Ploidy level of citrus rootstocks affects the carbon and nitrogen metabolism in the leaves of Chromium-stressed Kinnow mandarin plants

Abstract

This study compared the effects of chromium toxicity on the carbon and nitrogen metabolism in Kinnow mandarin (Citrus nobilis Lour x Citrus deliciosa Ten) plants grafted on the diploid (2x) and tetraploid (4x) of Poncirus trifoliata [L.], Citrus reshni, and Citrus limonia Osbeck. Plants were grown under controlled conditions and fertigated with complete nutrient solution supplemented with chromium (0.75 mM). After 120-d of growth under chromium (Cr) stress, carbon (C) and nitrogen (N) metabolism in leaves were studied. In the C metabolism study sucrose, glucose, fructose, starch contents, and the enzymatic activities of acid invertase (AI), neutral invertase (NI), sucrose synthetase (SS), sucrose phosphatesynthetase (SPS), fructokinase (FK), hexokinase (HK), phosphofructokinase (PFK), pyruvate kinase (PK) were investigated. Nitrogen metabolism study was comprised of determination of nitrate, nitrite and ammonium content and the nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthetase (GOGAT), and glutamate dehydrogenase (GDH) activities. Basically, in the C metabolism study the Cr enhanced the enzymatic activities and sugar contents, and decreased starch. Chromium stress caused a decrease in nitrate and nitrite, and an increase in ammonia; it also inhibited all measured enzymatic activities except GDH which was increased. The responses of Kinnow mandarin plants grafted on 2x or 4x for any rootstock followed a similar pattern, although the effects of Cr toxicity were more drastic in 2x grafted plants. Glycolysis was the only metabolic process which had a different behavior between 2x and 4x grafted plants under Cr stress: FK and HK activities under Cr stress were significantly increased in 4x grafted plants than in 2x grafted plants. These data suggest that in Kinnow mandarin plants grafted on 4x rootstocks the C metabolism is modified thereby strengthening the plants to adapt Cr stress, but N metabolism was inhibited by Cr toxicity.