Plant indicators of wheat and soybean crop water stress

Summary

The onset of water stress within a crop is defined as the time at which the rate of water loss declines below that of a well watered crop in the same locality. The relation to the onset of water stress and soil water status of several readily measured plant parameters was investigated in crops of wheat and soybeans over three years. Evapotranspiration ET was monitored with weighing lysimeters. A noticeable decline in the rate of ET for both wheat and soybeans was detected once 20% to 30% of the total plant available water PAW remained in the 1 m deep lysimeter soil profile. Extension growth of wheat declined when PAW was 33% and 34% in two years of measurement. In soybeans, the decline in the rate of leaf extension coincided with the decline in the rate of ET. Midmorning measurement of exposed leaf water potential ψ_L, covered leaf water potential ψ_CL and covered plant leaf water potential ψ_CP yielded similar results for both wheat and soybeans. Day-to-day variability was least in ψ_CP and most in ψ_L. Values of ψ_CP, ψ_L and ψ_CL decreased rapidly with PAW < 30%. Daily values of leaf diffusive conductance were variable but there was a general decline in conductance with PAW < 30%. It is suggested that ψ_CL may be the easiest and most reliable parameter to monitor as a means of detecting the onset of stress. The results indicated that PAW levels in the root zone of 50% for wheat and 30% for soybean probably do not affect extension growth or plant water status parameters and can thus be used as criteria for irrigation scheduling.