Photosynthetic acclimation of young sweet orange trees to elevated growth CO2 and temperature

Two-year old trees of orange, a hybrid of tangerine (Citrus reticulate Blanco) and tangelo (C. paradisi Macf. x C. reticulate), were grown for twenty-nine months under two daytime [CO2] of 360 (ambient) and 720 (elevated) mumol mol(-1), and at two temperatures of 1.5 and 6.0degreesC above ambient temperature. The objectives were to characterize the physiology and biochemistry of citrus photosynthesis in response to both elevated [CO2] and temperature, and to test if the photosynthetic capacity of sweet orange, in terms of rubisco activity and protein concentration, was down-regulated under long-term elevated growth [CO2]. Both mature (old) and expanding (new) leaves of trees grown under elevated [CO2] had higher photosynthetic rates, lower transpiration and conductance, and higher water-use efficiency (WUE), compared to those grown under ambient [CO2]. Although leaf WUE was reduced by high temperature, elevated [CO2] compensated for adverse effect of high temperature on leaf WUE. Activity and protein concentration of rubisco were down-regulated in both new and old leaves at elevated [CO2]. In contrast, the amount of total leaf soluble protein was not affected by elevated [CO2] and high temperature. Down-regulation of photosynthetic capacity was greater for the old leaves, although activity and protein concentration of rubisco in the new leaves were higher. Contents of soluble sugars and starch in all leaves sampled, which were higher under elevated [CO2], were generally not affected by high temperature. Within each specific CO2-temperature treatment and leaf type, total soluble sugars remained relatively unchanged throughout the day, as did the starch content of early morning and midday samples, and only a moderate increase in starch for the old leaves at late afternoon sampling was observed. In contrast, starch content in the new leaves increased substantially at late afternoon. Activities of sucrose-P synthase and adenosine 5'-diphosphoglucose pyrophosphorylase were reduced at elevated [CO2] in the old leaves, but not in the new leaves. The photosynthetic acclimation of Ambersweet orange leaves at elevated [CO2] allowed an optimization of nitrogen use by reallocation/redistribution of the nitrogen resources away from rubisco. Thus, in the absence of other environmental stresses, citrus photosynthesis would perform well under rising atmospheric [CO2] and temperature as predicted for this century.