Relationships between photosynthetic activity and silica accumulation with ages of leaf in Sasa veitchii (Poaceae, Bambusoideae)

• Background and Aims: Bamboos have long-lived, evergreen leaves that continue to accumulate silica throughout their life. Silica accumulation has been suggested to suppress their photosynthetic activity. However, nitrogen content per unit leaf area (N_area), an important determinant of maximum photosynthetic capacity per unit leaf area (P_max), decreases as leaves age and senescence. In many species, P_max decreases in parallel with the leaf nitrogen content. It is hypothesized that if silica accumulation affects photosynthesis, then P_max would decrease faster than N_area, leading to a decrease in photosynthetic rate per unit leaf nitrogen (photosynthetic nitrogen use efficiency, PNUE) with increasing silica content in leaves. • Methods: The hypothesis was tested in leaves of Sasa veitchii, which have a life span of 2 years and accumulate silica up to 41 % of dry mass. Seasonal changes in P_max, stomatal conductance, N_area and silica content were measured for leaves of different ages. • Key Results: Although P_max and PNUE were negatively related with silica content across leaves of different ages, the relationship between PNUE and silica differed depending on leaf age. In second-year leaves, PNUE was almost constant although there was a large increase in silica content, suggesting that leaf nitrogen was a primary factor determining the variation in P_max and that silica accumulation did not affect photosynthesis. PNUE was strongly and negatively correlated with silica content in third-year leaves, suggesting that silica accumulation affected photosynthesis of older leaves. • Conclusions: Silica accumulation in long-lived leaves of bamboo did not affect photosynthesis when the silica concentration of a leaf was less than 25 % of dry mass. Silica may be actively transported to epidermal cells rather than chlorenchyma cells, avoiding inhibition of CO₂ diffusion from the intercellular space to chloroplasts. However, in older leaves with a larger silica content, silica was also deposited in chlorenchyma cells, which may relate to the decrease in PNUE.