Intracellular Water Dynamics Are Associated with Contrasting Drought Responses in Broussonetia papyrifera (L.) Vent. and Morus alba L.

Drought severely constrains plant growth and physiological functions, particularly in karst ecosystems where shallow and heterogeneous soils further exacerbate water limitations. However, changes in intracellular water dynamics during progressive drought and their relationship with photosynthetic responses remain poorly understood. Here, we subjected seedlings of Broussonetia papyrifera (L.) Vent. and Morus alba L. to progressive drought for 16 days under two simulated habitat conditions: rock outcrop (RO) and non-rock outcrop (NRO). At nine progressive drought stages, we measured soil water content (SWC), leaf water potential (Ψ), net photosynthetic rate (Pn), stomatal conductance (gsw), transpiration rate (E), and four impedance-derived intracellular water parameters: intracellular water-holding capacity (IWHC), intracellular water transport rate (IWTR), intracellular water-holding time (IWHT), and intracellular water-use efficiency (IWUE). During progressive drought, SWC declined and Ψ became more negative. IWHC, IWTR, Pn, gsw, and E also declined, whereas IWHT generally increased; IWUE showed species-specific temporal patterns. M. alba L. exhibited earlier declines in Ψ, IWHC, IWTR, and photosynthetic traits, whereas B. papyrifera L. showed more gradual physiological changes and maintained comparatively higher water status. IWHC and IWTR were positively correlated with photosynthetic traits, whereas IWHT showed negative associations. These correlations suggest that intracellular water dynamics may complement conventional measurements for characterizing species-specific responses to drought. In particular, IWHT increased during drought and may reflect a temporal dimension of intracellular water regulation; however, causality and predictive capacity remain to be tested.