Most documented experiments concerning cotton responses to nitrogen relate nitrogen application rates to yield, yield components or other growth parameters. Some of those to the effects of N on specific physiological processes. None gave quantitative relations between leaf nitrogen and growth or development. The objective of this experiment is to develop quantitative relations between leaf nitrogen and growth of cotton.

The experiment was conducted in controlled environment chambers at 30/22°C day/night temperatures. Cotton plants (G. hirsutum L., cv. DES 119) were grown in 12 L PVC pots. Plants were exposed to two different CO₂ treatments from emergence, ambient (350 µL L^-1) and twice ambient (700 µL L^-1). The nitrogen treatments of 0, 1, 2, 6 and 10 mM were imposed at 17 days after emergence, DAE, to 49 DAE, through modified half strength Hoagland's nutrient solutions. Sulphate and chloride ions were used to substitute nitrate ion in nitrogen deficient treatments. Plant heights and the lengths of mainstem leaves were measured daily. Dry weights of individual organs were taken from weekly harvests. Photosynthesis was measured with portable photosynthetic system, LiCor-6200, at regular intervals. Nitrogen content of each organ was analyzed with Kjeldahl method on each sample.

Relative leaf expansion rate and stem elongation rate decreased quadratically with decreasing leaf nitrogen. The decrease in relative leaf expansion was similar in both CO₂ environments, but stem elongation of high CO₂ grown plants was more than that of low CO₂ grown plants in all leaf nitrogen levels. By expressing stem elongation as a fraction of maximum elongation, all the data in both CO₂ environments fit one quadratic equation. Fraction of maximum stem elongation is equal to -2.94 + 3.19*N(g m^-2) -0.646*N₂. The decrease in photosynthesis also fits this equation, indicating that main stem elongation at nitrogen deficiency was primarily limited by available photosynthate.

Leaf expansion is more sensitive to leaf-N than photosynthesis. This indicates that leaf expansion is limited in nitrogen deficient environments by both carbon and nitrogen. The reduction factor for leaf expansion due to nitrogen was calculated by subtracting the leaf expansion-nitrogen-response curve from the photosynthesis-nitrogen-response curve. This provided a measure of the direct effect of leaf nitrogen on leaf expansion, which follows the equation : Fraction of maximum expansion = -2.623 + 3.096*N - 0.66*N₂.

The rate of node addition and the length of newly unfolded leaves were also quadratically reduced with leaf nitrogen. The fact that newly unfolded leaves were always larger in high CO₂ than that in low CO₂ grown plants indicates that length of newly unfolded leaves are partly limited by carbon.

There is an interactive effect between leaf nitrogen and plant age on partitioning of assimilate between shoot and roots. The allocation changed linearly with the change in leaf nitrogen. Plants with lower leaf nitrogen allocated more carbon to roots. The change is more in young plants.