ADAPTATION OF THE PHOTOSYNTHETIC APPARATUS IN MAIZE LEAVES AS A RESULT OF NITROGEN LIMITATION - RELATIONSHIPS BETWEEN ELECTRON-TRANSPORT AND CARBON ASSIMILATION

In maize (Zea mays L., cv Confessa), nitrogen (NO3-) limitation resulted in a reduction in shoot growth and photosynthetic capacity and in an increase in the leaf zeaxanthin contents. Nitrogen deficiency had only a small effect on the quantum yield of CO2 assimilation but a large effect on the light-saturated rate of photosynthesis. Linear relationships persisted between the quantum yield of CO2 assimilation and that of photosystem II photochemistry in all circumstances. At high irradiances, large differences in photochemical quenching and nonphotochemical quenching of Chl a fluorescence as well as the ratio of variable to maximal fluorescence (Fv/Fm) were apparent between nitrogen-deficient plants and nitrogen-replete controls, whereas at low irradiances these parameters were comparable in all plants. Light intensity-dependent increases in nonphotochemical quenching were greatest in nitrogen-deficient plants as were the decreases in Fv/Fm ratio. In nitrogen-deficient plants, photochemical quenching decreased with increasing irradiance but remained higher than in controls at high irradiances. Thermal dissipative processes were enhanced as a result of nitrogen deficiency (nonphotochemical quenching was elevated and Fv/ Fm was lowered) allowing PSII to remain relatively oxidised even when carbon metabolism was limited via nitrogen limitation.