Quercus variabilis Blume. Source-sink dynamics: Mechanistic responses to photosynthate feedback inhibition

Changes in source-sink relationships significantly affect photosynthetic efficiency and subsequent growth. However, understanding the alleviation or exacerbation of source strength to influence function remains limited under light suppression. To investigate photosynthetic feedback mechanisms in Quercus variabilis under varying source-sink dynamics, we subjected two-year-old seedlings to gradient pruning treatments. Under photo inhibition, excessive leaf starch accumulation significantly reduced the net photosynthetic rate (P_n), maximal photochemical efficiency of PSII in the dark (F_v/F_m), and maximum carboxylation rate of Rubisco (V_cmax) (p < 0.05). In contrast, 40 % leaf removal enhanced soluble sugar transport to the stems and roots, increasing P_n (p < 0.05). At 80 % pruning, excessive source loss impaired light reactions (34 % reduction in stomatal conductance) and caused abnormal root starch accumulation, thereby disrupting assimilation metabolism. We conclude that under intense light stress, this leads to photodamage in source leaves and excessive starch accumulation in chloroplasts, and under extreme source leaf loss, stomatal limitation occurs. Appropriately regulating the source-sink relationship helps to optimize the distribution of assimilates, thereby mitigating the effects of high light stress. However, an extreme loss of source strength exceeds the plant's compensation threshold, causing the photosynthetic assimilation system to collapse. This highlights source-sink coordination as a key physiological adaptation mechanism for maintaining the carbon balance under environmental disturbances.