Root Traits Differentiates Osmotic Stress Tolerant and Susceptible Wheat Genotypes under PEG-Treatment ^†

Abstract

Wheat is an important cereal crop that often suffers from osmotic stress under various growing conditions. The objective of this study was to investigate the effects of PEG-induced osmotic stress at the phytomer level on root growth and root hair morphology of 22 hydroponically grown wheat varieties. Two treatments, 0% and 10% PEG, were imposed on 20-day old wheat seedlings for a duration of 15 days. PEG stress significantly reduced plant height, number of live leaves per tiller, chlorophyll content, shoot dry weights, number of root bearing phytomers and roots per tiller. By contrast, PEG stress significantly increased leaf injury scores, root dry weight, main axis length and diameter of developed roots, length and diameter and density of both first and second order lateral roots, and the density and length of root hairs. An increase in root dry weight in PEG stress tolerant wheat genotypes was achieved through an increase in the length and diameter of main axis and lateral roots.

1. Introduction

Wheat is one of the major cereal crops grown in the winter season in Bangladesh [1]. Wheat crops in Bangladesh face a moisture deficit in water sensitive stages such as booting and flowering [2]. Deficit irrigation may increase wheat production by 11% to 136% [2]. However, a number of traits related to the yield of wheat shows an adaptive mechanism under drought stress [3]. In a recent study, wheat cultivars showed variability in tolerance level based on leaf morphological traits [4]. In earlier studies, different sets of wheat genotypes showed genetic diversity based on days to maturity and yield contributing traits in Bangladesh [5,6]. None of those studies involved measuring root traits, although tolerant crop plants exhibited root adaptive traits under various abiotic stress conditions [7,8]. The present study was therefore planned to investigate detailed root traits of wheat genotypes under osmotic stress.

2. Experiments

A total of 22 wheat genotypes, including landraces, obsolete varieties, and high-yielding cultivars, were selected to impose osmotic stress. Plants were hydroponically cultured following Robin et al. [9,10] for 20 days before imposing 10% PEG-induced osmotic stress along with control (Figure 1a). Plants were exposed to 15 days under a stressed condition to allow the plants to reach the sufficient stress necessary to record the morphological difference in root growth (Figure 1b). The chlorophyll content of leaves under control and osmotic stress was recorded (Figure 1c) and leaf injury scores were scored (Figure 1d). A number of root traits, including length, diameter and density of root axes, lateral roots and root hairs were measured under a light microscope (Figure 1e,f, [9,10]). In addition, a number of live leaves and roots per tiller were recorded at the destructive harvest.

Figure 1. Hydroponic culture of wheat genotypes and relevant measurements. (a) two weeks old wheat plants ready for treatment imposition; (b) Root growth of what plants under 10% PEG-induced osmotic stress; (c) Measurements of chlorophyll content using SPAD meter; (d) scoring of damaged leaves under osmotic stress; (e) diameter of a root axis and (f) length of a root hair.

3. Results

3.1. Effect of PEG Stress

PEG-induced osmotic stress significantly reduced plant height, number of live leaves per tiller, and chlorophyll content in the leaf tissues (Table 1). Leaf injury scores increased at the 5th live leaves (Table 1). Osmotic stress also reduced shoot dry weight per tiller but strikingly root dry weight per tiller increased (Table 1). Despite an increase in root dry weight per tiller, the number of root bearing phytomers and the total number of root per tiller decreased (Table 1). Osmotic stress reduced the number of seminal roots per tiller, but the length of individual seminal roots increased. With the increase in root dry weight per tiller the length, diameter and density of lateral roots and root hairs also increased (Table 1).

Table 1. Difference between control and 10% PEG-induced osmotic stress among wheat genotypes. Pr, phytomer; MA, main axis; FLR, first order lateral roots; SLR, second order lateral roots; RH, root hairs; d, diameter; Dn, density; L, length.

