| Acetyl salicylic acid and24-epibrassinolide have been reported as heat stress ameliorating agents. The effect of different concentrations of24-epibrassinolide and acetyl salicylic acid on heat tolerance of tomato plants was evaluated in growth chamber as well as in field conditions. The research work comprised of five experiments as summarized below.1. In first experiment the effect of exogenous application of varying levels of24-epibrassinolide (0.75,1.5and3μM) and acetyl salicylic acid (0.25,0.75and1.25mM) were applied and their effect on photosynthesis, chlorophyll content, membrane integrity and survival in four weeks old tomato (cultivar:Mei Jie Lo) seedlings under high temperature stress (46℃/4h daily) for21days was investigated. The daily heat stress treatment had severe effects on seedlings but chemical treatments significantly reduced the magnitude of deleterious impacts to different extents. Epibrassinolide (3μM) was over all the best treatment to improve survival (86.11%), photosynthesis (39.4%), and chlorophyll contents (26.12%) accompanied with initiation of flower buds and improved vegetative growth. Whereas acetyl salicylic acid (1.25mM) best improved photo synthetic activity (40.6%) as compared to the untreated heat stressed control seedlings. Moreover,24-epibrassinolide (3μM) and acetyl salicylic acid (0.75mM) attenuated cell membrane injury to8.3and6.9%respectively from22.4%in heat stressed control seedlings. However, in the given experimental conditions, lower doses of acetyl salicylic acid (0.25and0.75mM) had slight (5.6and12.8%) inhibition effect on the photosynthesis than the heat stressed controls. Certain concentrations of both acetyl salicylic acid and24-epibrassinolide positively affected most of the studied parameters under daily heat shock by up regulation of heat tolerance in tomato seedlings and different concentrations demonstrated different effect.2. In the second experiment, the effect of exogenous application of varying levels of24-epibrassinolide (0.75,1.5and3μM) and acetyl salicylic acid (0.25,0.75and1.25mM) on root activity in terms of2,3,5-triphenyl tetrazolium chloride reduction and root morphological features was evaluated in four-weeks-old tomato seedlings (cultivar:Mei Jie Lo) grown under high temperature stress (46℃/4h per day) for21days. The daily heat stress treatment almost ceased the root growth of chemically untreated seedlings. However, both24-epibrassinolide and acetyl salicylic acid significantly attenuated the deleterious impacts of heat stress to different extents regarding root activity, total root length, surface area, volume, number of nodes and connections.24-epibrassinolide (3μM) was over all the best treatment to improve root activity whereas acetyl salicylic acid (0.25mM) best enhanced root architecture (net length, volume and area) as compared to the untreated heat stressed controls. However,24-epibrassinolide (3μM) and acetyl salicylic acid (1.25mM) slightly inhibited mean root diameter. It is concluded that under high temperature conditions, the exogenous24-epibrassinolide and acetyl salicylic acid in studied doses improve root morphological features and root activity, hence enhance heat stress tolerance.3. The third experiment was designed to test the potential of these heat ameliorative molecules in growing tomato plants under acute heat stress conditions in field. The tomato plants (MJL cultivar) were grown in small plastic tunnels in summer exogenously sprayed with24-epibrassinolide and acetyl salicylic acid. It was observed that the24-epibrassinolide and ASA significantly up-regulated vegetative (plant height, number of leaves, leaf area, stem thickness) as well as reproductive parameters (number of flowers, reduced flower shed and greater number of fruits per plant and yield) to different extents as compared with the controls. The improved plant performance was achieved due to enhanced heat tolerance by virtue of improved antioxidant enzyme system, enhanced chlorophyll contents, better protected photosynthetic machinery and its performance. The highest yield per plant (1611.7g) was contributed by acetyl salicylic acid (0.25mM) followed by (0.75mg·L-1)24-epibrassinolide (1540g) which were88%and79%higher than the controls (856.9g), respectively. Other studied treatments had also significantly higher yields per plant but lower than Al and E3.4. In fourth exeprment, it was observed that when heat acclimated tomato seedlings (38±1℃for5h per day for two weeks) were subjected to sudden heat stress (46±1℃for5h per day for three days), the acclimated seedlings demonstrated remarkably improved plant survival, ability to recover and enhanced membrane thermostability against sudden heat stress as compared with the un-acclimated controls. Moreover, another group of tomato seedlings, heat acclimated at high temperatures (at30and33℃for2days each, and then at38±1℃for5h per day for three weeks) demonstrated significantly enhanced adaptation to sunlight, higher photosynthesis rate, improved plant growth and yield parameters under moderate heat stress (Tmax.37-40℃, Tmin.16～19℃). However, under chronic heat stress conditions in plastic tunnel (Tmax.38～55℃, Tmin.20～28℃) the heat acclimation treatment was not fruitful.5. In the fifth experiment, it was observed that pollen germination medium supplemented with24-epbrassinolide (0.75and1.25mg·L-1) showed ameliorative role in pollen germination at25,35,40and45℃, however,24-epibrassinolide (0.75mg·L-1) had over all best performance and contributed by increasing percent pollen germination and pollen tube length, and reducing percent pollen burst at all temperatures in tomato; Mei Jie Lo cultivar.6. Based upon these observations, it is suggested that,24-epibrassinolide and acetyl salicylic acid have a positive role towards heat stress amelioration in indoor experiments as well as in field conditions. However, the effect of both chemicals depends upon concentration as well as growth stage. To better exploit this potential, studies involving different concentrations of given chemical agent at different growth stages are proposed, that may lead to development of growth stage specific products. It is suggested that heat acclimation at seedling stage can be exploited as a potential tool for induction of heat tolerance and improved tomato plant performance under moderately heat stressed field conditions. Thus it is concluded that both chemical agents can be suitable candidates in practical agriculture for extension of tomato growth period in summer by virtue of their heat stress amelioration ability. However, keeping in mind the small scale of present study, wide scale field studies are proposed to further implicate these results. |
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