The effects of drought on barley: soil and plant water relations

Abstract

SUMMARYIn a field experiment on the effects of drought on spring barley, the crop was protected from rainfall by automatic rain shelters; a range of drought treatments was achieved by irrigating various plots according to a predetermined schedule. There were 12 treatments which ranged from no irrigation to full irrigation from emergence to harvest; results from seven treatments are discussed in this paper.The rate of water uptake was determined for four soil horizons centred at 0·15, 0·50, 0·80 and 1·10 m. For all treatments, the rate of uptake in each horizon decreased as the soil dried, and although there were large differences in root density between horizons, maximum rates of uptake were similar in all horizons down to 0·80 m. Treatment effects showed that prolonged drought decreased the rate of uptake from the 0·80 and 1·10 m horizons: root density at and below 1·0 m probably differed between treatments.Differences between treatments in leaf water potential (ψL) and osmotic potential (πL) were small, and there was no evidence that osmotic adjustment contributed to the drought response of this crop. Near anthesis, pre-dawn ψL was near zero for irrigated treatments and between – 3 and – 5 bar for unirrigated. During the day, ψL decreased to a minimum of – 15 to – 18 bar for irrigated plants, and was generally 3 bar lower for unirrigated. For all treatments, ψL was greater than π for the major part of the day, i.e. positive turgor was maintained; however, turgor was usually greater for irrigated than for unirrigated plants. The relationship, for leaf 8, between ψL and transpiration flux density was markedly non-linear, and was of a similar form for irrigated and vinirrigated plants. As the form of this relationship was independent of treatment, the non-linearity could not have been caused by variations in soil water potential through the profile.Stomatal resistance differed markedly between treatments. A detailed analysis is presented, relating measured resistance for leaf 8 to ψL and to environmental variables: irradiance (I), water vapour pressure deficit (vpd), and temperature (T). The analysis showed no significant dependence of resistance on ψL or T, but marked dependence on I and vpd; a mathematical model combining a hyperbolic response function for I and an exponential function for vpd fitted the data well. The responses of abaxial and adaxial surface resistances to vpd were similar, but their light responses differed because of their different exposures to incident irradiance.