Relationship between crop water stress index and other physiological plant water status indicators in guayule

Abstract

Guayule ( Parthenium argentatum Gray, cv. N565 II) was grown at Phoenix, Arizona, in well-watered and water-stressed conditions. The relationships between the crop water stress index ( CWSI) and other physiological indicators of plant water status were examined. The drought treatment lasted for 70 days (29 May through 5 August 1986) during which time the available soil moisture content declined from 100 to 0%, while the well-watered plants were maintained at greater than 70% available soil moisture. The CWSI results were normalized to reduce environmental effects by subtracting the values obtained for the well-watered plants from those of the water-stressed plants. Significant ( P<0.01) linear relationships were obtained between the normalized CWSI and the similarly normalized values of leaf water potential ( r 2=0.75), leaf osmotic potential ( r 2=0.70), and stomatal conductance ( r 2=0.62). Turgor potentials of the plants in both treatments were similar until their difference in CWSI reached approximately 0.6, above which the water-stressed plants were no longer able to maintain turgor. No difference in net photosynthesis was observed between the wet and dry plots even though their difference in CWSI was as great as 0.80. During an 11-day period following irrigation of the water-stressed plants, their CWSI values remained significantly higher than the well-watered plants even though the soil moisture content of the two treatments was similar.