Differentiating day from night effects of high ambient [CO2] on the gas exchange and 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 micromoles 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 micromoles mol-1 decreased Rd in both control and salinized plants (by 20% at 1000, and by 50% at 2000 micromoles 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 micromoles mol-1 throughout the day, to 31% in those grown continuously in 900 micromoles mol-1 [CO2]. The effect of [CO2] at night on salinized plants depended on the daytime CO2 concentration. Under 350 micromoles mol-1 day-time [CO2], 900 micromoles mol-1 at night reduced growth over a 4-week period by 9% (P < 0.05) and 1700 micromoles mol-1 reduced it by 14% (P < 0.01). However, under 900 micromoles mol-1 day-time [CO2], 900 vs. 350 micromoles mol-1 [CO2] at night increased growth by 17% (P < 0.01). It is concluded that 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.