Chlorophyll a fluorescence of NO 3 -FED vs N 2 -fixing soybean grown under two water treatments

Diurnal courses of chlorophyll a fluorescence of horizontally restrained and heliotropic leaves of soybean plants ( Glycine max (L.) Merr., cv. Cumberland), inoculated with N 2 -fixing Bradyrhizobium species and grown under conditions of high (HW) and low (LW) water availability and with supplemental application of 15 mM (NO 3 -fed) or 0.2 mM (N 2 -fixing) nitrate, were measured with a non-modulated fluorimeter for 3 consecutive days. Earlier studies have shown increased paraheliotropism in water-stressed and low nitrogen-grown soybean plants. Since increased paraheliotropism results in lower irradiances, we hypothesized that midday fluorescence quenching would be reduced in oriented leaves compared to horizontal leaves. A midday depression in the ratio of variable fluorescence to maximum fluorescence ( F v / F m ) was measured in all treatments. Of plants in the same nitrogen and water treatments, restrained leaves had a greater reduction in midday F v / F m compared to orienting leaves. Within leaves of the same nitrogen source, orienting leaves of LW plants had higher leaf angles and less reduction in F v / F m during midday periods relative to those of HW plants. In contrast, restrained leaves of LW plants had a greater midday depression in F v / F m relative to restrained leaves of HW plants. Horizontally restrained leaves of NO 3 -fed/LW plants, N 2 -fixing/HW and N 2 -fixing/LW plants had a progressive decline in early morning F v / F m. After 2 days of being held horizontal, restrained leaves of N 2 -fixing/LW plants had significantly lower early morning F v / F m than those of NO 3 -fed/LW plants. The minimum fluorescence ( F 0 ) of restrained leaves of NO 3 -fed/LW, N 2 -fixing/HW, and N 2 -fixing/LW plants increased after 2 days of being held horizontal. The results supported our initial hypothesis, and indicated that horizontal leaf orientation during high light, high temperature midday conditions may result in cumulative damaging effects upon leaf metabolism in soybean.