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The influence of shading intensity on growth, morphology and leaf gas exchange of mangosteen (Garcinia mangostana L.) seedlings was investigated over a 2-year period. Diurnal gas exchange studies revealed significantly higher carbon gain for leaves grown in 20 or 50% shade compared to leaves grown in 80% shade. Seedlings grown in 20 or 50% shade accumulated significantly more dry weight than seedlings grown in 80% shade during the 2-year study period. Seedlings grown in decreased shade showed decreased leaf size, increased leaf thickness, lower specific leaf area (SLA) and higher stomatal frequency. Less shaded seedlings also allocated relatively more dry matter to roots than shaded seedlings and exhibited a significant reduction in leaf area relative to total plant dry weight (leaf area ratio). Increased leaf number, enhanced branching and shorter internodes resulted in a more compact appearance of less shaded seedlings. Irrespective of light conditions, mangosteen seedlings exhibited inherently slow growth because of low photosynthetic rates per unit leaf area, low SLA, low leaf area ratios and inefficient root systems.

Gas exchange responses of mangosteen (Garcinia mangostana L.) leaves to photosynthetic photon flux density (PPFD), internal CO(2) concentration (C(i)), leaf-air vapor pressure deficit (VPD), leaf temperature (T(l)) and time of day were investigated in plants grown in three shade treatments. Maximal photosynthetic rate (P(n(max))) per unit leaf area at light saturation did not differ significantly among plants grown in the different shade treatments despite significant morphological differences. Light compensation point (9-15 micro mol m(-2) s(-1)) and quantum yield (0.022-0.023) did not differ significantly among treatments, whereas light saturation point was significantly higher for leaves grown in 20% shade than for leaves grown in 50 or 80% shade (951, 645 and 555 micro mol m(-2) s(-1), respectively). Shade treatments significantly affected assimilation responses to varying CO(2) concentrations. At CO(2) concentrations between 600 and 1000 micro mol mol(-1), leaves from the 20% shade treatment recorded higher P(n) (6.44 micro mol m(-2) s(-1)) than leaves from the 80% shade treatment (4.57 micro mol m(-2) s(-1)). Stomatal conductance (g(s)) decreased with increasing CO(2) concentrations. Vapor pressure deficits higher than 2.5 kPa significantly decreased P(n) and g(s), whereas P(n) remained steady over a 24-33 degrees C temperature range in leaves in 80% shade and over a 27-36 degrees C range in leaves in 20 and 50% shade. Highest carbon gain during a 12-hour photoperiod was observed for leaves grown in 50% shade. The results imply that mangosteen is a shade-tolerant, lower canopy tree adapted to humid tropical lowlands. Providing low VPD in the nursery may result in maximal growth. However leaves of seedlings grown in 50 or 20% shade tolerated high VPD and temperatures better than leaves of seedlings grown in 80% shade, indicating some adaptability to the seasonally wet and dry tropics of northern Australia.

Seasonal changes in the high temperature limit for photosynthesis of desert winter annuals growing under natural conditions in Death Valley, California were studied using an assay based upon chlorophyll fluorescence. All species of this group were 6 to 9 degrees C more tolerant of high temperature at the end of the growing season (May) than at its beginning (February). Over this same time period, the mean daily maximum air temperatures increased by 12 degrees C. Laboratory studies have demonstrated that increases in thermal tolerance could be induced by increasing growth temperature alone. For plants growing under field conditions there was also a good correlation between the thermal tolerance of leaves and the osmotic potential of leaf water, indicating that increases in the concentrations of some small molecules might also confer increased thermal tolerance. Isolated chloroplast thylakoids subjected to increasing concentrations of sorbitol could be demonstrated to have increased thermal tolerance.

