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 H oagland nutrient solution, increased dry weight per unit area of Xanth ium strumarium L. leaves by 19%, and chlorophyll by 45% compared to pl ants grown without added NaCl at ambient (350 mu mol mol(-1)) CO2 conc entration. Photosynthesis, per unit leaf area, was almost unaffected. Even so, over a 4-week period, growth (dry weight increment) was reduc ed in the salt treatment by 50%. This could be ascribed to a large red uction in leaf area ( >60%) and to an approx. 20% increase in the rate of dark respiration (Rd). Raising ambient [CO2] from zero to 2000 mu mol mol(-1) decreased Rd in both control and salinized plants (by 20% at 1000, and by 50% at 2000 mu mol mol(-1) CO2 concentration) compared to Rd in the absence of ambient CO2. High night-time [CO2] had no sig nificant 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 mu mol mol(-1) throughout the day, to 31% i n those grown continuously in 900 mu mol mol(-1) [CO2]. The effect of [CO2] at night on salinized plants depended on the daytime CO2 concent ration. Under 350 mu mol mol(-1) day-time [CO2], 900 mu mol mol(-1) at night reduced growth over a 4-week period by 9% (P <0.05) and 1700 mu mol mol(-1) reduced it by 14% (P <0.01). However, under 900 mu mol mo l(-1) day-time [CO2], 900 vs. 350 mu mol mol(-1) [CO2] at night increa sed growth by 17% (P <0.01). It is concluded that there is both a func tional and an otiose (functionless) component to Rd, which is increase d by salt. Under conditions of low photosynthesis (such as here, in th e low day-time [CO2] regime) the otiose component is small and high ni ght-time [CO2] partly suppresses functional Rd, thereby reducing salt tolerance. In plants growing under conditions which stimulate photosyn thesis (e.g. with increased daytime [CO2]), elevated [CO2] at night su ppresses mainly the otiose component of respiration, thus increasing g rowth. Consequently, in regions of adequate water and sunlight, the pr edicted further elevation of the world atmospheric [CO2] may increase plant salinity tolerance. (C) 1997 Annals of Botany Company.