Leaf electrophysiological readouts reveal bicarbonate-associated shifts in early water-deficit response modes in Broussonetia papyrifera and Morus alba.

Abstract

Karst carbonate regions are characterized by shallow soils and high bicarbonate availability, and seedlings during establishment may experience episodic water deficit. However, it remains unclear whether bicarbonate co-occurrence alters early response modes to short-term water deficit in a species-dependent manner. Here, we compared Broussonetia papyrifera and Morus alba under three treatments for 10 d: control (CK), polyethylene glycol-induced water deficit (WD), and WD with bicarbonate addition (WD + Bic). By integrating photosynthetic traits with non-destructive leaf electrophysiological signals, we evaluated early responses at both leaf and cellular scales. Under WD + Bic, the net photosynthetic rate, electrophysiologically derived nutrient-use-efficiency, and metabolic-energy indices of B. papyrifera were closer to the CK than those under WD. In contrast, M. alba deviated less from CK under WD alone, yet net photosynthetic rate and energy-related indices remained below CK under WD + Bic, accompanied by a stepwise decline in intrinsic water conductivity (CK > WD > WD + Bic). Overall, WD + Bic elicited species-specific shifts in early response trajectories relative to WD alone, rather than a uniform intensification of stress responses. These electrophysiological readouts helped distinguish distinct response modes, supporting electrophysiological phenotyping as a non-destructive candidate approach for early screening and species-habitat matching under carbonate-rich conditions.