Assessment of seed storability parameters: Vigour Index-I, Vigour Index-II, electric conductivity and enzyme activities of soybean as influenced by different methods of threshing and processing

Introduction

Soybean (Glycine max L.) is a globally important annual crop, often called the "wonder crop" due to its high nutritional and economic value. It contains about 40-45% protein and 19-20% oil, making it a major source of edible oil and plant-based protein, especially for vegetarians. Because of its affordability, it is popularly known as the "poor man's meat." Soybean is widely used in food products such as soya milk, flour, biscuits, and snacks, as well as in industrial products like paints and varnishes. It also improves soil fertility by fixing atmospheric nitrogen, benefiting subsequent crops. Globally, countries like Brazil, the United States, and Argentina dominate soybean production. In India, soybean is mainly grown during the rainy season in states like Madhya Pradesh, Maharashtra, and Rajasthan, contributing significantly to farmers' income and cropping intensity. Seed quality plays a crucial role in soybean productivity, with good quality seeds increasing yields by 15-20%. However, soybean seeds have poor storability due to high oil content and sensitivity to environmental conditions. Proper harvesting at the right stage of maturity is essential to maintain seed viability and ensure better germination and overall crop performance.

Materials and Methods

The seeds obtained by use of different threshing methods viz., T 1 , Multi-crop thresher with 300 rpm; T 2 , Multi-crop thresher with 500 rpm and T 3 , Threshing by combine harvester and same seeds as per assigned treatments subjected to processing operations viz., P 1 , Air screen cleaner; P 2 , Air screen cleaner followed by gravity separator; and P 3 , Air screen cleaner followed by gravity separator and spiral separator were stored in small gunny bags and placed in the seed storage cabinet at the Department of Seed Science and Technology, College of Agriculture, Raichur under ambient conditions. These seeds were used for further testing of seed quality parameters. The experiment was conducted in laboratory using Factorial Completely Randomized Design with three replications. The seeds were used for recording parameters viz., 100 seed weight, bulk density, seed germination, shoot length, root www.extensionjournal.com length, seedling dry weight, seedling vigour index -I, seedling vigour index -II, electrical conductivity, dehydrogenase enzyme activity and peroxidase activity. All of the above parameters were recorded for every month, with the exception of peroxidase and dehydrogenase enzyme activity, which were recorded every three months.

Results and Discussion

Seedling vigour index I

The results of seedling vigour index I as influenced by different threshing methods, processing operations and their interaction effects at different months of storage period are presented in Table 1. The seedling vigour index decreased with the advancement of storage period in all the treatments. Whereas, initially higher seedling vigour index I was recorded in T 1 (3037), P 3 (2840) and T 1 P 3 (3159) and lowest was in T 3 (2396), P 1 (2631) and T 3 P 1 (2310). At the end of nine months of storage period, highest seedling vigour index I was recorded in T 1 (2121), P 3 (2005) and T 1 P 3 (2211) while, significantly lowest seedling vigour index I was recorded in T 3 (1734), P 1 (1851) and T 3 P 1 (1670).

