Photosynthesis under osmotic stress

Abstract

1. Photosynthesis of leaf slices, mesophyll protoplasts, and intact chloroplasts of spinach was inhibited in hypertonic sorbitol solutions. Sorbitol could be replaced by other nonpenetrating osmotica such as sucrose or glycinebetaine. As a penetrating solute, ethyleneglycol was also inhibitory, but osmolarities required for inhibition of photosynthesis were considerably higher than in the case of non-penetrating osmotica.-2. With leaf slices and protoplasts, 50% inhibition by sorbitol was usually observed at osmotic potentials between 25 and 40 bar. With isolated intact chloroplasts, the osmotic potentials producing 50% inhibition varied considerably. Depending on the growth conditions of the plant material, 50% inhibition occurred between 14 and 40 bar. The integrity of the chloroplast envelope as measured by the accessibility of the thylakoid system for ferricyanide was not affected by osmotic stress.-3. Quantum requirements for CO2 assimilation and reduction of 3-phosphoglycerate or nitrite by intact chloroplasts increased under osmotic stress. The increase was larger for CO2 reduction than for reduction of 3-phosphoglycerate or nitrite.-4. In intact chloroplasts, electron transport to methylviologen was not much affected by osmotic stress. Basal electron transport was not stimulated, suggesting absence of uncoupling.-5. The increase in ATP/ADP ratios on illumination of intact chloroplasts was slower at an osmotic potential of 36 bar than at 11 bar.-6. The results indicate that inhibition of photosynthesis is not caused by the sensitivity of a single photosynthetic reaction to increased osmotic potentials. Rather, several reactions are sensitive to water stress. Osmotic stress acts on the photosynthetic apparatus mainly at the level of dark reactions and ATP synthesis, and much less on primary photoreactions or electron transport, between water and the primary oxidant of photosystem I.-7. The different sensitivity of chloroplasts to penetrating and non-penetrating solutes and the observed variability of chloroplast sensitivity to stress suggests that the reduction in water potential is not directly responsible for damage to the photosynthetic apparatus during osmotic stress. Rather, the composition of the chloroplasts appears to be a decisive factor which determines sensitivity or resistance to osmotic stress.

Causes of decreased photosynthetic rate and metabolic capacity in water-deficient leaf cells: a critical evaluation of mechanisms and integration of processes

“…Evaluation of the effects of WD on A pot has come from assessing the response of A to increasing external CO 2 concentration (C a ) and calculated C i . The view that g s determines A, even at substantial WD (70 -60 % RWC or less), rests on studies where removing the epidermis (and thus g s ) or increasing C a overcomes the limiting g s (Kaiser and Heber, 1981;Dietz and Heber, 1983;Kaiser 1984Kaiser , 1987Quick et al, 1992;Tourneux and Peltier, 1995;Cornic, 2000). However, a pattern is apparent (see Table 1, and references therein): these studies used plants grown at low irradiance with WD that developed quickly in no or weak light before measurement.…”

“…(1) Plants are grown under glasshouse or controlledenvironment conditions, often at low light, and samples of leaf are taken and subjected to WD under no or low light, resulting in rapid stress (Kaiser and Heber, 1981;Dietz and Heber, 1983;Kaiser 1984Kaiser , 1987Renou et al, 1990;Tourneux and Peltier, 1995).…”

Effects of Drought on Primary Photosynthetic Processes of Cotton Leaves

“…However, recent studies (2,16) of drought on in vivo fluorescence transients from cotton leaves. Indeed fluorescence induction is a difficult but powerful method of determining the functioning of photosynthesis in vivo (for a critical review, see [18]).…”

Chloride involvement in the synthesis, functioning and repair of the photosynthetic apparatusin vivo

“…Kaiser et al . (1981) also found a relatively low quantum requirement of 12 mol absorbed photons per mol O 2 produced by the isolated chloroplasts. Earlier work by Larkum (1968) used chloroplasts extracted by nonaqueous techniques from Beta vulgaris , from the same family (Amaranthaceae) as S. oleracea .…”

“…Earlier work found higher (60-100 mol m À3 ) Cl À concentrations in aqueously isolated chloroplasts of S. oleracea (Kaiser et al, 1983;Robinson et al, 1983;Robinson & Downton, 1984;Demmig & Winter, 1986); these higher Cl À concentrations were shown to be a result of interference of chloroplast components with amperometric titration assays of Cl À (Schr€ oppel-Meier & Kaiser, 1987). The methodology of aqueous chloroplast extraction produces chloroplasts with CO 2 -and light-saturated rates on a Chl basis that were at least as high as those found for leaf slices of S. oleracea (Kaiser et al, 1981). Kaiser et al (1981) also found a relatively low quantum requirement of 12 mol absorbed photons per mol O 2 produced by the isolated chloroplasts.…”