THE RELATIONSHIP BETWEEN PHOTOSYNTHESIS AND NITROGEN ASSIMILATION IN MUSTARD PLANTS EXPOSED TO ELEVATED NITRATE RATES IN NUTRIENT SOLUTIONS

Photosynthesis, nitrogen metabolism, and growth of brown mustard (Bras sica juncea L.) plants were investigated as a function of nitrate cont ent in Knop nutrient solution (1-9 rates, with 1 rate equivalent to 14 .4 mM NO3-). The plants were able to assimilate nitrate at a wide conc entration range (up to 5 rates) without loss of homeostasis, which ens ured normal growth and accumulation of dry matter. Further elevation o f nitrate concentration in the nutrient solution to 7-9 rates was stre ssful for the plants. The increase in the nitrate content above the op timum led to a substantial suppression of apparent and potential photo synthesis, reduced starch accumulation and plant productivity, and mod ified the pathways of nitrogen assimilation in leaves. The leaves of s tressed plants contained high amounts of glutamic acid, glycene, proli ne, asparagine, and glutamine. It is concluded that high nitrate conce ntrations, exceeding the reaction norm of plants, disturb the coordina tion between carbon and nitrogen assimilation.