Jimei Han

Rising atmospheric CO₂ concentration and global warming jointly affect plant photosynthesis and water use, but how elevated CO₂ regulates photosynthetic short-term thermal response and its implications for leaf carbon-water balance remain poorly understood. We investigated the temperature responses (25–45 °C) of net photosynthetic rate (A_n), intrinsic water use efficiency (iWUE), stomatal conductance (g_s), and mesophyll conductance (g_m) in Liriodendron tulipifera L., a fast-growing and economically important hardwood species sensitive to heat stress, under five CO₂ concentrations (150–800 μmol mol⁻¹), and quantified the partitioning of stomatal, mesophyll, and biochemical limitations. Elevated CO₂ increased the optimal temperature (T_opt) of A_n, iWUE, and g_m, but decreased that of g_s, creating a divergent thermal response pattern that decoupled carbon acquisition from water loss. The upward shift in the T_opt of g_m enhanced CO₂ supply to chloroplast carboxylation sites at high temperatures, sustaining higher carboxylation rates, indicating that improved mesophyll diffusion, rather than biochemical capacity, drove photosynthetic short-term thermal response. The downward shift in T_opt of g_s aligned with optimal stomatal behavior theory, promoting earlier stomatal closure to conserve water, which together with enhanced mesophyll diffusion resulted in a net improvement in water use efficiency below 40 °C. At 45 °C, however, mesophyll limitation (l_m) became dominant (~70% of total limitation), indicating that elevated CO₂ cannot prevent photosynthetic breakdown under extreme heat, and the water-saving benefit was lost. We suggest that terrestrial biosphere models incorporating separate temperature dependencies of g_m and g_s would improve predictions of tree carbon-water balance under future climates.

Photosynthesis in higher plants is highly sensitive to drought and salinity. While studies have examined the individual effects of drought or salt stress on photosynthesis, their combined impact remains poorly understood. In this study, we investigated the diurnal dynamics and primary limiting factors (stomatal, mesophyll, and biochemical) affecting the net photosynthetic rate (A_n) in Ginkgo (G.) biloba under drought, salt, and combined drought–salt stress. The results revealed that G. biloba exhibited a bimodal pattern of A_n under control conditions, primarily driven by mesophyll conductance (g_m). Under drought, this pattern shifted, with stomatal limitations dominant in the late afternoon. In contrast, salt and combined stress induced a unimodal A_n pattern due to a flattened g_m curve and reduced correlation between g_m and A_n. Interestingly, combined stress caused significantly lower mesophyll limitations than salt stress alone, compensating for increased stomatal limitations and leading to a higher A_n. Our findings reveal a dynamic shift in the limiting factors over time and stress types, suggesting that G. biloba has mechanisms to mitigate combined drought–salt stress. These insights deepen our understanding of plant resilience under complex environmental conditions.

Nitrogen nutrition monitoring is crucial in agriculture and forestry. With the development of Unmanned Aerial Vehicle (UAV) imaging technology, its application in nitrogen nutrition monitoring has gained attention. Traditional regression methods often struggle to accurately capture the nonlinear relationships between image features and nitrogen nutrition parameters. This study introduces Gaussian regression models to better model the relationship between UAV image features and nitrogen nutrition in Ginkgo. UAV RGB imagery of three-year-old Ginkgo biloba L. seedlings was used to extract nitrogen-related image features. Gaussian regression models were employed to select and model these features, creating regression models for nitrogen accumulation and nitrogen content in Ginkgo. The accuracy of the models was validated. Results indicated that the optimal canopy type for monitoring nitrogen accumulation in Ginkgo was the shadowed canopy, with the color feature BMR being the most important feature. For monitoring nitrogen content, sunlight and shadow canopy types were suitable, with BMR and b* being the key features. Gaussian regression demonstrated superior accuracy and robustness compared to traditional regression models. This study emphasizes the potential of Gaussian regression models to improve nitrogen monitoring through UAV imagery, offering valuable applications in precision agriculture and forestry management, particularly in supporting nitrogen fertilization and nutrition management for Ginkgo.

Balance among the sequential photophysical, photochemical, and biochemical reactions of photosynthesis is needed for converting fleeting energy in light to stable energy in chemical bonds. Any imbalance acts as either a bottleneck for limiting photosynthetic efficiency or an agent for inducing structural and functional damage to photosynthetic apparatus. Not only must each reaction be carefully regulated, but regulatory processes must also be coordinated across the reactions. However, regulations of different stages of photosynthesis have rarely been studied jointly. Non‐photochemical quenching ( NPQ ) and stomatal conductance ( g _s ) are key regulators of photophysical and biochemical reactions, respectively. Existing evidence suggests that the redox state of plastoquinone regulates g _s and that the photochemical reactions are partially regulated by the ultrastructural dynamics of thylakoids induced by osmotic water fluxes in chloroplasts of land plants. To examine how these regulations are coordinated and feedback to each other, we simultaneously measured NPQ and g _s and inferred the redox state of plastoquinone and the light‐induced thylakoid swelling/shrinking on numerous C ₃ and C ₄ species. For all species measured, NPQ and g _s covary with the redox states of the electron transport chain, particularly plastoquinone, and increase as thylakoid swelling is inferred. NPQ has the maximal sensitivity at the light intensity at which thylakoid is inferred to be fully swollen. Our findings suggest that plant energy and water use strategies are intimately linked by evolution, and studying the regulations of different photosynthetic stages as a whole can lead to new insights of the functioning of photosynthetic machinery in dynamic environments.

