Responses of Nicotiana tabacum to CO2 enrichment at low-photon flux density

Bean plants ( Phaseolus vulgaris ) grown in phosphate-deficient (–P) medium display deficiency symptoms after about 2 weeks of culture. A decrease in inorganic phosphate level in roots was observed after 10 days of culture, and after 17 days it was more than 30 times less than control. The dry weight of the shoots of phosphate-deficient plants was lower whereas the dry weight of the roots was higher as compared to the control roots. After 2 weeks of culture, the sucrose level in –P roots almost doubled compared to control roots. An increase in glucose and fructose was observed earlier than that of sucrose, and after 10 days of culture in phosphate-deficient medium the glucose level in –P roots was about 3 times higher than that in +P roots. At the same time, a decrease in hexose-phosphate level was observed. This decrease may reflect a higher drain from the hexose phosphate pool, or it may be due to the lower capacity for hexose phosphorylation of phosphate-deficient plants, as judged by the lower hexokinase and fructokinase activities. The ratio of non-phosphorylated to phosphorylated sugars in –P roots was about 5 times higher as compared to control roots. We propose that glucose and fructose accumulation in phosphate-deficient roots represents a non-metabolic, probably vacuolar pool which is not utilized for growth and metabolism of the roots.