The potential of measurements of chlorophyll fluorescence in vivo to detect cellular responses to salinity and degrees of salt stress in leaves was investigated for three crop plants. Sugar beet (Beta vulgaris L.) (salt tolerant), sunflower (Heliandtws annuus L.) (moderately salt tolerant), and bean (Phaseohis Vulgaris L. cv Canadian Wonder) (salt intolerant) were grown in pots and watered with mineral nutrient solution containing 100 millimolar NaCI. The fast rise in variable chlorophyll fluorescence yield that is correlated with photoreduction of photosystem II acceptors increased in leaves of sugar beet plants treated with salt suggesting stimulation of photosystem II activity relative to photosystem I. In sunflower, this fast rise was depressed by approximately 25% and the subsequent slow rate of quenching of the chlorophyll fluorescence was stimulated. These differences were more marked in the older mature leaves indicating an increasing gradient of salt response down the plant. The salt effect in vivo was reversible since chloroplasts isolated from mature leaves of salt-treated and control sunflower plants gave similar photosystem II activities. Unlike in sugar beet and sunflower, leaves of salt-treated bean progressively lost chlorophyll. The rate of slow quenching of chlorophyll fluorescence decreased indicating development of a partial block after photosystem II and possible initial stimulation of photosystem II activity. With further loss of chlorophyll photosystem II activity declined. It was concluded that measurements of chlorophyll fluorescence in vivo can provide a rapid means of detecting salt stress in leaves, including instances where photosynthesis is reduced in the absence of visible symptoms. The possible application to screening for salt tolerance is discussed.Increasing salinity in soil and water and its effect on crop plants has become a vast problem for agriculture in arid and semiarid regions that depend on irrigation. It has been estimated that agriculture in one-third of the land irrigated worldwide is already plagued by excess salinity (9). Further, problems of salinity are not confined to irrigated fields, but now extend to large areas of nonirrigated farmlands across the plains of North American and southern Australia. Although much effort is being expended in water control schemes and engineering projects to improve saline environments, as pointed out by Epstein et al. (9), these serve to minimize the problem of salinity but cannot eliminate it and the need to develop salt-tolerant crops at the same time has been emphasized (9, 21).Visible symptoms, frequently leaf burns, are rather late manifestations of severe salt stress, and except in a few instances, e.g. citrus (19), the salt content of leaves or roots is not a reliable guide to salt tolerance. New methods are required to monitor salt stress in physiological studies, to detect adverse effects of salt on fieldgrown crops that reduce yield without producing visible symptoms, and in the selection of salt-tolerant strain...

Tomato leaves were detached and stored at 0 C for various periods of time. Chloroplasts were isolated from the leaves and their photoreductive activities were determined. Comparisons were made between two altitudinal forms of the wild tomato Lycopersicon hirsutum Humb. and Bonpl. (a tropical lowlands form and a highlands form adapted to growth at 3,100 meters), and two cultivars of the domestic tomato L. esculentum Mill. In each case the capacity of the isolated chloroplasts to photoreduce ferricyanide declined linearly with time of storage of the leaves at 0 C, but not at 10 C. This injury developed more slowly in the high altitudinal form of the wild tomato compared with the low altitudinal form and the two domestic cultivars indicating an enhanced resistance toward chilling injury in the tomato from 3,100 meters. Chioroplast activity declined in green tomato fruit held at 0 C, at about the same rate as in the chilled leaves.Measurements of photochemical activities in the isolated chloroplasts and in vivo measurements of cytochrome-554 photooxidation in chilled leaves showed that the site of action of the chilling effect was water donation to photosystem II.The chilling-induced impairment of photoreductive activity in chioroplasts provides a useful assay for detecting and measuring differences in the susceptibility of plants to chilling injury.Plant physiological studies of chilling resistance in plants, as well as the breeding and selection of varieties with enhanced cold tolerance, are hampered by the lack of assays which can detect and measure chilling-induced cellular changes well before obvious tissue damage occurs. Cellular membranes appear to be especially sensitive to chilling temperatures (6) and indeed the membranelocalized Hill reaction activity of chloroplasts declines rapidly in chilled leaves of chilling-sensitive plants (3,5,7,8). This suggests the possibility of using measurements of Hill activity to detect early changes caused by chilling and to measure differences in resistance to chilling injury in closely related species.Tomatoes, in common with most plants of tropical origin, are damaged by prolonged exposure to chilling temperatures. All stages of the plant's development are affected by chilling temperatures, including germination, growth, and fruit set (4). The harvested fruit are also susceptible to chilling injury and low temperatures can disrupt the normal ripening process. We have examined changes in the photoreductive activity of chloroplasts isolated from chilled, detached leaves of two cultivars of the domestic tomato, Lycopersicon esculentum, and high and low altitudinal forms of the wild tomato, L. hirsutum. In all cases activities declined linearly with the time that the leaves were exposed to a chilling temperature of 0 C. There was a clear distinction between the rate of decline in the high and low altitudinal forms of the wild tomato indicating a considerable adaptation to low temperature. A chilling-induced decline in photoreductive activity also took place in green...

