Stomatal conductance is the main limitation to photosynthesis in sugar beet plants treated with Zn excess
The effects of high Zn concentrations in growth and photosynthetic parameters of sugar beet (Beta vulgaris L.) plants grown in hydroponics were investigated. Zinc toxicity (100 and 300 µM) resulted in large reductions in biomass accumulation (>50%) and photosynthetic rates (40-50%). It was known that high Zn concentrations usually lead to decreases in net photosynthesis, but the effects of excess Zn on each of the possible factors limiting photosynthesis, including photochemistry, stomatal conductance and mesophyll conductance, had not been studied in detail until now. Leaf photochemistry was not affected by Zn, and reduced photosynthesis was due mostly to marked decreases in stomatal conductance, whereas mesophyll conductance decreased to a lower extent. Stomata were study using scanning electron microscopy (SEM) and large structural differences were observed in stomatal frequency, size and shape between 300 µM Zn and control plants. In high Zn plants stomatal density was lower than in the controls, and stomata were smaller and had a more rounded shape than those present in control plants. The presence of these morphological changes could be associated to the reductions in photosynthetic rates under excess Zn.