A comparative analysis of stomatal traits and photosynthetic responses in closely related halophytic and glycophytic species under saline conditions

To understand the adaptive strategies employed by plants to deal with saline conditions, two halophytic species [Chenopodium quinoa (quinoa) and Beta maritima (sea beet)] and their glycophytic relatives [Chenopodium album and Beta vulgaris (sugar beet)] were grown under 0−500 mM salt concentrations followed by the comprehensive assessment of their agronomical, ionic, gas exchange characteristics. Salinity levels up to 300 mM NaCl had no adverse effect on quinoa biomass and 200 mM NaCl stimulated sea beet growth. Stomatal conductance decreased in a dose-dependent manner in all species with increasing NaCl concentrations. However, CO₂ assimilation rates remained constant or displayed higher values at the medium level of salinity (100−200 mM NaCl) in quinoa, sugar beet and sea beet. High maximum carboxylation rate of Rubisco (V_cmax) and higher rate of electron transport through photosystem II (J) were responsible for the high photosynthetic rates and biomass productions under these conditions. Both characteristics were much higher in halophytic species for the given external NaCl level. Stomatal densities were intrinsically lower in halophytic species (14–34%); these increased with increasing salinity levels in the sugar beet and sea beet while C. album and quinoa stomata remained less dense under saline conditions. Stomata responses to environmental stimuli were much faster in halophytes (16.6–49.7%), and substitution of K⁺ by Na⁺ resulted in promotion of stomatal opening under 50 mM NaCl and 50 mM KCl in quinoa. It is concluded that superior salinity tolerance in halophytes is achieved by significantly faster stomatal opening and closure, their ability to discriminate K⁺ over Na⁺, and uncoupling of CO₂ assimilation from changes in stomatal aperture.