Cotton and maize primary root growth responses to water deficit : comparative physiological and metabolic analysis
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Cotton (Gossypium hirsutum L.) and maize (Zea mays L.) are important economic crops that can suffer significant yield loss under drought. Cotton and maize root systems have fundamentally different architectures, in addition to cotton being a dicotyledonous perennial plant while maize is a monocotyledonous annual plant. However, both species exhibit relative maintenance of elongation of the primary root at low water potentials compared with the shoot. Previous studies on the mechanisms of primary root growth maintenance were conducted mainly in maize but have not been conducted in cotton. The objective of this dissertation was to compare the metabolic responses to water deficit of the primary root growth zone in the two species to determine whether similarities or differences occur. A series of experiments was conducted to establish the foundation for directly comparing the metabolic responses to water deficit of cotton and maize primary roots. A modified vermiculite-culture seedling system was developed to achieve stable growth rates for cotton roots under different water stress conditions. With this system, a cotton genotype (cv. AU90810) was selected and the spatial pattern of cell elongation within the primary root growth zone was determined using a kinematic approach. These results showed that the cotton primary root exhibits a similar spatial growth pattern to a drought-tolerant maize line (cv. FR697) when compared at equivalent root tissue water potentials, allowing for direct comparisons of the metabolic responses to water deficit in the two species. Experiments for comprehensive metabolite analysis of the responses to water stress in the primary root growth zone in both species were conducted. Commonalities were found primarily in the metabolites that function as osmolytes. However, there were significant differences that have important implications for the control of growth in water-stressed roots of the two species. Anti-oxidative mechanisms and sulfur metabolism exhibited the most striking differences that separate the responses of the two species to water deficit. In the water-stressed treatments, glutathione and sulfate decreased in abundance in cotton whereas they increased in abundance in maize, which indicated that sulfur limitation may occur in cotton roots under water-stressed conditions. Further investigations into the abundance of glutathione and hydrogen peroxide verified the quantitative differences in the glutathione response of the two species but showed relatively low hydrogen peroxide levels in cotton, suggesting that cotton may have alternative anti-oxidative mechanisms that are independent of glutathione. A transcriptomics study of the cotton primary root growth zone that was focused on sulfur metabolism and anti-oxidative mechanisms further confirmed the alterations of these metabolic pathways in response to water-deficit stress. To test the hypothesis that water-stressed cotton roots are sulfur-limited, the effects of sulfur supplementation on root growth and plant-water relations in responses to water stress were examined in a simulated field condition, but the results demonstrated that growth limitation of the root system by exposure to water deficits was not relieved by sulfur supplementation. Future studies will focus on investigating the roles of alternative anti-oxidative mechanisms as well as alterations in sulfur metabolism in cotton under water-deficit conditions, with the aim of generating novel strategies for cotton yield improvement under water-limited conditions.
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