Yield and vegetative growth as related to plant water potential of cotton irrigated with a moving sprinkler system at different frequencies and wetting depths

Seed-cotton yield, yield components and vegetative growth were determined under different irrigation frequencies and wetting depths with a self-propelled moving-irrigation-system (MSIS) in 1986 and 1987. Irrigation timing was determined in both years by pre-irrigation, mid-day plant water potential (ψ_w). The amount of water to be applied was determined by measuring the soil moisture deficit. In 1987, the effect of a change from one irrigation frequency and wetting depth to another at mid-flowering was also examined. Linear responses of relative seed-cotton yield to the amount of evapotranspiration (ET) were found for both years with similar slopes but different intercepts. Significant positive regressions were obtained between pre-irrigation plant ψ_w and relative seed-cotton yield, and vegetative growth during the linear growth stage. Seed-cotton yield was affected by both wetting depth and pre-irrigation plant ψ_w. The deeper the irrigation the higher was the seed-cotton yield for each pre-irrigation plant ψ_w. Irrigation frequencies which maintained plant ψ_w above -1.5 MPa during vegetative growth, flowering and boll-filling resulted in maximum production. The boll filling stage appeared to be a very sensitive one, as boll weight was found to be the main yield component responding to irrigation treatments. At a wetting depth of 120 cm, higher seed-cotton yields were obtained than at a more shallow wetting. Different irrigation managements resulted in different turgor potentials (ψ_t) mainly during mid-day. Both leaf water vapour conductance and net assimilation rate were sensitive to leaf ψ_w.