Effect of saline irrigation on plant water traits, photosynthesis and ionic balance in durum wheat genotypes.

In the era of climate change, decreased precipitation and increased evapo-transpiration hampers the yield of several cereal crops along with the soil salinity and poor ground water resource. Wheat being the moderately tolerant crop face many challenges in the arid and semi-arid regions under irrigated agriculture. In view of this, the study was planned to explore the potential of durum wheat genotypes under salinity on the basis of physiological traits. Experiment was designed as RBD in three replications to evaluate 15 wheat genotypes with moderate saline irrigation (ECiw - 6 dS m-1) and extreme saline irrigation (ECiw - 10 dS m-1) along with one set of control (Best available water). Different physiological traits such as water potential (ψp), osmotic potential (ψs), relative water content (RWC), Na+ and K+ content were recorded in roots as well as shoots at the reproductive stage whereas photosynthetic rate and chlorophyll content were measured in the flag leaves. A significant variability (p < 0.001) was noted among the genotypes under different stress environments and it was observed that durum genotype HI 8728 and HI 8737 showed less reduction in plant water traits (RWC, ψp and ψs) than the salinity tolerant checks of bread wheat KRL 99 and KRL 3-4. HD 4728 and HI 8708 maintained higher photosynthetic rate as well as higher chlorophyll content under the extreme salinity level of ECiw - 10 dSm-1. No significant differences were found in root Na+ in genotypes KRL 99 (3.17g), KRL 3-4 (3.34g) and HI 8737 (3.41g) while in shoots, lowest accumulation was seen in KRL 99, MACS 3949 and KRL 3-4 at ECiw - 10 dSm-1. The mean range of K+ content was 7.60-9.74% in roots and 4.21-6.61% in shoots under control environment which decreased to 50.77% in roots and 46.05% in shoots under extreme salinity condition of ECiw - 10 dSm-1. At ECiw - 10 dSm-1, KRL 99 maintained highest K+/Na+ in both root and shoot followed by KRL 3-4, HI 8737, MACS 3949, HD 4728 in roots and MACS 3949, KRL 3-4, MACS 4020, HD 4758, MACS 3972 and HI 8713 in shoots. The differential response of durum wheat genotypes under salinity particularly for physiological traits, confer their adaptability towards stress environments and exhibit their potential as genetic sources in breeding programs for improving salt stress tolerance.

Introduction

Wheat is one of the most important food crops cultivated in India to meet the food demand of burgeoning population of the country. Apart from the regular seasonal environmental constraints, in many parts of the country, wheat crop is often subjected to periods of soil and atmospheric water deficits as well as high soil salinity. These limitations are likely to increase in future as climatic change is expected to decrease precipitation and increase evapo-transpiration (World Bank, 2007, Lobell et al., 2008). Among various factors, salinity of soil and ground water resource is a major problem for irrigated agriculture. In India, salt affected soils are predicted to increase from the current 6.73 M ha to 16.2 M ha by 2050 due to planned expansion in irrigation network, use of poor quality water in irrigation and climate change impacts (Sharma and Singh, 2015, Singh et al., 2018). Globally, wheat ranks second after maize in cereal crops (Datta et al., 2009) which is consumed approximately by 36% of the population. Salinity stress negatively affects the wheat productivity and yield starts declining when ECe value exceeds 6 dS m−1 in the soil solution (Chinnusamy et al., 2005, Shahzad et al., 2016). Increasing attention to the mechanisms of salinity stress response is being emphasized now due to threats of climate change and loss of arable land during urbanization and environmental degradation.

