| Drought stress is a main meteorological disaster restricting crop production worldwide. China, as a large agricultural country, drought stress is seriously limiting crop yield. One of the cost-effective and practical substitute approaches to fully utilize natural resource is to develop drought tolerant crop cultivars. To breed crop cultivars tolerant to drought stress, it is important to identify genetic resources in potential of drought tolerance and fully understand its mechanism. Barley is the fourth most important cereal crop in the world. However, much of the genetic diversity for improving abiotic stress tolerance has been lost during the process of modern barley breeding. Tibetan annual wild barley (H. vulgare ssp. spontaneum and ssp. agriocrithum) posses rich in abiotic-tolerant/resistant related specific genes, which has been attracting more attention in international barley study. The present study was carried out, using drought-resistant Tibetan wild barley germplasm (XZ5) identified in our previous research, to elucidate physiological mechanism for its specifically tolerance by examining fluorescence parameters, ultrastructure, proline metabolism, potassium content and other physio-chemical responses, combined with gene chip results. The main results are as follows:1. Pot experiments were undertaken, using Tibetan wild barley XZ5(drought-tolerant) and XZ54(drought-sensitive) and drought resistant cultivar Tadmor and cv ZAU3as control, to study the genotypic difference in response to drought stress in growth and production. There were two treatments:(1) Control, in which soils in the pots were kept humid (30-40%water holding capacity) throughout;(2) Drought stress, seedlings were subjected to drought stress for35d by withholding irrigation until the soil moisture content reduced to around4%. On the following day, the treated pots were re-watered to keep humid for10d recovery, then normal cultured until harvest. The results showed that a strong drought tolerance was observed in genotype XZ5without any yellow or curly leaves but wilt slightly, whereas genotype XZ54showed yellow, dead, curling leaves and severe wilting. Compared with barleys normally watered, drought stress significantly decreased the ratios (fresh weight/dry weight) in shoot, with XZ5and Tadmor being less reduction than these in XZ54and ZAU3. As for yield per plant, no significant difference was determined in genotype XZ5, but the other three genotypes declined significantly.2. Genotypic differences in the key enzymes in proline metabolism in response to drought stress were investigated. When soil moisture content decreased to15%, no significant difference was detected between stressed and control plants of four genotypes in proline content and its key enzymes. When soil moisture content decreased to4%, ProDH activity and Pro accumulation were significant higher than the control in cv Tadmor and ZAU3. P5CS activity was lower than the control and no obvious difference was determined in the other two parameters in drought-tolerant XZ5. After10d recovery, there was a significant difference between drought treatment and control in these three parameters in Tadmor and ZAU3. However, no notable difference was detected between treated and control plants in Tibetan wild barley (XZ5and XZ54), but Pro content remarkably increased.3. The effects of drought stress on photosynthetic characteristics, cell ultrastructure and potassium concentration in Tibetan wild barley and its genotypic difference were studied. As soil moisture content decreased to4%, grana stack, grana stack lamination layer, the number of stroma lamellae notably decreased, some lamellar even disintegrated, osmiophilic particles increased extremely significant and a remarkable significant decrease detected in the maximal photochemical efficiency of PS â…¡ (Fv/Fm) in XZ54and cv ZAU3stressed plants in comparison with control. However, XZ5and Tadmor did not show these variances, but Fv/Fm significantly increased. Compared with control, there was a significant decline in net photosynthetic rate, transpiration rate and stomatal conductance in XZ54, ZAU3and XZ5. Besides, no significant difference was detected in intercellular CO2concentration between stressed and unstressed plants in XZ5but it escalated clearly in XZ54and ZAU3. Furthermore, the least decrease of net photosynthetic rate was recorded in XZ5(c.f. significantly decreased by3.2-folds in XZ5and10-folds in XZ54and ZAU3). In the case of potassium content in shoots, stressed seedlings of XZ54and ZAU3reduced obviously compared to control, whereas a very significant increase was determined in XZ5.4. DNA microarray was carried out to study influences of severe drought stress on gene expression prolfiles of XZ5, Tadmor and XZ54. Severe drought stress lead to several related to drought tolerance genes expression, such as GPX54Hv, GRPs, DHN, Pro, ProDH, P5CS. The results showed that cells, cell part, metabolic processes, organelles, metabolic processes, binding, and response to stimulus were closely related to tolerance to drought in Tibetan wild barley. |
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