Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 4, 2001
Publication Date: N/A
Interpretive Summary: Citrus is one of the major fruit crops in world trade today. Current knowledge of citrus photosynthesis in response to global climate change, i.e. rising atmospheric carbon dioxide (CO2) and temperature, is very limited. In this study, by USDA, ARS scientists at the Center for Medical, Agricultural and Veterinary Entomology and University of Florida scientists in Gainesville, FL, young sweet orange trees were grown for 29 months at ambient or twice-ambient CO2, along with either near- ambient or 4.5C above ambient temperature. The purpose was to characterize the photosynthetic mechanisms of sweet orange under elevated growth CO2 and temperature. Photosynthesis is a process by which green leaves absorb atmospheric CO2 to make compounds necessary for plant growth and yield. The results showed that sweet orange trees grown under elevated CO2 at both temperatures had greater photosynthetic rates and higher water-use efficiency than their counterparts at ambient CO2. In addition, levels of soluble sugars and starch in the leaves were greatly enhanced by elevated CO2, and these enhancements were beneficial for tree growth and development. Also, the photosynthetic acclimation of sweet orange grown at elevated CO2 allowed an optimization in allocating and using the nitrogen resources. Thus, in the absence of other stresses, citrus would perform well under rising atmospheric CO2 and temperatures predicted for this century.
Two-year old sweet orange trees were grown for 29 months at ambient or twice-ambient CO2 and at 1.5 and 4.5C above outside temperature. Both mature and expanding leaves of trees grown at elevated CO2 had higher photosynthetic rate, lower transpiration and conductance, and higher water-use efficiency (WUE). Although high temperature per se reduced WUE, increase in WUE by elevated CO2 was greater at high temperature. Rubisco activity and protein content were down-regulated at high CO2. Total leaf soluble protein level, however, was not affected by high CO2 or temperature. The down-regulation in Rubisco was greater for the mature leaves, although Rubisco activity and protein content in the expanding leaves were higher. Contents of soluble sugars and starch in both leaf types were substantially higher under elevated CO2, but were not affected by high temperature. At high CO2, leaf photosynthetic acclimation by reallocating the N resources away from Rubisco allowed an optimization of N use in citrus. Also the greater accumulation of primary photosynthetic products at high CO2 were beneficial for citrus tree growth. Thus, in the absence of other environmental stresses, citrus photosynthesis would perform well under rising atmospheric CO2 and temperature as predicted for this century.