Salt injury involves both osmotic effects and specific ion effects. In most crop plants, the main toxic components of salinity are Na+ and Cl−. These toxic ions interfere with the normal physiological processes causing membrane damage, nutrient imbalance, altered levels of growth regulators, enzymatic inhibition and metabolic dysfunction, including photosynthesis which ultimately leads to plant death (Mahajan and Tuteja, 2005, Kumar et al., 2018a, Kumar et al., 2019). Thus the plant growth is impaired by osmotic stress in a first phase and ionic stress in second phase (accumulation of high concentrations of salts (toxic ions) within the plant which damages cell functions and structure) and finally suppress the yield (Kumar et al., 2018b, Mann et al., 2019a). These negative effects include interference of root function in absorbing water, as well as the prevention of physiological and biochemical processes such as uptake of nutrient like Ca++, Mg++ and K+ and their assimilation (Carillo et al., 2011). The roots are the first plant organs that control the uptake and translocation of nutrients and salts throughout the life cycle of plant (Lata et al., 2019). In spite of the direct exposure of roots to saline environment, their growth is less vulnerable to salt than that of the shoots (Munns, 2002). Wheat is one of the world’s major crops and has been subjected to intensive breeding and selection for about a century. The bulk of the selection effort to date has been directed to improving grain yield, end use quality, and disease resistance. Bread wheat (Triticum aestivum) is always considered as a more salt-tolerant species than durum wheat (Triticum turgidum) (Munns and James, 2003, Munns and Tester, 2008); this difference has been largely attributed to its superior ability to maintain lower Na+ accumulation in the leaf/shoot (Colmer et al., 2006, Cuin et al., 2010, James et al., 2011, Munns et al., 2012). With increasing soil salinization and shortage of water supply, a major shift is now underway to improve its level of abiotic tolerance. So, to study the differential physiological and biochemical responses of roots and shoots along with mechanism of ion transport among different genotypes could serve as a selection criteria or the possible strategy to identify tolerant genotype.

Material and methods

Plant material, growth conditions and experimental details

The experiment was designed as Randomised Block Design (RBD) in three replications in net house of Division of Crop Improvement, ICAR-Central Soil Salinity Research Institute, Karnal, Haryana, India. Fifteen wheat genotypes were evaluated under three different environments i.e. control (best available water), moderate saline irrigation (ECiw – 6 dS m−1) and extreme saline irrigation (ECiw – 10 dS m−1). Natural saline water collected from Nain experimental farm, Panipat (Table 1) was diluted to the desired saline levels and stress treatment was imposed prior to the seed sowing. Twelve durum wheat genotypes (HD 4728, HD 4730, HI- 8708, HI-8713, HI-8758, HI-8759, HD-4758, MACS-3949, MACS-4020, MACS- 3972, MACS 4028 and HI- 8737) were collected from wheat repository, ICAR-Central Soil Salinity Research Institute, Karnal along with salt tolerant checks (KRL-99 and KRL 3–4) and salt sensitive, HD 2009. Seeds of each genotype were surface sterilized with 0.1% Bavistin and sown in 20 kg capacity clay/porcelain pots filled with sand. Nutrients were supplied with Hoagland nutrient solution. The region witnesses subtropical and sub-humid climate with hot summers. The net house was covered with a high quality transparent polythene sheet to avoid the rain water and maintain the salinity treatments.

Table 1

Effect of saline water irrigation on water potential (ψp) of roots and shoots in wheat genotypes.

Source Variation	Water potential root (−MPa)			Water potential shoot (−Mpa)
	df	Mean of Square	F-ratio	df	Mean of Square	F-ratio
Rep	2	0.0048	1.94	2	0.0014	0.7704
Genotype	14	0.8865**	361.05	14	0.582**	318.36
Trt	2	27.4349**	11763.74	2	14.95**	9803.16
Genotype × trt	28	0.2212	94.84	28	0.147**	96.52
Treatments/Genotypes	Control	ECiw – 6 dS m−1	ECiw – 10 dS m−1	Control	ECiw – 6 dS m−1	ECiw −10 dS m−1
MACS 3949	1.08bcde	1.97bc	2.61e	0.87D	1.06F	2.25C
MACS 4020	0.96defgh	1.73de	2.93c	0.93CD	0.95F	1.54HI
HI 8737	0.9gh	1.12h	1.94g	1.11AB	1.43BC	1.86E
KRL – 99	0.99cdefg	1.17h	1.67h	1.06AB	1.24DE	1.65GH
HD 4728	1.17ab	1.62ef	2.74de	1.02BC	1.2E	1.51I
MACS 4028	1.09bcd	1.86cd	3.14ab	0.75EF	1.02F	2.05D
KRL 3–4	1.04bcdef	1.17h	1.71h	0.62G	0.75G	1.56HI
HD 4730	1.27a	2.04b	3.12b	0.59G	1.71A	2.51B
HI 8759	0.94fgh	1.22h	2.95c	0.64FG	1.26DE	2.67A
HI 8758	0.85h	1.18h	1.87g	0.85DE	1.42C	1.73FG
HD 2009	1.12bc	2.35a	3.27a	0.83DE	1.35CD	2.24C
HD 4758	0.94efgh	1.41g	2.75d	1.04ABC	1.26DE	1.84EF
MACS 3972	0.86gh	1.21h	2.47f	1.07AB	1.67A	2.08D
HI 8708	1.05bcdef	1.73de	2.71de	1.13AB	1.71A	2.72A
HI 8713	0.98defgh	1.59f	2.45f	1.14A	1.54B	2.47B

