Enhancement by sodium on the growth of the xerophyte Zygophyllum xanthoxylum is achieved by maintaining efficient photosynthesis when stomatal aperture is minimized

In most terrestrial plants, large quantities of water inevitably are diffused into the air via open stomata for CO2 absorption during photosynthesis. It is difficult to reduce such an intrinsic demand of water for transpiration but still maintain photosynthesis. Interestingly, the xerophyte Zygophyllum xanthoxylum evolves an outstanding ability to resolve this dilemma by accumulating Na+ to stimulate growth while reducing transpiration. However, the underlying mechanisms are still unclear. Two-week-old Z. xanthoxylum plants were treated with 50 mM NaCl (Na) for 7 days to investigate their growth, gas-exchange related parameters, chlorophyll fluorescence parameters, leaf anatomical characteristics, and related biochemical traits. NaCl treatment reduced the stomatal opening and transpiration but markedly enhanced the plant biomass. Analysis showed that a higher efficiency of intrinsic water use (WUEi) and photosynthetic chlorophyll (P-Chl) in leaves of Na-treated plants. A lower CO2 partial pressure in substomatal cavities, mainly accounting for higher WUEi, which efficiently facilitates CO2 extraction from air via partially opening stomata in leaves of Na-treated plants, may be largely due to increased internal cell surface and reduced CO2 diffusion path between chloroplasts and corresponding cell wall, in addition to increased expression of aquaporin and induced C-4 or CAM-like metabolism. Increased leaves area, reduced chlorophyll content per unit weight, and enhanced operating efficiency of Photosystem II allowed more efficient light harvesting, deeper light penetration and more light using efficiency, and so would also benefit this water-saving photosynthesis. Our data demonstrate that Z. xanthoxylum efficiently rematches both CO2 intake and light use to fulfill vigorous growth under significantly reduced stomatal aperture and markedly decreased water consumption. The mechanisms that Z. xanthoxylum uses to achieve more biomass while using less water give a valuable clue for future improvement of agricultural production in water-limited areas.

Automated summary

The study demonstrates that Zygophyllum xanthoxylum efficiently enhances growth, reduces transpiration, and improves intrinsic water use efficiency and photosynthetic chlorophyll by accumulating Na+. By adjusting stomatal opening, CO2 intake, and light utilization efficiency, Z. xanthoxylum achieves more biomass production while using less water, providing valuable insights for enhancing agricultural production in water-limited areas.