Traits	Control	10% PEG	p Value
Plant height (PH)	42.5 ± 0.71	36.7 ± 0.79	<0.001
Live leaves per tiler (LL)	4.76 ± 0.10	3.88 ± 0.08	<0.001
Chlorophyll content	29.8 ± 0.46	27.7 ± 0.49	<0.001
Leaf injury scores at 5th leaves	5.06 ± 0.19	6.94 ± 0.19	<0.001
Shoot dry weight (SDW)	0.46 ± 0.02	0.32 ± 0.01	<0.001
Root dry weight (RDW)	0.05 ± 0.00	0.07 ± 0.00	<0.001
Total no. of phytomer (TPr)	7.18 ± 0.16	5.70 ± 0.16	<0.001
Total no. of roots per tillers (TRt)	9.34 ± 0.36	7.32 ± 0.34	<0.001
No. of seminal roots (SR)	5.59 ± 0.16	4.48 ± 0.17	0.008
Length of seminal roots (LSR)	2.55 ± 0.13	3.23 ± 0.17	<0.001
Main axis length at Pr 4 (MALPr4)	47.6 ± 1.17	53.2 ± 1.3	<0.001
Diameter of main axis (dMA)	0.36 ± 0.01	0.50 ± 0.01	<0.001
Length of FLR (LFLR, cm)	1.48 ± 0.08	3.00 ± 0.14	<0.001
Diameter of FLR (dFLR, mm)	0.19 ± 0.00	0.27 ± 0.01	<0.001
Density of FLR (DnFLR)	4.95 ± 0.07	6.60 ± 0.08	<0.001
Length of SLR (LSLR, cm)	0.39 ± 0.09	0.57 ± 0.03	<0.001
Diameter of SLR (dSLR, mm)	3.35 ± 0.09	5.13 ± 0.09	<0.001
Density of SLR (DnSLR)	6.58 ± 0.22	8.18 ± 0.23	<0.001
Density of root hairs of MA (DnRHMA)	6.70 ± 0.22	8.41 ± 0.20	<0.001
Density of root hairs of SLR (DnRHSLR)	7.06 ± 0.20	9.15 ± 0.20	<0.001
Length of root hairs of FLR (LRHFLR)	420.5 ± 16.5	503.6 ± 12.71	<0.001

3.2. Correlation among Selected Root Traits

In spite of the increasing trend of all root traits upon PEG-induced osmotic stress, only the main axis length showed a significant association with root dry weight (Table 2). The number of root bearing phytomers, number of roots per tiller and the number of seminal roots per tiller showed a strong positive association among each other (Table 2). The total number of roots per tiller showed a negative association with the density of primary lateral roots per unit main axis length (Table 2).

Table 2. Correlation among root dry weight (RDW), total number of root bearing phytomers (TPr) and total number of roots (TR) per tiller and with other root morphological traits of wheat genotypes under PEG-induced osmotic stress.

Traits	RDW	TPr	TR
Total no. of phytomer (TPr)	0.379 NS
Total no. of roots per tillers (TRt)	0.492 NS	0.92 ***
No. of seminal roots (NSR)	0.405 NS	0.754 **	0.867 ***
Main axis length at Pr 4 (MALPr4)	0.664 *	−0.101 NS	0.091 NS
Diameter of main axis (dMA)	0.34 NS	−0.383 NS	−0.358 NS
Length of FLR (LFLR, cm)	0.481 NS	−0.381 NS	−0.149 NS
Diameter of FLR (dFLR, mm)	0.073 NS	−0.612 *	−0.528 NS
Density of FLR (DnFLR)	−0.145 NS	−0.759 **	−0.634 *
Length of SLR (LSLR, cm)	0.162 NS	−0.067 NS	−0.068 NS
Diameter of SLR (dSLR, mm)	0.257 NS	0.309 NS	0.266 NS
Density of SLR (DnSLR)	0.448 NS	−0.337 NS	−0.262 NS
Density of root hairs of MA (DnRHMA)	0.392 NS	−0.032 NS	−0.113 NS
Density of root hairs of SLR (DnRHSLR)	0.205 NS	−0.186 NS	−0.125 NS
Length of root hairs of FLR (LRHFLR)	0.334 NS	−0.374 NS	−0.447 NS

*, ** and *** indicate significance at 5%, 1% and 0.1% level of significance, respectively.

Thus, the genotypes produced higher root dry weight and the main axis length were more tolerant to osmotic stress compared to contrasting genotypes.

4. Discussion

The production of a large root system associated with the elongation of main axis and lateral branches under drought stress conditions is believed to be a characteristic feature of drought tolerant plants [8]. Similar to this study, one previous study reported that all fescue plants produce extensive root hairs under drought stress, but that the root dry weight per plant was decreased under stress condition [11]. In corn, a kind of genotypic diversity was observed where increasing root dry weight in the tolerant genotypes was associated with yield [12] and these results are consistent with our observations. However, unlike this study, none of the previous studies have observed root traits at the phytomer level in detail.

5. Conclusions

This study explored the effects of PEG-induced osmotic stress on root development at the phytomer level. A strong positive association between root dry weight and main axis length was observed. The results indicated that tolerant wheat genotypes increase the length and density of main axis and lateral branches as an adaptive mechanism to cope with osmotic stress.