Observations and data analysis

The observations on different physiological parameters such as water potential (ψp), osmotic potential (ψs), relative water content (RWC), Na+ and K+ content were recorded in roots as well as shoots at the reproductive stage. Photosynthetic rate and chlorophyll content were also measured in the flag leaves. Water potential (ψp) was measured by chilled-mirror dew point technique using WP4C Dewpoint Potentia Meter (METER Group, Inc. USA) and expressed in -MPa. Osmotic potential was measured using Vapour Pressure Osmometer (Model 5600, ELITech Group, Belgium) and quantified as mmol kg−1. Relative water content in roots and shoots was measured by the method of Weatherley (1950). Net photosynthesis (Pn) was measured between 10:00 AM and 12.00 PM in the flag leaves using the portable photosynthetic system (Li 6800, Li-Cor Biosciences, USA). Chlorophyll content was estimated according to the method of Hiscox & Israelstam (1979) using dimethyl sulfoxide (DMSO). The leaves used for estimation of chlorophyll were tagged for measuring photosynthesis also. Chlorophyll content was expressed in mg g−1 FW according to (Welburn, 1994) formula. For ionic analysis, the plants were uprooted and washed with distilled water to remove dust and salt particles. Oven dried and finely ground root and shoot (100 mg each) were digested separately with 10 ml of HNO3:HClO4 (3:1) di-acid mixture and measurements were taken on flame photometer (Systronics Flame Photometer 128) using standard NaCl and KCl. Data was analysed using factorial RBD for two factors. For critical difference (CD), treatments and genotypes were compared at 5% level of significance using SAS (Version 9.3, SAS Institute Inc., Cary, NC, USA).

Results and discussion

Five plants were randomly selected and tagged for recording observations for all the growth and development studies during the crop duration. Flag leaves were used for taking different observations at reproductive stage after seeing the visible effect of stresses (tip burning/yellowing of leaves). Relative water content (RWC) depicts the level of cellular and tissue hydration, important for the physiological plant metabolism and explains how plants control or maintain the hydration of cells up to an optimal level under stress conditions (Pooja et al., 2019, Sheoran et al., 2021). In the present study, root RWC was not much affected at moderate stress conditions of ECiw – 6 dS m−1 but further increase in stress level caused increase in water content and genotypes MACS 3949, MACS 4028, KRL 3–4 and HD 4730 showed more than 25 per cent increase in root RWC (Fig. 1A). It has been reported that roots showed higher RWC by sequestering toxic ions to mitigate the negative effect of stress and to maintain water balance in the cell, crucial for plant survival under osmotic stress conditions generated by high salinity (Annunziata et al., 2017). Durum wheat roots maintain plant water homeostasis under osmotic stress. On the other hand, relative water content in shoot decreased with increase of stress level and higher reduction was observed at salinity of ECiw – 10 dS m−1 (Fig. 1B). The maximum reduction in shoot RWC was recorded in genotype MACS 3949 (37.87%) followed by HI 8759 (37.12%) and HD 2009 (36.7%) and minimum reduction in MACS 4020 (15.6%), KRL 99 (19.83%) and KRL 3–4 (20.57%). Salt stress induced a reduction in the relative water content of the leaves, which indicates a loss of turgor that resulted in limited water availability for cell extension process and thus dehydration at cellular level (Kumar et al., 2018b, Makarana et al., 2019, Yadav et al., 2020). Genotype-dependent reduction trends in leaf RWC have been reported in Australian durum wheat under water deficiency and heat during reproductive stage (Liu et al., 2019). Leaf traits associated with better physiological performance are preferably used in phenotyping for breeding experiments.

Fig. 1

Effect of saline water irrigation on relative water content (%) of roots (A) and shoots (B) in wheat genotypes.

