CROP WATER-STRESS INDEX FOR SEED ALFALFA - INFLUENCES OF WITHIN-SEASON CHANGES IN PLANT MORPHOLOGY

A field study was conducted to determine the utility of the crop water stress index (CWSI) for assessing plant water status of seed alfalfa (Medicago sativa L.). Six irrigation treatments consisting of combinations of three rates of irrigation and two irrigation frequencies were used in assessing the influence of vegetative, flowering, and pod-filling periods on non-stressed canopy-air temperature differences versus vapour pressure deficit baselines and calculated CWSI values. Three distinct non-stressed baselines were determined and found to be related to the following crop morphological stages: (1) fully vegetative (few or no flowers); (2) flowering (few drying pods); and (3) pod development and maturation. Crop water stress index values across all treatments were highly and linearly correlated with stem water potentials. A less distinct curvilinear relationship existed between CWSI and leaf conductance. Use of the vegetative period non-stressed baseline to calculate CWSI values during flowering and pod-filling resulted in CWSI values 0.1 to 0.2 higher than those determined using growth-stage-dependent baselines. For irrigation scheduling, use of two baselines is suggested, one for the vegetative period and another baseline determined using pooled data from the flowering and podfilling periods.