| Drought is one of the most influential abiotic stresses that are detrimental to the growth and development of plants. Perennial woody plants are frequently exposed to drought and re-watering scenarios in their long life cycle, leading to productivity reduction. Populus is an important woody tree species for preferable biomass production at the expense of consuming a large amount of water, thus sensitive to water deficit. Nevertheless, great variations of acclimation capacity and tolerance to drought exist among different poplar species because of their inter- and intra-specific genetic diversity. Although there have been numerous studies on drought tolerance of poplars, yet their physiological mechanisms to drought and re-watering remain unclear. This study investigated the variations in growth, water use efficiency, photosynthesis and anatomy of six poplar genotypes, and further analyzed the relationships between water use efficiency and photosynthesis and anatomy related parameters; subsequently, the two genotypes with contrasting water use efficiency were chosen to perform drought and re-watering treatments. Methodologies of electron microscopy, plant physiology and quantitative real-time PCR were employed to comprehensively analyze the anatomical, physiological and transcriptional responses of poplars to drought and re-watering conditions. The common responses of acclimation to drought and re-watering were generalized, and the responsive differences between the two poplar genotypes were also discussed. The main results obtained are as follows:Growth performance, photosynthesis, intrinsic WUE(WUEi), stable carbon isotope composition(?13C) of mature leaves and anatomical properties of leaf and basal xylem were analyzed in plants from six poplar genotypes grown in a botanical garden. The relationships between photosynthesis?WUE and anatomical properties of leaf and xylem were examined. Significant differences were found in growth, photosynthesis, WUEi and anatomical properties among the examined genotypes. The results showed that Populus cathayana was the clone with the fastest growth and the lowest WUEi??13C, whereas P. × euramericana had a considerable growth increment and the highest WUEi??13C. Among the analyzed poplar genotypes, the highest total stomatal density in P. cathayana was correlated with its highest stomatal conductance(gs) and lowest WUEi??13C. Moreover, significant correlations were observed between WUEi and abaxial stomatal density and stem vessel lumen area. These data suggest that photosynthesis, WUEi and ?13C are associated with leaf and xylem anatomy and there are tradeoffs between growth and WUEi. It is anticipated that some poplar genotypes, e.g. P. × euramericana, are better candidates for water-limited regions and others, e.g. P. cathayana, may be better for water abundant areas.One-year old saplings of Populus cathayana(Pc) and Populus × euramericana(Pe) were exposed to moderate stress(60% field capacity) or severe stress(40% field capacity) for 26 days and subsequently re-watered for 13 days. In acclimation to drought, both poplar species demonstrated declined height growth, altered anatomical properties in leaf and xylem. The thickness of lower epidermis, palisade tissue, spongy tissue and total leaf thickness in both genotypes were markedly increased. The vessel frequency and vessel element length in xylem were increased, the vessel lumen diameter and area, whereras the predicted conductivity of xylem maintained basically the same level to the control. Those anatomical changes indicate that poplar species reduce water loss by leaf thickening and maintain water transport capacity from roots to shoots through xylem anatomy modification in response to low water availability. Most of the plasma membrane intrinsic proteins were greatly up-regulated in response to drought but down-regulated upon re-watering. The content of c ABA was significantly induced in roots and leaves of both genotypes, accompanied by the up-regulation of NCED3, which is the key enzyme involved in the ABA biosynthesis, up to 10-fold change. Meanwhile, the transcript level of PP2 C involving in ABA signaling was also dramatically increased. C and N metabolism was altered and ?13C and ?15N were increased. To acclimate to drought, WUEi was increased greatly to decreased water loss and maintain CO2 fixation. Furthermore, the homeostasis of ROS production and scavenging was shifted in response to drought stress, and these changes could be recovered upon re-watering. Despite of the common acclimation to drought, distinct physiological and transcriptional differences existed between Pc and Pe. Compared to Pc, Pe had higher water content and a stronger responsiveness of transcriptional regulation of PIPs, indicating its better capacity to finetune water tranport via plasma membrane. Pe had higher total C, starch accumulation, ?13C, ?15N and WUEi in roots and leaves than Pc did. Less oxidative stress induced by drought was found in leaves of Pe. These data indicate that Pe is better acclimated to drought stress than Pc and that anatomical, physiological and transcriptional acclimations to decreasing soil water availability and re-watering is essential for poplars to survive and grow under projected dry climate scenarios in the future. |
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