الفهرس 
GENERAL INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Temperature stresses are the major environmental factors limiting growth of crop plants. Rapid recovery from these stresses is important for maintain the productivity of crop plants. The objective of the present investigation was to study the physiological and molecular traits in response to paclobutrazol (PBZ) pretreatment (100 mg L-1) in mitigation of the adverse effects of temperature stresses of 15-day-old wheat (Triticum aestivum L., cv. Sakha 93) seedlings. The main experimental results can be briefly summarized in the following:
1- Short term exposure (3 hrs) of wheat seedlings to low or high temperature stresses significantly reduced the fresh and dry weights of shoots and roots as well as the relative water content. PBZ pretreatment nullified the reduction in the previously mentioned parameters of chilled and heat stressed wheat seedlings. A strong recovery of stressed-wheat seedlings was noticed after 24 hrs from removing the temperature stress.
2- The levels of chlorophylls a and b as well as carotenoid contents were significantly decreased in the leaves of stressed-wheat seedlings, particularly heat stressed ones. PBZ pretreatment induced significant increases in the contents of chlorophylls a and b as well as carotenoids of temperature stressed wheat seedlings. By returning plants to the optimum temperature (25°C), both chlorophyll and carotenoid contents of the stressed plants tended to increase.
3- The accumulation of total soluble sugar content were significantly increased in stressed wheat seedlings under both chilling and heat stresses. However, PBZ pretreatment as well as the recovery period induced further accumulation in the levels of total soluble sugar in chilled and heat stressed wheat shoots.
4- Accumulation of total free amino acids in response to both chilling and heat stress which was concomitant with the decline in soluble protein content. PBZ pretreatment induced reduction in the amino acids content concomitant with increases in the total soluble protein of both chilled and heat stressed wheat seedlings. The accumulation of total free amino acids significantly decreased parallel with significant increases in the total soluble protein of both untreated and PBZ-treated stressed shoots returned to the optimum growth conditions.
5- Either chilling or heat stress induced the accumulation of proline and GB in stressed wheat shoots. PBZ pretreatment as well as the 24 hrs recovery period induced further increase in proline accumulation concomitant with reduction in GB levels in the stressed-wheat shoots.
6- Temperature stresses (low and high) resulted in significant increases in the SOD, CAT, POX, PPO, APX and GR activities as well as the endogenous levels of ASA and GSH levels, while reduced the activity of ASO. PBZ pretreatment enhanced SOD and APX activities as well as ASA and GSH levels in stressed wheat seedlings. Moreover, a recovery period (24hr) imposed enduring declines in SOD, CAT, POX, APX and GR activities as well as the ASA and GSH levels.
7- Temperature stress induced the accumulation of total free phenols in stressed wheat seedlings. PBZ pretreatment reduced the levels of phenols in stressed wheat shoots. The recovery period of 24 hrs following temperature stresses obviously decreased the accumulation of phenols in stressed-wheat seedlings.
8- The endogenous level of H2O2 was significantly increased in leaves of both untreated and PBZ-treated wheat seedling exposed to low and high temperatures. However, the recovery period of 24 hrs following imposition of temperature stresses reduced the accumulation of H2O2 in both untreated and PBZ-treated wheat leaves.
9- Wheat seedlings exhibited time-dependent increase in lipid peroxidation and enhanced ion leakage under low and high temperature stresses. PBZ-treated wheat seedlings as well as the recovered ones exhibited much less MDA content which would hence indicate lower lipid peroxidation and lesser leakage of ions.
10- Protein profile has been analyzed via SDS electrophoresis of wheat leaf extracts. Three types of modifications are observed in the protein patterns of temperature stressed wheat shoots, disappearance of some protein bands (150 KDa), overexertion of some proteins (50 and 70 KDa) and generation of new proteins were induced (heat shock proteins; 17, 26, 90 and 101KDa). Such responses were related to the treatment period of chilling and heat as well as paclobutrazol treatment.
11- The mRNA expression analysis showed time-dependent induction of the transcription factor (HSFA2), HSP16.9, SP26.6, HSP70, HSP90 and HSP101 genes in cold (5°C) and heat (45°C) stressed-wheat shoots. The HSFA2 as well as the five investigated HSPs gene expression were significantly up-regulated under either cold or heat stresses with variable differences. PBZ pretreatment significantly enhanced the mRNA expression of the HSFA2, HSP16.9, HSP26.6, HSP90 and HSP101 genes under heat stress as well as HSP70 expression under chilling stress compared to the untreated shoots.
12-The results obtained by western blotting analysis for HSP70 protein expression revealed that, a rapid induction and accumulation of HSP70 was observed at a low temperature stress. In contrast, at high temperature stress, a somewhat smaller, delayed and short-lasting induction was observed. PBZ pretreatment caused a rapid and somewhat more persistent induction of HSP70 in stressed-wheat shoots.
Finally we can conclude that, PBZ pretreatment was efficient in mitigation of temperature stress hazards of wheat plant by rapid resumption of photosynthetic pigments, increasing of cell membrane stability, antioxidant activities and induction of heat shock proteins which were the most important factors contributing for increasing of wheat tolerance to the temperature stresses.
In addition, a field experiment was conducted to evaluate the effect of paclobutrazol on the yield and yield components as well as the nutritional quality of wheat sown at different sowing dates. In this study, the photosynthetic capacity, the morphological and anatomical criteria at anthesis as well as the yield components, the nutritional quality of wheat grains and the chemical composition of wheat straw in PBZ-treated and untreated plants were studied.
1- Both early and late sowing resulted in a significant reduction in wheat leaf area; stem diameter, fresh and dry weights as well as spike yield. However, PBZ pretreatment nullified the injurious effects of temperature stresses by increasing, fresh and dry weights as well as spike yield of wheat plants grown at the different sowing dates.
2- The anatomical studies showed increments in the epicuticular wax, number of stomata, starch and crystals (druses) as well as number of xylem vessels in PBZ-pretreated wheat leaves and stems. Likewise, the intercellular spaces, epidermal cell length and width stem pith, vessels width and thickness were markedly reduced in wheat plants pretreated with paclobutrazol.
3- PBZ pretreatment mitigated the adverse effects of temperature stress induced by manipulating the sowing date and increased the photosynthetic pigments as well as reduced the fluorescence ratio F680/F730. Therefore, enhance the photosynthetic capacity.
4- Both early and late sowing showed a significant reduction in all yield components in terms of number of spikelets per spike, number of grains per spike, 1000 grain weight, biological yield, grain yield and the harvest index of wheat plants. PBZ pretreatment markedly nullified the observed decline in yield components at both early and late sowing dates.
5- PBZ pretreatment improved the nutritional quality in terms of soluble protein, soluble carbohydrate, oil, mineral and phytic acid contents as well as the fiber of the yielded grains and straw which were influenced by the manipulating the date of wheat sowing.
6- PBZ pretreatment significantly increased the accumulation of total free phenols and flavonoids in yielded wheat grains which may be caused further enhancing of wheat antioxidant capacity.