Physiological and molecular characterization of black pepper genotypes subjected to limited water availability

Abstract

The cultivation of black pepper (Piper nigrum L.) requires specific weather conditions, such as warm temperatures, high humidity, and uniform rainfall, which are typical of tropical climates. However, its cultivation faces significant impacts under drought stress. Hence, the present study ("Physiological and Molecular Characterization of Black Pepper Genotypes Subjected to Limited Water Availability.") focused on the identification of black pepper genotypes with more drought-tolerant characteristics from a selected pool of forty genotypes, which were maintained at the National Active Germplasm Site of ICAR-IISR. Physiological and biochemical assessments, coupled with transcriptomic studies, revealed the adaptive mechanisms employed by drought-tolerant genotypes. Based on our experience and observation, morphological and physiological parameters, lower leaf length, leaf width, leaf area, petiole length, internodal length, and stomata number, and higher wax content were considered favorable traits for drought tolerance. Further evaluation of 21 genotypes from the mentioned forty genotypes was characterized by yield attributing traits and quality traits employing principal component analysis which indicated that 73.88% cumulative variation across five PCs is notably influenced by yield-related traits. The genotypes assumed to be drought- tolerant exhibited distinct trait patterns, especially in leaf area, stomatal density, and wax content. The overall yield, determined by berry size, spike length, and test weight, was emphasized in the findings of Pearson correlation analysis. Accessions 7211 (cluster 2), 1495, 1343, and 4132 (cluster 3) exhibited drought tolerance traits based on extensive research conducted at IISR over many years. . Accessions, 5717 and 4064 (cluster 1) exhibited drought-susceptible characteristics. In the next step, moisture stress was imposed in the genotypes which showed drought tolerant and susceptible traits (7211, 1495, 1343, 4132, 5717, and 4064) by withholding irrigation for 28 days to decipher the mechanism of drought tolerance. The desirable physiological traits, such as higher relative water content, photosynthetic pigments, and lower membrane leakage and stomatal opening, helped the plants to tolerate water stress, as demonstrated by the accessions 7211, 1495, 1343, and 4132, in comparison to the accessions 5717 and 4064. Accessions 7211, 1495, 1343, and 4132 showed higher proline, minerals, ABA content and sugar accumulation, lower hydrogen peroxide and malondialdehyde levels, decreased starch and protein degradation, and improved antioxidant enzymatic activity. When evaluated for yield, yield attributing traits, and quality characteristics, genotypes 1343, 1495, 4064, and 4132 exhibited lower yield reductions, while 5717 and 7211 had the highest reductions. These genotypes also showed comparatively lesser reduction in major yield-related traits such as spike length, number of berries per spike, berry size, and 100-berry fresh weight. GC-MS profiling revealed significant variations in elemental composition under imposed stress, particularly in the 7211, 1495, 1343, 4132, and 5717 genotypes, aiding in discrimination in terms of tolerance. Morphological characterization of shoots and roots demonstrated that genotypes 1343 and 1495 had minimal reductions in biomass and efficient resource allocation. A comparative gene expression analysis of sixteen drought-responsive genes, including DHN, OSM, DREB, GST, HSP70, PX5, PX12, NAC, and SOD CuZn, specificallydiscriminates genotypes 7211, 1495, 1343, and 4132 as possessing drought tolerance traits compared to 5717 and 4064 under different field capacity levels of 25%, 50%, and 100%. At the same moisture levels, 7211 and 1343 exhibited the highest root-to- shoot ratio, followed by 4132 and 1495, as supported by molecular findings. Overall, genotypes 1343 and 4132 consistently showed desirable traits for drought tolerance compared to other genotypes while the genotypes 4064 and 5717 showed the least desirable traits among the genotypes.