Effects of water deficit and its interaction with CO2 supply on the biochemistry and physiology of photosynthesis in sunflower

Photosynthetic responses of sunflower plants grown for 52 d in ambient and elevated CO2 (A=350 or E=700 mumol mol(-1), respectively) and subjected to no (control), mild or severe water deficits after 45 d were analysed to determine if E modifies responses to water deficiency. Relative water content, leaf water potential (Psi(w)) and osmotic potential decreased with water deficiency, but there were no effects of E. Growth in E decreased stomatal conductance (g(s)) and thereby transpiration, but increased net CO2 assimilation rate (P-n, short-term measurements); therefore, water-use efficiency increased by 230% (control plants) and 380% (severe stress). Growth in E did not affect the response of P-n to intercellular CO2 concentration, despite a reduction of 25% in Rubisco content, because this was compensated by a 32% increase in Rubisco activity. Analysis of chlorophyll a fluorescence showed that changes in energy metabolism associated with E were small, despite the decreased Rubisco content. Water deficits decreased g(s) and P-n: metabolic limitation was greater than stomatal at mild and severe deficit and was not overcome by elevated CO2. The decrease in P-n with water deficiency was related to lower Rubisco activity rather than to ATP and RuBP contents. Thus, there were no important interactions between CO2 during growth and water deficit with respect to photosynthetic metabolism. Elevated CO2 will benefit sunflower growing under water deficit by marginally increasing P-n, and by slowing transpiration, which will decrease the rate and severity of water deficits, with limited effects on metabolism.