Intrinsic morphology and spatial distribution of non-structural carbohydrates contribute to drought resistance of two mulberry cultivars.

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Drought is one of the most adverse environmental stresses limiting plant growth and productivity. However, the underlying mechanisms regarding non-structural carbohydrates (NSCs) metabolism in source and sink organs are still not fully elucidated in woody trees. Saplings of mulberry cultivars Zhongshen1 and Wubu were subjected to a 15-day progressive drought stress. NSCs levels and gene expressions involved in NSCs metabolism were investigated in roots and leaves. The growth performance and photosynthesis, leaf stomatal morphology, and other physiological parameters were also analyzed. Under well-watered conditions, Wubu had higher root-to-shoot ratio (R/S) with higher NSCs in leaves than roots, while Zhongshen1 exhibited lower R/S with higher NSCs in roots than leaves. Under drought stress, Zhongshen1 showed decreased productivity and increased concentrations of proline, abscisic acid, reactive oxygen species and activities of antioxidant enzymes, while Wubu sustained comparable productivity and photosynthesis. Interestingly, drought resulted in decreased levels of starch and slightly increased soluble sugars in leaves of Wubu, accompanied by notable downregulation of starch-synthesizing genes and upregulation of starch-degrading genes. The similar patterns in NSCs levels and relevant gene expressions were also observed in roots of Zhongshen1. Concurrently decreased soluble sugars and unchanged starch in roots of Wubu and leaves of Zhongshen1 were found. However, the gene expressions of starch metabolism in roots of Wubu were unaltered, but in leaves of Zhongshen1 the starch metabolism was largely activated. These findings revealed that intrinsic R/S and the spatial distribution of NSCs in roots and leaves concomitantly contributed to drought resistance in mulberry.