Physiological and molecular basis of selective fertilization for high temperature stress tolerance in tomato (Solanum lycopersicum L.)

Abstract

The study entitled “Physiological and molecular basis of selective fertilization for high temperature stress tolerance in tomato (Solanum lycopersicum L.)”, was undertaken with the objective to screen the critical temperature for pollen germination of tomato genotypes and to evaluate the thermotolerance of selectively fertilized tomato hybrids through physiological and molecular techniques. The experiments were conducted using the Open field & Rainout shelter (ROS) facility at the Department of Plant Physiology, College of Agriculture, Vellayani during 2019-2024. The first experiment was devised to advance the selectively fertilized hybrid of Anagha and Manuprabha by the method of selective selfing by pollen selection up to F4 to stabilize the thermotolerance. Pollen germination %, photosynthetic rate, RWC, percentage leakage, chlorophyll content, CSI and stomatal conductance were observed in all generations. In F2 generation, T3 and T10 line showed comparatively better performance than the other lines on the basis of phenotypic evaluation and the molecular markers SSR136 and SSR630 were also present in these lines. The seeds from T3 and T10 line selected for next generation and F3 was developed. In F3 generation, T6 and T8 line were selected for next generation i.e., F4 and the molecular markers SSR134 and SSR630 were present in these lines. In F4 generation, T4 and T6 were the best lines. The second experiment was designed as a confirmation study of the selective fertilization technique in which ten popular varieties of tomato namely Anagha, Vellayani Vijay, Manuprabha, Nandi, Arka Saurabh, Arka Rakshak, Pusa Rohini, IIHR 26372, Arka Vikas and Akshaya were used for the identification of critical temperature for pollen germination. Mature pollen grains from the fully opened flowers were collected and incubated at different temperatures (320C to 400C) for two hours in the pollen germination media. The temperature of 36oC identified as the critical temperature for pollen germination and Anagha showed high pollen germination percentage (25.88 %) and Arka Saurabh showed least pollen germination percentage (16.34 %) at this critical temperature. In the third experiment, based on the critical sterility temperature six varieties were selected for pollen selection and selective fertilization. The variety Anagha, Vellayani Vijay and IIHR26372 with highest critical temperature for pollen germination percentage were selected as tolerant genotypes and the variety Manuprabha, Arka Saurabh and Pusa Rohini with low critical temperature were selected as the susceptible genotype. Selective fertilization (SF) was done between the six genotypes. Reciprocal cross between the tolerant and susceptible genotypes was also done by selective fertilization as well as normal cross for comparison. Growth performance of the selectively fertilized tomato plants was evaluated in ROS. The experiment was laid out in CRD with three replications. Floral characters (number of clusters and number of flowers per cluster) were noted significantly high in crosses with susceptible genotype as male parent. Among the crosses, T23 recorded the highest no. of clusters (18) and number of flowers per cluster (10). The flower characters like percentage of flowers with exerted stigma and length of stigma exertion were found high under high temperature but the selectively fertilized crosses recorded less value than the normal crosses. T16 (20.63%) showed lesser stigma exertion. The length of exerted stigma was recorded least in T7 (0.13 mm) which was on par with T2, T11, T17, T16, T23, T28, T26, T24, T19, T5, T4, T25, T34, T10, T1, T22 and T8. The no. of days to first flowering after sowing were recorded, T30 took the shortest period (37 days) for flowering, and T36 took the longest (50 days). The Normal and SF crosses with tolerant as male parent flowered comparatively earlier than its reciprocal crossed ones. Physiological parameters like photosynthetic rate and relative water content were observed high in selectively fertilized crosses than the normal crosses under higher temperature, the crosses of SF plants with tolerant genotype as male parent show significantly higher photosynthetic rate while it was SF plants with susceptible genotype as male parent in case of RWC. The photosynthetic rate was recorded higher in T13 (10.73 μmol CO2 m-2 s-1) and the relative water content was recorded highest in T29 (64.09%). Among the temperature tolerance characters like membrane integrity (% leakage), SOD activity and pollen viability, % leakage and SOD activity recorded higher in selectively fertilized plants than the normal plants while the pollen viability recorded high in normal crosses. The T27 (1.78 %) recorded the least percentage leakage, which was on par with T35 and T30. The highest value of SOD activity was recorded in T28 (0.35 activity g-1 min-1) and pollen viability was observed highest in T1 (100 %). Vitamin C content of leaves were recorded highest in normal crossed hybrid with tolerant as male parent, T9 (413.02 μg g-1) showed highest value. The number of fruits/cluster and yield were high in selectively fertilized crosses than the normal crosses, specifically the SF crosses with tolerant genotype as male parent. T10 (4.67) recorded highest number of fruits per cluster which was on par with T16. The yield was very less in ROS condition and the highest value was also observed in the treatment T10 (226 g plant-1). In the first experiment, the selfing of plant through SF improves its performance in each generation. The mean performance of F4 plants were better than F2 and F3. In the second experiment among the ten varieties of tomato, Anagha was found as the best with high germination percentage with critical temperature 360 C and Arka saurabh as the susceptible one with lowest pollen germination percentage. In the third experiment selective fertilization technique was found to have an advantageous influence on the physiological and yield attributes as it increased the photosynthetic efficiency and high temperature stress tolerance mechanism. Among the various cross combinations, selectively fertilized cross of tolerant and susceptible parent having tolerant genotype as male parent was found to be the best at high temperature. Hence this study has importance in improving the high temperature tolerance mechanism in tomato under increasing daily mean temperature.