Plant water relations, chlorophyll content, photosynthetic rate and ion content are important parameters to determine plant growth/physiological efficiency of crop plants under stress conditions. Water potential (ψp) and Osmotic potential (ψs) are the physiological parameters used for quantifying the stress level in plants (Kumar et al., 2012, Pooja et al., 2019, Lata et al., 2019). The water potential (ψp) of roots and shoots decreased significantly in all the wheat genotypes. Significant variations were observed among the genotypes over the treatments for roots and shoots, respectively. Higher reductions were noted in the values of ψp under stress condition of ECiw – 10 dSm−1 (Table 1). Maximum reduction in root ψp were observed in the genotype MACS 4028 (−3.14 MPa) and HI 4730 (−3.12 MPa) at ECiw – 10 dSm−1 than the respective control i.e. −1.09 MPa and −1.27 MPa. The genotypes, KRL 99, KRL 3–4, HI 8758 and HI 8737 showed the lowest values of root ψp under highest stress level of ECiw – 10 dSm−1. It has been reported earlier that at low ψw, root elongation takes place, although at a reduced rate, whereas shoot elongation was completely inhibited (Sharp et al., 1988, Wu and Cosgrove, 2000). Decrease in the root ψw was caused by specific toxic effects due to the accumulation of Na+ and Cl– ions in root tissues, or by the imbalance in the acquisition of other nutrients under high salinity conditions (Aroca et al., 2012). Similar results of decreasing ψp in wheat genotypes with increase of stress level were also recorded for shoot ψp but the decrease in value of ψp is less in comparison to roots (Table 1). In case of shoots, maximum decrease were observed in genotype HI 8708 (−2.72 MPa) followed by HI 8759 (−2.67 MPa) and HI 4730 (−2.51 MPa) at ECiw – 10 dSm−1 whereas genotypes showing lower values were HI 4728 (−1.51 MPa), MACS 4020 (−1.54 MPa), KRL 3–4 (−1.56 MPa) and KRL 99 (−1.65 MPa). Increase of salt in the root medium can lead to a decrease in water potential due to reduction in root hydraulic conductivity resulting in decreased water flow from roots to shoot and this decrease in water flow due to stress may cause a lowering in leaf water content (Neto et al., 2004, Nandwal et al., 2007, Pooja et al., 2019).

Osmotic potential (ψs) is another important physiological parameter used for recording the extent of stress level in plants. Generally in saline soils, plants cannot take up enough water to meet their evaporative demands and thus, low turgor creates a decrease in osmotic potential. From the results obtained, it was observed that roots showed higher values of osmotic potential (ψs) than the shoots (Table 2). Higher values of ψs (mmol kg−1) depicted lower osmotic potential in -Mpa.

Table 2

Effect of saline water irrigation on osmotic potential (mmol kg−1) of roots and shoots in wheat genotypes.

Source variation	Osmotic potential root (mmol kg−1)			Osmotic potential shoot (mmol kg−1)
	df	Mean of Square	F-ratio	df	Mean of Square	F-ratio
Rep	2	13.655	1.467	2	14.73	2.297
Genotype	14	4841.821**	520.298	14	1273.55**	198.552
Trt	2	106147.05**	8413.968	2	18991.82**	4229.826
Genotype × Trt	28	815.317**	64.627	28	290.8**	64.758
Treatments/Genotypes	Control	ECiw – 6 dS m−1	ECiw – 10 dS m−1	Control	ECiw – 6 dS m−1	ECiw – 10 dS m−1
MACS 3949	75.0de	138.8c	174e	71.5a	83cd	119c
MACS 4020	51.0h	98f	162g	54ef	62f	127b
HI 8737	71.0defg	103f	164fg	74a	84cd	104.5de
KRL – 99	91.0c	104f	149h	59cde	67ef	78gh
HD 4728	102ab	169a	283a	61cd	63.5f	81gh
MACS 4028	107a	152b	185cd	74a	86bc	107de
KRL 3–4	73.5defg	101f	122j	53ef	62f	76h
HD 4730	79d	116e	189bcd	70ab	91ab	128b
HI 8759	65fg	86g	137j	65bc	93a	101e
HI 8758	97bc	127d	187bcd	65bc	78d	109d
HD 2009	105ab	136cd	195b	76a	95a	144a
HD 4758	69efg	101f	181de	64bc	86bc	102e
MACS 3972	74def	97f	163g	57.5de	71e	83g
HI 8708	92c	129cd	193bc	61cd	65ef	107de
HI 8713	50f	62f	92f	50f	62f	92f

