Field Studies of Cereal Leaf Growth IV. WINTER WHEAT LEAF EXTENSION IN RELATION TO TEMPERATURE AND LEAF WATER STATUS

Throughout winter and early spring, rule and auxanometer measurements showed that leaf extension rate (Rf) was directly related to temperature and stopped at about 0 °C. During this period, both night and day time RE responded similarly to temperature. Bright sunshine in late April and May caused fast transpiration which was associated with low leaf water potential (y/) and slow Re. When bright sun was obscured by cloud, RE increased but this did not compensate for previous slow Re. Leaf turgor potential, calculated as the difference between y/ and leaf osmotic potential, was large (0-6-1-8 MPa) and bore little relation to RE. Low yt was associated with slower RE than would have been expected without water stress, but the relation was not unique. On a bright day in May, adaptation to low y/ occurred and during the afternoon RE was faster than at similar values of y/ and meristem temperatures before noon. The response of RE and duration of leaf extension to temperature suggested that for any particular leaf grown under field conditions, variation in mean growing temperature would affect final leaf length only slightly. Because severe water stress slows Re without affecting the duration of leaf extension markedly, it decreases final leaf size. INTRODUCTION The previous paper in this series showed the importance of temperature and leaf water potential in governing the rate at which leaves of spring barley extend (Gallagher and Biscoe, 1979). The experiment on winter wheat described in this paper was designed to investigate leaf extension at low temperatures. In addition, more detailed measurements of leaf water status enabled the influence of turgor potential on extension rate to be studied. MATERIALS AND METHODS The winter wheat crop (Triticum aestivum, var. Huntsman) was sown on 30 October 1974 at a rate of 14 g m'2 (field) with 35 g m-2 of a 30:0:0 fertilizer applied on 1 May 1975. Further details about the site, crop, and weather have been published (Gallagher, 1979a). The auxanometers used in this study and the rule measurement technique were described by 1 Present address: Botany Department, Rothamsted Experimental Station, Harpenden, Herts. AL5 2JQ, England. 2 Present address: Broom's Barn Experimental Station, Higham, Bury St. Edmunds, Suffolk, England. 3 Present address: Institute of Hydrology, Crowmarsh Gifford, Wallingford, Oxfordshire, OXIO 8BB. This content downloaded from 207.46. 13.128 on Wed, 07 Sep 2016 06:32:10 UTC All use subject to http://about.jstor. org/terms 658 Gallagher, Biscoe, and Wallace—Cereal Leaf Growth Gallagher, Biscoe, and Saffell (1976). Frequency of rule measurements was determined by the time taken for leaves to extend about 3 mm, i.e. 2-4 d during winter to twice daily in late spring. As well as detailed measurements of soil and air temperatures (Gallagher and Biscoe, 1979), plant tissue temperatures were monitored with thermocouple thermometers. These were made from 0-2 mm diameter copper and constantan wire, varnished to prevent corrosion, and referenced to the constant temperature block of a computer-controlled data-logging system. Meristem temperature was measured by inserting thermocouples between the leaf sheath adjacent to the position of the apical meristem and the main body of the stem. Three of these thermometers were used from 1 May until 31 July and the tissue temperatures (Tt) recorded usually agreed to within ±0-5 °C. During December and January, when the data logger was not functioning continuously, mean air temperature recorded at a meteorological station 1 km south of the field site was assumed equal to mean air temperature of the plants between rule measurements. Over periods of a few days, differences between these two mean air temperatures were usually less than 1 °C (Gallagher, 1976). Leaf water potential (y/) was measured using a pressure chamber and leaf osmotic potential (y/s) with a Wescor dewpoint hygrometer (Campbell, Campbell, and Barlow, 1973). Leaf turgor potential (t//p) was estimated as the difference t//—y/s. Further details about the apparatus and technique were given by Biscoe, Cohen, and Wallace (1976). Because of limitations of time and equipment it was only possible to measure 2-hourly mean values of y/s. Estimates of crop evaporation rate were made using a Bowen ratio micrometeorological method (Biscoe, Clark, Gregson, McGowan, Monteith, and Scott, 1975). The relative contribution of laminar, sheath, and internode extension to that measured by the auxanometer was described in detail by Gallagher (19796). For leaves 1-7 extension measured by auxanometer or rule consisted almost entirely of laminar extension; for leaves 8-10 about 50% of the measured extension was attributable to the sheath and for leaves 11 and 12 laminar, sheath and internode accounted for about 25, 35, and 35% respectively of total measured extension. RESULTS Cool temperatures From December until March, rule measurements of the youngest main stem leaves showed a close relation between RE and temperature (Fig. 1). During this period leaves 2-6 extended. Although it was impossible to make any rule measurements for Re calculation on any one leaf, RE could still be related to temperature for individual leaves. Figure 2 shows a typical example, for leaf 4, in which RE increased linearly with temperature. However, for any one leaf, the range of