1. Plants are often exposed to multiple abiotic and biotic environmental variations in nature, but studies are very scarce on how plant respond to biotic variations, or interactive effects of abiotic and biotic factors. 2. Our objective was to investigate effects of light conditions on responses of plant biomass allocation to conspecific or heterospecific neighbors, and effects of these neighbors on plant response to shading vs. full light conditions. 3. We subjected plants of Buddleja lindleyana and Bidens pilosa to three treatments of solitary growth (control), intra- and interspecific interaction, under 50% shading and full light conditions, and measured a series of biomass and morphological traits on them. 4. Full light relative to shading decreased total mass, root mass and root:shoot ratio of B. pilosa under intraspecific competition, but increased them for both species grown alone and with heterospecific neighbors. Compared to those grown alone, intraspecific interaction increased mean total mass for B. lindleyana and increased root mass and root: shoot ratio for both species in shading, but reduced total mass and root:shoot ratio of B. pilosa under full light, with no effects of interspecific interaction. 5. Results suggested conspecific neighbors will more likely interfere with plant acquiring resources, making it more difficult or less efficient for plants to utilize the resources. Plants will adjust the strategy of biomass allocation for maximizing growth depending on both resource availability and accessibility, to enhance the efficiency of resource acquiring under severe environmental challenges. 6. Synthesis. By investigating responses of plants to variations in abiotic conditions and plant interactions simultaneously, we not only provided direct evidence for responses of plants to complex environmental factors, but also revised the optimal partitioning theory by emphasizing the importance of resource accessibility.