Transpiration-dependent passive silica accumulation in cucumber (Cucumis sativus) under varying soil silicon availability

Regulation of the uptake of silicon (Si) varies among plant species; some species may passively transport Si, through transpiration, from soils to shoots, while others actively transport silica and deposit it in leaf tissues at high concentrations. Cucumber ( Cucumis sativus L.) accumulates moderate amounts of silica in leaves, but the relative importance of passive and active processes pertaining to silica accumulation is poorly understood. In a factorial experiment with cucumber seedlings, we manipulated transpiration rates by changing humidity and air movements around pot-grown plants receiving a daily supply of solutions containing 0, 1, 1.5, and 2 mmol Si·L–1. Higher transpiration rates resulted in significantly greater Si per unit leaf mass after 4 days, suggesting that passive processes affect the rate of silica accumulation. Actual silica accumulation during the 4-day period was significantly higher than the expected accumulation attributable to passive transport alone in 1 and 1.5 mmol Si·L–1 treatments, while passive processes alone could account for the actual silica accumulation at 2 mmol Si·L–1. We conclude that the relative importance of active and passive processes in silica deposition in cucumber leaves depends on transpiration rates and the balance between soil Si availability and plants’ demands for Si.