Integrating Chlorophyll a Fluorescence and Enzymatic Profiling to Reveal the Wheat Responses to Nano-ZnO Stress

Abstract

It has been shown that increased concentrations of zinc oxide nanoparticles (nano-ZnO) in the soil are harmful to plant growth. However, the sensitivity of different wheat cultivars to nano-ZnO stress is still unclear. To detect the physiological response process of wheat varieties with different tolerance to nano-ZnO stress, four wheat cultivars (viz., cv. TS1, ZM18, JM22, and LM6) with different responses to nano-ZnO stress were selected, depending on previous nano-ZnO stress trials with 120 wheat cultivars in China. The results found that nano-ZnO exposure reduced chlorophyll concentrations and photosynthetic electron transport efficiency, along with the depressed carbohydrate metabolism enzyme activities, and limited plant growth. Meanwhile, the genotypic variation in photosynthetic carbon assimilation under nano-ZnO stress was found in wheat plants. Wheat cv. JM22 and LM6 possessed relatively lower Zn concentrations and higher leaf nitrogen per area, less reductions in their net photosynthetic rate, a maximum quantum yield of the PS II (F_v/F_m), electron transport flux per cross-section (ETo/CSm), trapped energy flux per cross-section (TRo/CSm), and total soluble sugar and sucrose concentrations under nano-ZnO stress, showing a better tolerance to nano-ZnO stress than wheat cv. TS1 and ZM18. In addition, the chlorophyll a fluorescence parameters F_v/F_m, ETo/CSm, and TRo/CSm could be used to rapidly screen wheat varieties resistant to nano-ZnO stress. The results here provide a new approach for solving the issues of crop yield decline in regions polluted by heavy metal nanoparticles and promoting the sustainable utilization of farmland with heavy metal pollution.

Keywords: carbon assimilation; nano-ZnO; net photosynthetic rate; total soluble sugar; wheat.

Figure 1

Net photosynthetic rate (An, (a)) and relative chlorophyll content (SPAD, (b)) in different wheat cultivars as affected by zinc oxide nanoparticles (nano-ZnO). Vertical bars indicate mean ± SE (n = 3). Non-nano-ZnO stress, Control; nano-ZnO stress, Nano-ZnO; wheat cultivars, TS1, ZM18, JM22, and LM6; **, p < 0.01; and ***, p < 0.001; ns, no significant difference.

Figure 2

Changes in the maximum quantum yield of the PS II (Fv/Fm, (a)), electron transport flux per cross-section (ETo/CSm, (b)), trapped energy flux per cross-section (TRo/CSm, (c)), and performance index on absorption basis (PIabs, (d)) in different wheat cultivars as affected by zinc oxide nanoparticles (nano-ZnO). Vertical bars indicate mean ± SE (n = 3). Non-nano-ZnO stress, Control; nano-ZnO stress, Nano-ZnO; wheat cultivars, TS1, ZM18, JM22, and LM6; *, p < 0.05; **, p < 0.01; ***, and p < 0.001; ns, no significant difference.

Figure 3

Activities of initial Rubisco (a) and total Rubisco (b), Rubisco activation (c), activities of Ca²⁺-ATPase (d) and Mg²⁺-ATPase (e), and ATP concentration (f) in different wheat cultivars as affected by zinc oxide nanoparticles (nano-ZnO). Vertical bars indicate mean ± SE (n = 3). Non-nano-ZnO stress, Control; nano-ZnO stress, Nano-ZnO; and wheat cultivars, TS1, ZM18, JM22, and LM6; *, p < 0.05; **, p < 0.01; and ***, p < 0.001; ns, no significant difference.

Figure 4

Heat map of key carbohydrate metabolism enzyme activities in different wheat cultivars as affected by zinc oxide nanoparticles (nano-ZnO). The difference in activity for a given enzyme among the different treatments was normalized and converted to a color scale. Vertical bars indicate mean ± SE (n = 3). Non-nano-ZnO stress, Control; nano-ZnO stress, Nano-ZnO; wheat cultivars, TS1, ZM18, JM22, and LM6; cytoplasmic invertase, cytInv; hexokinase, HXK; fructokinase, FK; phosphoglucomutase, PGM; UDP-glucose pyrophosphyorylase, UGPase; ADP-glucose pyrophosphorylase, AGPase; glucose-6-phosphate dehydrogenase, G6PDH; phosphoglucoisomerase, PGI; phosphofructokinase, PFK; sucrose synthase, Susy; cell wall invertase, cwInv; aldolase, Ald; and vacuolar invertase, vacInv.

Figure 5

The concentrations of leaf nitrogen per area (a), leaf total soluble sugar (b), shoot dry matter (Shoot DM, (c)), leaf Zn (d), leaf sucrose (e), and root dry matter (Root DM, (f)) in different wheat cultivars as affected by zinc oxide nanoparticles (nano-ZnO). Vertical bars indicate mean ± SE (n = 3). Non-nano-ZnO stress, Control; nano-ZnO stress, Nano-ZnO; and wheat cultivars, TS1, ZM18, JM22, LM6; *, p < 0.05; **, p < 0.01; and ***, p < 0.001; ns, no significant difference.

Figure 6

A comprehensive description of the response of wheat plants (e.g., ZM18) to nano-ZnO stress. The green up and red down arrows indicate positive and negative effects of nano-ZnO on the physiological processes of wheat plants. Hexokinase, HXK; phosphoglucoisomerase, PGI; glucose-6-phosphate dehydrogenase, G6PDH; fructokinasem, FK; phosphofructokinase, PFK; phosphoglucomutase, PGM; UDP-glucose, pyrophosphyorylase, UGPase; aldolase, Ald; ADP-glucose pyrophosphorylase, AGPase; net photosynthetic rate, An; stomatal conductance, g_s; maximum quantum efficiency of photosystem II, F_v/F_m; photosystem I, PSI; photosystem II, PS II; ribulose-1,5-bisphosphate, RuBP; adenosine diphosphate, ADP; adenosine triphosphate, ATP; and nicotinamide adenine dinucleotide phosphate, NADPH.