A Study On Xylem Anatomy And Drought Resistance Mechanism Of Different Mulberry Varieties

Water is the fundamental medium to guarantee the normal physiological and metabolic activities in plant cells,drought stress is one of the main abiotic stresses that reduce the yield of commercial crops.Mulberry is the foundation of sericulture industry,perennial mulberry trees are often threatened by drought stress in their life cycle,leading to an enormous yield loss.With the development of"moving east mulberry to west"project,breeding mulberry varieties with stronger drought resistance has become one of the most urgent issues to sustain sericulture development.China is rich in mulberry germplasm resources,and considerable variations exist in drought tolerance in different varities.However,the physiological and transcriptional differences in response to drought stress among different varieties are still unclear.In this study,the relationship between petiole or stem xylem anatomy and plant water relations in mulberry was studied by analyzing the anatomical characteristics of xylem from seven mulberry varieties originated from different habitations.To uncover the physiological and transcriptional mechanisms of drought stress tolerance among different mulberry varieties,3-month-old saplings of Wubu,Yu711 and7307 were exposed to soil drought stress by withholding water.The physiological responses and the transcriptional expression levels of the plasma membrane aquaporin were determined in wood and bark tissue at different time points.The main results are as follows:1.The xylem characteristics of the petiole and stem among the seven examined practical mulberry varieties showed that the variety Wubu had long and narrow vessel elements that are more resistant to drought-induced embolism.It also had the highest capacity of water transport and water storage,which can sustain high-efficiency long-distance water transport in xylem.In contrast,the vessel elements were long and wide,it is inferred that wider vessels are more efficient for short-distance water transport from stem xlym to leaf veins.These results indicate that variety Wubu is less prone to be affected by embolism,and thereby could be more drought tolerant than the other examined varieties.Variety Yu711 was lower in xylem hydraulic conductivity because of its small vessel lumen diameter and area,while variety Liaolu 11 had the lowest xylem vessel frequency in stem and area ratio of vascular bundle in petiole,which lead to its lowest hydraulic conductivity as well.In addition,both varieties Liaolu 11 and Yu711 had shorter fiber element length,thinner fiber wall thickness and thinner collenchyma which suggest that these varieties might be more prone to wilting under drought stress,and they are speculated to have relatively low drought resistance compared with the other tested varieties.2.Under drought stress,the water content in xylem of Wubu,Yu711 and 7307 was overall decreased with the aggravation of stress degree.Osmoregulation substances,such as soluble sugar,soluble protein and free proline generally increased as the stress continued.However,the contents of MDA,H₂O₂ and O₂^·-were not significantly different at different time points in the stem among all the three mulberry varieties.At the early stage of drought stress,the content of carotenoids in the stem of mulberry increased significantly.Proline content in bark and wood of 7307 responded to drought stress faster than Wubu and Yu711did.Furthermore,higher membrance lipid peroxidation was found in the xlym of variety7307 than varieties Wubu and Yu711,and the activities of antioxidant enzymes were less affected by drought for variety Wubu.Based on these results,it is anticipated that Wubu is the most drought tolerant variety among the tested varieties,followed by Yu711 and 7307which is the most drought sensitive variety.3.The transcriptional expression levels of PIPs in Wubu,Yu711 and 7307 were analyzed.Changes in the transcriptional abundance of PIP1;1,PIP1;2,PIP1;3 and PIP2;3in bark of Yu711 were more significant than these in Wubu and 7307,so it was speculated that the plasma membrane aquaporin in Yu711 might be more sensitive t to water stress,and it can regulate the water transport in xylem through PIPs transcription.Under drought stress,the different types of aquaporins displayed different pattern of changes in response to drought stress.For varieties Wubu and 7307,the transcript abundance of PIP1;2,PIP2;1and PIP2;4 was up-regulated in general,while PIP1;3 and PIP2;3 was down-regulated.In addition,changes of the transcript levels of the same PIP showed different pattern,as detected that the mRNA levels of PIP1;1 was continuously up-regulated in the bark,while down-regulated in the wood with the stress level and stress time increasing,indicating that subfamily members of PIP could have different function in intercellular water transport.