Relation between drought-exposed photosynthetic apparatus and tree water deficit derived from stem diameter variations in Norway spruce seedlings

Ten five-year-old seedlings of Norway spruce (Picea abies L. Karst) from the Western Carpathians were subjected to drought (30 days without irrigation) in laboratory conditions (D trees). The control group (C trees) were irrigated regularly. Parameters such as stem diameter variations (SDV), soil water potential (Ψ_S), gas exchange and chlorophyll a fluorescence were measured. Stem growth of D trees was significantly reduced below Ψ_S = –0.3 MPa, and completely stopped below Ψ_S = –1.1 MPa. Tree water deficit (TWD, calculated from SDV) of D trees started to increase substantially below the threshold Ψ_S = –0.9 MPa, and closely correlated with Ψ_S. Photosynthetic traits of D trees reacted synchronously with TWD during drought, and recovered after rehydration. Gas exchange and most of the chlorophyll a fluorescence parameters were tightly positively related to TWD. Growth and TWD parameters, as well as those of gas exchange and most of the chlorophyll a fluorescence parameters differed against C trees during drought. Thus, the parameters derived from SDV may serve as indicators of the functionality of photosynthetic apparatus.