Effects of nitrogen nutrition on photosynthesis in Cd-treated sunflower plants

Abstract

Increased nitrogen supply stimulates plant growth and photosynthesis. Since it was shown that heavy metals may cause deficiencies of essential nutrients in plants the potential reversal of cadmium toxicity by increased N nutrition was investigated. The effects on photosynthesis of low Cd (0, 0·5, 2 or 5 mmol m-3) combined with three N treatments (2, 7·5 or 10 mol m-3) were examined in young sunflower plants. Chlorophyll fluorescence quenching parameters were determined at ambient CO2 and at 100 or 800 μmol quanta m-2 s-1 The vitality index (R(fd)) decreased approx. three-times in response to 5 mmol m-3 Cd, at 2 and 10 mol m-3 N. The maximum photochemical efficiency of PSII reaction centres (F(v)/F(m)) was not influenced by Cd or N treatment. The highest Cd concentration decreased quantum efficiency of PSII electron transport (Φ(II)) by 30 %, at 2 and 10 mol m-3 N, mostly due to increased closure of PSII reaction centres (q(p)). Photosynthetic oxygen evolution rates at saturating CO2 were decreased in plants treated with 5 mmol m-3 Cd, at all N concentrations. The results indicate that Cd treatment affected the ribulose-l,5-bisphosphate (RuBP) regeneration capacity of the Calvin cycle more than other processes. At the same time, the amounts of soluble and ribulose-l,5-bisphosphate carboxylase/oxygenase (Rubisco) protein increased with Cd treatment. Decreased photosynthesis, but substantially increased Rubisco content, in sunflower leaves under Cd stress indicate that a significant amount of Rubisco protein is not active in photosynthesis and could have another function. It is shown that optimal nitrogen nutrition decreases the inhibitory effects of Cd in young sunflower plants. (C) 2000 Annals of Botany Company.