The proposition is examined that measurements of chorophyll fluorescence in vivo can be used to monitor cellular injury caused Damage to crop plants and reductions in yields are often brought about by unfavorable environmental conditions; these include extremes of temperature, lack or surfeit of H20, high solar UV radiation, excess soil salinity, and the presence of toxic pollutants. Looking to the future, however, it seems that some environmental stresses will inevitably intensify, and this, coupled with such practices as utilizing land marginally suited to agriculture and growing desirable crop plants in climates for which the plants are ill adapted, e.g. maize in high latitudes (6) and potatoes in the semitropics (3), are likely to create an increasing need for new stress-tolerant varieties. Presently, the incidence of salt contamination of soil is increasing (10) and there also is concern that temperatures worldwide may rise because of increasing concentrations of CO2 and methane in the atmosphere, and solar UV radiation reaching the earth may intensify (22)

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Photoinhibition resulting from exposure at 70C to a moderate photon flux density (300 micromoles per square meter per second, 400-700 nanometers) for 20 hours was measured in leaves of annual crops differing widely in chilling tolerance. The incidence of photoinhibition, determined as the decrease in the ratio of induced to total chlorophyll fluorescence emission at 693 nanometers (F,/F,,,) (5), maize (12), bean (18), and tomato (15). Up to a point, photoinhibition is completely reversible (1,4), and this condition may be thought of as an adaptive mechanism allowing some adjustment of photosynthetic processes to suit prevailing conditions of light, temperature and other environmental factors. However, prolonged exposure to light and low chilling temperatures can be debilitating and associated with severe and irreversible photoinhibition followed by photooxidative destruction of Chl (1, 28) and ultimately cell death.Chilling-dependent photoinhibition has mainly been studied in plants well known to be intolerant of chilling temperatures, and this raises the question of whether or not adverse effects of chilling and light on photosynthetic productivity are primarily problems associated with chilling-sensitive plants. Photoinhibition ofphotosynthesis arising from the interaction of chilling and light has been shown to occur in leaves of the chilling-sensitive annual crops, bean (4, 18), cotton (18), cucumber (9, 28), maize (12), rice (5), and tomato (15, 29). Olive, an evergreen crop of warm temperate climates, is also vulnerable to photoinhibition during chilling at 5°C (1). In contrast, spinach, a chilling-resistant plant, was reported to be insensitive to inhibition of photosynthetic processes in light at 4°C (3). However, some recent studies have rendered this distinction less clear. Two plants of temperate climates, Lemna minor and L. gibba were shown to be photoinhibited at chilling temperatures (1 1, 16), although it is to be noted that vegetative growth of these plants is largely restricted to the warm months of the year and over-wintering is accomplished by producing resting buds (turions) that sink to the bottom of ponds during winter. Photoinhibition during chill-ing was also demonstrated in two species of potato that are somewhat more tolerant of chilling than the chilling-sensitive plants mentioned above (26). Two relevant studies have been 1609www.plantphysiol.org on May 10, 2018 -Published by Downloaded from

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