Environmental effects of Protective Cropping Systems on cherry tree ecophysiology and fruit quality
Creators: name: Stone, Cameron
Publisher: name: University of Tasmania
Year: 2024

description: Protected cropping systems (PCS) are commonly used in intensive horticulture, and particularly in sweet cherry (<i>Prunus avium</i>) production to alleviate the risk of cherry cracking caused by late season rainfall. Coupled with advances in tree training systems that can be optimised for different climatic conditions and tree growth habits, the advent of modern, vented PCS provides new growing environments that, if properly understood, will enable quantum shifts in fruit quality and marketable yield. This study examined the microclimates under vented PCS and the effects on tree water uptake and fruit quality along with the influence of different planar canopy training systems on light interception and penetration, and fruit quality.<br>Research was conducted over the 2019-20 and 2020-21 seasons on commercial orchards in Tasmania, Australia. In the first season daily tree sap flow was measured using the compensation heat-pulse method with sap flow sensors set up in ‘Lapins’ sweet cherry trees at various locations under a vented rain cover PCS and a bird exclusion netted block at Grove in the Huon Valley. Climate data (temperature, relative humidity, solar radiation and wind speed) and tree sap flow were measured every 30-minutes over the growing season. Analysis of the influence of individual climate predictors on tree sap flow was undertaken using generalised additive models (GAMs). This form of modelling enabled the investigation of each climate predictor (temperature, relative humidity, solar radiation and wind) individually, while keeping all others constant, revealing their individual effects on tree sap flow rates and estimates of tree water uptake. In the second season, sap flow and climate data were measured at 30-minute intervals at four separate locations of varying distances and elevations from the PCS boundary under a large (21 ha) PCS at Rosegarland in the Derwent Valley.<br>Average and maximum temperatures in the first season at Grove under vented PCS were higher (14.7°C and 22.9°C) in contrast to netted (13.9°C, 21.3°C), while total daily solar radiation and average wind speed were consistently lower (12.9 MJ m<sup>-2</sup> and 0.2 m s<sup>-1</sup>) in the vented PCS block than the netted block (13.7 MJ m<sup>-2</sup> and 0.9 m s-<sup>1</sup>). Time alone in the model accounted for approximately 90% of tree water use, however the GAMs highlighted the influence of PCS on the buffering of wind speeds and increasing average and maximum daily temperatures which impacted average daily tree water uptake rates. A three-fold reduction in daily tree water uptake was observed under PCS (3.9 L day<sup>-1</sup>) compared to bird netting (12.2 L day<sup>-1</sup>). Modelling of individual climatic predictors revealed that tree water uptake increased exponentially with increasing temperature up to 40°C in both the PCS and netted blocks. However, tree water uptake was three times greater in the netted block in contrast to PCS at the same temperature. Similarly, solar radiation responded in a parabolic fashion in both the PCS and netted block with maximum tree water uptake occurring at approximately 1.2 MJ m<sup>-2</sup> before declining at higher light intensities. The GAMs also highlighted that whilst RH less than 60% had little effect on tree water uptake, a significant reduction in water uptake occurred when RH levels were greater than 60%. A parabolic response was also seen for water uptake under PCS with increasing wind speeds up to 2 m s<sup>-1</sup>, above which water uptake declined. In contrast there was little to no response in water uptake for trees under bird netting with increasing wind speeds, probably due to acclimation to the relatively exposed conditions. Very poor fruit set did not allow for investigation of the influence of these findings on fruit quality. This and further investigation of the critical role of wind in tree water uptake became a focus for further study in the 2020-21 season.