Interactive Effects of Light Intensity and Temperature on Photosynthesis, Chlorophyll Fluorescence and Leaf Ultrastructure in the Precious Water Lily Nymphaea hybrid

The precious aquatic plant Nymphaea hybrid, is valued for its ornamental and economic importance but is highly sensitive to environmental fluctuations. However, the interactive effects of light and temperature, two critical abiotic factors, on its photosynthetic performance and underlying structural basis remain poorly understood. In this paper, we conducted a two-factorial experiment in which N. hybrid plants were exposed to five light–temperature regimes for 9 days: control (T0, 800 μmol·m−2·s−1 + 25/20 °C); low light–low temperature—LLLT (T1, 200 μmol·m−2·s−1 + 15/10 °C); low light–high temperature—LLHT (T2, 200 μmol·m−2·s−1 + 35/30 °C); high light–low temperature—HLLT (T3, 1400 μmol·m−2·s−1 + 15/10 °C); and high light–high temperature—HLHT (T4, 1400 μmol·m−2·s−1 + 35/30 °C). We systematically investigated changes in photosynthetic pigments, gas exchange, chlorophyll fluorescence, and leaf ultrastructure by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that: The HLLT treatment (T3) inflicted the most severe damage, triggering a rapid decline in net photosynthetic rate (Pn), maximal photochemical efficiency (Fv/Fm), and chlorophyll content, coupled with a significant increase in intercellular CO2 concentration (Ci). Ultrastructurally, T3 caused stomatal closure, disintegration of thylakoid membranes, and accumulation of large osmophilic granules, indicating severe photo-oxidative stress. In contrast, the LLHT treatment (T2) demonstrated remarkable resilience, with physiological and structural parameters closely resembling the control. The HLHT treatment (T4) caused intermediate damage, primarily through non-stomatal limitations over time. Our findings demonstrate a significant light–temperature interaction in N. hybrid. The thermophilic nature of N. hybrid was evident: high temperature mitigated the negative effects of low light, whereas low temperature, especially in combination with high light, acted synergistically to cause catastrophic damage to the photosynthetic apparatus. This study provides a mechanistic understanding of N. hybrid’s environmental adaptability, offering critical insights for its cultivation management under changing climate conditions, especially in high altitude and high latitude areas.