Mesophyll conductance (gm) has been proved to be one of the important factors limiting photosynthesis and thus affects the estimation of plant productivity and terrestrial carbon balance. However, beyond the leaf scale, gm is usually assumed to be infinite because of the unavailability of the estimating technology. In this study, we first verified the important role of gm on photosynthesis by utilizing a wide range of ginkgo (Ginkgo biloba L.) families. Then, the dataset was adopted to establish a random forest-based gm estimation approach with the drivers being selected under the guidance of several mechanistic models (e.g. Farquhar, von Caemmerer, Berry model, the mechanistic light reaction model of photosynthesis). This model exhibited high predictive accuracy, utilizing both the measured fraction of open reaction centers in PSII (qL) (R2 = 0.71, RMSE = 0.008) and the estimated qL (R2 = 0.70, RMSE = 0.008) as inputs. Since qL, a key physiological driver in the model, can be obtained from chlorophyll fluorescence of PSII (SIFPSII) using the open-closed (OC) redox model of photosynthetic electron transport, this leaf-scale model could potentially be applied beyond the leaf scale, provided that environmental data are available. Direct measurements also confirmed the close relationship between qL and gm under ambient CO2 concentration and saturated light conditions. Our findings pave the way for additional attempts to estimate gm across a variety of scales.

Leaf nitrogen allocation plays a crucial role in determining both photosynthetic function and structural development of plants. However, the effects of drought, salt stress, and their combination on leaf nitrogen allocation, and how these affect mesophyll conductance (g_m) and photosynthesis, remain poorly understood. In this study, we first investigated variations in photosynthetic characteristics and leaf nitrogen allocation, and analyzed the relationship between g_m and leaf nitrogen allocation ratios in Ginkgo biloba under drought, salt and combined drought-salt stress. The results showed that all stress treatments significantly reduced the photosynthesis in G. biloba, with the combined drought-salt stress having the most significant inhibitory effect on the plant’s physiological characteristics. Under combined drought-salt stress, the limitation of photosynthesis due to g_m (MC_L) was significantly greater than under individual drought or salt stress. In contrast, the limitation due to stomatal conductance (S_L) was similar to that observed under drought but higher than under salt stress. No significant differences in biochemical limitations (B_L) were found across all stress treatments. Further research suggests that the increase in MC_L under combined drought-stress treatment may be linked to a greater allocation of leaf nitrogen to non-photosynthetic apparatus (e.g., cell structure) and a smaller allocation to photosynthetic enzymes (i.e., ribulose-1,5-bisphosphate carboxylase/oxygenase, Rubisco). This is supported by the positive correlation between g_m and the proportion of nitrogen allocated to the carboxylation system (P_r), as well as the negative correlation with the non-photosynthetic nitrogen ratio (P_np). These findings help to advance our understanding of the mechanisms of photosynthesis and plant adaptability under combined drought-salt stress.

A photochemical model of photosynthetic electron transport (PET) is needed to integrate photophysics, photochemistry, and biochemistry to determine redox conditions of electron carriers and enzymes for plant stress assessment and mechanistically link sun‐induced chlorophyll fluorescence to carbon assimilation for remotely sensing photosynthesis. Towards this goal, we derived photochemical equations governing the states and redox reactions of complexes and electron carriers along the PET chain. These equations allow the redox conditions of the mobile plastoquinone pool and the cytochrome b₆f complex (Cyt) to be inferred with typical fluorometry. The equations agreed well with fluorometry measurements from diverse C₃/C₄species across environments in the relationship between the PET rate and fraction of open photosystem II reaction centres. We found the oxidation of plastoquinol by Cyt is the bottleneck of PET, and genetically improving the oxidation of plastoquinol by Cyt may enhance the efficiency of PET and photosynthesis across species. Redox reactions and photochemical and biochemical interactions are highly redundant in their complex controls of PET. Although individual reaction rate constants cannot be resolved, they appear in parameter groups which can be collectively inferred with fluorometry measurements for broad applications. The new photochemical model developed enables advances in different fronts of photosynthesis research.

Summary: In higher plants, photosystems II and I are found in grana stacks and unstacked stroma lamellae, respectively. To connect them, electron carriers negotiate tortuous multi‐media paths and are subject to macromolecular blocking. Why does evolution select an apparently unnecessary, inefficient bipartition? Here we systematically explain this perplexing phenomenon. We propose that grana stacks, acting like bellows in accordions, increase the degree of ultrastructural control on photosynthesis through thylakoid swelling/shrinking induced by osmotic water fluxes. This control coordinates with variations in stomatal conductance and the turgor of guard cells, which act like an accordion's air button. Thylakoid ultrastructural dynamics regulate macromolecular blocking/collision probability, direct diffusional pathlengths, division of function of Cytochrome b₆f complex between linear and cyclic electron transport, luminal pH via osmotic water fluxes, and the separation of pH dynamics between granal and lamellar lumens in response to environmental variations. With the two functionally asymmetrical photosystems located distantly from each other, the ultrastructural control, nonphotochemical quenching, and carbon‐reaction feedbacks maximally cooperate to balance electron transport with gas exchange, provide homeostasis in fluctuating light environments, and protect photosystems in drought. Grana stacks represent a dry/high irradiance adaptation of photosynthetic machinery to improve fitness in challenging land environments. Our theory unifies many well‐known but seemingly unconnected phenomena of thylakoid structure and function in higher plants.