Residual Effect of Boron and Sodium Chloride Applied to Wheat on Growth and Biochemical Parameters of Green Gram (Vigna radiata) Grown in Subsequent Season

Pulse being a major source of proteins (three times more than cereals) contains low fat and is rich in fibre; they also contain vitamins and minerals like iron, potassium and folate. It is also rich in sulphur, calories and vitamins especially B-complex. It has important role in human diet as well as in farm economy of our country (Swaminathan et al., 2021). They contribute in maintaining the soil fertility as well as mitigating climate change through atmospheric nitrogen fixation. India is the leading producer of pulses in the world accounting for the production of 22.40 million tons in an area of 29.28 million ha (Adhana and Yadav, 2019). India stands first in the production of green gram, contributing about 75% of world’s production (Taunk et al., 2012). Green gram is one of the major kharif pulse crop covering 31.15 lakh ha (Pavithra et al., 2021). It contains 24-25% protein in seeds. It is generally grown in rainy and summer seasons in central part of India.
       
Boron (B) is an essential nutrient and its availability in soil and irrigation water is a key determinant of agricultural production (Nable, 1988). Boron toxicity affects a wide range of processes in vascular plants, including distorted metabolism, decreased root cell division, reduced leaf chlorophyll contents and photosynthetic rates (Nable et al., 1997).  Availability of boron is greatly influenced by the soil characteristics like pH, electrical conductivity, organic matter, cation exchange capacity, calcium carbonate and texture of soil (Evans and Sparks, 1983). Sources of high B include over-fertilization and irrigation with water containing high concentrations of B (Nable et al., 1997).
       
Sodium chloride induced salinity affects the growth of plants. It has an effect on nitrogen metabolism and the urea cycle; as a result, the leaf may turn yellow due to an accumulation of urea in the leaf, which is known as urea burning. Simultaneous boron toxicity and salinity stress can occur when plants are grown in soils with naturally high levels of salts and boron, which is prevalent in semi-arid and arid regions with low annual precipitation and inadequate drainage (Nable et al., 1997).
               
Once salinity or boron toxicity develops in field, their effects persist to subsequent crops. Current study aims to study the effect of both on morpho-physiological and biochemical parameters of green gram.

The experiments were conducted in pots during the kharif 2018 at the Agricultural farm of the Institute of Agricultural Sciences Banaras Hindu University, Varanasi, India. During rabi 2017-18 wheat variety HUW 234 was grown in earthen pots containg .10 .kg soil missed with FYM (4:1). Pots were supplied with following 9 treatment combinations: T₁: Normal soil, T₂: Normal soil+2.0 mg B kg^-1 soil, T₃: Normal soil+3.5 mg B kg^-1 soil, T₄: Normal soil+2.50 g NaCl kg^-1soil, T₅: Normal soil+5.0 g NaCl kg^-1 soil, T₆: Normal soil+2.0 mg B kg^-1+2.50 g NaCl kg^-1 soil, T₇: Normal soil+3.5 mg B kg^-1+2.50 g NaCl kg^-1 soil, T₈: Normal soil+2.0 mg B kg^-1+5.0 g NaCl kg^-1 soil, T₉:Normal soil+3.5 mg B kg^-1+5.0 g NaCl kg^-1 soil. There was no drainage in the pots. After harvesting wheat, in the same pots green gram variety HUM-2 was grown in triplicate by adopting the completely randomized block design (CRD) in the subsequent kharif under recommended package of practices.
       
Shoot length, root length, leaves plant^-1, leaf area plant^-1 and dry weight plant^-1 were determined following standard practices at regular intervals after sowing. Using these data relative growth rate (RGR), net assimilation rate (NAR) (Radford, 1967) and leaf area ratio (LAR) (Watson, 1952).were calculated.
               
Chlorophyll a, b, total chlorophyll and carotenoid contents (Hiskox and Israelstam, 1979), soluble sugars and starch (Dubios et al., 1956), free amino acids (Yemm et al., 1995), proline (Bates et al., 1973) and  malondialdehyde (MDA) content (Heath and Packer 1968) were determined in first fully expanded leaves from top at 20, 40, 60 days after sowing. At harvest total seed weight plant^-1, number of pods plant^-1, number of seeds plant^-1, totals seeds pod^-1, test weight (100 seed weight) and harvest index (Nichiporovitch, 1960) were determined. All biochemical parameters were measured in first fully expanded leaf from top.

