Light-nutrient interaction orchestrates leaf dynamics, nitrogen assimilation, and cellular energetics in Agastache rugosa (Fisch. & C.A.Mey.) Kuntze

Light and nutrients are vital environmental factors shaping plant growth and metabolism, yet their interactive effects on leaf dynamics, nitrogen assimilation, and cellular energetics remain largely unexplored. We aimed to investigate these processes in Agastache rugosa (Fisch. & C.A.Mey.) Kuntze under two light levels; high-light (HL, 0 % shade) and low-light (LL, 50 % shade) combined with four nutrient levels; low (NPK1, 40 mg kg⁻¹), moderate (NPK2, 80 mg kg⁻¹), high (NPK3, 120 mg kg⁻¹) and very high (NPK4, 160 mg kg⁻¹). High-light conditions and high-nutrient levels (HL-NPK3) synergistically enhanced leaf mass area by 44 % with net photosynthesis rates and nitrate reductase activity increasing by up to 17.62 ± 0.89 µmol CO₂ m⁻² s⁻¹ and 0.34 ± 0.02 μmol NO₂ cm⁻² h⁻¹ each. Low-light and moderate-nutrient levels (LL-NPK2) triggered a 42 % increase in specific leaf area and threefold higher photosynthetic nitrogen use efficiency. Unexpectedly, high-light and moderate-nutrient levels (HL-NPK2) elicited peak vacuolar H⁺-ATPase and H⁺-pyrophosphatase activities at 15.6 % and 53.1 % each. This study also found significant positive correlations between chlorophyll content, nitrate reductase (r = 0.62, P < 0.01), and vacuolar H⁺-ATPase activity (r = 0.58, P < 0.01), suggesting a mechanism for maintaining high photosynthetic capacity and efficient nitrogen assimilation. The clustering of leaf area index, specific leaf area, and photosynthetic nitrogen use efficiency (similarity of > 70 %) suggests optimized leaf structure and nitrogen use in light-limited but nutrient-rich environments. Our findings show how A. rugosa adjusts its physiology in response to environmental conditions, with implications for understanding plant adaptation and improving cultivation practices.