CARBON DIOXIDE ENRICHMENT AND TEMPERATURE EFFECTS ON ROOT GROWTH IN COTTON

Understanding crop response to climate change requires knowledge of how roots respond to changes in the aerial environment. Changes in rate and amount of root growth could affect the root distribution in the soil profile and absorption of water and nutrients. Root growth and distribution in cotton (Gossypium hirsutum L.) were examined at day/night temperatures of 15/7, 20/12, 25/17, 30/22, and 35/27℃ and at CO_2 concentrations of 350 and 700μLL^<-1> Plants were grown in controlled-environment chambers with a perspex top under nearly natural daylight. At least twice each week root observations were made on one 2m^2 glass side of the soil bin. Root weight was significantly greater in the 700μLL^<-1> CO_2 treatment at all depths and at all temperatures. Number of roots increased with increasing temperature up to 25/17℃ but was not affected by the CO_2 treatment. Roots in the 350μLL^<-1> CO_2 treatment were longer (root length per root axis) and penetrated the soil profile faster at the lower temperatures. In the 700μLL^<-1> CO_2 treatment, roots were more evenly distributed down the soil profile than in the ambient [CO_2] treatment. Root growth was depressed 63 days after emergence (DAE) in virtually all treatments when fruits (bolls) were developing. The optimum temperature for root growth was also the optimum temperature for shoot growth (30/22℃). The effect of elevated [CO_2] was to make roots heavier, but there was no evidence that this translated into a root system with increased length. Roots were shorter in elevated [CO_2], penetrating the soil profile less rapidly but perhaps more thoroughly