Temperatures producing heat injury in leaves of 30 species were measured by following heat-induced changes in fluorescence of the leaf chlorophyll. Except for six alpine plants and four cereals, mainly leafy vegetables and leaves of young fruit trees were used. Dark-adapted leaves were heated at a rate of one Celsius degree per minute. Chlorophyll fluoroscence began to increase between 30 and 40°C and reached a peak around 50°C. The peak of fluorescence was reached at 47.1°C in an alpine plant Plantago glacialis, at 51.2°C in lettuce (Lactuca sativa) and at 57.6°C in pawpaw (Carica papaya). Similarly, in the cereals, the peak of fluorescence was reached at lower temperatures in the temperate ones (49.5°C in barley and 48.9°C in oats) than in the two from warm climates (55.2°C in maize and 53.5°C in sorghum). This difference in heat sensitivity between plants from the three groups did not hold for all plants surveyed and there was some overlapping between individual plants in different groups. However, the mean temperature, for the inital rise in fluorescence as well as for the peak of fluorescence, occurred at a lower temperature in the alpine group of plants and at a higher temperature in the tropical group of plants compared with the temperate group. The results demonstrate a relationship between heat resistance and temperature of the environment in which the plants evolved, and illustrate the potential use of the chlorophyll fluorescence method for in vivo measurements of heat sensitivity in green plants.

Relative susceptibilities of chilled leaves to photoinhibition were determined for 15 species of crop annuals showing a wide range of chilling tolerance. Leaf tissue at 7°C was exposed to a moderate photon irradiance of 300 �mol m-2 s-1 and photoinhibition was measured by the decrease in chlorophyll fluorescence (Fv/Fm) measured at 77K. All chilling-resistant plants surveyed (barley, broad bean, oat, pea and wheat) were photoinhibited at chilling temperatures. The chilling-sensitive plants (bean, cucumber, lablab, maize, pearl millet, pigeon pea, sesame, sorghum and tomato) were more susceptible, the mean of values for susceptibility to photoinhibition being twice that of the chilling-resistant plants. Rice, however, showed a tolerance to photoinhibition at 7°C comparable to that of some of the chilling- resistant plants. Indica rices were more susceptible than japonica rices. Photoinhibition increased with decreasing temperature and with increasing photon irradiance in both the chilling-resistant and sensitive plants. In pea and cucumber, photoinhibition at 7°C was correlated linearly with the decrease in photosystem II activity assayed in chloroplast thylakoids isolated from similarly treated tissue. Relative tolerances of leaves of the same 15 species to chilling injury in the dark were also measured. No linear correlation was found between susceptibility of chilled leaves to photoinhibition and susceptibility to dark chilling injury. The pattern of differences between species for photoinhibition at 7°C was largely preserved when photoinhibitory treatments were given at a non-chilling temperature (21°C) by increasing the photon irradiance to 900 �mol m-2 s-1. We conclude that, while the chilling-sensitive plants were generally more susceptible than the chilling-resistant ones to photoinhibition at low temperatures, this arose from a greater sensitivity to the irradiance rather than from the chilling sensitivity. Photoinhibition associated with low temperatures was also demonstrated in the tropical fruit species, banana, pawpaw and Monstera. Low values of Fv/Fm recorded in leaves exposed to full sunlight during the winter month of July (range 0.39-0.56 compared with 0.70-0.79 in January) indicated that photoinhibition could adversely affect some tropical perennial fruit species cultivated in semitropical or warm temperate areas experiencing recurrent cool to cold winters.

S ings of barey (Hordeum ulgareL. cv. Abyssinian) were grown at constant temperature and light intensity and the properties and structure of chloroplasts in the primary leaf were examined. Seventeen growth temperatures ranging from 2 to 37 C were employed. Three Temperature, light, and other environmental factors can affect the course of chloroplast development in higher plants and algae. High temperatures (above 32 C) inhibit the production of normal chloroplasts in Euglena (18) and in higher plants (9), while low temperatures (10-16 C) can inhibit chloroplast development in chilling-sensitive plants such as sugarcane (7), sorghum (20), and maize (13). Temperatures intermediate between these extremes may also affect differentiation of the photosynthetic apparatus. Aside from considerations ofgenotypic adaptation to temperature, many plants show an ability to acclimate to the prevailing temperature regime during their growth. The rate and temperature optimum of photosynthesis in leaves and the leaf anatomy can vary according to the growth temperature and light intensity (2,3,11,21). Little is known about changes in the photosynthetic electron transfer system of chloroplast membranes as the result of acclimation to temperature. To study this aspect, as well as the effect of growth temperature on chloroplast differentiation, barley seedlings were grown at temperatures ranging from 2 to 36 C under constant light intensity. The plants were then examined for differences in chloroplast structure and activity. MATERIALS AND METHODSPlant Material and Growth Conditions. Seeds of barley (Hordeum vulgare cv. Abyssinian) were washed in running tap water for 24 hr and then planted in Vermiculite, previously soaked in Hoagland solution and drained, in weighted glass tubes (4 x 30 cm). The top ofeach tube was covered with Gladwrap film (Union Carbide) which allows gas exchange while reducing water loss and immersed to within 1 to 2 cm of the top of the tube in water contained in an insulated water bath. The baths were maintained at temperatures from 11 to 37 C (± 0.10 C) and continuous overhead illumination of 4,500 lux at seed level was provided by white fluorescent lights. The plants were harvested when they had reached a height of 17 to 20 cm (6-21 days depending on the temperature). Plants grown at 2 and 5 C were raised under similar conditions except that temperature-controlled rooms were used instead of water baths.Chloroplast Isolation and Assay. Chloroplast thylakoids were isolated from 1 to 5 g fresh wt of leaf tissue as previously described (16). The top half of the primary leaf less 1 cm of the leaf tip was used.The photoreduction of ferricyanide was assayed spectrophotometrically in a reaction mixture (1.5 ml) containing chloroplasts (4 ,ug of Chl ml-'), 50 mm S0rensen's phosphate buffer (pH 7.5), 50 mM NaCl, 0.05% (w/v) BSA, gramicidin D (4,g ml-'), and 0.34 mm K-ferricyanide. Actinic light (11 x I04 ergs cm-' sec-1) filtered through a Coming 2-60 red cut-off filter was supplied from a 150-w tungsten-ha...