Elevated atmospheric partial pressure of CO and plant growth : I. Interactions of nitrogen nutrition and photosynthetic capacity in C and C plants.

Cotton and maize plants were grown under full sunlight in glass houses containing normal ambient partial pressure of CO (330±20 μbar) and enriched partial pressure of CO (640 ±15 μbar) with four levels of nitrogen nutrient. In 40 day old cotton plants grown in high CO, there was a 2-fold increase in day weight and a 1.6-fold increase in leaf area compared with plants grown in ambient CO. In 30 day old maize plants there was only 20% increase in dry weight in plants grown in 640 μbar CO compared with plants grown in 330 μbar and no significant increase in leaf area. In both species, at both CO treatments, dry weight and leaf area decreased in similar proportion with decreased nitrogen nutrient.The increase of leaf area in cotton plants at high CO caused a reduction of total nitrogen on a dry weight basis. In cotton assimilation rate increased 1.5 fold when plants were grown with high nitrogen and high CO. The increase was less at lower levels of nitrate nutrient. There was a 1.2 fold increase in assimilation rate in maize grown at high CO with high nitrate nutrient.Cotton and maize grown in high CO had a lower assimilation rate in ambient CO compared to plants grown in normal ambient air. This difference was due to the reduction in RuBP carboxylase activity. Water use efficiency was doubled in both cotton and maize plants grown at high CO in all nutrient treatments. However, this increase in water use efficiency was due primarily to reduced transpiration in some treatments and to increased assimilation in others. These data show that plant responses to elevated atmospheric partial pressure of CO depend on complex of partially compensatory processes which are not readily predictable.