Plant photosynthesis assimilates CO₂ from the atmosphere, and CO₂ diffusion efficiency is mainly constrained by stomatal and mesophyll resistance.  The stomatal and mesophyll conductance of plants are sensitive to abiotic stress factors, which affect the CO₂ concentrations at carboxylation sites to control photosynthetic rates.  Early studies conducted relevant reviews on the responses of stomatal conductance to the environment and the limitations of mesophyll conductance by internal structure and biochemical factors.  However, reviews on the abiotic stress factors that systematically regulate plant CO₂ diffusion are rare.  Therefore, in this review, the rapid and long-term responses of stomatal and mesophyll conductance to abiotic stress factors (such as light intensity, drought, CO₂ concentration and temperature) and their physiological mechanisms are summarized.  Finally, future research trends are also investigated.

Soybean is one of the major oil seed crops, which is usually intercropped with other crops to increase soybean production area and yield.  However, soybean is highly sensitive to shading.  It is unclear if soybean morphology responds to shading (i.e., shade tolerance or avoidance) and which features may be suitable as screening materials in relay strip intercropping.  Therefore, in this study, various agronomic characteristics of different soybean genotypes were analyzed under relay intercropping conditions.  The soybean materials used in this study exhibited genetic diversity, and the coefficient of variations of the agronomic parameters ranged from 13.84 to 72.08% during the shade period and from 6.44 to 52.49% during the maturity period.  The ratios of shading to full irradiance in stem mass fraction (SMF) were almost greater than 1, whereas opposite results were found in the leaves.  Compared with full irradiance, the average stem length (SL), leaf area ratio (LAR) and specific leaf area (SLA) for the two years (2013 and 2014) increased by 0.78, 0.47 and 0.65 under shady conditions, respectively.  However, the stem diameter (SD), total biomass (TB), leaf area (LA), number of nodes (NN) on the main stem, and number of branches (BN) all decreased.  During the shady period, the SL and SMF exhibited a significant negative correlation with yield, and the SD exhibited a significant positive correlation with yield.  The correlation between the soybean yield and agronomic parameters during the mature period, except for SL, the first pod height (FPH), 100-seed weight (100-SW), and reproductive growth period (RGP), were significant (P<0.01), especially for seed weight per branch (SWB), pods per plant (PP), BN, and vegetative growth period (VGP).  These results provide an insight into screening the shade tolerance of soybean varieties and can be useful in targeted breeding programs of relay intercropped soybeans.