Elevated CO2 reduces stomatal and metabolic limitations on photosynthesis caused by salinity in Hordeum vulgare

Abstract

The future environment may be altered by high concentrations of salt in the soil and elevated [CO(2)] in the atmosphere. These have opposite effects on photosynthesis. Generally, salt stress inhibits photosynthesis by stomatal and non-stomatal mechanisms; in contrast, elevated [CO(2)] stimulates photosynthesis by increasing CO(2) availability in the Rubisco carboxylating site and by reducing photorespiration. However, few studies have focused on the interactive effects of these factors on photosynthesis. To elucidate this knowledge gap, we grew the barley plant, Hordeum vulgare (cv. Iranis), with and without salt stress at either ambient or elevated atmospheric [CO(2)] (350 or 700 μmol mol(-1) CO(2), respectively). We measured growth, several photosynthetic and fluorescence parameters, and carbohydrate content. Under saline conditions, the photosynthetic rate decreased, mostly because of stomatal limitations. Increasing salinity progressively increased metabolic (photochemical and biochemical) limitation; this included an increase in non-photochemical quenching and a reduction in the PSII quantum yield. When salinity was combined with elevated CO(2), the rate of CO(2) diffusion to the carboxylating site increased, despite lower stomatal and internal conductance. The greater CO(2) availability increased the electron sink capacity, which alleviated the salt-induced metabolic limitations on the photosynthetic rate. Consequently, elevated CO(2) partially mitigated the saline effects on photosynthesis by maintaining favorable biochemistry and photochemistry in barley leaves.

Physiological factors involved in positive effects of elevated carbon dioxide concentration on Bermudagrass tolerance to salinity stress

“…Elevated CO 2 concentration has been shown to increase carboxylation rate of ribulose carboxylase/ oxygenase enzyme (Rubisco) or alleviation of metabolic limitations controlling photosynthesis in some plant species (Yelle et al, 1989;Yu et al, 2014). The alleviation of salt-indced metabolic limitations on P n was mainly through the increase in electron sink capacity (Pérez-López et al, 2012). In addition, other parameters involved in Calvin cycle were also improved by elevated CO 2 concentration, such as photosynthetic electron transport rate (J max ) and the ratio of C to N .…”

<b>Salinity reduces carbon assimilation and the harvest index of cassava plants (<i>Manihot esculenta</i> Crantz)

“…This result imply that the restriction of CO 2 diffusion from the outside air into the chloroplast was one of the factors responsible for lower A, as was the case of the newly expanded leaves of the plants grown with 40 mM NaCl, which showed lower intercellularCO 2 concentrations. All other saline treatments showed an increase in intercellular CO 2 concentration, clearly indicating that in addition to lower stomatal conductance, salinity may have increased mesophilic resistance to the ingress of atmospheric CO 2 into carboxylation sites and/or decreased the enzymatic activities associated with photosynthetic carbon metabolism, a common effect in plants grown under saline conditions (Chaves et al, 2009;Pérez-López et al, 2012). Another important aspect, as deduced from a lower HI, is that the tuberous roots' ability to metabolize sucrose unloaded from phloem could have been impaired, leading sugar accumulation in the source leaves, which can determine photosynthesis inhibition, a process known as "photosynthetic repression feedback".…”

“…In fact, the growth reduction induced by NaCl has also been correlated with low photosynthate supply to the sink organs as a consequence of the reduction in the photosynthetic capacity under these conditions (Eisa, Hussin, Geissler, & Koyro, 2012). Decreases in photosynthesis can occur by two main mechanisms: (i) restricted diffusion (CO 2 flux to carboxylation sites) caused by decreases in stomatal and internal conductance; and (ii) the inhibition of the metabolic potential for photosynthesis (Pérez-López, Robredo, Lacuesta, Mena-Petite, & Muñoz-Rueda, 2012). Plants develop several physiological and biochemical mechanisms in order to survive in soil with high salt concentrations.…”

Response of Wheat Seedlings to Combined Effect of Drought and Salinity

“…This was shown in different plant species (Chen et al 2010;Pérez-L opez et al 2012). The simultaneous application of NaCl and PEG in nutrient solution, however, increases restoration of maximal photosynthetic intensity and stomatal conductivity after stress.…”