Biochemical and photochemical constraints to photosynthesis under boron excess: Evidence from physiological and...

This study investigated the effects of high boron (B) concentrations on photosynthetic performance in Citrus macrophylla. Six-month-old plants were grown under controlled conditions and irrigated for 30 days with nutrient solutions containing 0.11 mg·L^-¹ (control), 5 mg·L^-¹ (B5), and 10 mg·L^-¹ (B10) B. Excess B reduced stomatal conductance (g_s) and net CO₂ assimilation (A_CO2). However, the stronger decline in A_CO2 relative to g_s, together with stable intercellular CO₂ concentration (C_i), indicated that photosynthetic inhibition was not exclusively driven by stomatal closure. Instead, non-stomatal limitations played a key role. Chlorophyll fluorescence analysis showed progressive declines in F_v/F_m and ΦPSII with increasing B accumulation, indicating PSII photoinhibition and impaired electron transport. At moderate B levels (B5), regulated non-photochemical energy dissipation (ΦNPQ) was activated. However, at excessive B accumulation (B10), photoprotective mechanisms became insufficient, resulting in increased lipid peroxidation. Photosynthetic capacity analysis revealed a significant decrease in V_cmax under high B, while J_max and TPU remained unaffected. Total soluble sugars and starch concentrations were also not significantly affected by B supply, indicating that the decline in apparent carboxylation capacity occurred without detectable changes in bulk leaf carbohydrate pools. These results are consistent with a predominant biochemical limitation involving reduced apparent Rubisco carboxylation capacity. Additionally, chlorophyll concentration decreased under excess B, further compromising photosynthetic efficiency. At the gene expression level, processes related to cellular functions aiming to maintain the structural cell integrity and plant defence responses are modified, which may take precedence over the preservation of photosynthetic efficiency. Overall, B toxicity in C. macrophylla induced a shift from stomatal to dominant non-stomatal limitations in photosynthesis, involving Rubisco impairment and PSII photochemistry. These findings provide new insights into the mechanisms underlying B-induced photosynthetic decline and highlight the vulnerability of citrus to B toxicity.