Water status response of corn and cotton to altered irrigation

Water is a primary limiting factor to crop production and thus crop water status is essential information for management decisions. Corn and cotton were grown in the field under two constant water regimes. The low water level (WL) was 0.662PET (potential evapotranspiration) in corn and rainfall for cotton. The high water level (WH) was 1.02PET for both crops. Two transient water treatments in each crop began as the two constant water level treatments but then the water inputs were reversed and the change in water status was monitored. When the transient water treatments were initiated, corn was at the V14 and V16 growth stages in the WL and WH treatments, respectively, and cotton was 2 weeks past first bloom for both water levels. The purpose of the experiment was to compare the sensitivity of leaf water potential (LWP) and crop canopy temperature to changes in irrigation rate. The transient water treatment of each crop that relieved water stress (TLH) changed from WL to WH and the treatment which induced water stress changed from WH to WL (THL). The LWP values of the transient water treatments reversed 5 and 8 days after reversing water input rates to corn in 1998 and 1999, respectively, and after 3 days in both years for cotton. A reversal in canopy temperatures, expressed as the amount of daily time that the temperature was above 28°C (DST), was not detected between the TLH and THL treatments of corn after 25 days in 1998 or after 13 days in 1999. The DST values of the cotton transient water treatments reversed after 4 days in 1998 and 5 days in 1999, when the values of THL became greater than for TLH. Corn tassels, which apparently transpire less than leaves, were forming at the beginning of the transient water treatments and their presence in the view of the infrared thermocouples may have reduced the apparent radiometric temperature difference between the transient water treatments. During the water status adjustment period following the initiation of the transient water treatments, there were significant linear relationships between LWP and DST in cotton in both years but only in 1998 in corn. Cotton canopy temperature could be used to rapidly monitor an entire field in contrast to LWP which accurately measures plant water status but cannot provide automated measurements across a large area.