Water status effects on net photosynthesis in field-grown alfalfa

Net photosynthesis (Pn) typically declines with drought. Partitioning of this decline into stomatal and nonstomatal factors determines the seasonal performance and stress adaptation of the leaves. Here stomatal and nonstomatal limitations to Pn in alfalfa (Mediago sutiva L.) are evaluated to determine their relative importance during drought. Photosynthesis of the upper leaves of irrigated and nonirrigated alfalfa (cv. Sabina) was determined in 1985 and 1986 during spring regrowth. In both years, Pn was inhibited more than 35% in nonimgated compared with irrigated plants, which had higher leaf water potential. Internal CO2 partial pressure (pi) indicated that stomatal limitation was not the only reason for reduced Pn, stomatal and nonstomatal effects were about equally important in the inhibition of Pn in leaves of nonirrigated plants in 1986. In vitro ribulose- 1,s-bisphosphate carboxylase activity (RuBPCO) per unit leaf area was higher in the nonirrigated treatment, suggesting that the enzyme was not responsible for the nonstomatal component of inhibition of Pn. In 1986, O2 evolution, measured in detached leaves with saturating CO2, was lower in the nonirrigated treatment compared to the irrigated one, implying that limitations to ribulose-1,Sbisphosphate (RuBP) regeneration may play a significant role. The carbon isotope composition of total leaves of plants collected at the end of the experiment in 1985 and 1986 showed a lower discrimination against 13C in nonirrigated compared with the irrigated leaves, indicating a reduced conductance to C02 transfer. It is concluded that both stomatal and nonstomatal mechanisms contributed to reduced Pn in droughted alfalfa.