Mitigation of Heat Stress in Solanum lycopersicum L. by Using Heat Tolerant Plant Growth Promoting Bacteria

Abstract

Bacteria can be evaluated for their capabilities of heat tolerance and plant growth promotion in sustainable agriculture. Three planned studies were piloted to assess the potential of heat tolerant plant growth promoting bacteria (PGPB) to mitigate heat stress in tomato plant. Samples were collected including tomato plants and rhizospheric soil from Larkana, Sindh, Pakistan during the year 2015. Seventy isolates were isolated, screened and characterized for plant growth promoting activities, extracellular enzymes activities and heat tolerance potential. These isolates were positively confirmed for indole acetic acid (IAA), phosphate, ammonia, siderophores, hydrogen cyanide, protease, amylase, pectinase, catalase, ACC deaminase and exopolysaccharide production. The strains were screened at high temperature which was maintained at 600C. The Five promising potential heat tolerant isolates were identified through 16S rRNA gene sequencing technique. Acds gene was successfuly amplified from these promising bacterial strains. Morphological characterization revealed that four strains were Gram positive and one was Gram negative. Bacillus safensis (SCAL1) strain revealed maximum production of IAA (0.52 µg/ml) and Bacillus safensis (T6) strain, showed higher quantity of Giberellic acid (8.73 µg/ml) and Kinetin (34.8 µg/ml) under heat stress condition Impact of plant growth promoting bacteria application on key physiological and biochemical analysis were studied under normal and high temperature stress conditions in green house. The results of morpho-physiological parameters revealed significant affect of heat on un-inoculated and inoculated tomato plants under high temperature stress. From all observation of experimental results, we found the best impact of inoculation of characterized bacteria Bacillus safensis SCAL1, significantly enhanced all agronomic parameters of both varities (Riogrande and Sweetie) including root length (39.6 and 64.4%), shoot length, (37.1 and 61.4%) fresh weight (55.4 and 80.2%) and dry weight (22.1 and 60.04%) and leaf surface area (33.2 and 63.2%) and number of flowers (51.6 and 63.9%) and fruit (55.7 and 77.8%) under normal and heat stress conditions. A total of three field experiments were conducted at National Agriculture Research Centre, Islamabad, Pakistan, (NARC) during 2018, 2019 and climatically xvii important district Muzaffargarh, Punjab, Pakistan. As bacterial strains gave promising results in greenhouse experiment, multi-year/multi-location field trials were conducted to extend the heat mitigations effects at larger scale. In all three field experiments, plant growth promoting bacteria (PGPB) enhanced agronomic and yield parameters of tomato plants under heat stress conditions. First year 2018 field trial at National Agriculture Research Centre, Islamabad, Pakistan showed signifcant changes in different physiological and agronomic parameters. However, the parameters of number of flowers and number of fruits were of prime importance as it is main indicator of yield. Bacillus safensis (SCAL1) produced maximum number of flower’s (47%) and fruits (31.1%), followed by Bacillus safensis T6 (40.7 and 24.8%), Bacillus safensis BT (20.07 and 11.75%), Bacillus cereus KTES (30.01 and 8.56%) and Klebsiella variicola TR3 (17.1 and 9.48%) respectively. All inoculated plants grown under heat stress condition enhanced the flowers and fruits per plant which can be regarded as major in terms of yield along with improvement in other parameters such as root and shoot length, fresh and dry weight, chlorophyll content, leaf area and number of flowers as compare to un-inoculated heat stressed plants. In the second year, 2019, NARC, Islamabad, Pakistan field data expressed changes in results of inoculated compared with un-inoculated under heat stress. Among all the applied bacterial strains, Bacillus safensis (SCAL1) showed the best performance under field conditions as it was noticed to be more heat tolerant than Bacillus cereus (KTES) and Klebsiella variicola (TR3). Bacillus safensis SCAL1 produced maximum number of flowers (44.1%) and fruits (20.9%), followed by Bacillus safensis T6 (43 and 11.03%), Bacillus safensis BT (24.5 and 6.27%), Bacillus cereus KTES (35.5 and 4.76%) and Klebsiella variicola TR3 (20.1 and 0.7%) respectively. The study year 2019, District Muzfargarh, Punjab, Pakistan field data revealed improvement in parameters under the inoculation of bacterial consortia. The important parameters of number of flower and fruits showed significant improvement as reported in the previous field studies of NARC, study years of 2018-19. The consortia improved the percentage of number of flowers and fruits by 16.9 and 52.1 % respectively. xviii The study comprehensively exhibited the role of plant growth promoting bacteria in the mitigation of heat stress. Up to the best of our knowledge, this is the very first study claiming the potential of isolated bacterial strains to mitigate the heat stress and plant growth promotion under greenhouse and field conditions from Pakistan. We also report that in our best of our knowledge for the first-time, field studies demonstrating the mitigation of effects of heat stress in tomato plant by inoculation of thermotolerant plant growth promoting bacteria.