Nitrate reductase activity, nitrogenase activity and photosynthesis of black alder exposed to chilling temperatures

Abstract

Actinorhizal (Frankia‐nodulated) black alder [Alnus glutinosa (L.) Gaertn.] seedlings fertilized with 0.36 mM nitrate (low nitrate fertilizer treatment) or 7.14 mM nitrate (high nitrate fertilizer treatment) and acclimated in a growth chamber for 2 weeks were exposed to 2.5 h of night‐time chilling temperatures of −1 to 4°C. Cold treatment decreased nitrogenase activity (acetylene reduction activity) 33% for low nitrate fertilized plants and 41% for high nitrate fertilized plants. Recovery of nitrogenase activity occurred within 7 days after chilling treatment. In contrast, in vivo nitrate reductase (NR) activities of leaves and fine roots increased immediately after chilling then decreased as nitrogenase activities recovered. Fine roots of alder seedlings exhibited NR activities proportional to the amounts of nitrate in the rooting medium. In contrast, the NR activities of leaves were independent of substrate and tissue nitrate levels and corresponded to nitrogenase activity in the root nodules. In a separate experiment, net photosynthesis (PS) of similarly treated black alder seedlings was measured before and after chilling treatments. Net PS declined in response to chilling by 17% for plants receiving low nitrate fertilizer and 19% for plants receiving high nitrate fertilizer. After chilling, stomatal conductance (gs) decreased by 39% and internal CO2 concentration (ci) decreased by 5% in plants receiving the high nitrate fertilizer, whereas plants receiving the low nitrate fertilizer showed no change in gs and a 13% increase in ci. Results indicate that chilling stimulates stomatal closure only at the high nitrate level and that interference with biochemical functions is probably the major impact of chilling on PS.