Annals of Botany

Photosynthesis saved by specific Si siting

It is well known that grasses, especially the bamboos, accumulate silica in their leaves. Indeed, the highest recorded Si concentrations are seen in the leaves of the bamboo, Sasa veitchii, as discussed by Motomura et al., at Sendai, Japan (pp. 463–468). Silicon is accumulated via uptake of silicic acid from the soil and transported in the transpiration stream to the leaves. A question then arises: do these extensive deposits of Si interfere with other physiological processes such as photosynthesis? To answer this question, the authors measured Si accumulation during the 3-year life time of individual leaves. Si is accumulated in all three seasons over a period from spring through to autumn, reaching up to 15 % of dry weight in year 1, up to 25 % in year 2 and up to an astonishing 41 % in year 3. Relationships between Si content and photosynthesis are complex. In year 1, photosynthetic capacity (Pmax) increases as Si content goes up, but in years 2 and 3 Pmax declines markedly when plotted against Si content. However, this does not necessarily mean that Si inhibits photosynthesis. Photosynthesis expressed as a function of N-content (PNUE) fell dramatically in year 1, whether expressed over time or in relation to Si; this was a result of the significant increase in N content. PNUE was approximately constant in year 2 and fell again in year 3, while N content fell. Nitrogen content thus also affects photosynthesis and the authors conclude that Si accumulation only inhibits photosynthesis in year 3, i.e. in the later stages of the leaf’s life. The changing effects of Si probably reflect its deposition pattern in the leaves: in years 1 and 2, Si seems to be directed to the epidermal cells; CO2 diffusion remaining unimpeded. However, when the Si concentration exceeds 25 % it is also deposited in the chlorenchyma; CO2 diffusion is now impeded and photosynthesis inhibited thereby.