Monitoring and Predicting Low Temperature and Low Irradiance Stress in Strawberries Using Combined Morphological and Physiological Features

Low temperature and low irradiance (LTLI) stress severely limits strawberry growth and productivity during winter protected cultivation. This study investigated the physiological responses of the short-day strawberry cultivar ‘Benihoppe’ to individual and combined LTLI stress and developed a quantitative damage evaluation index. Seedlings were exposed to four treatments for 20 d: control (25/15 °C, 600 μmol m−2 s−1), single low temperature (LT: 15/5 °C), single low irradiance (LI: 100 μmol m−2 s−1), and combined stress (LTLI: 15/5 °C, 100 μmol m−2 s−1). Compared to isolated stress factors, combined LTLI treatment exhibited a statistically verified synergistic damaging effect (two-factor ANOVA, LT × LI p < 0.01) on leaf structure and function. LTLI-treated plants showed severe reductions in leaf area, palisade tissue thickness, chlorophyll content, and net photosynthetic rate (Pn), alongside elevated malondialdehyde (MDA) accumulation. Chlorophyll a fluorescence (OJIP) analysis revealed that LTLI stress strongly blocked the electron transport chain at the PSII acceptor side, increasing the J-step relative variable fluorescence (Vj) and suppressing the performance index (PI). To quantify these impacts, a Low Temperature and Low Irradiance Damage Index (LTLDI) was derived from 12 core physiological and morphological variables. The LTLDI scores demonstrated that LTLI induced severe damage by day 10 (score: 0.69) and extremely severe damage by day 20 (0.87), which were substantially higher than the damage caused by LT (0.58 at 20 d) and LI (0.63 at 20 d) alone. The index reliability was confirmed by its strong correlation (r > 0.9) with key stress markers (Fv/Fm, Pn, and MDA). Overall, combined LTLI stress exacerbates structural degradation and PSII photoinhibition in strawberry leaves. The proposed LTLDI offers a practical, standardized tool for evaluating stress severity, facilitating timely environmental management in greenhouse strawberry production.