Cadmium-induced inhibition of photosynthesis and long-term acclimation to cadmium stress in the hyperaccumulator Thlaspi caerulescens

Abstract

• Acclimation of hyperaccumulators to heavy metal-induced stress is crucial for phytoremediation and was investigated using the hyperaccumulator Thlaspi caerulescens and the non-accumulators T. fendleri and T. ochroleucum.• The main technique was the measurement of spatially and spectrally resolved kinetics of in vivo absorbance and fluorescence with a novel fluorescence kinetic microscope.• Key Results: While at the beginning of growth on cadmium all species suffered from toxicity, T. caerulescens completely recovered later. During stress, a few mesophyll cells in T. caerulescens became more inhibited and accumulated more Cd than the majority. This heterogeneity disappeared during acclimation. Chlorophyll fluorescence parameters related to photochemistry were more strongly affected by Cd-stress than non-photochemical parameters, and only photochemistry showed acclimation. In healthy plants, maximal PSII efficiency (Fv/Fm) was homogeneous from 650-790 nm. Cd-stress reduced Fv/Fm and ΦPSII stronger at 670-740nm compared to >740nm and <670nm. Cd enhanced nonphotochemical quenching mainly <690nm.• Main Conclusions: Cd-acclimation in the Cd-hyperaccumulator T. caerulescens shows that part of its Cd tolerance is inducible and involves transient physiological heterogeneity as an emergency defence mechanism. Differential effects of Cd-stress on photochemical vs. non-photochemical parameters indicate that Cd inhibits the photosynthetic light reactions more than the Calvin-Benson cycle. Differential spectral distribution of Cd-effects on photochemical vs. non-photochemical quenching shows that Cd inhibits at least two different targets in/around PSII. Spectrally homogeneous Fv/Fm suggests that in healthy T. caerulescens all chlorophylls fluorescing at room temperature are PSII-associated.