LEAF SENESCENCE IN FERN - EFFECT OF DURATION, INTENSITY AND QUALITY OF LIGHT

Continuous white light at an intensity of 12 W/m2 causes a mild retarding effect on chlorophyll loss during fern leaf senescence. Pigment loss is robustly retarded by continuous light of 6 W/m2 and daily 1 or 4 hr pulses of light of 12 W/m2. A 5-min red light pulse at 24 hr intervals retarded chlorophyll loss; this red light response was nullified with far-red pulses, suggesting the participation of phytochrome. Blue light pulses were similarly effective; the response was partially reversed by far-red pulses. This suggests that the blue light photoreceptor might be partially acting through phytochrome. Addition of sucrose during pulses of red or blue light did not result in any additional effect over the light pulses alone, suggesting non-involvement of photosynthesis in regulating chloroplast degradation. Since fern is a shade plant, its response to different light intensities is rather complex. The data suggest that moderate intensity light (12W/m2) protects chloroplast degradation through phytochrome and blue light photoreceptors. In addition, light at this intensity may also photo-oxidize some pigments. This postulate appears to be supported by the relatively high level of thylakoid lipid peroxidation as indicated by the accumulation of MDA (malondialdehyde) during senescence of leaves exposed to 12W/m2 light intensity. The apparent mild effect of continuous white light of 12W/m2 to retard pigment loss, therefore, is due to a balance between photoprotection and light-induced oxidative processes. © 1990.