Carbon and water relations are fundamental to plant life and strongly interact. Under drought, the ability of plants to assimilate carbon is reduced, which increases their consumption of stored labile carbon in the form of non-structural carbohydrates (NSC). Stored NSC depletion may impair plant water relations, but mechanisms are not clear, and we do not know if its effects are independent of water deficit. If so, carbon costs of fungal symbionts could also indirectly influence plant drought tolerance through stored NSC depletion. We connected well-watered Pinus ponderosa seedling pairs via ectomycorrhizal (EM) networks where one seedling was shaded and the other experienced full light and compared responses to seedling pairs in the light. We measured plant water relations and traced carbon movements using 13CO2 to explore the mechanisms linking stored NSC to water relations, and to identify potential tradeoffs between the ability to endure low water potentials and maintaining EM fungi under carbon-limiting conditions. Even in the absence of drought, mild NSC depletion impaired osmoregulation capacity and turgor maintenance, a critical strategy to tolerate drought. This demonstrates that NSC storage influences plant water relations independently of plant water status. We also found that EM networks propagated NSC depletion and its negative effects on water relations from carbon stressed hosts to non-stressed hosts. These results highlight carbon allocation tradeoffs between supporting fungal symbionts and retaining water via stored NSC and have implications for biotic interactions and forest drought responses.

中文翻译：

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植物碳水化合物的消耗会削弱水分关系，并通过外生菌根网络传播

碳和水的关系是植物生命的基础，并且相互作用密切。在干旱下，植物吸收碳的能力降低，从而增加了其以非结构性碳水化合物（NSC）形式储存的不稳定碳的消耗。储存的NSC枯竭可能会损害植物的水分关系，但机制尚不清楚，我们不知道其影响是否与水分亏缺无关。如果是这样，真菌共生体的碳成本也可以通过储存的NSC消耗间接影响植物的耐旱性。我们通过外生菌根（EM）网络连接了浇水良好的黄松幼苗对，其中一棵幼苗被遮蔽，另一棵幼苗处于全光照，并在光照下比较了对幼苗的响应。我们使用13CO2测量了植物的水分关系并追踪了碳的运动，以探索将存储的NSC与水分关系联系起来的机制，并确定了在碳限制条件下承受低水势和维持EM真菌的能力之间的潜在权衡。即使在没有干旱的情况下，轻度的NSC消耗也会损害渗透调节能力和保持充沛的体力，这是耐受干旱的重要策略。这表明NSC的存储独立于植物水状况而影响植物水关系。我们还发现，EM网络传播了NSC枯竭及其对水关系的负面影响，从碳胁迫寄主到非胁迫寄主。