Significant variations were noted among the genotypes (p < 0.001) and it was found that root ψs ranged between (50 mmol kg−1 to 107 mmol kg−1) in control conditions. With the increase in salinity level the root ψs values were increased and maximum increase was observed in genotype HD 4728 (283 mmol kg−1) followed by HD 2009 (195 mmol kg−1) and HI 8708 (193 mmol kg−1) but the per cent increase was maximum in MACS 4020 (217.64%) at ECiw – 10 dSm−1 (Table 2). This showed that with the increase in intensity and duration of stress, the genotypes earmarked higher decrease in root ψs. This decrease of osmotic potential is considered to be an osmotic adaptation and is one of the defense strategies against salt stress (Nandwal et al., 2007, Hajlaoui et al., 2010). Mild and moderate salinity levels induced a significant reduction in leaf osmotic potential but the reductions were lower than the roots. Some genotype showed lesser increase in ψs at ECiw – 10 dSm−1 viz. KRL 99 (32.2%), HI 4728 (32.78%), HI 8737 (41.21%) and KRL 3–4 (43.39%) whereas MACS 4020 showed maximum increase in shoot ψs i.e. 135.18% followed by HD 2009 (89.47%) in comparison to their respective controls. The possible reason for decreasing Ψs is that plants adjust to physiological drought conditions caused by salinity to maintain pressure potential (Wright et al., 1997, Kumar et al., 2008, Kumar et al., 2012). It was also reported in literature that degree of decline in ψw and ψs depends upon the tolerance ability of genotypes which were able to absorb water from the rhizosphere (Singh, 2010) and these parameters (ψw and ψs) could be useful in determining and developing appropriate irrigation management to improve crop production in saline areas.

Photosynthesis, as one of the most important physiological processes, provides 90% of the plant dry matter (Steduto et al., 2000, Sheoran et al., 2021). Photosynthetic pigments particularly chlorophyll allows plants to absorb energy from light and acts as a key factor or an indicator of the photosynthetic capacity of plants (Taiz and Zeiger, 2006, Kumar et al., 2016a). Fig. 2A showed that moderate to extreme salinity stress levels, lead to decrease in chlorophyll content in all the genotypes. More than 30 per cent reduction in the chlorophyll content was observed in genotypes HI 8758, HD 2009, MACS 4028 and HD 4730 at ECiw – 10 dSm−1 whereas KRL 99 showed minimum reduction of 15.39 per cent followed by HI 8737 (18.99%) and MACS 4020 (19.53%) under highest salinity of ECiw – 10 dSm−1 (Fig. 2A). This decrease in chlorophyll content might be due to reduced activity of ALA synthase enzyme responsible for chlorophyll synthesis or due to increased activity of chlorophyll degrading enzyme, chlorophyllase (Garg and Singla, 2004, Singh et al., 2016; Mann et al., 2018) or the excessive accumulation of Na+ in the leaf tissues resulted in an alteration of the chlorophyll pigments, and/or a restriction of its biosynthesis which translates into a chlorosis of leaves (Yadav et al., 2020).

Fig. 2

Effect of saline water irrigation on chlorophyll content (A) and photosynthetic rate (B) in wheat genotypes.

Significant variability was observed among different wheat genotypes for chlorophyll content and photosynthetic rate under the salt stress conditions. Gas exchange particularly CO2 exchange was regarded as an important indicator of the growth of plants, because of its direct link to net productivity (Ashraf, 2004). The decrease in chlorophyll content is directly correlated with a decrease in the photosynthetic activity of plants (Su et al., 2016). Under moderate stress of ECiw – 6 dSm−1, net reduction in photosynthesis was in order of MACS 3972 (36.01%) followed by HD 2009 (35.77%), MACS 4020 (35.47%) and MACS 4028 (33.63%) (Fig. 2B). Further elevation in the stress condition leads to much higher reduction i.e. 63.6% in MACS 3972, 60.71% in HD 2009 and 59.44% in HI 8713. These reductions in the net photosynthesis (Pn) might be attributed to reduced efficiency of ribulose-1, 5-bisphosphate (RuBP) carboxylase, to a reduction in RuBP regeneration capacity, or to the sensitivity of PSII to excessive accumulation of Na+ in the leaf tissues (Kumar et al., 2016b). Such inhibition in the photosynthetic rate might coincide with a strong decrease in ψp and ψs contributing to a positive water balance. While some genotype showed little resistance in terms of per cent reduction towards the effect of salinity stress on photosynthetic rate (Fig. 2B) i.e. genotypes HD 4728, KRL 99 and KRL 3–4 showed reduction of less than 25 per cent under the extreme salinity level of ECiw – 10 dSm−1. Perturbation in gas exchange attribute could be associated with decreased utilization efficiency of light, stomatal closure and the resulting CO2 deficit in the chloroplasts, which was the main cause of decreased photosynthesis under mild and moderate stresses (Kumar et al., 2016a).

