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 le af conductance (g1) in the control of crop transpiration depends prima rily on their sensitivity to soil water deficits. The aim of this pape r was to quantify the responses of LER and g1 to soil water deficits i n sunflower (Helianthus annuus L.) under conditions of moderate (sprin g) and high (summer) evaporative demand. Soil water content, g1, and L ER were measured in dryland (DRY) and daily-irrigated (WET) crops esta blished on a deep sandy-loam (Typic Xerofluvent) in a Mediterranean en vironment. 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 d id not respond to water deficit in a ten-day period in which LER of DR Y 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 c ontrol of sunflower transpiration in the pre-anthesis period and confi rms the importance of leaf expansion in the regulation of gas exchange of expanding canopies subjected to soil water deficits.