Abstract
Photosynthesis plays an essential role in plant growth and crop yield, and the mechanisms of the effects of cadmium (Cd) on photosynthetic performance require more attention. The acute toxicity of Cd in soil to the photosynthetic capacity of Hybrid Pennisetum was evaluated using gas exchange parameters, A/Ci curves, light response curves, and chlorophyll a fluorescence transients after exposure to elevated Cd concentrations (0, 10, 20, 50, 70, and 100 mg kg−1) for a 3-month period. The results indicated that leaf Cd concentration in Hybrid Pennisetum increased with the strength of soil Cd stress and ranged from 4.9 to 15.8 μg g−1 DW. The accumulation of leaf Cd severely restricted photosynthesis and its non-stomatal limitation in regulating the photosynthetic performance of Hybrid Pennisetum. The leaf chloroplasts at 10 and 20 mg kg−1 Cd concentrations showed no noticeable change, but the chlorophyll content significantly decreased by 9.0–20.4% at 50–100 mg kg−1 Cd concentrations. The Cd treatments also decreased plant ribulose-1,5-bisphosphate (RuBP) activity (Vcmax) and regeneration capacity (Jmax), triose phosphate utilization (TPU), light-saturated photosynthesis (Amax), apparent quantum yield (AQY), light saturation point (LSP), and dark respiration (Rday), but Cd treatment increased the light compensation point (LCP). The shape of chlorophyll a fluorescence transients in leaves was altered under different Cd treatments. The increased OJ phase and the decreased IP phase in fluorescence induction curves suggested that Cd toxicity inhibited both light use efficiency and photodamage avoidance ability. These results suggested that the decrease in photosynthesis through exposure to Cd may be a result of the decrease in leaf chlorophyll content, Rubisco activity, and RuBP regeneration, inhibition of triose phosphate utilization, reduction of the ability to use light and provide energy, and restrictions on electron transport in PSII.
Highlights
Among all the physiological and biochemical processes in plants, photosynthesis, as a central carbon anabolic pathway providing energy-rich organic compounds, plays an essential role in maintaining the equilibrium between the light energy absorbed by photosystems and the chemical energy consumed by metabolic sinks (Brestic et al, 2015)
With the increase in Cd added into the soil, the Cd concentration in leaves showed a sharp increase
The results indicated that Hybrid Pennisetum has a high ability to uptake Cd metal and quickly transfer Cd2+ from roots to leaves
Summary
With the increasing anthropogenic activities of phosphate fertilizer abuse and sewage sludge, herbicide and pesticide application, excessive industrial and aquaculture wastewater for being used for irrigation, as well as a high frequency of mining and smelting, contaminated soil polluted by heavy metals is attracting increasing attention worldwide (Mcintyre, 2003; Rizwan et al, 2017).Cadmium Toxicity Harms Plant PhotosynthesisCadmium (Cd), as one of the most harmful heavy metals in soils, has been widely accepted to be an extremely dangerous pollutant due to its acute toxicity, high water solubility, non-degradability, and persistence inside most live organisms (Ghosh and Singh, 2005; Groppa et al, 2012; Parmar et al, 2013). Excessive accumulation of Cd2+ in plant tissues could cause serious phytotoxicity (Groppa et al, 2007) and numerous morphological, physiological, and biochemical toxic effects on plant growth and development (Lutts and Lefèvre, 2015; Wali et al, 2015), such as destroying leaf chlorophyll structure (Santos et al, 2018), depressing photosynthesis and respiration (Hendrik et al, 2010; Lee et al, 2010; Zhang et al, 2015), disturbing the uptake and translocation of mineral nutrients (Erdal and Turk, 2016), accumulating reactive oxygen species (ROS) (Gallego et al, 2012), restraining protein synthesis and enzymatic activity (Fornazier, 2002; Wu et al, 2015), dwarfing plants (Rascio et al, 2008), inhibiting the growth of roots (Vartika et al, 2005), decreasing biomass (Rascio et al, 2008), and even leading to plant death if Cd exceeds their resistance values (Clemens et al, 2013; Shanying et al, 2017).
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