Effect of Water-deficit Stress on Cotton During Reproductive Development

Water deficit is a major abiotic factor limiting plant growth and crop productivity around the world. Cotton (Gossypium hirsutum L.) is considered to be relatively tolerant to drought and the effects of water stress on leaf physiology and metabolism have been extensively documented. However, information is lacking on the effect of water-deficit stress on the cotton flower. It was hypothesized that water-deficit stress would impair gas exchange functions which consequently would result in perturbation of carbohydrates of cotton reproductive units. To investigate this hypothesis growth room studies and field studies were conducted with the objectives being to document the physiological and biochemical changes that take place in cotton flowers and their subtending leaves when subjected to limited water supply. Additionally, the effect of the ethylene inhibitor 1-Methylcyclopropene under conditions of water stress was investigated as well as the response of leaf and ovary polyamine metabolism of two cotton cultivars differing in drought tolerance. Results indicated that water-deficit stress during flowering significantly compromised leaf gas exchange functions resulting in decreased stomatal conductance, photosynthesis, respiration and water potential. However, cotton reproductive units appeared to be less drought-sensitive compared to the leaves possibly due to higher water potential and glutathione reductase activity. Limited supply of water significantly affected carbohydrate metabolism of both leaf and pistil resulting in carbohydrate accumulation. Contrary to expectations, application of the ethylene inhibitor 1-MCP had no effect on leaf gas exchange function, however, it reversed the effect of water stress on pistil sucrose concentrations. Finally, water-deficit stress during flowering had a significant effect on polyamine metabolism of both leaf and pistil, resulting in increases in putrescine, spermidine and spermine in drought-sensitive cultivars. The differential response of polyamine metabolism between drought-sensitive and tolerant cultivars suggests that polyamines could be effective tools not only in selection of drought-tolerant cultivars, but also in drought tolerance engineering, however further research is needed in order to elucidate the exact pathways of their action