Stem Photosynthesis in ‘Hybrid Poplar 275’ Remains Stable Following Defoliation Induced by Severe Drought

Drought is a major stressor affecting tree physiology and is expected to intensify under climate extremes. Stems, partly due to their photosynthetic capacity, tend to be more drought-resilient than leaves. This study aimed to assess stem photosynthetic and its impact on carbon balance in leafless stems under drought conditions. Severe drought caused a marked decline in stem and root water potential (Ψ) and reduced stem water vapor conductance (gtw) by about 40%. Despite this, stems retained the capacity for active gas exchange: though with reduced stem CO2 efflux (ECO2) and enhanced CO2 refixation, which increased from about 40% under control conditions to ~55%–60% after drought, accompanied by a twofold increase in intrinsic water use efficiency (iWUE). Chlorophyll a fluorescence and pigment analyses indicated that the integrity of photosystem II (PSII) was preserved under drought, supporting sustained corticular photosynthesis. Concentrations of chloride, malate, and citrate in the xylem sap did not change significantly under drought, indicating a high capacity of stems to maintain homeostasis. Stable isotope analyses revealed drought-induced shifts in δ13C, consistent with altered carbon allocation following leaf abscission. These results confirm that stem photosynthesis and CO2 reassimilation contribute significantly to stem metabolic resilience, mitigating drought-induced carbon losses and helping to preserve plant survival.