Water deficit tolerance in cowpea plants is associated with coordinated metabolic and physiological mechanisms involving roots and leaves

Abstract

Drought reduces the productivity of agricultural crops and restricts the use of extensive areas for agricultural production. Several studies have reported that cowpea exhibits a variety of physiological, biochemical, and phenotypic adaptations that enable it to survive and maintain productivity under drought conditions, making it a suitable model for studies associated with plant responses to water deficit. The most commonly reported acclimatizations include the regulation of stomatal conductance, the accumulation of compatible osmolytes, and adjustments in leaf morphology. On the other hand, the underlying mechanisms that allow cowpea to modify root system architecture under water deficit conditions have been poorly studied. Therefore, in the present thesis, through an integrated approach, we investigated water deficit tolerance mechanisms in cowpea plants, emphasizing the importance of metabolic and physiological responses. We delved into the interactions between the plant's capacity to adjust root growth and changes in the metabolic profile, as well as other biochemical and physiological adjustments under water deficit conditions. For this purpose, 21-day-old cowpea plants were subjected to progressive irrigation withdrawal, resulting in three water regimes: a well-watered control (75% of substrate water-holding capacity), moderate water deficit (25% of substrate water-holding capacity), and severe water deficit (0% of substrate water-holding capacity) for seven days. After five days of water withdrawal, moderately stressed plants were rewatered and allowed to recover to control conditions for two consecutive days. Leaf and root samples were collected for GC-MS-based metabolomic analyses, including amino acids, soluble sugars, and organic acids. In addition, the activities of enzymes related to nitrogen metabolism, including glutamine synthetase, glutamate synthase, and glutamate dehydrogenase, were determined. Physiological analyses were also performed, including gas exchange, osmotic adjustment capacity, root and shoot biomass, and indicators of water deficit, such as relative water content, membrane damage, and water potential. The results revealed progressive reductions in photosynthesis, stomatal conductance, and water potential under increasing levels of water deficit, accompanied by contrasting responses between roots and shoots. Whereas shoot growth was markedly reduced, roots exhibited increased

biomass under moderate water deficit, suggesting a redistribution of resources to enhance water acquisition. Following rehydration, root biomass decreased and shoot growth resumed, indicating a functional reorganization of the root system during recovery. Enzymatic responses, particularly those involving glutamine synthetase and glutamate dehydrogenase, indicated a coordinated reorganization of nitrogen metabolism in leaves and roots under water deficit conditions. In parallel, plants exhibited osmotic adjustment in both organs, associated with the accumulation of amino acids, soluble sugars, and glycine betaine. Although proline accumulated substantially during stress, its relative contribution to osmotic adjustment was limited, indicating that maintenance of tissue turgor depends on the coordinated action of multiple osmolytes. Integrated physiological and metabolomic analyses revealed strong associations among

plant water status, nitrogen metabolism, amino acid accumulation, and growth- related traits. Consistent alterations in 2-oxoglutarate and other organic acids

suggest the involvement of these metabolites in the integration of carbon and nitrogen metabolism during water deficit. Furthermore, the high recovery capacity observed for several physiological and biochemical traits demonstrates that drought tolerance in cowpea depends on coordinated responses between roots and leaves involving physiological, biochemical, and metabolic adjustments that operate in an integrated manner during both stress and recovery.