Modelling photosynthetic responses by day and night during initial water stress in <i>Pulmonaria vallarsae</i>

The relationships between photosynthesis and initial water deficit stress were investigated by chlorophyll fluorescence analysis in Pulmonaria vallarsae, a shade tolerant, perennial C3 herb by following changes of light response curves (LRCs) in single leaves during water shortage. We devised an LRC model based on two interacting rectangular hyperbolae (DH model) for the low (H1) and the high irradiance regions (H2), characterized by two parameters: maximum extrapolated ETR (V1, V2) and half-saturation irradiance (K1, K2). While H1 is assumed to represent an ETR-related function, H2 may reflect Rubisco activity. Plants were subjected to four days of water restriction in summer and tested every 12 h. While daytime values remained relatively stable, increasing water stress gradually induced a night depression of photosynthesis mainly dependent on decreasing ETR with concomitant reduction of PSII-dependent parameters (Φ_PSII) and fluorescence-related functions, while nonphotochemical quenching (NPQ) was strongly enhanced. In terms of the DH model, the night depression of photosynthesis featured a night drop of V2 and K2 followed by decreases of V1 and K1. The H2 hyperbola was more stress responsive than H1 and frequently showed a reversible decrease of nocturnal H2 parameters (bright illumination constraint, BIC). Pulmonaria plants tested during winter with very low water stress displayed LRCs resembling rectangular hyperbolae, similar during day and night. The DH model is shown to yield accurate and perspicuous photosynthetic parameters representing the principal components of an LRC and to be well suitable to document the day/night divergence of photosynthetic capacity during a water deficit stress.