Significant variation (p < 0.001) was observed for Na+ under different saline treatments among the wheat genotypes. Higher Na+ accumulation was recorded in roots as compared to shoots. Under control conditions, root Na+ content ranged from 1.06 to 1.79% among the genotypes which increased to 2.02–4.69% under moderate salinity and 3.17–6.64% under extreme salinity (Table 3). Maximum accumulation was observed in genotypes MACS 4028 (6.64%) followed by HD 4730 (6.56%), HD 2009 (6.5%) and MACS 4020 (6.31%) under highest stress of ECiw – 10 dSm−1 (Table 3) whereas non significant differences were noted for genotypes KRL 99 (3.17g), KRL 3–4 (3.34g) and HI 8737 (3.41g) which had accumulated lower Na+ in their roots at ECiw – 10 dSm−1. Ionic toxicity is one of the main components of salt stress that commences with the accumulation of injurious concentrations of ions such as Na+ and Cl– in the plant cells. The cytosol of plant cells normally contains 100–200 mM K+ and 1–10 mM Na+; this Na+/K+ ratio is optimal for many metabolic cell functions (Kader et al., 2006). Both Na+ and K+ ions compete for entry into plant root cells and the replacement of K+ by Na+ often leads to nutritional imbalances (Singh et al., 2018). Maintenance of low leaf Na+ concentration and lower Na+/K+ ratio is an important aspect of stress tolerance (Kumar et al., 2016b). The over-accumulation of Na+ may create detrimental effects on the availability of water in a root medium that can lead to cell dehydration and reduced turgor (Flowers et al., 1991).

Table 3

Effect of saline water irrigation on Na+ content of roots and shoots in wheat genotypes.

Source variation	Na+ content root (% DW)					Na+ content shoot (% DW)
	df	Mean of Square		F-ratio		df	Mean of Square		F-ratio
Rep	2	0.0004		0.0493		2	0.0002		0.057
Genotype	14	3.856		493.4013		14	1.0980		311.809
Trt	2	140.94		16221.1702		2	30.74		10340.4682
Genotype × Trt	28	1.138		131.0597		28	0.3374		113.4965
Treatments/Genotypes	Control	ECiw – 6 dS m−1		ECiw – 10 dS m−1		Control	ECiw – 6 dS m−1		ECiw – 10 dS m−1
		Absolute value	% change	Absolute value	% change		Absolute value	% change	Absolute value	% change
MACS 3949	1.42bc	3.13c	120.42	4.75d	234.50	0.69ef	1.34hi	94.20	1.86h	169.56
MACS 4020	1.08ef	2.46d	127.7	6.31b	484.25	1.54a	2.05abc	33.11	2.67g	73.37
HI 8737	1.06f	2.01e	89.62	3.41g	221.69	1.22b	2.01abcd	64.75	3.51b	187.70
KRL – 99	1.16def	2.06e	77.58	3.17g	173.27	1de	1.23i	23.0	1.3i	30.0
HD 4728	1.55ab	2.97c	91.61	5.41c	249.03	1.14bcd	1.91bcde	67.54	2.62g	129.82
MACS 4028	1.44bc	3.21c	122.9	6.64a	361.11	1.18bc	1.85def	56.77	3.15cd	166.94
KRL 3–4	1.34bcde	2.51d	87.31	3.34g	149.25	1de	1.21i	21.0	1.77h	77.0
HD 4730	1.79a	4.69a	162.01	6.56ab	266.48	1.26b	2.07ab	64.28	2.86f	126.98
HI 8759	1.29bcdef	3.11c	141.08	5.54c	329.45	1.28b	1.67g	30.46	2.65g	107.03
HI 8758	1.22cdef	2.55d	109.01	4.08f	234.42	1.29b	2.04abc	58.13	3.83a	196.89
HD 2009	1.45bc	4.14b	185.51	6.5ab	348.27	0.76f	1.9cde	150.0	3.1d	307.89
HD 4758	1.25cdef	2.35d	88.0	4.29ef	243.20	1.03cde	1.78efg	72.81	2.92ef	183.49
MACS 3972	1.29bcdef	2.52d	95.34	4.13f	220.15	1de	1.73fg	73.0	3.04de	204.0
HI 8708	1.76a	3.22c	82.95	4.86d	176.13	1.3b	2.16a	66.15	3.26c	150.76
HI 8713	1.36bcd	3.11c	128.67	4.48e	229.41	0.97e	1.4h	44.32	2.92ef	201.03

