Development of rice mutants with enhanced resilience to drought and brown spot (Bipolaris oryzae) and their physiological and multivariate analysis

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Abstract
Rice serves as staple food for four billion people but encounters yield reductions because of natural as well as biological stress factors. An analysis on 65 rice mutants alongside RICF-160 parent and Kainat commercial rice variety was conducted to measure disease resistance against brown spot using both normal and water-stress conditions as study parameters. Selection of the thirteen mutant lines [NMSF]-1, [NMSF]-8, [NMSF]-18, [NMSF]-2, [NMSF]-7, [NMSF]-13, [NMSF]-62, [NMSF]-3, [NMSF]-4, [NMSF]-10, [NMSF]-16, [NMSF]-56 and [NMSF]-65 as drought tolerant lines demonstrated less reduction in photosynthetic rate under stress along with higher shoot lengths, increased chlorophyll levels and relative water content while showing decreased stomatal conductance and transpiration rates. The disease index evaluations were performed twice during the seedling stage under controlled conditions together with water-deficient environments. The brown spot disease resistance level of [NMSF]-1, [NMSF]-8, [NMSF]-10, [NMSF]-16, [NMSF]-56 and [NMSF]-59 mutants was considered moderate. The correlation analysis based on Pearson's method showed that growth, yield performance shared significant statistical relation with physio-biochemical traits. The control tests showed that shoot length had a positive link with photosynthesis rate as well as stress tolerance indexes. The relative water content and photosynthetic rate showed a positive relationship with chlorophyll content while the chlorophyll value had an inverse relationship with stomatal conductance. The level of stress in plants resulted in correlations where relative water content and SPAD measurements exhibited high values but chlorophyll stomatal conductance and transpiration rate decreased simultaneously. Eight principal components explained variability 83.025% when operating under both conditions. However, the PC-I contributed 22.8% of total variability. Under the testing conditions, six of the nine analyzed components revealed positive loadings in PC-1. The stress tolerance indexes together with brown spot disease index along with rate of transpiration and rate of photosynthesis in stress condition showed greatest influence on PC-I. Analysis of present studies demonstrated five clusters among mutants and their parent line and commercial variety. The cluster-I contained fifty-four mutants and cluster-II consisted of six mutants and cluster-III consisted of four genotypes followed by two mutants alongside one genotype which enabled crucial research for rice breeding to enhance stress tolerance and disease resistance.

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