Summary: Recent progress in observing sun‐induced Chl fluorescence (SIF) provides an unprecedented opportunity to advance photosynthesis research in natural environments. However, we still lack an analytical framework to guide SIF studies and integration with the well‐developed active fluorescence approaches. Here, we derive a set of coupled fundamental equations to describe the dynamics of SIF and its relationship with C₃ and C₄ photosynthesis. These equations show that, although SIF is dynamically as complex as photosynthesis, the measured SIF simplifies photosynthetic modeling from the perspective of light reactions by integrating over the dynamic complexities of photosynthesis. Specifically, the measured SIF contains direct information about the actual electron transport from photosystem II to photosystem I, giving a quantifiable link between light and dark reactions. With much‐reduced requirements on inputs and parameters, the light‐reactions‐centric, SIF‐based biophysical model complements the traditional, dark‐reactions‐centric biochemical model of photosynthesis. The SIF–photosynthesis relationship, however, is nonlinear. This is because photosynthesis saturates at high light whereas SIF has a stronger tendency to keep increasing, as fluorescence quantum yield has a relatively muted sensitivity to light levels. Successful applications of the SIF‐based model of photosynthesis will depend on a predictive understanding of several previously underexplored physiological and biophysical processes. Advances can be facilitated by coordinated efforts in plant physiology, remote sensing, and eddy covariance flux observations.

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.

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.

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.