Higher Na+ accumulated in shoots with the increase in salinity but to a lesser extent than roots. In shoots (particularly flag leaves), Na+ content ranged from 0.77% to 1.55% under control conditions which increased to 1.38–3.84% at ECiw – 10 dSm−1 (Table 3). Under moderate saline conditions, only 2–3 genotypes showed higher Na+ accumulation i.e. (<75% increase over control). But under extreme salinity of ECiw – 10 dSm−1 only KRL 99, MACS 3949 and KRL 3–4 showed lowest accumulation i.e. 30.0%, 73.37% and 77.0% respectively (Table 3). High Na+ reduces the amounts of available K+, Mg++ and Ca++ for plants resulting in Na+ toxicity on one hand and deficiencies of essential cations on the other hand (Kumar et al., 2018a, Lata et al., 2019).

Potassium (K+) is an essential macronutrient in plants comprising generally 4–6% of its dry matter and is recognized as a rate-limiting factor that plays important functions related to enzyme activation, osmotic adjustment and turgor generation, regulation of membrane potential, and cytoplasmatic pH homeostasis (PPI, 1998, Barragan et al., 2012, Zorb et al., 2014). Results showed that K+ content either in roots or shoots decreased with increasing salinity stress (Table 4) and significant variations were noted among the genotypes for K+ accumulation. In case of roots, K+ content ranged from 7.60 to 9.74% under the control conditions which decreased to 5.18–8.13% at ECiw – 6 dSm−1 and 2.49–6.22% at ECiw – 10 dSm−1 (Table 4) and the genotypes, KRL 99, KRL 3–4 and HD 4728 displayed the lowest decrease and the observed performance might be due to a stronger osmotic adjustment, which is considered to be important for plant adaptation to salt stress since it contributes to the maintenance of turgor and cell volume (Lata et al., 2017; Mann et al., 2019b). While in comparison to roots, shoots (flag leaves) had lower K+ accumulation. The range of K+ is 4.21–6.61% under control environment with a mean value of 5.21% among the genotypes which decreased by 28.8 per cent at ECiw – 6 dSm−1 and 46.05 per cent at ECiw −10 dSm−1 (Table 4). The lowest decrease was exhibited by KRL 99 (26.53%) followed by KRL 3–4 (29.75%), HI 8713 (29.76%) and HD 4758 (30.33%) among all the genotypes. Similar results of decrease in K+ content in both roots and shoots were also noted by Chen et al. (2005) in seven barley cultivars. Reducing salt induced K+ efflux would allow its contribution towards osmoregulation, negating the need for a high investment into the production of organic solutes and allowing the critical maintenance of optimal cytosolic K+/Na+ ratio (Cuin et al., 2010). Shabala, 2000, Shabala et al., 2003 also suggested that a cell's ability to retain K+ is at least as important for plant salt tolerance as its ability to exclude or compartmentalize toxic Na+.

Table 4

Effect of saline water irrigation on K+ content of roots and shoots in wheat genotypes.

