Differentiating day from night effects of high ambient [CO₂] on the gas exchange and growth of Xanthium strumarium L. exposed to salinity stress

Sodium chloride, at a concentration of 88 mol m^-3 in half strength Hoagland nutrient solution, increased dry weight per unit area of Xanthium strumarium L. leaves by 19%, and chlorophyll by 45%, compared to plants grown without added NaCl at ambient (350 μmol mol^-1) CO₂ concentration. Photosynthesis, per unit leaf area, was almost unaffected. Even so, over a 4-week period, growth (dry weight increment) was reduced in the salt treatment by 50%. This could be ascribed to a large reduction in leaf area (> 60%) and to an approx. 20% increase in the rate of dark respiration (Rd). Raising ambient [CO₂] from zero to 2000 μmol mol^-1 decreased Rd in both control and salinized plants (by 20% at 1000, and by 50% at 2000 μmol mol^-1 CO₂ concentration) compared to Rd in the absence of ambient CO₂. High night-time [CO₂] had no significant effect on growth of non-salinized plants, irrespective of day-time ambient [CO₂]. Growth reduction caused by salt was reduced from 51% in plants grown in 350 μmol mol^-1 throughout the day, to 31% in those grown continuously in 900 μmol mol^-1 [CO₂]. The effect of [CO₂] at night on salinized plants depended on the daytime CO₂ concentration. Under 350 μmol mol^-1 day-time [CO₂], 900 μmol mol^-1 at night reduced growth over a 4-week period by 9% (P < 0.05) and 1700 μmol mol^-1 reduced it by 14% (P < 0.01). However, under 900 μmol mol^-1 day-time [CO₂], 900 vs. 350 μmol mol^-1 [CO₂] at night increased growth by 17% (P < 0.01). It is concluded thai there is both a functional and an otiose (functionless) component to Rd, which is increased by salt. Under conditions of low photosynthesis (such as here, in the low day-time [CO₂] regime) the otiose component is small and high night-time [CO₂] partly suppresses functional Rd, thereby reducing salt tolerance. In plants growing under conditions which stimulate photosynthesis (e.g. with increased daytime [CO₂]), elevated [CO₂] at night suppresses mainly the otiose component of respiration, thus increasing growth. Consequently, in regions of adequate water and sunlight, the predicted further elevation of the world atmospheric [CO₂] may increase plant salinity tolerance.