Photosynthetic limitations and yield potential in two barley genotypes (Hordeum vulgare L.) subjected to drought

Abstract

The photosynthetic productivity of plants is determined by the quantity of photosynthetically active radiation intercepted and the efficiency with which it is utilised for net dry matter production. We examine the effects of drought on the photosynthetic capacity of five durum wheat (Triticum turgidum L. var durum) genotypes, grown in a greenhouse. Drought was imposed on five-week-old plants by withholding water. During the onset of water stress, the response of net carbon assimilation by leaves to their intercellular CO2 concentration (A/ci curves, with an LICOR 6400, NB, USA) was measured to examine the limitation imposed by stomata (l), carboxylation velocity (Vc,max) and capacity for ribulose-1,5-bisphosphate regeneration (Jmax) on photosynthesis. Furthermore, simultaneous measurements of modulated chlorophyll fluorescence (MiniPAM, Walz, Effeltrich, Germany) were used to identify changes in the efficiency of light utilisation for electron transport and the occurrence of photoinhibition of photosystem II. At moderated water stress (y w » -1 MPa and RWC ³ 65%), the light-saturated net CO2 assimilation rate (Asat) decreased by ca. 56% in water-stressed plants of all genotypes, accompanied by decreases in Vc,max and Jmax by ca. 35% and 45%, respectively, and increases in stomatal limitation (by ca. 30%), indicating inactivation or loss of Rubisco and other key Calvin cycle enzymes (i.e. sedoheptulose-1,7-bisphosphate), and stomatal closure. As water-stress progressed (y w £ -2 MPa and RWC £ 65%), further decreases were found in Asat accompanied with large decreases on in Vc,max and Jmax (and increases in stomatal limitation) for all varieties. The exponential decline of Asat vs. y w were fitted by non-linear regression using the model y = a*e-b(x), where b can be taken as an index of sensitivity to drought. However, no significant differences were found in this parameter for any genotype studied. Furthermore, decreases in the quantum yield of photosystem II were accompanied with decreases in the efficiency of energy capture by open PSII reaction centres (Fv¿/Fm¿) without major changes in the photochemical quenching (qp) in water-stressed plants of all varieties, indicating down regulation of electron transport.