Source variation	K+ content root (% DW)					K+ content shoot (% DW)
	df	Mean of Square		F-ratio		df	Mean of Square		F-ratio
Rep	2	0.0231		1.0321		2	0.0048		0.439
Genotype	14	5.344**		238.5963		14	1.6276**		150.2015
Trt	2	214.069**		5408.9679		2	66.2004**		4705.3809
Genotype × Trt	28	1.398**		35.319		28	0.6713**		47.7136
Treatments/Genotypes	Control	ECiw – 6 dS m−1		ECiw – 10 dS m−1		Control	ECiw – 6 dS m−1		ECiw – 10 dS m−1
		Absolute value	% change	Absolute value	% change		Absolute value	% change	Absolute value	% change
MACS 3949	9.04b	6.9de	−23.67	5.36b	−40.70	6.35a	5.13a	−19.21	4.06a	−36.06
MACS 4020	8.52bcde	7.79ab	−8.56	3.19ef	−62.55	4.75fg	3.44ef	−27.57	2.26h	−52.42
HI 8737	9.71a	6.57efg	−32.33	5.08b	−47.68	5.68b	3.69cde	−35.03	2.63efg	−53.69
KRL – 99	8.62bcde	7.47bc	−13.34	6.07a	−29.58	4.56g	3.92bc	−14.03	3.35bc	−26.53
HD 4728	8.96b	7.83ab	−12.61	5.97a	−33.37	5.21cde	3.31fg	−36.46	2.46fgh	−52.78
MACS 4028	8.15ef	6.83def	−16.19	4.51c	−44.66	5.29cde	3.31fg	−37.42	2.31gh	−56.33
KRL 3–4	8.73bcd	8.03a	−8.01	6.22a	−28.75	5.21cde	4.21b	−19.19	3.66b	−29.75
HD 4730	9.74a	6.61efg	−32.13	3.62de	−62.83	5.05ef	3.88c	−23.16	2.77def	−45.14
HI 8759	8.31de	6.32fgh	−23.94	2.73fg	−67.14	5.52bc	3.77cd	−31.70	2.39gh	−56.70
HI 8758	7.63fg	5.94hi	−22.14	2.48g	−67.49	4.85fg	3.54def	−27.01	2.28h	−52.98
HD 2009	8.41cde	7.18cd	−14.62	3.46e	−58.85	5.45bcd	3.25fg	−40.36	2.42gh	−55.59
HD 4758	7.6g	5.59ij	−26.44	4.07cd	−46.44	4.22h	3.53def	−16.35	2.94de	−30.33
MACS 3972	8.93bc	6.63efg	−25.75	3.99cd	−55.31	5.18de	3.73cde	−27.99	3.08cd	−40.54
HI 8708	8.22de	6.24g	−24.08	3.47e	−57.78	6.61a	3.1g	−53.10	2.57fgh	−61.11
HI 8713	7.68fg	5.17j	−32.68	2.9fg	−62.23	4.2h	3.8cd	−9.52	2.95de	−29.76

One of the key features of plant salt tolerance is the ability of plant cells to maintain optimal K+/Na+ ratio in the cytosol (Maathuis and Amtmann, 1999, Tester and Davenport, 2003, Kumar et al., 2016b) and different genotypes showed variability‘s (p < 0.001). It was observed from the results that all the genotypes exhibited higher K+/Na+ under control conditions in roots as well as shoot tissues which declined under mild and extreme stress condition (Fig. 3). Excessive Na+ accumulation in the cytosol and higher K+ leakage from the cell might be responsible for such drop down in K+/Na+ (Mann et al., 2015, Kumar et al., 2016b). Highest K+/Na+ (>1) was shown by genotypes KRL 99 followed by KRL 3–4, HI 8737, MACS 3949, HD 4728 in roots and KRL 99 followed by MACS 3949, KRL 3–4, MACS 4020, HD 4758, MACS 3972 and HI 8713 in shoots in comparison with their respective mean at ECiw – 10 dSm−1. It is reported in literature that the optimal value of K+/Na+ is about one which could be used as determinative trait in salt tolerance (Maathuis and Amtmann, 1999; Kumar et al., 2018c). Thus, it seems that any genotypes to be defined as salt tolerant might have the ability to retain K+ efficiently with the ability to prevent excessive accumulation of Na+ (Chen et al., 2007, Mann et al., 2019c).

Fig. 3

Effect of saline water irrigation on K+/Na+ of roots and shoots in wheat genotypes.

Conclusion

Plant adaptive responses to salinity stress require orchestrated regulation of a plethora of physiological mechanisms and thus imply efficient root to shoot communication. Bread wheat is generally considered as a more salt-tolerant species than durum wheat but some genotypes have the potential (traits specific) to survive under saline conditions such as genotypes MACS 3949, HI 8737, MACS 4020, HD 4758 and HI 8713 maintained their K+/Na+ higher under extreme salinity of ECiw – 10 dSm−1 than the optimal value reported in literature. HD 4728 and HI 8708 showed better performance in term of photosynthesis by maintaining higher chlorophyll content. HI 8728 and HI 8737 showed less reduction in plant water traits (RWC, ψp and ψs). So based on these results, it was concluded that durum wheat also have some potential in terms of physiological traits governing the crop growth and yield under saline conditions and could be useful in crop improvement programme. The positive correlations between yield parameters and physiological traits under salt stress can pinpoint the tolerance mechanism and lead to defined strategies for breeding programmes.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.