Two sides of the same coin: the intertwined relationship between photosynthesis and light stresses

Light is a crucial factor in plant growth, development, and productivity. While light primarily serves as the energy source to drive photosynthesis, environmental light fluctuations like vegetative shade and high-light intensity also function as signaling cues that regulate plant morphology, physiology and development. Under vegetative shade, the depletion of red and blue wavelengths reduces photosynthetic rate and triggers light signaling pathways that cause shade avoidance response (SAR). SAR promotes elongation growth and reduces photosynthetic pigment content, thereby compromising overall photosynthetic efficiency. In contrast, high-light stress occurs when light exposure exceeds the capacity of the photosynthetic apparatus, resulting in chlorophyll overexcitation, excessive production of reactive oxygen species (ROS), and subsequent oxidative damage and photoinhibition. To mitigate high-light stress, plants have developed diverse photoprotective mechanisms such as leaf and chloroplast movements, non-photochemical quenching of overexcited chlorophylls, ROS scavenging, and ATP/NADPH homeostasis. This review summarizes the molecular and physiological responses of plants to shade and high-light stress, with a particular focus on their impacts on photosynthesis. We further discuss current approaches for managing light stresses and optimizing photosynthetic performance in agriculture.