The Role of Photosynthetic Daily Light Integral in Plant Response to Extended Photoperiods

Under continuous lighting (CL, 24 h photoperiod), both the energy and signaling components of light are continuously supplied to plants. In response to CL sensitive plant species exhibit leaf chlorosis and necrosis, down-regulation of photosynthesis, accumulation of carbohydrates and accelerated leaf senescence. Current research was undertaken to test if the CL-induced injury truly results from CL per se or from the excessive light received by the plant due to high daily light integral (DLI). Experimental design included combinations of light intensity and photoperiods providing either different (Experiment 1) or equal (Experiment 2) DLI for tomato and cucumber plants grown under CL or 14 h photoperiod. In experiments where equal DLI was provided CL-treated plants developed injury symptoms similar to those in CL treatments with higher DLI. Plants exhibited an entire spectrum of responses to increasing day length-epinastic movements of leaves and petioles, chlorosis, the significant decline in values of potential quantum yield of photochemical activity of PSII (F-v/F-m) and effective quantum yield of PSII (Phi PSII), elevation of the ratio of Chl a/b, decrease of Chl/Car ratio, reduction of LHCII, increase of lipid peroxidation degree measured in terms of the malondialdehyde (MDA) levels and enhanced activities of antioxidant enzymes (catalase, superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase). Data obtained have shown that a higher DLI is not needed to trigger CL-induced injury. The continuity of light signaling, photosynthesis and photooxidative pressure may induce injury even if DLI values are not high. Extended photoperiod itself may also be a reason of the excess of absorbed light even if DLI is not higher than normally required by plants under shorter photoperiods.