Osmotic Adjustment by Intact Isolated Chloroplasts in Response to Osmotic Stress and Its Effect on Photosynthesis and Chloroplast Volume

Abstract

Spinach leaf chloroplasts isolated in isotonic media (330 millimolar sorbitol, -1.0 megapascals osmotic potential) had optimum rates of photosynthesis when assayed at -1.0 megapascals. When chloroplasts were isolated in hypertonic media (720 millimolar sorbitol, -2.0 megapascals osmotic potential) the optimum osmotic potential for photosynthesis was shifted to -1.8 megapascals and the chloroplasts had higher rates of COrdependent O2 evolution than chloroplasts isolated in 330 millimolar sorbitol when both were assayed at high solute concentrations.Transfer of chloroplasts isolated in 330 millimolar sorbitol to 720 millimolar sorbitol resulted in decreased chloroplast volume but this shrinkage was only transient and the chloroplasts subsequently swelled so that within 2 to 3 minutes at 20°C the chloroplast volume had returned to near the original value. Thus, actual steady state chloroplast volume was not decreased in hypertonic media. In isotonic media, there was a slow but significant uptake of sorbitol by chloroplasts (10 to 20 micromoles per milligram chlorophyll per hour at 20C). Transfer of chloroplasts from 330 millimolar sorbitol to 720 millimolar sorbitol resulted in rapid uptake of sorbitol (up to 280 micromoles per milligram chlorophyll per hour at 20C) and after 5 minutes the concentration of sorbitol inside the chloroplasts exceeded 500 millimolar. This uptake of sorbitol resulted in a significant underestimation of chloroplast volume unless 1'4Clsorbitol was added just prior to centrifuging the chloroplasts through silicone oil. There is already some experimental evidence for osmotic adjustment by chloroplasts in response to stress. Spinach plants exposed to salt-stress have decreased osmotic potentials, yet photosynthetic capacity is not decreased (12,14), suggesting that osmotic adjustment in the chloroplasts somehow prevents the inhibition which is observed when chloroplasts are exposed to decreased osmotic potentials in vitro (2, 3, 5, 1 1). The concentration of inorganic ions increased in the leaf in response to salt stress but not in the chloroplast (12, 14) suggesting accumulation of organic solutes in the chloroplasts. Indeed, chloroplasts isolated from the salt-stressed plants had increased levels of sugars and amino acids (12) although these were not sufficient to account fully for the decrease observed in cell sap osmotic potential. Chloroplasts isolated from Mesembryanthemum grown under high salt had optimal photosynthetic rates at higher sorbitol concentrations in vitro than did chloroplasts grown in low salt (5). These observations are consistent with the accumulation of compatible solutes in chloroplasts to provide osmotic adjustment in response to salt stress.In this paper, I have investigated the possibility of inducing osmotic adjustment in chloroplasts in vitro in an attempt to determine the influence of such osmoregulation on photosynthesis. The

Leaf Water Relations and Maintenance of Gas Exchange in Coffee Cultivars Grown in Drying Soil

“…Recently, a correlation between protoplast volume and photosynthetic capacity has been demonstrated in intact leaves of growth chamber-grown wheat (21). The existence of an apparent optimum protoplast volume for maximal photosynthetic activity in coffee is in agreement with results from in vitro studies cited above (7,20). The optimal relative symplast volume for maximum A in coffee was approximately 0.94, above which A declined by about 40% at maximum symplast volume attained at midday under field conditions (Fig.…”

“…Previous in vitro studies carried out with isolated chloroplasts (2,20) and leaf slices (7) have led others to conclude that inhibition of photosynthesis in water-stressed plants may be largely attributable to reduced chloroplast volume. It has also been suggested that drought-induced alterations in the protoplast, and therefore chloroplast volume-IL relationship may serve to maintain photosynthetic activity during drought (23).…”

Assessment of the requirement for aquaporins in the thylakoid membrane of plant chloroplasts to sustain photosynthetic water oxidation

“…Some considerations are summarized below. First, measurement of the ion content for dark-adapted chloroplasts indicated K + being the major monovalent cation (160-200 mM), which could be replaced by Na + (40-70 mM), and Cl À as the major anion (100 mM) [35]. In the same report, the major fraction of these ions was found in the stroma since only 10%, 5-10% and 10-20% of the above concentrations of K + , Na + and Cl À were found inside the thylakoid lumen.…”

Plastid Osmotic Stress Activates Cellular Stress Responses in Arabidopsis

“…The same is presumably true of C S across organelle membranes, where metabolism and transport activities in both the organelle and the cytoplasm can produce substantial differences in solute content. Organelles have been shown to change shape and volume in response to alterations in environmental osmolarity and during metabolic activity (Robinson, 1985;Gupta and Berkowitz, 1987;McCain, 1995;Boustany et al, 2002;Finan et al, 2009), suggesting that they can undergo osmoregulation from within the cell cytoplasm. However, the molecular details of organellar osmoregulation and how it might be integrated into the larger Figure 6.…”