Physiological Effect of Thallium in the Facultative Hyperaccumulator <i>Silene latifolia</i>

The metallicolous populations of the facultative Tl hyperaccumulator Silene latifolia are extraordinarily tolerant and capable of accumulating up to 80,000 μg Tl g^-1 in nature. A growth stimulatory effect of Tl was observed, and this study set out to determine possible mechanisms. Plants from non-metallicolous and metallicolous populations were subjected to hydroponics dosing experiments at 2.5 and 10 μM Tl. Metal impact on stomatal and non-stomatal photosynthetic constraints, light energy conversion processes and plant anatomy/ultrastructure was assessed over time. Photosynthetic rates improved in 10 μM Tl-treated metallicolous plants by 20% compared to controls, partly due to increased stomatal conductance. The latter was mainly driven by Tl-induced anatomical changes, such as increased central cylinder area and stomatal density, likely to enhance water uptake/translocation and, consequently, leaf metal accumulation. The apparently Tl-favoured CO₂ trafficking resulted in ameliorated maximal photosynthetic capacity. The first signs of photosynthetic declines appeared only at very high Tl leaf concentrations (15,000 μg Tl g^-1), with limitations involving stomatal and biochemical factors; whereas the photochemical reactions remained functional. The observed Tl-induced stimulatory response in growth and net photosynthetic rate in metallicolous plants shows that Tl improves physiological performance in Silene latifolia, mainly through improved stomatal conductance.