Effect of different levels of boron and NaCl on shoot length (cm), root length (cm), number of leaves plant ^-1 , leaf area (cm² plant^-1) and dry weight (g plant^-1) was recorded at different stages of growth (Table 1). At different stages of growth, the response of plant to different treatments in terms of shoot length was significant and at 45 DAS the highest shoot length was recorded in plants under treatment T₆ (40.97 cm), which was at par with T₅  and T₁. At 45 DAS the highest root length was recorded in plants under treatment T₁, which was at par with T₂, T₄, T₇ and T₈. T₁ (39.40 cm²/plant^-1) again registered the maximum leaf area plant^-1 . The dry weight plant^-1 was noted maximum at T₄ (1.75 g/ plant^-1), which was at par with T₃ and T₂. Shoot-root length decreases with increased concentrations of salt and boron (Lata et al., 2017). Although due to strong stimulation of defense sometimes plant shows improved dry weight as compared to control under stress. 
 

       
Effect of different levels of boron and NaCl on amino acids, proline, soluble sugars, starch and malondialdehyde was recorded at different stages of growth (Table 2). At 60 DAS, plants under T₃ (4.931 mg g^-1 fresh weight) contained significantly higher amount of amino acids. The maximum amount of proline was recorded in plants under T₃ upto 40 DAS and at 60 DAS T₉ (0.214 mg g^-1 fresh weight) showed significantly higher amount. Samet and Çikili (2019) found significant proline accumulation under excess B, which indicates that B is contributing to the osmotic stress. However, Mann et al., (2019) observed increase in proline content with increasing levels of salinity. Plants under T₁ (32.67 mg g^-1 fresh weight) showed significantly higher amount of soluble sugars and the minimum amount in plants under T₉ (24.38 mg g^-1 fresh weight)at 40 and 60 DAS. The amount of starch content at 60 DAS under T₁ (102.4 mg g^-1 fresh weight) showed significantly higher and minimum amounts in plants under T₉. A similar result has been reported by Wang et al., (2021). The maximum MDA content was found in plants under T₈ (2.636 µ mole g^-1 fresh weight) and minimum amount of MDA was found in the plants under T₁. Similar result is reported by Samet and Çikili (2019).
 

       
It is reported that under salinity or boron toxicity, photosynthetic rate is reduced and one of the reasons for reduction in photosynthetic rate is due to reduction in the level of photosynthetic pigments (Carillo et al., 2011). Hegazi et al., 2018 reported that mild concentrations of boron can increase the photosynthetic pigments. The amount of chlorophyll a, chlorophyll b, their ratio, total chlorophyll and carotenoids in first fully mature leaf from top was measured at 20, 40, 60 DAS (Table 3). The chlorophyll a and b content was high in T₂, which was at par with T₁, however the ratio was high in T₆ (2.164). The carotenoids content was higher in T₇ (0.375 mg g^-1 fresh weight).
 

       
The total dry weight plant^-1, total seed weight plant^-1, test weight, number of pods plant^-1, total number of seeds pod^-1 and total number of seeds plant^-1 showed significant difference between the treatments (Table 4). The entire above mentioned yield parameters were observed maximum in plants under T₁. The harvest index obtained showed significant difference between the treatments .The maximum harvest index was observed in plants under T₂, which is at par with T₆. Salinity and/or nutrient toxicity result in reduction in yield and yield attributes of crop (Mauromicale, 2010)
 

 
The RGR, NAR, LAR were measured among different stages (Table 5). Between 15-30 days stage the plants under T₃ showed highest RGR. When RGR was calculated between 45 DAS and harvest stage, plants under T₁ had significantly higher RGR. The NAR measured was found higher under T₂ between 15-30 DAS and T₄ between 30-45 DAS. The LAR showed the highest value under T₆ between 15-30 days stage. Between 30-45 DAS it was significantly higher under T₇. Major effect of salinity and boron toxicity has been reported to decrease photosynthetic processes of plant (Lovatt, 1984).
 

It has been found that differing amounts of boron or NaCl stress, either used singly or in combination, have a substantial impact on plant growth and development by changing several biochemical processes, ultimately lowering the economic yield. When treated to rabi planted wheat, the 2.0 mg B kg^-1 soil, 2.5 g NaCl kg^-1 soil, 3.5 mg B kg^-1 soil and 5.0 g NaCl kg^-1 soil, or their combinations, showed substantial residual toxic effects on growth, development, biochemical parameters, yield and yield characteristics. As a result, it is advised that on soils that are somewhat saline or where boron is present in higher amounts in the soil or when boron is sprayed for enhancing wheat yield during rabi, green gram cultivation may be used in the following kharif season.

None.