Pakistan Journal of Biological Sciences

Year: 1998 | Volume: 1 | Issue: 3 | Page No.: 167-169
DOI: 10.3923/pjbs.1998.167.169

Combined Effect of Salinity and Hypoxia on Growth, Ionic Composition and Yield of Wheat Line 234-1

M. Saqib and R. H. Qureshi

Abstract: The present experiment was designed to study the response of wheat line 234-1 to the combined stress of NaCI salinity and hypoxia in soil culture in pots. All the growth parameters (shoot fresh and dry weights, shoot length, number of tillers, CO. assimilation and transpiration rates) and yield components (number of tillers, total biomass, number of spikes, 100 grain weight and grain yield) except number of spikelets were significantly affected under saline hypoxic conditions. This combined effect of salinity and hypoxia was more negative than any of the individual stresses. The ionic composition of leaves was also disturbed. The concentration of Na⁺ and Cl‾ was generally increased while K⁺ concentration decreased. Hypoxia increased the grain yield significantly, however, salinity alone caused a non significant reduction while a significant reduction was observed under saline hypoxic conditions. Grain yield was 5.15 g, 4.24 g, 3.52 g and 1.45 g under hypoxic, controlled saline and saline hypoxic conditions, respectively.

Keywords: Salinity, Hypoxia and Wheat

Introduction

In dense saline and saline sodic soils with low water permeability, crops face surface ponding or temporary water logging at the time of flood irrigation or after heavy rainfall. The incidence of salinity and hypoxia (water logging induced oxygen deficiency) in top soil is a common feature of irrigated agriculture. In Pakistan, salinity and waterlogging co-exist in an area of 1.16 x 10⁶ ha (Qureshi et al., 1993). Wheat is the most important staple food of Pakistan and is grown extensively in rotation with rice. In many instances moderate salinity and sodicity is present in rice tract and land is usually low lying. Here, the wheat crop typically faces a dual stress of hypoxia and moderate salinity with devastating effects on yield. Despite their usual common occurrence, very little work has been directed to study the consequences of these joint stresses. Low oxygen tension in root zone inhibits oxidative phosphorylation which, inturn, restricts the energy available for the ion pumps involved in exclusion of salts from roots (Barrett-Lennard, 1986). In wheat, differences in varietal response to salt stress and hypoxia and various physiological parameters related to such differences have been studied earlier by Akhtar et al. (1994). The present study was designed to obtain information on response of wheat line 234-I to the interactive effects of salinity and hypoxia.

Materials and Methods

Wheat line 234-I was sown in pots each containing 10 kg soil, collected from a normal field and sieved through a 2 mm sieve. Four treatments viz. control, hypoxia, saline and saline- hypokia were employed and replicated, thrice in Completely Randomized Design. The saline treatment had EC_e = 12 dS m‾¹ which was developed by mixing required amount of NaCI in soil. For non saline treatments soil was Used as such. Pots were filled with soil and irrigated with canal water. Ten seeds were sown per pot at "wattar" condition. Plants were thinned out to 6 per pot at two leaf stage. Hypoxic conditions were created twice for about 15 days each time, i.e. at tillering stage and later at booting stage by maintaining a water level of 5-7 cm above the soil surface. Non hypoxic pots were given irrigation as and when required. Recommended doses of fertilizer (NPK) were also uniformly applied to all pots.

Photosynthetic and transpiration rates of standing plants were recorded during sixth week of salt stress by Infra Red Gas Analyser (IRGA). Three plants from each pot were uprooted at the age of about 8 weeks and data regarding the shoot fresh weight, shoot dry weight, shoot length and number of tillers were recorded. Young and old leaves were washed with distilled water and stored in separate eppendorf tubes in the refrigerator. Leaf sap was collected by crushing with steel rod after thawing the frozen leaves. After centrifuging the sap at 6500 rpm for about 10 minutes the supernatant sap was collected. This tissue sap was diluted as required and sodium and potassium were determined using Jenway PFP 7 Flame Photometer (Richards, 1954). Chloride in the tissue sap was determined by titrating the sap against 0.005 N AgNO₃ in the presence of K₂Cr0₄ as an indicator (Richards, 1954). At maturity data regarding number of tillers, number of spikes, number of spikelets, grain yield and total above ground biomass were also recorded. Data were subjected to statistical analysis using Completely Randomized Design in factorial classification (Steel and Torrie, 1980).

Results

Plant growth: Salinity and hypoxia alone as well as in combination caused a significant reduction in all the growth parameters, viz. shoot fresh weight, shoot dry weight, shoot length, number of tillers, CO₂, assimilation rate and transpiration rate (Table 1). Hypoxia alone and in combination with salinity caused a significantly higher reduction (p<0.05) in growth parameters than salinity alone. However, difference between the effects of hypoxia alone and that of saline hypoxia could reach the level of significance only in the case of transpiration rate. The maximum growth was observed in the case of control, followed by saline, hypoxic and saline hypoxic treatments in a descending order.

