Concurrent Temperature and Light Intensity Fluctuations Promote Stomatal Opening and Non-Steady State Photosynthesis

In nature, fluctuations in light intensity (FL) are tightly coupled to rapid changes in leaf temperature (T_leaf), yet the short-term physiological effects of these concurrent drivers remain largely unresolved. Here, we combined rapid infrared induced T_leaf fluctuations with controlled step changes in light intensity for leaf gas exchange measurements to disentangle how each factor, and their interaction, affects stomatal conductance (g_s) and net CO₂ assimilation (A) in cucumber plants grown under constant light or FL. Modest but rapid increases in T_leaf (~3°C) alone triggered a pronounced wrong‑way-like stomatal response, leaf movements due to change in epidermal cell turgor, and transient (but long-lasting) decrease in A. This transient decrease in A disappeared under low O₂, indicating a photorespiratory origin. When T_leaf and light increased simultaneously, both g_s and A responded faster and more strongly than to light alone, with cumulative enhancement across successive cycles. Plants acclimated to FL displayed larger transient increases in A and maintained higher integrated carbon gain under combined fluctuations. Our findings demonstrate that realistic, rapid T_leaf dynamics exert strong biomechanical and biochemical influences on dynamic gas exchange that are distinct from steady-state responses and should be explicitly considered in experimental design and dynamic photosynthesis models.