Water is one of the most limiting factors to Australian cotton production. Improved irrigation scheduling efficient water use is central to the sustainability of the Australian irrigated cotton industry. Irrigation scheduling is a two-fold process where-by the amount and frequency of water applied to a plant is determined. Producers must aim to optimise crop water use through timely irrigation scheduling and efficient utilisation of in-crop rainfall. Currently, furrow irrigation is the dominant form of irrigation delivery and cotton farmers use a limited range of methods to make irrigation decisions. A combination of the cost, accuracy and complexity of these methods has limited their effective use in commercial production. In this study a potentially simpler method based on crop canopy temperatures and the thermal optimum concept was investigated.
Compared to well-watered plants, water stressed plants exhibit elevated canopy temperatures. This is a consequence of the closing of stomata, in response to soil water deficits. The closure of stomata results in a decrease in transpiration and consequently a reduction in latent energy flux, leading to a rise in canopy temperatures. However, ambient conditions can have a large influence on canopy temperatures; thus canopy temperatures are a reflection of both plant and environmental factors. In order to develop indicators of the early onset of water and temperature stress, research conducted in the USA developed a theory that defined optimal plant temperatures with respect to the thermal dependence of the Michaelis-Menten constant of an enzyme (Km). The optimal enzymatic function was restricted to a range of ambient temperatures that was termed the thermal kinetic window (TKW), which is an indicator of the optimal temperature range of a plant species. Using alternative diagnostic methodologies of chlorophyll fluorescence recovery rates and analysis of plant physiological function under field conditions, the optimal temperature of an Australian cultivar was identified to be ~28 °C. Although this was consistent with values obtained from US cotton cultivars, and average day-time canopy temperatures that were achieved in the field at close to optimal water applications, it was important to verify this as Australian cotton cultivars are genetically different to US cultivars and the combined effect of different genetics and ecological adaptations may potentially influence the optimal temperature of biochemistry.
The TKW theory was used as the basis for the BIOTIC (Biologically Identified Optimal Temperature Interactive Console) protocol. This protocol was developed by researchers at the USDA-ARS, and uses the relationship between canopy temperature (Tc) and plant water status to schedule irrigation using a temperature-time threshold system. Irrigations are commanded when the crop’s Tc exceeds an optimal temperature threshold for a pre-determined period of time. Using the BIOTIC system as a basis, this study aims to assess the physiological base and utility of the thermal optimal approach to schedule irrigation, with particular emphasis on its use in precision application and large soil water deficit irrigation systems of the Australian cotton industry. Deficit irrigation is an optimisation strategy where full crop water requirements are not necessarily provided, improving water-use efficiency (WUE). The thermal optimal approach was studied previously; however, its use was limited to irrigation systems that provide full water requirements at high irrigation frequencies and low irrigation volumes. Hence, its application to deficit and furrow irrigation systems was unknown.
The physiological basis of the principles underlying the thermal optimum concept for irrigation scheduling was examined through the monitoring of Tc of the commercial cotton cultivar Sicot 70BRF at ‘Myall Vale’ Narrabri Australia. Surface drip irrigation experiments were conducted in the 2007/08 and 2008/09 seasons, where irrigation treatments were based on daily crop evapotranspiration (ETC) rates calculated using the FAO56 protocol with a locally calibrated crop coefficient. A furrow-irrigated experiment was conducted in the 2008/09 season, where irrigation treatments were based on plant available soil water deficits (mm) from field capacity calculated from neutron attenuation data. The objectives of this research were to: (1) confirm that the optimum temperature (Topt) of a current commercial Australian cotton cultivar (Sicot 70BRF) is the same as other measured USA cotton cultivars; (2) determine if Tc can define plant water stress by comparison with soil and atmospheric conditions; and (3) determine the potential of the thermal optimum approach to scheduling irrigation in Australian cotton systems.
The hypothesis that Tc provides sufficient information for irrigation scheduling was investigated in the surface drip and furrow irrigated cotton. Irrigation treatments resulted in differences in lint yield, plant architecture, growth, biomass accumulation and Tc. Canopy temperatures were correlated with crop lint yield and the volume of water applied to the crop. Peak lint yields occurred at average day-time (Rn > 300 W m-2) Tc of 26.4 ± 1.7 °C and total water of 108% calculated ETC under surface drip conditions, and at Tc of 28.6 °C ± 0.6 °C and water supplies of 99% calculated ETC under furrow irrigated conditions. Acclimation of Tc due to the wetting and drying cycles of furrow irrigation did not occur and the combination of both furrow and drip irrigated data showed a single relationship where peak lint yields occurred at Tc of 28 °C. This highlights the benefits of maintaining average canopy temperatures close to 28 °C, and supports the potential utility of the thermal optimum concept in Australian drip and furrow irrigated cotton.
Although lint yield is proportional to the thermal optimum, the physiological limitations of a plant can mean that a well-watered plant’s Tc can still exceed the thermal optimum. This gives rise to the stress time (ST) concept, where ST represents the average daily period of time that a well-watered crop’s Tc can exceed its optimum temperature. The ST concept was tested and adapted to Australian field-based drip and furrow irrigation systems. Peak lint yields and crop WUE (the ratio of lint yield produced per hectare to the cumulative amount of water used by the crop through evapotranspiration) in drip-irrigated cotton occurred at 4.5 h ST, considerably higher than the empirically calculated threshold of 2.8 h. A thermal optimum protocol was developed to schedule furrow irrigation events through a cumulative ST approach, where one ST h represents 0.6 mm plant available soil water depletion, enabling a producer to determine the desired soil water deficit and schedule irrigations based on cumulative ST. An integrated approach to stress detection was also proposed. This approach, the sum of cumulative ST, is theoretically advantageous as it considers both the degree and duration of time Tc exceeding the optimum.
The physiological principle underlying a thermal optimal approach to irrigation scheduling were analysed in this thesis. An independently estimated optimal temperature was determined to be 28 °C. This optimal temperature was correlated with peak lint yields, and Tc was responsive to irrigation. A stress time threshold producing peak lint yield was developed in surface drip irrigation systems, and a cumulative stress time threshold for soil water deficits was outlined for furrow irrigation systems. These modified stress time thresholds provided the information required to detect water stress for irrigation scheduling. The practical implication of this research is that temperature-time thresholds in a thermal optimal irrigation scheduling system have utility in the irrigated Australian cotton industry. However, the time thresholds that were determined in this study were developed by monitoring cotton crops with infra red thermometers, and irrigations were not scheduled with a thermal optimum protocol in this study. With field validation, these irrigation protocols could be used as the basis for a modified BIOTIC system and be adopted by the commercial cotton industry, as it is a simple, cost effective irrigation scheduling system.