Spinach (Spinacia oleracea) plants were subjected to salt stress by adding NaCl to the nutrient solution in increments of 25 millimolar per day to a final concentration of 200 millimolar. Plants were harvested 3 weeks after starting NaCl treatment. Fresh and dry weight of both shoots and roots was decreased more than 50% compared to control plants but the salt-stressed plants appeared healthy and were still actively growing. The salt-stressed plants had much thicker leaves. The salt-treated plants osmotically adjusted to maintain leaf turgor. Leaf K(+) was decreased but Na(+) and Cl(-) were greatly increased.The potential photosynthetic capacity of the leaves was measured at saturating CO(2) to overcome any stomatal limitation. Photosynthesis of salt-stressed plants varied only by about 10% from the controls when expressed on a leaf area or chlorophyll basis. The yield of variable chlorophyll a fluorescence from leaves was not affected by salt stress. Stomatal conductance decreased 70% in response to salt treatment.Uncoupled rates of electron transport by isolated intact chloroplasts and by thylakoids were only 10 to 20% below those for control plants. CO(2)-dependent O(2) evolution was decreased by 20% in chloroplasts isolated from salt-stressed plants. The concentration of K(+) in the chloroplast decreased by 50% in the salt-stressed plants, Na(+) increased by 70%, and Cl(-) increased by less than 20% despite large increases in leaf Na(+) and Cl(-).It is concluded that, for spinach, salt stress does not result in any major decrease in the photosynthetic potential of the leaf. Actual photosynthesis by the plant may be reduced by other factors such as decreased stomatal conductance and decreased leaf area. Effective compartmentation of ions within the cell may prevent the accumulation of inhibitory levels of Na(+) and Cl(-) in the chloroplast.

Winter- and summertime-active desert annual species were grown at different temperatures to assess their capacity for photosynthetic acclimation. Thermal stability of photosynthesis was determined from responses of chlorophyll fluorescence to increased temperature. Photosynthesis in winter ephemerals grown at 28 degrees C/21 degrees C became unstable close to 41 degrees C in contrast to the summer annuals which were stable up to about 46 degrees C. Growth at higher temperature (43 degrees C/32 degrees C) resulted in increases in thermal stability of 5 to 7 degrees C for the winter annuals and 3 to 4 degrees C for the summer annuals, showing that temperature can provide the primary stimulus for acclimation of the photosynthetic apparatus. The magnitude of these changes was very similar to the range of field values observed for the respective floras, indicating that the thermal acclimation response under field conditions was qualitatively similar to that occurring under controlled growth conditions. Perennial species, co-existing with these annuals in the desert, were on average more thermostable. The cacti were exceptionally heat stable, the threshold for fluorescence increase averaging 55 degrees C.

The gas exchange of spinach plants, salt-stressed by adding NaCl to the nutrient solution in increments of 25 millimolar per day to a final concentration of 200 millimolar, was studied 3 weeks after starting NaCl treatment. Photosynthesis became light saturated at 1100 to 1400 micromoles per square meter per second in salt-treated plants and at approximately 2000 micromoles per square meter per second in control plants. Photosynthetic capacity of the mesophyll measured as a function of intercellular partial pressure of CO(2) at the light intensity prevailing during growth and at light saturation were both decreased in the salttreated plants. The CO(2) compensation points and relative enhancements of photosynthesis at low O(2) were not affected by salinity. The lower photosynthetic rates in salt-treated leaves at 450 micromoles per square meter per second were associated with a 70% reduction in stomatal conductance and low intercellular CO(2) (219 microbars; cf. 285 microbars for controls). Increasing photon flux density to light saturation extended the linear portions of the CO(2) response curves, increased stomatal conductances, increased intercellular CO(2) in the salt-treated plants, but lowered it in controls, and accentuated differences in photosynthetic rate (area basis) between the treatments.Leaves from salt-treated plants were thicker but contained about 73% of the chlorophyll per unit area of control plants. When photosynthetic rates were expressed on a chlorophyll basis there was no difference in initial slope of assimilation versus intercellular CO(2) between treatments. Photosynthetic rates (chlorophyll basis) at light saturation differed only by 20% which was also observed earlier with isolated, intact chloroplasts (Robinson et al. 1983 Plant Physiol 73: 238-242).Measurement of carbon isotope ratio revealed less discrimination against (13)C with salt treatment and confirmed the persistence of low intercellular partial pressures of CO(2) during plant growth. The development of a thicker leaf with less chlorophyll per unit area during salt treatment permitted stomatal conductance and intercellular partial pressure of CO(2) to decline without restricting photosynthesis and had the benefit of greatly increasing water use efficiency.

