Above and below: soil moisture and soil temperature interact to alter grapevine water relations

Abstract

The combined effect of soil moisture and soil temperature on grapevine physiology is gaining interest in the context of global warming. However, the interaction between these factors and their impact on grapevine water relations remain poorly understood. Grapevines can adapt their root water uptake in response to environmental changes by regulating the expression of root aquaporin genes. Drought and lower rootzone temperatures have been observed to reduce their expression, decrease water uptake, and root hydraulic conductance. Based on data from a field irrigation trial where irrigation induced a localized reduction of 3°C in the soil that lasted for three days, we investigated the interaction of soil moisture and soil temperature on gas exchange, vine water status, whole-plant hydraulic conductance (Kplant), and root aquaporin differential gene expression in potted vines. Due to the important role that aquaporins play in root hydraulic conductance changes in response to the environment, we also analyzed the relative expression of four Vitis vinifera PIP aquaporins (VvPIP1;1, VvPIP2;1, VvPIP2;2, VvPIP2;4N) known for their confirmed presence in grapevine root tips. We hypothesized that warmer root zone temperatures would increase Kplant by upregulating VvPIP gene expression in roots, possibly accelerating the onset of drought stress. We manipulated soil moisture and soil temperature in potted, own-rooted vines by recirculating through a custom-built system either chilled glycol or hot water while controlling the irrigation input. Leaf gas exchange and leaf water potential (predawn and midday Ψleaf) were measured, and Kplant was estimated. Root samples were collected after seven days for RT-qPCR analysis of PIP gene expression. RNA was extracted from actively growing roots following the DEPC-CTAB protocol and used for differential gene expression after performing RT-qPCR. The results showed that the leaf-level responses were dominated by the influence of soil moisture, while aquaporin gene expression in the roots was more sensitive to the effects of soil temperature. Soil moisture and, to a lesser degree, soil temperature were correlated with Ψleaf and Kplant, supporting the hypothesis of a possible interaction between these factors. Cooler root zone temperatures also reduced Kplant, stomatal conductance and the expression of aquaporin genes in the roots.