Contrasting crop species responses to CO2 and temperature: rice,soybean and citrus

The continuing increase in atmospheric carbon dioxide concentration (CO₂]) and projections of possible future increases in global air temperatures have stimulated interest in the effects of these climate variables on plants and, in particular, on agriculturally important food crops. Mounting evidence from many different experiments suggests that the magnitude and even direction of crop responses to CO₂] and temperature is almost certain to be species dependent and very likely, within a species, to be cultivar dependent. Over the last decade, CO₂] and temperature experiments have been conducted on several crop species in the outdoor, naturally-sunlit, environmentally controlled, plant growth chambers by USDA-ARS and the University of Florida, at Gainesville, Florida, USA. The objectives for this paper are to summarize some of the major findings of these experiments and further to compare and contrast species responses to CO₂] and temperature for three diverse crop species: rice (Oryza sativa, L.), soybean (Glycine max, L.) and citrus (various species). Citrus had the lowest growth and photosynthetic rates but under CO₂] enrichment displayed the greatest percentage increases over ambient CO₂] control treatments. In all three species the direct effect of CO₂] enrichment was always an increase in photosynthetic rate. In soybean, photosynthetic rate depended on current CO₂] regardless of the long-term CO₂] history of the crop. In rice, photosynthetic rate measured at a common CO₂], decreased with increasing long-term CO₂] growth treatment due to a corresponding decline in RuBP carboxylase content and activity. Rice specific respiration decreased from subambient to ambient and superambient CO₂] due to a decrease in plant tissue nitrogen content and a decline in specific maintenance respiration rate. In all three species, crop water use decreased with CO₂] enrichment but increased with increases in temperature. For both rice and soybean, CO₂] enrichment increased growth and grain yield. Rice grain yields declined by roughly 10 % per each 1 °C rise in day/night temperature above 28/21 °C.