Modulation of Progressive Leaf Senescence by the Red:Far-Red Ratio of Incident Light

Unifoliate soybean leaves were irradiated with different red:far-red ratios (R:FR) while the shoot above the leaves was kept under a constant R:FR. The R:FR was changed by lowering the red photon fluence rate. An R:FR of 0.07 accelerated chlorophyll and protein loss and enhanced the rate of exudation of amino acids from the detached irradiated leaves. Different photon fluence rates of far-red were added to a background of cool white fluorescent light, to lower R:FR without significantly changing photosynthetically active radiation. R:FR of 0.09-0.18 accelerated chlorophyll loss. These results are consistent with the view that the R:FR may modulate progressive senescence and that low R:FR occurring in shadelight may promote senescence.