Linking detrimental effects of salinity on leaf photosynthesis with ion transport in leaf mesophyll

Soil salinity is defined as a situation when the electrical conductivity of the saturated paste extract from the upper layers of the soil is in excess of 4 dSm-1 (which equates to ~40 mM NaCl). Soil salinisation is an ever-growing problem, resulting in vast losses of agricultural land and costing industry in excess of $12 billion in the lost production globally. The only practical way of dealing with increasing land salinisation is to develop ways of growing plants in these saline environments, either through breeding/ engineering of salinity tolerant plants or through the application of chemicals that have ameliorative effect on plant performance under saline conditions. Both of these approaches have been tried in the past, but with a rather limited success. To increase the efficiency of breeding/ engineering and the development of chemical applications, the processes by which salinity affects key intracellular structures and processes, and physiological mechanisms behind salinity tolerance, should be understood. Salinity causes osmotic and ionic limitations to growth. While osmotic effects causing reduced growth within minutes, ionic limitations take longer but are predominate long lasting effects. Less tolerant plants (glycophytes) tend to be Na+ excluders while naturally more tolerant plants (halophytes) tend to include Na+. Na+ has long been believed to be toxic in the cytosols of cells, however, its uptake into the shoot is necessary to provide low carbon-cost osmotic adjustment, which is essential for plants to maintain tissue turgor and expansion growth under saline conditions. The exact biochemical/ physiological targets of Na+ stress in the cytosol are not well understood, despite many years of research. It has long been suggested that the maintenance of high K+/Na+ ratios in the cytosol is more important than the Na+ concentration itself. Despite being a long-standing paradigm in salinity tolerance, this notion has not been well tested either in vitro or in vivo. The major aim of this work was to investigate mechanisms of non-stomatal limitation to photosynthesis caused by salinity stress and link them with ionic relations and ion transport across mesophyll cell plasma and chloroplast membranes. By doing this the following objectives were addressed: ‚Äö To understand effects of salinity on ionic homeostasis and light-induced changes in ion transport in leaf mesophyll, in the context of leaf photosynthetic and growth responses. ‚Äö To elucidate effects of altered ionic homeostasis and cytosolic osmolality on photosynthetic performance in chloroplasts. ‚Äö To reveal differences between chloroplast responses to salinity from species contrasting in salinity tolerance, e.g. halophytes and glycophytes. ‚Äö To the link ion transport in the chloroplast and their resilience to perturbed ionic homeostasis. ‚Äö To investigate effects of ameliorative chemicals (polyamines, compatible solutes and antioxidants) on salinity-induced disturbance to leaf photosynthetic machinery, at the chloroplast level. ‚Äö To understand the differences in mesophyll ion transport between species with contrasting salinity tolerance and link them with primary photosynthetic processes in green leaf tissues. For plants to be truly salinity tolerant they need to be able to handle elevated Na+ concentrations in leaves. The ability to handle Na+ in leaves is called tissue tolerance. Using non-invasive microelectrode ion flux measuring (the MIFE) technique it was found that an increase in salinity within the apoplast of bean leaves results in a large K+ efflux from mesophyll tissue. This efflux was mediated predominately by K+ outward rectifying channels (84%), with the remainder of the efflux being through non-selective cation channels. The reduction in K+ concentration associated with this efflux was linked to a decline in the photochemical efficiency of photosystem II (chlorophyll fluorescence FV/FM values). In addition to K+ efflux, Na+ has also induced a vanadate sensitive H+ efflux presumably mediated by the plasma membrane H+-ATPase. This H+ efflux is essential for the maintenance of membrane potential and ion homeostasis in the cytoplasm of bean mesophyll. Salinity also caused reductions in the ability of bean mesophyll tissue to respond to light with both K+ and H+ fluxes. This decline in response was both time- and NaCl concentrationdependant. The effects of salinity-induced altered ionic conditions in the cytoplasm of mesophyll cells on photosynthesis were assessed.