Seedling vigour index II

The results of seedling vigour index II as influenced by different threshing methods, processing operations and their interaction effects at different months of storage period are presented in Table 2. The seedling vigour index decreased with the advancement of storage period in all the treatments. Whereas, initially higher seedling vigour index II was recorded in T 1 (54460), P 3 (52535) and T 1 P 3 (56284) and lower was in T 3 (48094), P 1 (50282) and T 3 P 1 (47257). At the end of nine months of storage period, highest seedling vigour index II was recorded in T 1 (44368), P 3 (43275) and T 1 P 3 (45939) while, significantly lower seedling vigour index II was recorded in T 3 (40627), P 1 (41315) and T 3 P 1 (39857). Differences in vigour index among the processed seeds may be due to the difference in seed size and extent of mechanical injury. This was supported by various findings by Hesse and Peske (1981) [14] , Moreano et al. (2018) [18] in soybean. The soybean crop threshed by multi-crop thresher with 300 rpm and seeds processed using air screen cleaner followed by gravity separator, and spiral separator (T 1 P 3 ) recorded significantly highest seedling length, dry matter content and vigour index than other combinations. This could be attributed to less percentage of mechanical damage to these seeds. Mechanical damage to the seed may be one of the causes for reduction in length of seedlings of soybean. The threshing produces breaks, cracks, bruises and abrasions in seeds which in turn results in abnormal seedlings of questionable planting value. These results are in conformity with the findings of Kausal et al. (2006) [16] in soybean. It was noticed that seedling length, dry matter content and vigour index were decreased as the storage period advanced. The decrease in length of seedlings could be ascribed to the ageing or deterioration of seed, which is progressive process, accompanied by accumulation of metabolites and progressively decrease germination and growth of seedlings with increased age (Floris, 1970) [12] and ultimately reducing the dry matter and vigour of soybean seed during storage. Seedling growth is considered to be an important tool that can be used for assessing the magnitude of deterioration (Toole et al., 1957) [25] .

The data on electrical conductivity of seed leachate as influenced by different threshing methods, processing operations and their interaction effects at different months of storage period are presented in Table 3. Electrical conductivity of seed leachate was increased gradually as the storage period advanced. Initially, Lowest EC of seed leachates was observed in T 1 (0.396 dSm -1 ), P 3 (0.459 dSm - 1 ) and T 1 P 3 (0.379 dSm -1 ) and highest EC of seed leachates was observed in T 3 (0.539 dSm -1 ), P 1 (0.496 dSm -1 ) and T 3 P 1 (0.569 dSm -1 ). At the end of storage period significantly lowest EC of seed leachates was observed in T 1 (1.407 dSm -1 ), P 3 (1.632 dSm -1 ) and T 1 P 3 (1.347 dSm -1 ) and highest EC of seed leachates was observed in T 3 (1.916 dSm -1 ), P 1 (1.762 dSm -1 ) and T 3 P 1 (2.023 dSm -1 ). The electrical conductivity of soybean increased progressively with the advancement in storage period. A significant difference in electrical conductivity due to processing operations was observed throughout the storage period. The electrical conductivity increased with subsequent increase in storage period irrespective of processing operations. Lower electrical conductivity in seeds processed using air screen cleaner followed by gravity separator, and spiral separator (P 3 ) may be due to higher test weight, less mechanical damage and more physical purity. Increase in electrical conductivity along storage period might be caused by increase in permeability of membrane of deteriorated seed. Loss of membrane integrity of deteriorated seeds leaks more substances into the medium. This could be attributed to the high mechanical injury, poor membrane structure and leaky cells. These results in greater loss of electrolytes such as sugars, amino and organic acids from seeds and increased conductivity in the soak water and are in conformity with findings of Abdul Baki and Anderson, 1973 [1] and Agrawal, 1977 [2] in soybean.

Dehydrogenase enzyme activity

The data on dehydrogenase enzyme activity as influenced by different threshing methods, processing operations and their interaction effects at different months of storage period are presented in Table 4. Dehydrogenase enzyme activity decreased gradually as the storage period advanced. Initially, highest dehydrogenase enzyme activity was observed in T 1 (1.009 OD), P 3 (0.945 OD) and T 1 P 3 (1.032 OD) and lowest dehydrogenase enzyme activity was observed in T 3 (0.833 OD), P 1 (0.895 OD) and T 3 P 1 (0.815 OD). At the end of storage period significantly lowest dehydrogenase enzyme activity was observed in T 3 (0.434 OD), P 1 (0.466 OD) and T 3 P 1 (0.424 OD) and highest dehydrogenase enzyme activity was observed in T 1 (0.525 OD), P 3 (0.492 OD) and T 1 P 3 (0.537 OD). These results are in agreement with findings of Radha et al. (2014) [24] in maize, Chauhan et al. (2011) [8] in wheat and Pallavi et al. (2003) [21] in sunflower who observed reduction in dehydrogenase activity with the advancement of storage period, which could be due to enzymes undergo compositional changes by losing or gaining certain functional groups, by oxidation of sulf-hydral groups or by conversion of amino acids within the protein structure. The enzymes may also undergo configurational changes such as partial folding or unfolding of ultra structure, condensation to form polymers and degradation to sub units. The continual accumulation of free fatty acids culminates in reduction of cellular pH and is detrimental to normal cellular metabolism (Copeland and McDonald, 1995) [9] . Further, it denatures enzymes resulting in their loss of activity. A high level of correlation between was reported between the decrease in dehydrogenase activity and loss of seed viability by Mustafa et al. (2010) in onion. The amount of dehydrogenase activity in storage decreases with the progress in ageing as reported by Verma et al. (2003) [26] in Brassica spp.