Ionic composition: Salinity and hypoxia disturbed the uptake and accumulation of Na⁺, K⁺ and Cl‾ in wheat leaves. The disturbance was observed both under individual and combined stresses, However, the effects of combined stress were significantly more pronounced than that of the individual stresses (Table 2). These effects were true for the young as well as older leaves. The concentration of Na⁺ was increased significantly under increased salt concentrations and decreased supply of oxygen in the rooting media. The increse due to the combined stress of salinity and hypoxia was more than that of the individual stresses. In contrast to Na⁺, the concentration of K⁺ was significantly decreased under saline and saline hypoxic conditions. However, hypoxia alone produced a very interesting trend. It caused a significant increase in K⁺ concentration in old leaves while decreased it non-significantly in younger leaves. The ratio of K⁺/Na⁺ was also reduced significantly both by salinity and hypoxia. Saline hypoxic conditions also decreased this ratio but varied non-significantly from salinity treatment. The CI‾ content of leaves was also increased significantly under saline and saline hypoxic conditions while hypoxia alone produced a non-significant effect.

Yield performance: A severe effect of salinity and hypoxia is obvious on all the yield components except number of spikelets (Table 3). Salinity caused a significant reduction in all the yield components except number of spikes and spikelets and grain yield where this reduction was non-significant. Hypoxia increased the number of tillers, total biomass, number of spikes and total grain weight. However, number of spikelets were increased non-significantly while 100 grain weight was decreased significantly. Salinity and hypoxia in combination caused a severe decrease in all the yield components except number of spikelets compared with salinity alone. However, this decrease was significant only in the cases of total biomass, 100 grain weight and total grain yield/weight.

The concentration of Na⁺ and CI‾ in younger leaves showed a negative correlation while K⁺ and K⁺/ Na⁺ ratio showed a positive correlation with all the yield components (Table 4). In particular, the negative correlation of total grain yield was highly significant with Na⁺ as well as Cl‾ while that of 100 grain weight was only significant with Na⁺ and non significant with CI‾. Similarly, 100 grain weight exhibited a nonsignificant correlation with K⁺ and K⁺/Na⁺ ratio but total grain yield exhibited highly significant correlation only with K⁺ while non-significant correlation with K⁺/Na⁺ ratio.

Discussion

The sensitivity of crop ^-plants to abiotic stresses is usually manifested in reduced plant growth. In this study salinity and hypoxia both individually and in combination decreased the wheat growth compared with the control. Salinity might have reduced the shoot growth and tillering by causing water stress, salt ion toxicity or by disturbing the nutrient balance of plants. A decrease in these parameters have also been reported earlier by Nasim et al. (1993). Hypoxia might have reduced the growth mainly due to oxygen deficit for root respiration (Barrett-Lennard, 1986) and decreased nutrient uptake (Morard and Silvestre, 1996). The changed soil chemistry might also have contributed to damaged plant growth and development. Similar results have also been reported earlier by Shafqat (1994). Yeo et al. (1985) reported that high salt concentration in leaves reduce the photosynthesis and transpiration by causing damage to leaf tissues. However, hypoxia might have reduced the transpiration by reducing stomatal conductance (Bradford and Hsiao, 1982). The decreased transpiration will lead to decreased photosynthesis.

The increased Na⁺ concentration and decreased K⁺ concentration in leaves under hypoxic conditions can be attributed to the increased permeability for Na⁺ intake and decreased selectivity of K⁺ against Na⁺ at root membrane level due to decreased energy available because of depressed aerobic; respiration. The increased K⁺ concentration in older leaves under hypoxic conditions might be due to the improved K⁺ uptake during relief period after flooding as reported by Buwalda et al. (1988). Salinity might have increased the Na⁺ and Cl‾ concentration and decreased the K⁺ concentration because of the increased salt concentration in rooting media, damaged membrane permeability and selectivity as well as disrupted exclusion mechanisms. The increased Na⁺ concentration and decreased K⁺ concentration under stressed conditions lead to decreased K⁺/Na⁺ ratio in leaves. Similar effects of salinity and hypoxia on leaf ionic composition of wheat has also been reported earlier by Akhtar et al. (1994). The higher concentration of Na⁺ in older leaves and that of K⁺ in younger leaves may be considered a resistive mechanism developed by these plants.

The reduced grain yield under saline and saline hypoxic conditions has earlier been reported by Akhtar et al. (1994). The reduced 100 grain weight under saline and hypoxic conditions may be attributed to impaired grain filling and development because of reduced photosynthesis and transpiration as well as imbalanced nutrition and reduced water uptake under saline and hypoxic conditions. However, the increased grain yield reported here in this study may be attributed to improved growth because of increased nutrient uptake by plants during relief period as reported by Buwalda et al. (1988).

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