Detection of Cadmium Risk to the Photosynthetic Performance of Hybrid Pennisetum

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/C_i 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 (V_cmax ) and regeneration capacity (J_max ), triose phosphate utilization (TPU), light-saturated photosynthesis (A_max ), apparent quantum yield (AQY), light saturation point (LSP), and dark respiration (R_day ), 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.

Keywords: Hybrid Pennisetum; cadmium stress; electron transport; non-stomatal limitation; photosynthetic performance.

FIGURE 1

Effect of different concentrations of Cd concentration in leaves of Hybrid Pennisetum. Vertical bars represent ± SD of the mean (n = 3); different letters on the SD bars indicate significant differences among the Cd treatments (p < 0.05).

FIGURE 2

Boxplots of SPAD under different Cd stress. The solid circle in the box plots represents the mean values, n = 15.

FIGURE 3

Net photosynthetic rate (A), Stomatal conductance (B), Intercellular CO₂ concentration (C), Transpiration rate (D), and Water use efficiency (E) of Hybrid Pennisetum leaves treated with different doses of Cd concentration in soil. Vertical bars represent ± SD of the mean (n = 3); different letters on the SD bars indicate significant differences among the Cd treatments (p < 0.05).

FIGURE 4

Effects of Cd treatment on the maximum velocity of Rubisco carboxylation (A), the maximum velocity of RuBP regeneration (B), and the triose phosphate utilization (C) in leaves of Hybrid Pennisetum. Vertical bars represent ± SD of the mean (n = 3); different letters on the SD bars indicate significant differences among the Cd treatments (p < 0.05).

FIGURE 5

Photosynthetic light response curves in leaves of Hybrid Pennisetum under different soil Cd stress conditions.

FIGURE 6

The fluorescence intensity of the original fluorescence kinetic curve of Hybrid Pennisetum leaves under different Cd treatments.

FIGURE 7

Energy pipeline leaf model of phenomenological fluxes (per cross-section, CS) in last fully expanded leaves as affected by Cd stress. Data are means ± standard errors (n = 3). Each relative value is drawn by the width of the corresponding arrow, standing for a parameter. Empty and full black circles indicate, respectively, the percentage of active (QA reducing) and non-active (non-QA reducing) reaction centers of PS II. TRO/CSm – trapped energy flux per CS; ETO/CSm – electron transport flux per CS; ABS/CSm – absorption flux per CS; DIO/CSm – non-photochemical quenching per CS.

FIGURE 8

Schematic representation of the non-stomatal limitation in regulating the photosynthetic performance of Hybrid Pennisetum under Cd stress conditions. The figure shows that the decrease in leaf chlorophyll content, reduction of the ability to use light, and restrictions on carbon assimilation caused by high Cd content in leaves limit the capacity of Hybrid Pennisetum in carbon fixation.

FIGURE 9

The relationship between J_max and V_cmax in leaves of Hybrid Pennisetum.

FIGURE 10

The relationship between leaf Cd concentration and respiration rate in leaves of Hybrid Pennisetum.