Cluster Bean under Salinity, Drought and Their Combination: A Critical Review of Physiological and Molecular Responses

Abstract

Cluster bean or guar (Cyamopsis tetragonoloba (L.) Taub.) is an arid-zone legume whose seed endosperm supplies galactomannan gum for food, pharmaceutical and hydrocarbon industries. The crop is widely described as tolerant of drought and salinity, and this description has encouraged its promotion on saline soils and under restricted irrigation. The supporting evidence, however, is fragmented across agronomic trials, physiological experiments and a small number of transcriptomic and metabolomic studies, and the combined occurrence of salinity and water deficit has received very little direct attention. This critical narrative review examines how cluster bean responds to salinity, to drought and to their combination, and evaluates how far current evidence justifies the claim of stress tolerance. Peer-reviewed literature was identified through open scholarly indexes, citation tracking and targeted searches, and was appraised for experimental design, developmental stage, stress realism and replication. The evidence indicates that seed yield is relatively robust under moderate root-zone salinity, whereas vegetative growth, germination under strong salt stress and leaf chloride regulation are more vulnerable. Tolerant genotypes consistently restrict sodium movement to shoots and retain potassium, but chloride control appears weaker and the genetic basis of both traits remains polygenic and poorly resolved. Under water deficit, tolerant genotypes maintain leaf water status, photosynthetic capacity, antioxidant activity and carbon–nitrogen balance, and photosynthesis and nodulation recover rapidly after rewatering; field trials show that yield penalties depend strongly on the timing of deficit. Molecular studies have identified candidate transcription factors, transporters, wax and lignin pathways and metabolite markers, but most rely on single genotypes, short exposures and leaf tissue. Only one controlled experiment has imposed drought, salinity and heat simultaneously, and it showed damage exceeding that of any single stress. Confidence is therefore moderate for single-stress physiology and low for combined-stress mechanisms. Priorities include factorial salinity × drought experiments under field-relevant conditions, multi-environment genotype screening linked to seed and gum quality, and integration of the new chromosome-level genome assemblies with physiological phenotyping. A calibrated view of cluster bean as conditionally rather than universally stress tolerant would better guide breeding and deployment in water-scarce saline landscapes.