Responses of grain sorghum to varying irrigation frequency in the Ord irrigation area. III. Water relations

Abstract

The water potential (°l), diffusive conductance (gs) and extension rates of leaves were measured throughout the growth of grain sorghum in the dry season at Kimberley Research Station in northwestern Australia. Four cultivars (Quicksilver, Texas 610SR, E57 and 47844) were compared under three furrow irrigation regimens (irrigation every 7-days, 28-days or once-only-at-sowing). Pre-dawn °l, noon °l, noon gs and daily leaf extension rate, between floral initiation and crop physiological maturity, gradually diverged in response to irrigation regimens. There was little difference between cultivars, with pre-dawn °l departing from near zero only after anthesis. Noon °l and gs values fluctuated erratically from day-to-day, perhaps in response to variation in saturation deficit. The effect of irrigation treatment on noon °l, noon gs and leaf extension rate was not obvious until 40-60 days after sowing. Diurnal trends at four stages of growth were measured for the cultivar Texas 610SR which showed partial midday depression of gs, particularly in the drier treatments. Multiple regression analysis of these data showed that variation in °l could largely be explained in terms of radiation, temperature, evapotranspiration (Et) and soil water potential. The best relationship was provided by radiation, temperature and either soil water potential or actual Et (R2 = 0.833). Similarly, much of the variation in g, was related to radiation, temperature, vapour pressure deficit and E,. Multiple regression models, however, failed to describe low midday values of gS observed in water-stressed plants. Although °l, gs and leaf extension responses may be used to compare treatments, none of these measurements could be used as an aid to irrigation scheduling.