A pot experiment was conducted to evaluate the beneficial effect of foliar application of glycine betaine (10mM), grain presoaking in salicylic acid (0.05 M) and their interaction on drought tolerance of two wheat (Triticum aestivum L.) cultivars (sensitive, Sakha 94 and resistant, Sakha 93). The present study was carried out in a greenhouse at the Faculty of Science, Mansoura University, Egypt, through the two successive seasons (2005/2006) and (2006/2007). Water deficit was imposed by withholding water at the reproductive stage for 30 days within two periods: on the day 65 from planting (heading stage) and the day 80 from planting (anthesis stage). Each droughted pot received 500 ml water at the end of 1st stress period. At the end of the 2nd stress periods, rewatering to the field capacity was carried out. The control plants were irrigated to the field capacity during the stress period, and all plants were left to grow until grain maturation under normal irrigation with tap water. Samples were taken for measurements of growth vigor and physiological criteria at the end of each stress period before re-watering (after 80 and 95 days from planting). For the anatomical studies and the plant-water relationships, samples were taken at the end of the 2nd stress period.
The obtained results showed that water stress caused noticeable increases in root length, number of adventitious roots, soluble sugars and nitrogen. On the other hand, water stress caused a massive reduction in fresh and dry masses of root, growth vigor of shoot, leaf area, pigments content, polysaccharides, protein-N and total nitrogen in both wheat cultivars. Exogenous application of GB, SA or their interaction could counteract the adverse effects of drought by improvement of growth vigor of root and shoot, leaf area, retention of pigments content, increasing the concentration of organic solutes (soluble sugars and soluble nitrogen) as osmoprotectants, keeping out the polysaccharides concentration and/or stabilization of essential proteins in both wheat cultivars.
The application of GB, SA or their interaction induced some modifications in the anatomical features of the flag leaf and peduncle of main shoot which appeared to be an adaptive response to drought stress.
Drought induced marked decreases in diurnal and mean daily values of transpiration rate, stomatal pore areas (on upper and lower sides), relative water content, water use efficiency, indole-3-acetic acid (IAA), gibberellic acid (GA3) and cytokinins (CKs) but led to a significant increase in the abscisic acid (ABA) concentration in flag leaves of the two wheat cultivars.
Grain presoaking in salicylic acid (SA) or foliar application with glycine betaine (GB) alleviated the stress induced by drought by keeping water within leaves and consequently recover the turgidity of stressed plants by restricting the transpiration rate, stomatal closure, decreasing the ABA level and enhancing the growth promoters (IAA, GA3 & CKs) particularly with the sensitive cultivar. Furthermore, the effect was more pronounced with GB+SA treatment.
Water stress caused an increase in osmotic pressure, proline, total soluble nitrogen, total soluble sugars, organic acids, ions (Na+, K+,Ca+2, Mg+2 and Cl-) content as well as Na+/K+ ratio in cell sap of flag leaves in both wheat cultivars. The resistant variety had higher values of osmotic pressure, proline, organic acids and ions content than the sensitive one. On the other hand, the applied chemicals mitigated the effect of water stress, the effect was more pronounced with GB+SA treatment, on the used wheat cultivars. The used chemicals increased the osmotic pressure and the osmolytes concentrations. The osmotic pressure appeared to depend on proline, TSN, TSS, organic acids and the ions content.
The applied chemicals appeared to alleviate the effect of water stress on wheat yield (particularly the sensitive one) and the biochemical aspects of yielded grains. The effect was more pronounced with GB+SA treatment. This improvement would result from the beneficial effect of the provided chemicals on growth and metabolism of wheat plants under water deficit condition.
From growth criteria at the end of 2nd stress period the grain yield was strongly correlated with leaf dry mass, leaf fresh mass, leaf area, number of tillers, plant height, root dry mass, root fresh mass, shoot diameter, shoot fresh mass, shoot dry mass, shoot length and root/shoot ratio for the sensitive and the resistant wheat cultivars. In contrast, a negative correlation was realized between grain yield and degree of succulence, number of adventitious roots, root length and root plasticity.
For anatomical features, the economic yield for susceptible and resistant cultivars appeared to be positively correlated with the vascular bundle area, xylem area, and phloem area for leaf and peduncle.
Concerning the physiological criteria at the end of 2nd stress period, the grain yield was positively correlated with carotenoids, chlorophylls, total pigments, polysaccharides, total carbohydrates, and protein for the two wheat cultivars. On the other hand, a negative correlation appeared between the grain yield and soluble nitrogen and soluble sugars.
The grain yield of the sensitive and resistant wheat cultivars was positively correlated with IAA, GA3, zeatin-riboside, kinetin, benzyl adinine, total cytokinins, RWC, WUEG, and WUEB but negatively correlated with ABA, SWD, transpiration rate, and stomatal area.
The economic yield of the sensitive and resistant wheat cultivars appeared to be positively correlated with osmotic pressure, proline, and keto-acids and negatively correlated with citric acid, total soluble nitrogen and total soluble sugars.
In response to the applied water stress and the used chemicals, the grain yield of the sensitive and resistant wheat cultivars was strongly correlated with all the estimated yield components (shoot length, spike length, plant height, main spike weight, number of spikelets per main spike, 100 kernel weight, grain number per spike, grain weight per plant, straw weight per plant, crop yield per plant, harvest, mobilization and crop indices).