Peroxidase enzyme activity

The data on peroxidase enzyme activity as influenced by different threshing methods, processing operations and their interaction effects at different months of storage period are presented in Table 4. Peroxidase enzyme activity decreased gradually as the storage period advanced. Initially, highest peroxidase enzyme activity was observed in T 1 (1.415 µ moles ml -1 min -1 ), P 3 (1.254 µ moles ml -1 min -1 ) and T 1 P 3 (1.511 µ moles ml -1 min -1 ) and lowest peroxidase enzyme activity was observed in T 3 (0.966 µ moles ml -1 min -1 ), P 1 (1.109 µ moles ml -1 min -1 ) and T 3 P 1 (0.915 µ moles ml -1 min - 1 ). At the end of storage period significantly lowest peroxidase enzyme activity was observed in T 3 (0.400 µ moles ml -1 min -1 ), P 1 (0.460 µ moles ml -1 min -1 ) and T 3 P 1 (0.379 µ moles ml -1 min -1 ) and highest peroxidase enzyme activity was observed in T 1 (0.586 µ moles ml -1 min -1 ), P 3 (0.519 µ moles ml -1 min -1 ) and T 1 P 3 (0.626 µ moles ml -1 min -

1

). The decreased activity of peroxidase is due to accumulation of H 2 O 2 and this accumulation of hydrogen peroxide itself is detrimental to seeds (Gidrol et al., 1989) [13] reported that similar results were reported by Bhanuprakash et al. (2010) [6] in bell pepper and Demirkaya et al. (2010) [10] in onion seeds. The ageing period had a significant effect on peroxidase activity. The decline in scavenging enzymes particularly peroxidase could be due to lipid peroxidation and sugar hydrolysis (formation of reducing sugars) was coupled to the Maillard reactions during seed ageing. Carbonyl intermediates as a result of Millard reaction may attack the enzyme complex and modify the native structure of enzyme either by adding or drawing the certain functional groups from enzyme structure. Thus, alter the macromolecular structure and reduces the efficiency of peroxidases as described by Narayanamurthy and Sun (2000) [20] . Cyanide production is often associated with deleterious mechanisms. It will inhibit the activity of heme proteins from peroxidases (Ellis and Dunford, 1968; Job and Ricard, 1975) [11,15] and make it inefficient. Demirkaya et al. (2010) [19] studied that a high level of correlation between loss in seed viability and the decreases that occurred in antioxidant enzyme activity in onion, Bhanuprakash et al. (2010) [6] recorded change in enzyme activity due to ageing in bell pepper. Cakmak et al. (2010) [7] studied the decrease germination ability of aged legume seed were correlated with decrease in activity of enzymatic antioxidant studies. These results support the hypothesis of Bailly et al. (1996) [4] that a decrease in antioxidant enzymes is linked to an increased lipid peroxidation and accelerated ageing. Subsequently, Bailly et al. (2000 and 2002) [5] proposed a positive relationship between antioxidant enzyme capacity and the vigour of the seed.

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