Evaluation of wild Arachis species for abiotic stress tolerance: I. Thermal stress and leaf water relations

Abstract

The Indian groundnut cultivars have a narrow genetic base. Hence, it was of interest to investigate the genetic variability among wild Arachis species and their accessions for tolerance to thermal stress. A wide variation was observed in leaf morphological characters such as colour, shape, hairiness, length and width and thickness (SLA). The temperature and time required for 50% leaf injury was worked out with limited number of genotypes and was found to be 54°C for 50 min. Among 36 genotypes (having SLA in the range of 66 and 161 cm2 g−1) screened, the inherent potential for cold as well as heat tolerance in terms of relative leaf injury (RI) was observed. Thus, based on RI-values, A. glabrata 11824 and A. paraguariensis 12042 were identified as heat-tolerant and cold-tolerant genotypes, respectively while A. appresipila 11786 was found to be susceptible to both heat and cold. Correlation between SLA and RI values for heat (r = 0.38, P < 0.05) and cold (r = 0.52, P < 0.05) tolerance was positive, indicating that thicker the leaf the lower the injury or higher the tolerance. Among six species and 13 accessions, comprising both heat-tolerant and heat- susceptible genotypes, the concentrations of various leaf chemical constituents such as total protein, phenols, sugars, reducing sugar, amino acids, proline, epicuticular wax load and chlorophyll varied significantly. The epicuticular wax load ranged between 1.1 and 2.5 mg dm2 among 13 A.glabrata accessions. These accessions were categorized into two groups, i.e. high-wax (range: 2.0–2.5 mg dm2) and low-wax types (range: 1.1–1.6 mg dm2). The high-wax type showed a higher diffusion resistance (dr) as compared to low-wax type; though the transpiration rate (tr) in high-wax type was moderate (between 9.5 and 11.6 μg cm−2 s−1). Genetic variability in parameters such as canopy temperature, dr and tr was also distinct. The fully turgid leaves with relative water content ≥91%, showed leaf water potential (ψleaf) between −0.7 and −1.2 MPa. Results indicated that the plants with thicker leaves are better protected from heat injuries. Further, epicuticular wax load seems to help in maintaining stomatal regulation and leaf water relations, thus affording adaptation to wild Arachis species to thrive under water-limited environments. The sources of tolerance, as identified in this study, could be utilized to improve thermal tolerance of the groundnut cultivars by intra-specific hybridization, following either conventional breeding using embryo rescue techniques, if required or utilizing biotechnological tools.