Physiological Effect of Thallium in the Facultative Hyperaccumulator Silene latifolia

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.