<br>In the second season the effect of a large 21 ha vented rain cover PCS on tree water uptake and fruit quality for the cultivar ’Staccato‘ grafted to ‘Colt’ rootstock at various elevations and proximities to the PCS boundary was examined at Rosegarland in the Derwent Valley, Tasmania. A parabolic response was found for water uptake with increasing solar radiation up to 1.2 MJ m<sup>-2</sup>, above which the rate of predicted water uptake decreased when all other climate predictors were held constant. In contrast water uptake responded linearly with temperatures up to maximums of 40°C. Our results indicate that light levels, and not temperature, may be the dominant cue for stomatal closure. Locations furthest from the PCS boundaries were warmer with reduced wind speeds in contrast to locations closer to the PCS boundary and a correlation was observed with some fruit quality parameters. Fruit at higher elevations above sea level (asl) and furthest from the block boundary (125m asl and 105 m respectively) were significantly firmer (343.2 g mm<sup>-1</sup>) and had greater dry matter content and total soluble solids (19.2% and 18.5 °Brix) compared to fruit at lower elevations (102m asl) and within closer proximity to block boundary (50m) (16.2% and 15.5 °Brix). Therefore, higher average temperatures further from the PCS boundary were favourable to the development of fruit quality and not excessive or detrimental to fruit quality.<br>In a separate study, the effects of planar training systems on light interception and penetration, yield, and fruit quality for the lateral bearing cultivar ‘Kordia’ grafted to Krymsk 5 were investigated in a two-year study at a commercial orchard at Jericho in the southern midlands of Tasmania. Using a ceptometer (Accupar LP-80), a plant canopy analyser (LAI 2200C) and quantum sensors (Apogee SQ 420) this study investigated the effect of light interception and penetration on fruit quality at various heights in the canopy of the following training systems: Bibaum (BB), steep leader (SL), super spindle axe (SSA), tall spindle axe (TSA) and upright fruiting offshoot (UFO).<br>Average light interception over the two seasons was highest for the UFO training system (69%) followed by the dual leader BB system (66%). The highest yielding training system was the SSA (15.1 t ha<sup>-1</sup> ) followed by the SL and UFO training systems (14.5 and 12.7 t ha<sup>-1</sup> respectively). Higher crop loads were negatively correlated with fruit dry matter content (r<sup>2</sup> =0.67 and 0.84) and total soluble solids (r2 =0.92 and 0.42) in each season. Fruit quality results from this study did not highlight an overall advantage of any particular training system for five-year-old trees of the lateral bearing cultivar ‘Kordia’. However, results indicated that the BB, TSA and SL training systems provided sufficient space between trees or branch uprights within a tree for future lateral wood production and fruiting sites compared to the SSA and UFO training systems where there was greater competition for space and light.<br>Further research in the second season highlighted differences in light penetration at different heights within the canopies of the various training systems. As expected, light penetration on average was highest at the top of the canopies (2 m) for each of the training systems (52% or 1033 μmol m-2 s<sup>-1</sup> ) followed by mid canopy (34% or 658 μmol m-2 s<sup>-1</sup> at 1.2 m) and finally low in the canopy (20% or 400 μmol m-2 s<sup>-1</sup> at 0.4 m), but fruit quality differed significantly between low (0.4 m), medium (1.2 m) and high (2 m) canopy heights. Independent of training system, it was observed that trees flowered earlier lower in the canopy. Average fruit weight (12.1 g), TSS (19.3 °Brix) and stem pull force (1082 g) were significantly higher in fruit low in the canopy where average light penetration levels were lower yet still adequate for fruit development, in contrast to fruit at the top of the canopy that were under higher light environments (11.3 g, 18.1 °Brix and 999 g).<br>Results from this thesis highlight the significant impact of novel microclimates created by PCS on tree water uptake and fruit quality in a commercial orchard. The buffering of winds, attenuation of solar radiation and increases in average temperatures and relative humidity under vented PCS resulted in a significant reduction of tree water uptake in contrast to under bird netting. Similarly, elevation and distance from the nearest PCS boundary impacted orchard microclimates and subsequent fruit quality. Locations furthest from the block boundary that had higher elevation and were warmer throughout the season had more favourable growing microclimates and resulted in better quality fruit in contrast to lower elevation locations closer to the block boundary. Additionally, this study has highlighted that appropriate space is required between tree planting distances or within training system fruiting limbs for lateral bearing cultivars when trained to a planar system. Fruit quality, light interception and penetration improved when adequate space between branches was available, supporting lateral wood development for flowering and fruiting. While seasonal variations (i.e. climate and crop load) had a significant impact, fruit quality noticeably improved with greater canopy light interception.