REGULATION OF EVAPOTRANSPIRATION, AND ITS PARTITIONING BETWEEN TRANSPIRATION AND SOIL EVAPORATION BY SUNFLOWER CROPS - A COMPARISON BETWEEN HYBRIDS OF DIFFERENT STATURE

This study investigates the regulation of evapotranspiration (E(t)) and its partitioning between soil evaporation (E) and transpiration (T) by sunflower (Helianthus annuus L.) crops. A comparison was made of cvs. Cannon (standard height) and Beauty (semi-dwarf) over a full crop season. Three irrigation regimes were established to achieve well irrigated crops (I), and crops stressed before (D(pre)) and after anthesis (D(post)). Weekly E(t) was measured by hydrological balance, E using evaporimeters, and T calculated as the difference. Evaporimeters were renewed each week and calibrated by comparison with evaporation measured using the hydrological balance on bare plots. There was no difference between cultivars in either E(t) (range from 5 to 64 mm week-1) or the fraction of E(t) due to soil evaporation, i.e. EE(t)-1 (range from 0.12 to 0.70). The two cultivars achieved similar partitioning of E(t) in different ways. Despite smaller leaf area index (L), Cannon crops transpired the same proportion of E(t) as Beauty because of a compensatory greater TL-1. The greater TL-1 of Cannon was consistent with its greater interception of radiation (IR) per unit L. Irrespective of cultivar, the mechanisms involved in the regulation of transpiration depended on the timing of the water shortage. Lower IR (resulting from reduced leaf expansion) accounted for the smaller T in D(pre) crops. Transpiration of D(post) crops was reduced by both smaller canopy conductance and smaller IR caused by greater leaf senescence.