Physiomorphic and molecular-based evaluation of wheat germplasm under drought and heat stress

Abstract

Drought and heat stress are potential problems that can reduce wheat yield, particularly during the terminal growth stages in arid and semiarid regions of the world. The current study intended to examine the impact of individual and combined drought and heat stress on the biochemical contents (antioxidant enzymes, proline, soluble proteins, and soluble sugars), physiological parameters (chlorophyll content, cell membrane stability, photosynthesis, stomatal conductance, and transpiration), plant-water relations (relative water content, water potential, osmotic potential, and pressure potential), agronomic traits (flag leaf area, plant height, number of tillers per plant, spike length, grains per spike, and thousand-grain weight), and gene expression (TaHSF1a, TaWRKY-33, TaNAC2L, and TaGASR1) in four different thermostable and drought-tolerant wheat genotypes (i.e., Gold-16, HS-240, Suntop, and Hemai-13) collected from different countries. The tri-replicate experiment was conducted using two factorial arrangements in a randomized complete block design (RCBD). All measured traits, except total soluble sugars, proline, and cell membrane stability index, showed significant reduction under both combined and individual treatments. Furthermore, correlation analysis revealed a significant association between biochemical and physiological characteristics and crop agronomic productivity. Furthermore, principal component analysis (PCA) and heatmap analysis demonstrated significant levels of variation in traits according to the type of stress and nature of wheat genotype. The spectrographs and micrographs generated by scanning electron microscopy for the selected high- and low- tolerance samples revealed clear differences in mineral distribution and starch granulation. All studied genes showed comparatively high levels of relative expression under combined treatments of drought and heat stress in all wheat genotypes, but this expression was the highest in 'Gold-16' followed by 'HS-240', 'Suntop', and 'Hemai-13'. Overall, this study concluded that plants are proactive entities and they respond to stresses at all levels; however, the tolerant plants tend to retain the integrity of their biochemical, physiological, and molecular equilibrium.

Keywords: biochemical contents; gene expression; growth and yield traits; physiological parameters; plant–water relations.

Figure 1

Variation in biochemical contents of wheat cultivars due to individual and combined treatments of drought and heat stress. Dry weight (DW); fresh weight (FW); superoxide dismutase (SOD); catalase (CAT); peroxidase (POD); total soluble proteins (TSPs); total soluble sugars (TSSs). Values in the graph are means averaged after every 4 days of treatment application during the two-factorial tri-replicate experiment at a p-value ≤ 0.05. *Units: POD, CAT, and CAT activities (enzyme units); proline (μgg^–1FW); TSP (mgg^–1DW); TSS (mgg^–1DW).

Figure 2

Variation in physiological attributes of wheat cultivars due to individual and combined treatments of drought and heat stress. Photosynthesis rate (Pn); transpiration (Tr); cell membrane stability (CMS); stomatal conductance (Gs). Values in graph are means averaged after every 4 days of treatment application during the two-factorial tri-replicate experiment at a p-value ≤ 0.05. *Units: Total chlorophyl l content (g kg^–1); Pn (μm^–2S^–1); Tr (mmm^–2S^–1); Gs (mmm^–2S^–1).

Figure 3

Variation in plant–water relations of wheat genotypes due to individual and combined treatments of drought and heat stress. Leaf water potential (ψ_w); osmotic potential (ψ_s); pressure potential (ψ_p); relative water content (RWC). Values in the graph are means averaged after every 4 days of treatment application during the two-factorial tri-replicate experiment at a p-value ≤ 0.05. *Units: ψ_w (–MPa); ψ_s (MPa); ψ_p (MPa).

Figure 4

Variation in growth and yield traits of wheat cultivars due to individual and combined treatments of drought and heat stresses. Values in the graph are means averaged after harvesting during the two-factorial tri-replicate experiment at a p-value ≤ 0.05.

Figure 5

Correlation matrix showing the extent of association between the biochemical contents, physiological attributes, plant–water relations, and agronomic traits in wheat cultivars and individual and combined treatments of heat and drought stress. Superoxide dismutase (SOD); catalase (CAT); peroxidase (POD); total soluble proteins (TSPs); total soluble sugars (TSSs), photosynthesis rate (Pn); transpiration rate (Tr); cell membrane stability (CMS), stomatal conductance (Gs); water potential (Wp); osmotic potential (Sp); pressure potential (Pp); relative water content (RWC); flag leaf area (FLA); plant height (PH); number of tillers per plant (NTP); spike length (SL); number of grains per spike (GPS); and thousand-grain weight (TGW). ***= Significant at P ≤ 0.001; **= Significant at P ≤ 0.01; *= Significant at P ≤ 0.05.

Figure 6

Principal component analysis (PCA) vectors demonstrating the effect on the proximity association between biochemical contents, physiological attributes, plant–water relations, and agronomic traits, and wheat cultivars (right) and individual and combined treatments of heat and drought stress (left). Superoxide dismutase (SOD); catalase (CAT); peroxidase (POD); total soluble proteins (TSPs); total soluble sugars (TSSs), photosynthesis rate (Pn); transpiration rate(Tr); cell membrane stability (CMS); stomatal conductance (Gs); water potential (Wp); osmotic potential (Sp); pressure potential (Pp); relative water content (RWC); flag leaf area (FLA); plant height (PH); number of tillers per plant (NTP); spike length (SL); number of grains per spike (GPS); and thousand-grain weight (TGW).

Figure 7

Heatmap cluster analysis depicting varying expression and association patterns of biochemical contents, physiological attributes, plant–water relations, and agronomic traits of wheat cultivars (right) and individual and combined treatments of heat and drought stress (left). Superoxide dismutase (SOD); catalase (CAT); peroxidase (POD); total soluble proteins (TSPs); total soluble sugars (TSSs); photosynthesis rate (Pn); transpiration rate (Tr); cell membrane stability (CMS); stomatal conductance (Gs); water potential (Wp); osmotic potential (Sp); pressure potential (Pp); relative water content (RWC); flag leaf area (FLA); plant height (PH); number of tillers per plant (NTP); spike length (SL); number of grains per spike (GPS); and thousand-grain weight (TGW).

Figure 8

Scanning electron microscope (SEM) spectrographs and micrographs showing differential elemental distribution and starch granulation, respectively, in representative samples of wheat genotypes Gold-16 and Hemai-13, respectively, under (A) the control treatment and (B) combined drought and heat stress.

Figure 9

Varying expression patterns of drought- (TaWRKY-33 and TaGASR1) and heat- (TaHSF1a and TaNAC2L) associated genes under the control treatment and individual and combined applications of drought and heat stress in four different wheat genotypes (A–D).