Physiological and Metabolic Responses of Red Leaf Lettuce (<i>Lactuca sativa</i> L.) under Low Pressure Conditions

Understanding plant responses under low-pressure conditions is important for developing closed cultivation systems that simulate space environments. This study aimed to assess the effects of different pressure levels on growth, photosynthesis, and secondary metabolite accumulation in red leaf lettuce (Lactuca sativa L. var. ‘Super Caesar’s Red’). Plants were cultivated for three weeks in sealed chambers under 101 kPa (atmospheric pressure), 66 kPa (moderate low pressure), and 33 kPa (severe low pressure). Growth analysis showed that leaf length and leaf area decreased significantly with reduced pressure, while chlorophyll content and SPAD values increased gradually. Photosynthetic measurements indicated lower transpiration and stomatal conductance under low pressure relative to atmospheric conditions, consistent with reduced stomatal size and density observed by SEM. Secondary metabolite analysis showed strong induction of anthocyanins (41.3% at 66 kPa and 190.8% at 33 kPa), with significant increases in phenolic and flavonoid contents. Thus, low-pressure conditions may suppress morphological growth but promote secondary metabolite contents, offering potential advantages for quality-oriented cultivation strategies. This study provides fundamental insights into physiological adaptation under low pressure and practical implications for crop selection and management in space agriculture and other controlled environments.