Silicon alleviates mesophyll limitations of photosynthesis on rice leaves infected by Monographella albescens

Leaf scald, caused by Monographella albescens, is a key disease affecting rice production worldwide. Alternative methods for leaf scald management are demanded by the growers and silicon (Si) application emerges as a promising way to decrease severity not only of leaf scald but also of other relevant rice diseases. Some evidence suggests that Si may preserve the photosynthetic performance of plants upon pathogen infection but the mechanistic basis for this remain unresolved. In the present study, mesophyll conductance (g(m)) was calculated to suitably parameterize the responses of net carbon assimilation rate (A) to chloroplastidic CO2 concentration (C-c) and to resolve the relative contributions of stomatal, mesophyll, and biochemical drawbacks to photosynthesis in rice plants challenged with M. albescens and how all of these facts may be influenced by Si application. Rice plants (cultivar "Primavera") were hydroponically grown with 0 or 2 mM Si (- Si and + Si plants) and inoculated with M. albescens. Leaf scald-induced decreases in A were associated with roughly proportional decreases in CO2 diffusion (lower stomatal conductance and g(m)) and impaired photochemistry (e.g. reduced maximum electron transport rate) and biochemistry (e.g. RuBisCO activity) regardless of Si supply. The magnitude of these decreases were, overall, greater in - Si plants than in their + Si counterparts, and therefore a mitigating effect of Si on the preservation of the photosynthetic activity on diseased plants is evident. On infected leaves, g(m) was not scaled with maximum carboxylation rate (V-cmax) - C-c and, as a consequence, C-c increased accordingly, but only in + Si plants. In conclusion, the supply of Si to rice plants played a central role in decreasing leaf scald symptoms and, as such, preserving to a certain extent their photosynthetic performance. This preservation was not linked to differential impairments on the stomatal function or biochemical steps of photosynthesis but rather with increased CO2 diffusion throughout the mesophyll. This ultimately led to a lower photorespiration-to-gross photosynthesis ratio and less mesophyll limitations of photosynthesis in + Si plants than in their - Si counterparts during the infection process of M. albescens on rice leaves.