A mechanical auxanometer, suitable for measuring hourly rates of leaf extension of grass and cereal crops, is described. Several of these instruments were used to monitor leaf extension rates (RE) of a spring barley crop. RE of main stem and first leaf tillers responded similarly to environmental factors. During May, when soil water deficits were less than 50 mm, and on dull days later in the season, RE was directly related to meristem temperature with night and day measurements responding similarly. During the central 10 h of bright days in late May and June, RE was unrelated to temperature but slowed during bright sunshine and accelerated at the start of cloudy periods. Pressure chamber measurements of total leaf water potential (y) showed that bright sun caused i// to decrease rapidly and that this was associated with slow RE. Analysis of 2 h mean values of w and jRe indicated that, at any given temperature, RE slowed in direct proportion to decrease of i//. INTRODUCTION The first paper in this series (Gallagher, 1979a) considered how the growth rate of a crop was likely to be restricted if its leaf area was too small for full light inter ception. In temperate climates, temperature is often thought to be the primary determinant of leaf area expansion rate but controlled environment experiments have indicated that leaf extension rate also slows at apparently low levels of water stress as measured in terms of leaf water potential (Boyer, 1970; Acevedo, Hsiao, and Henderson, 1971). However, Watts (1974) working in the field was unable to demonstrate a consistent relation between leaf water potential and extension rate. To determine what factors limit the expansion of leaves on a crop plant it is important to work in the field but, as weather fluctuates rapidly, leaf extension must be monitored over short periods. Auxanometers registering hourly rates of leaf extension provide a practical solution to this problem. This paper reports the results of field experiments in which the extension rates of spring barley leaves were related to temperature, irradiance, and leaf water potential. 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. This content downloaded from 207.46.13.12 on Thu, 12 May 2016 04:37:32 UTC All use subject to http://about.jstor.org/terms 646 Gallagher and Biscoe—Cereal Leaf Growth MATERIALS AND METHODS The spring barley crop (Hordeum distichum var. Proctor) was sown on 18 March 1972 at a seed rate of 15 g m-2 with 25 g m-2 of a 20:10:10 fertilizer applied to the seed bed. Further details about the site crop and weather have already been published (Biscoe, Clark, Gregson, McGowan, Monteith, and Scott, 1975). Air temperature was measured at 15 levels between 0-2 and 2 m above the ground (Biscoe et ai, 1975). Unfortunately, thermistor thermometers installed to measure meristem temperatures proved unreliable. When the meristem emerged from the soil its temperature was assumed to equal that of the nearest measured canopy air temperature. When canopy temperatures were not available, meristem temperature was assumed to equal 2 m air temperature. Subsequent studies on winter wheat showed that differences between hourly mean meristem, canopy, and 2 m air temperatures are seldom greater than 2 °C (Gallagher, 1976). Total solar radiation above the canopy was measured with a Kipp solarimeter which was recorded every 5 min. Pen arm Adjustable counterweight