effect of carbon dioxide injection to root zone on the growth and nitrate accumulation in two cultivars of lettuce (lactuca sativa l. cv. capitata and sativa)

nitrate is an essential component for plants and vegetables are of the most important source of nitrate for human being. it is estimated that vegetables provide 92 % of nitrate for human nutrition. the positive and negative effects of elevated root zone co2 on growth and yield of some plants have been reported. this study has been conducted in order to examine the effect of root zone co2 on growth and nitrate accumulation in lettuce. the experiment was conducted as a completely randomized design with four levels of co2 (0, 200, 400 and 600 ml per minute) and with five replication on two cultivars of lettuce (sativa and capitata). the study performed in floating system and aeration was supplied by air pump. the results showed that injection of co2 had significant effect on the fresh and dry weight of leaves, stems and roots of plants. by increasing the concentration of co2 fresh and dry weight of leaves, stems and roots increased in both cultivars. photosynthesis efficiency increased in both cultivars in the treatment of 400 ml/min root zone co2. the influence root zone co2 significantly decreased nitrate content in leaves of lettuce. nitrate concentration in the leaves of lettuce was decreased by injection of root zone co2 in l.sativa. var. capitata in the treatment of 400 ml/min and in l.sativa. var sativa in the treatment 600 ml/min. respectively by 27% and 47.6%. with increasing input root zone co2 increased activity of nitrate reductase enzyme in both cultivars that showed highest content in treatment 400 ml/min.