Diurnal variation in leaf and boll temperatures of irrigated cotton grown under two soil moisture regimes in a semi-arid climate

Abstract

A procedure using thermocouples is described for obtaining continuous records of mean leaf and boll temperatures of cotton plants growing under field conditions. Results are shown for cotton plants growing in Australia under furrow irrigation in a semi-arid climate, mainly during cloudless days in summer, when peak short-wave radiation ranged from 75–88 cal/cm 2 h and the mean maximum ambient air temperature was 33°C. The effect of withholding irrigation water upon leaf and boll temperature was examined. In five experiments with the Empire variety, mean leaf temperature ranged from a pre-dawn minimum of 9.3°C to a daytime maximum of 29.0°C for irrigated plants, and from 9.4–33.3°C, for wilted, non-irrigated plants. The variety Pima, showed a similar fluctuation in leaf temperature and temperature response to water shortage. More limited results for bolls of Empire, gave ranges of 16–29°C for irrigated plants and 15–37°C for wilted plants, suggesting boll temperature is more sensitive to soil moisture stress than is leaf temperature. Leaves low down in the canopy of irrigated plants were cooler than the upper leaves, during the day, whereas on wilted plants the lower leaves were approximately the same temperature as the upper ones during the day. The differences between leaf or boll temperature and ambient air temperature were calculated. In both irrigated and non-irrigated treatments, leaf and air temperatures were similar during the early morning. Around mid-day and in the early afternoon, the leaves of wilted plants became warmer than the ambient air, whereas this was only the case for leaves of irrigated plants on cool or warm days. On hot or very hot days, irrigated leaf temperature either approximated to ambient air temperature or was less than it. Bolls of irrigated and wilted plants were respectively, cooler and warmer than ambient, during the central part of the day. In the irrigated and non-irrigated treatments, leaf and boll temperatures showed a marked fall relative to ambient air temperature, during the afternoon. As a result, leaves and bolls were appreciably cooler than the ambient air by early evening. The diurnal variation in the difference between ambient vapour pressure and that of water in the leaves of irrigated or non-irrigated plants, is shown for three experiments, and its significance is discussed.