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 compo nents of a single cotton (Gossypium hirsutum L.) leaf subtending the fruit during ontogeny were investigated under field conditions. A 20-d old leaf e xhibited the highest physiological activity characterized by net photosynth etic (P-N) and transpiration (E) rates, stomatal conductances to CO2 exchan ge (g(sCO2)) and, water vapor transfer (g(sH2O)) and nitrogen (N) content. With the advent of leaf senescence, the gas exchange rates declined as exhi bited by the 30-, 40-, and 60-d old leaves. Regression analysis indicated c lose 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 the y declined with leaf age. Thus, the declines in P-N were associated with st omatal closure and removal of N during leaf ontogeny.