Plant senescence processes and free radicals.

Free radicals acting at sensitive subcellular sites, appear to play a pivotal role in both the deleterious and beneficial effects of maturation and senescence of various plant organs--leaves, flowers, and fruit. As evidenced by ESR spectrometry, spin trapping, specific membrane phase transition studies and enzyme kinetics, an important factor in the above processes appears to be lipoxygenase activity producing polyunsaturated fatty acid (PUFA) hydroperoxides and subsequently several free radical species and senescence-promoting compounds such as ethylene, malondialdehyde and jasmonic acid. The most intensely investigated are the oxy-free radical species including O2-., .OH, RO., ROO., PUFA and semiquinone free radicals. Higher plants are equipped with ways and means to combat free radicals and these may be classified under two general headings; (a) direct scavengers including SOD, ascorbic acid, and alpha-tocopherol acting in concert (b) incipient preventative mechanisms against radical formation, these include xanthine oxidase inhibitors, strategies based on endogenous H2O2 disposal in the form of peroxidative enzymes and glutathione turnover, and Ca2+ channel blockers. The antisenescence phytohormone cytokinin appears to possess a dual effect and may act in both capacities. The special case of delayed free radical formation in comparatively dry biological systems such as seeds is detailed, and specific free radical-generating photosensitizer compounds are also discussed.