Root Growth of Cotton as Influenced by CO₂ and Temperature

Abstract

Atmospheric carbon dioxide concentration [CO₂] is increasing and it is important to know how this will affect crop growth. 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. Growth and distribution of cotton (Gossypium hirsutum L.) roots were examined at day/night temperatures of 15°/7°C, 20712°C, 25717°C, 30722°C and 35727°C and at [CO₂] of 350 and 700 μL. L^-1 in the shoot chambers. Plants were grown in controlled-environment chambers with a perspex top under nearly natural daylight. Twice a week root observations were made on one 2 m2 glass side of the soil bin. Root weight was significantly greater in the 700 μL. L^-1 CO₂ treatment at all depths and at all temperatures. Number of roots increased with increasing temperature up to 25717°C but was not affected by the CO₂ treatment. Roots in the 350 μL. L^-1 [CO₂] treatments were longer (root length per root axis) and penetrated the soil profile faster at the lower temperatures. In the 700 μL. L^-1 CO₂ treatment, roots were more evenly distributed down the soil profile than in the ambient [CO₂] 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 (30722°C). The effect of elevated [CO₂] was to make roots heavier, but there was no evidence that this was translated into a root system with increased length. Roots were shorter in elevated [CO₂], penetrating the soil profile less rapidly but perhaps more thoroughly.