Abstract
We provide new data on the mechanism of Noccaea caerulescens acclimation to Cd exposure by elucidating the process of photosystem II (PSII) acclimation by chlorophyll fluorescence imaging analysis. Seeds from the metallophyte N. caerulescens were grown in hydroponic culture for 12 weeks before exposure to 40 and 120 μM Cd for 3 and 4 days. At the beginning of exposure to 40 μM Cd, we observed a spatial leaf heterogeneity of decreased PSII photochemistry, that later recovered completely. This acclimation was achieved possibly through the reduced plastoquinone (PQ) pool signaling. Exposure to 120 μM Cd under the growth light did not affect PSII photochemistry, while under high light due to a photoprotective mechanism (regulated heat dissipation for protection) that down-regulated PSII quantum yield, the quantum yield of non-regulated energy loss in PSII (ΦNO) decreased even more than control values. Thus, N. caerulescens plants exposed to 120 μM Cd for 4 days exhibited lower reactive oxygen species (ROS) production as singlet oxygen (1O2). The response of N. caerulescens to Cd exposure fits the ‘Threshold for Tolerance Model’, with a lag time of 4 d and a threshold concentration of 40 μM Cd required for the induction of the acclimation mechanism.
Highlights
Cadmium is a non-essential heavy metal that can occur in the environment in high concentrations as a consequence of numerous human activities, becoming toxic to all organisms [1,2,3,4,5]
The response of N. caerulescens to Cd exposure fits the ‘Threshold for Tolerance Model’, with a lag time of 4 d and a threshold concentration of 40 μM Cd required for the induction of the acclimation mechanism
Two light intensities were selected for chlorophyll fluorescence measurements, a low light intensity that was similar to the growth light (300 μmol photons m−2 s−1, GL) and a high light intensity (1000 μmol photons m−2 s−1, High light (HL), more than three times that of the growth light)
Summary
Cadmium is a non-essential heavy metal that can occur in the environment in high concentrations as a consequence of numerous human activities, becoming toxic to all organisms [1,2,3,4,5]. Hyperaccumulators are plant species that vigorously take up heavy metals, translocate them into the above-ground parts and isolate them into a risk-free state [4,11]. These plants can accumulate several percent of heavy metals in their dry mass [4]. Hyperaccumulators have to stock the absorbed heavy metal in a manner that is not detrimental to vital enzymes and especially photosynthesis [12,13]. Hyperaccumulators can be used for phytoremediation and for Materials 2018, 11, 2580; doi:10.3390/ma11122580 www.mdpi.com/journal/materials
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