Photo-oxidative stress in coconut seedlings: early events to leaf scorching and seedling death

Abstract

Experiments were conducted on coconut seedlings to delineate events of photo-oxidative stress damage. Studies on chlorophyll fluorescence indicated a clear case of excess light energy under high light conditions causing stress to coconut seedlings raised under coconut palms. Quantum yield of photo-chemistry of leaflets exposed to high light was significantly less than those under shade. Seedlings exposed to high light and then shifted to shade have shown significant improvement in quantum yield. Excess light energy harvested by chlorophyll antenna caused high non-photochemical quenching resulting in production of biologically toxic super oxide, hydrogen peroxide and hydroxyl radicals. It is apparent that photoinhibition of photosynthesis takes place due to i) PSII down regulation and ii) damage to PS II system in initial stages of exposure to excess light and under prolonged exposures inhibition is caused due to iii) chlorophyll bleaching and iv) damage to chloroplast and cell membrane integrity, followed by reduction in photosynthetically active leaf area because of scorching thus reducing canopy photosynthesis. Protein concentration in leaf tissue was higher in seedlings in high light conditions. Three distinct low molecular weight proteins with pI of 4.9, 8.4 and 10.15 having Mr less than 20,000 were found in seedlings exposed to high light intensities. Results clearly demonstrate the events that take place at early stage to subsequent cascading effects leading to the scorching and death of leaf and even seedling death under severe conditions.