in a clonal line with relatively high fructan accumulation, plant growth was reduced, leaf photosynthesis was decreased by 60%, and carbohydrate contents were reduced. The results are discussed in the context of manipulating soluble carbohydrate composition in pasture species to improve nutritive quality for grazing animals.