• Species-specific soil moisture ranges maximizing PSII efficiency were identified. • Chlorophyll fluorescence gives a practical framework to optimize water-use strategies • The C3 species kept a broad moisture window with moderate NPQ adjustment • The C4 species showed a narrow optimal range with strong decrease in Fv/Fm • Despite higher water-use efficiency, the C4 plant lost PSII capacity under drought • The CAM species displayed high ΦPSII even at very low soil moisture Soil water availability strongly constrains photosynthetic performance, and the moisture ranges that optimize PSII efficiency and energy partitioning may differ among photosynthetic types. Here, we used chlorophyll fluorescence analysis to identify soil moisture intervals that maximize photochemical performance and minimize stress-induced energy dissipation in representative C3, C4, and CAM species. Maximum PSII efficiency (Fv/Fm), effective quantum yield (ΦPSII), regulated non-photochemical quenching (ΦNPQ), constitutive dissipation (ΦC), and OJIP-derived flux parameters were evaluated across a gradient of soil water content. Under well-watered and intermediate conditions, all species maintained high Fv/Fm values, indicating preserved PSII integrity. However, their responses diverged under declining moisture. Vinca minor (C3) sustained a relatively broad optimal window, with moderate increases in ΦNPQ compensating for reduced ΦPSII. Axonopus compressus (C4) exhibited a narrower functional range, as severe water deficit caused marked declines in Fv/Fm and performance index (PI ABS ), suggesting an earlier transition from regulatory photoprotection to structural photoinhibition. In contrast, Kalanchoe pinnata (CAM) maintained comparatively high ΦPSII at lower soil moisture and showed delayed activation of thermal dissipation, consistent with their temporally decoupled carbon assimilation strategy. These results suggest that optimal soil moisture ranges vary with the type of photosynthetic CO₂ assimilation metabolism and that baseline energy partitioning patterns are linked to PSII resilience under hydric stress. Chlorophyll fluorescence provides a robust physiological framework to define species-specific irrigation thresholds and to guide crop selection and water-use optimization under increasing water scarcity, highlighting its potential as a powerful tool for precision agriculture.
Ospina‐Calvo et al. (2026) studied this question.