The temperature threshold for the onset of irreversible loss of photosynthetic capacity of leaves was examined in studies of net CO(2) exchange and by chlorophyll fluorescence techniques. Close agreement was found between the temperature threshold for a dramatic increase in the fluorescence of chlorophyll from intact leaves and the leaf temperature at which the capacity for photosynthetic CO(2) fixation (measured at rate saturating light intensity by infrared gas analysis) began to be temperature unstable (i.e. decline with time of exposure to a constant temperature). This decline in CO(2) uptake was not a result of a stomatal response yielding a reduced intercellular CO(2) concentration at high temperature, and it is interpreted as an indication of progressive damage to some essential component(s) of the leaf. The temperature-dependent change in chlorophyll fluorescence apparently also indicated the onset of this damage. The fluorescence assay could be conducted with discs of leaves collected from remote locations and kept moist while they were transported to a central location, allowing assessment of the high temperature tolerance of leaves which developed under natural field conditions. These assays were verified using a mobile laboratory to study gas exchange of attached leaves in situ. The high temperature sensitivity of leaves of plants growing under natural conditions were similar to those of the same species grown in controlled environments of similar thermal regimes. High temperature in controlled environment studies brought about acclimation responses which increased the threshold for high temperature damage as measured by gas exchange. Studies of fluorescence versus temperature confirmed that this method could be used to quantify these responses, and permitted the kinetics of the acclimation response to be examined. Gas exchange studies, while providing similar estimates of thermal stability, required more time, more elaborate instrumentation, and are particularly difficult to conduct with field plants growing in situ.

Intact chloroplasts were isolated from spinach, sugarbeet, and pea leaves by a rapid procedure which included purification on a two-step Percoll gradient. The content of K+, Na+, and Cl- was determined together with the chloroplast volume from which the apparent concentration of these ions in the chloroplast was calculated. It was demonstrated by successive washing that K+ leaked out of the chloroplasts during isolation whereas Na+ and Cl- did not. For spinach and sugarbeet chloroplasts K+ was the predominant monovalent cation (160-200 mM) with Na+ present in lesser amounts (40-70 mM). Pea chloroplasts contained equal amounts of K+ and Na+ (40 mM). The Cl- concentration in the chloroplast was similar for the three species and was calculated to be near 100 mM. The Cl- concentration in the chloroplast was well above that of the leaf tissue implying accumulation of Cl- in the chloroplast. Qualitatively similar results were obtained for chloroplasts isolated by rapidly fractionating protoplasts of wheat and barley although the concentration of ions was lower than for the above three species. Spinach grown in 200 mM NaCl accumulated high levels of Na+ and Cl- in the leaves but K+ was decreased. The replacement of K+ by Na+ was also observed in the chloroplasts but the level of Cl- in the chloroplast was not greatly increased even at leaf Cl- in excess of 200 mM. Replacement of K+ by Na+ in the nutrient solution also resulted in replacement of K+ by Na+ both in the leaf and in the chloroplast. Spinach grown under Cl- deficiency showed a 70% decrease in growth and a similar decrease in leaf Cl- but the concentration of Cl- in the chloroplast was not significantly decreased. It is concluded that in most instances K+ is the dominant monovalent cation in chloroplasts, although replacement by Na+ can occur to some extent. Chloride is a major anion, being accumulated within the chloroplast at low leaf Cl- and excluded from the chloroplast at high leaf Cl-. Ionic compartmentation within the leaf cell appears to be regulated to maintain a relatively constant level of Cl- in the chloroplast.

A method is described for separating mesophyll and bundle sheath chloroplasts from the leaves of C(4) plants. The agranal bundle sheath chloroplasts are inactive in the Hill reaction, whereas granal bundle sheath and granal mesophyll chloroplasts exhibit normal photosystem II activity. The agranal bundle sheath chloroplasts are deficient in photosystem II; they lack cytochrome b-559 and the fluorescence bands associated with photosystem II. All the chloroplasts exhibit photosystem I activity.

Some previous studies of photorespiration and glycolate oxidation were re-examined and correlated by infra-red CO(2) analysis. Data about rate of photosynthesis and oxygen sensitivity indicated that complete inhibition of photosynthesis with 3-(3,4-dichlorophenyl)-1,1 dimethyl urea (DCMU) allowed dark respiration to continue in the light. Photorespiration was also inhibited. The oxygen sensitivity of glycolate-stimulated CO(2) production was found to be compatible with the proposal that glycolate is a substrate of photorespiration. Both ;in vivo' and ;in vitro' studies of the alga Nitella flexilis have revealed a pathway of glycolate oxidation similar to that of higher plants. DCMU inhibition of photosynthesis by Nitella gave results similar to those for the monocotyledons tested. Under very low light intensity, carbon dioxide compensation in corn was measurable but was not sensitive to high oxygen concentration. It appears that the lack of photorespiration in this plant is not the end result of efficient internal recycling of CO(2) to photosynthesis.