Regresar

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

Abstract:

Photosynthetic responses of sunflower plants grown for 52 d in ambient and elevated CO<inf>2</inf> (A=350 or E=700 μmol mol<sup>-1</sup>, 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 (Ψ<inf>w</inf>) and osmotic potential decreased with water deficiency, but there were no effects of E. Growth in E decreased stomatal conductance (g<inf>s</inf>) and thereby transpiration, but increased net CO<inf>2</inf> assimilation rate (P<inf>n</inf>, 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<inf>n</inf> to intercellular CO<inf>2</inf> 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<inf>s</inf> and P<inf>n</inf>: metabolic limitation was greater than stomatal at mild and severe deficit and was not overcome by elevated CO<inf>2</inf>. The decrease in P<inf>n</inf> with water deficiency was related to lower Rubisco activity rather than to ATP and RuBP contents. Thus, there were no important interactions between CO<inf>2</inf> during growth and water deficit with respect to photosynthetic metabolism. Elevated CO<inf>2</inf> will benefit sunflower growing under water deficit by marginally increasing P<inf>n</inf>, and by slowing transpiration, which will decrease the rate and severity of water deficits, with limited effects on metabolism.