Light-NPK Synergy Increases Biomass, Photosynthetic Pigment and Nitrogen Content in Agastache rugosa (Fisch. & C.A.Mey.) Kuntze

Agastache rugosa (Fisch. & C.A.Mey.) is a medicinal herb native to subtropical and temperate climates which is highly valued for its essential oils and phytochemicals in the pharmaceutical, cosmetic, and food industries. This study examined the adaptive strategies of A. rugosa in response to varying light and NPK levels. The treatments were four NPK levels which were 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) nested under two light levels namely high-light (HL, 0% shade) and low-light (LL, 50% shade). We uncovered a resource allocation mechanism that optimises growth and photosynthetic efficiency through a multidimensional analysis involving biomass, photosynthetic pigment, and nitrogen contents. High-light promoted greater biomass and pigment content across NPK treatments, with the most pronounced effects under low NPK levels, indicating enhanced nitrogen use efficiency. This suggested that strategic management of light levels could compensate for nutrient deficiencies in this valuable herb. High-light increased leaf nitrogen content in a non-linear way, implying shifts in resource allocation, which are crucial for optimising fertilisation. Principal component analysis exhibited distinct clustering patterns, highlighting the dominant effect of light on overall physiology, with NPK levels introducing a secondary gradient of variation. Our results demonstrated the complex interplay between light and NPK availability in shaping plant responses, challenging simplistic notions of productivity and proving the importance of considering multiple environmental factors in tandem. These insights advance our understanding of plant adaptation to environmental changes and offer valuable guidance for optimising cultivation practices in medicinal herb production.