LEAF RESPONSES TO SOIL-WATER DEFICITS - COMPARATIVE SENSITIVITY OF LEAF EXPANSION RATE AND LEAF CONDUCTANCE IN FIELD-GROWN SUNFLOWER (HELIANTHUS-ANNUUS L)

The relative importance of changes in leaf expansion rate (LER) and leaf conductance (g1) in the control of crop transpiration depends primarily on their sensitivity to soil water deficits. The aim of this paper was to quantify the responses of LER and g1 to soil water deficits in sunflower (Helianthus annuus L.) under conditions of moderate (spring) and high (summer) evaporative demand. Soil water content, g1, and LER were measured in dryland (DRY) and daily-irrigated (WET) crops established on a deep sandy-loam (Typic Xerofluvent) in a Mediterranean environment. There was no difference between g1 of DRY and WET plants (p > 0.20) in contrast with a highly significant difference in LER (p < 0.001). Even under the harsh conditions of the summer experiment, g1 did not respond to water deficit in a ten-day period in which LER of DRY plants was reduced to approx. 30% of that measured in WET controls. This field study indicates that g1 plays at most a minor role in the control of sunflower transpiration in the pre-anthesis period and confirms the importance of leaf expansion in the regulation of gas exchange of expanding canopies subjected to soil water deficits.