Relationships between nitrogen content and net gas exchange components of a cotton leaf during ontogeny

Relationships between leaf nitrogen (N) content and leaf gas exchange components of a single cotton (Gossypium hirsutum L.) leaf subtending the fruit during ontogeny were investigated under field conditions. A 20-d old leaf exhibited the highest physiological activity characterized by net photosynthetic (P-N) and transpiration (E) rates, stomatal conductances to CO2 exchange (g(sCO2)) and, water vapor transfer (g(sH2O)) and nitrogen (N) content. With the advent of leaf senescence, the gas exchange rates declined as exhibited by the 30-, 40-, and 60-d old leaves. Regression analysis indicated close relationships between g(sCO2) and P-N, and g(sH2O) and E as the leaves advanced towards senescence. Both P-N and g(sCO2) were related to N as they declined with leaf age. Thus, the declines in P-N were associated with stomatal closure and removal of N during leaf ontogeny.