Photosynthetic Adaptations and Oxidative Stress Tolerance in Halophytes from Warm Subtropical Region

Abstract

This chapter attempts to synthesize recent work related to photosynthetic adaptations and oxidative stress mitigation mechanisms of perennial warm subtropical halophytes of Karachi, Pakistan. In most species the light-harvesting photochemical reactions appear less sensitive to increase in salinity treatments in comparison with those of carbon fixation. This was evident from unaltered F v /F m values which indicate the maximum photochemical efficiency and the degree of photoinhibition of PSII. However, there was a reduction in electron transport rate (ETR) and a rise in non-photochemical quenching (NPQ) by heat dissipation. Decrease in net photosynthetic rates (P N), stomatal conductance (gs), and transpiration

Effects of Salinity Stress on Chloroplast Structure and Function

“…Many halophytes are reported to exhibit reversible midday photoinhibition of PSII activity to limit excitation of the PSII reaction centers [130,134]. This mechanism also minimizes the possibility of ROS formation in salt-stressed plants under high light and is considered an important ecophysiological adaptation to salinity [31]. A decrease in the antennae size due to decreased chlorophyll content was also observed in Arthrocnemum macrostachyum to limit PSII excitation [132].…”

“…Under saline conditions, decreases in CO 2 assimilation via the Calvin cycle accompany a decrease in photochemical electron sink, which in the presence of light impacts the functioning/efficiency of photosystems [31]. In some sensitive plants such as olives, decreases in the F v /F m ratios indicate the incidence of photodamage under saline conditions [35].…”

“…Among C 3 plants, salinity reportedly resulted in poor PSII function in glycophytes such as rice and Arabidopsis [41,129] but not in the halophyte Arthrocnemum macrostachyum [130,131]. Several tolerant species, including halophytes such as Sarcocornia fruticosa [132] and Atriplex centralasiatica [133], also employ the xanthophyll cycle for non-photochemical quenching that dissipates excess excitation energy of PSII in the form of heat as a 'first line of defense' [31,133], thus preventing the formation of potentially cytotoxic reactive ROS. The xanthophyll cycle enzyme violaxanthin de-epoxidase consumes NADPH, which if accumulated may cause the over-reduction of reaction centers, and thereby enhance ROS (especially superoxide) formation [134].…”

“…Information on the effects of salinity on chloroplast CO 2 assimilation enzymes is limited among halophytes. Generally, CO 2 assimilation reactions are considered more sensitive to salinity than photochemical reactions of photosynthesis [31]. Several studies have reported that salinity generally inhibits many enzymes of the Calvin cycle [137,142,143].…”

“…Salt-induced toxicity negatively affects all these processes, resulting in poor plant growth and reduction in yield. Chloroplasts are also major reactive oxygen species (ROS) production sites at the reaction centers of PSII and PSI, where charge separation occurs, and the electron transport chain (ETC) from PSII to PSI are highly sensitive to salt-induced toxicity under which ROS production is further increased [31]. Higher concentrations of ROS cause oxidative damage to membranes, lipids, nucleic acids, proteins and some photosynthetic enzymes, resulting in reduced CO 2 fixation, slower plant growth and consequently low crop yields.…”