Irradiation and seed coating for enhancing storage life of grain cowpea (Vigna unguiculata (L.) Walp.)

Abstract

The present study entitled “Irradiation and seed coating for enhancing storage life of grain cowpea (Vigna unguiculata (L.) Walp.)” was carried out in the Department of Seed Science and Technology, College of Agriculture, Vellayani during 2020-2021, with an objective to standardize the dose of gamma rays for irradiation and concentration of chitosan for seed coating for enhancing the storage life of grain cowpea. The study was divided into two experiments which were conducted in Completely Randomized Block Design (CRD) with three replications. The first experiment was irradiation of cowpea seeds with gamma rays and the second experiment was seed coating of cowpea seeds with chitosan. The seeds were irradiated with five different doses (100 Gy to 500 Gy) at Indian Institute of Horticultural Research (IIHR), Bangalore. Another set of seeds were coated with chitosan at five different concentrations (1 % to 5 %) at two different quantities for each concentration as 1ml 50g-1 of seeds and 5ml 50g-1 of seeds. Coated seeds were then shade dried and packed in polythene bags and stored for six months along with control. In the first experiment, the cowpea seeds irradiated with 300 Gy, 400 Gy and 500 Gy gamma rays were not affected by pulse beetle infestation till the end of six months of storage. However, in control, the seed damage was observed which varied from 0.333% in first month to 56.333% in sixth month of storage with a seed weight loss of 28.182 per cent. The damage percentage recorded was 2.667 percent and 0.667 per cent in treatment T1 (100 Gy) and T2 (200 Gy) respectively in the sixth month of storage. Thus the gamma ray irradiation in all doses proved to be effective in controlling pulse beetle infestation. Germination parameters were studied in the undamaged seeds to study the effect of irradiation in seed aging. Among the different doses of gamma irradiation, T2 (200 Gy) recorded the highest seed germination percentage (84.33%), speed of germination (32.13 days), seedling shoot length (11.83 cm), seedling dry weight (0.703 g) and seedling vigour index I (2130.49) and II (59.29). All the germination parameters showed increased value at lower doses of gamma rays (100 Gy and 200 Gy) and declined at higher doses (300 Gy, 400 Gy, and 500 Gy) compared to control. Morphological evaluation of gamma irradiated seeds grown in field showed that the morphological parameters did not vary significantly from the control in treatments with gamma doses 100 Gy and 200 Gy. Gamma irradiation at 300 Gy also did not show variation in morphological parameters compared to control except for field germination percentage. But progressive decrease in all morphological parameters was observed for the treatments with gamma doses 400 Gy and 500 Gy. Reduction in germination percentage, plant height, number of pods plant-1 , number of seeds pod-1 and 100 seed test weight was observed when compared to control. Few crinkled leaves were observed in 400 Gy and 500 Gy irradiated treatments at earlier stages. No significant variation was observed in 100 Gy, 200 Gy and control. In the second experiment, among the different chitosan treatments, no seeds were observed with insects upto four months of storage. Although insect eggs and infestation were noticed in treatments such as T1 (1% 1 ml 50g-1 ), T2 (1% @ 5 ml 50g-1 ), T3 (2% @ 1 ml 50g1 ) and T5 (3% @ 1 ml 50g-1 ) at the end of storage period, the percentage of infestation decreased with an increase in concentration and quantity of chitosan used. The grain cowpea seeds coated with different concentrations of chitosan from lower to higher (1% to 5%) have different degrees of improvement in germination parameters compared to control. Among the different treatments of chitosan, T10 (5 % @ 5 ml 50g-1 ) recorded the highest seed germination percentage (89.37 %), maximum speed of germination (36.83), seedling shoot length (14.90 cm), seedling root length (17.53 cm), seedling dry weight (0.747 g). seedling vigour index I (2898.28) and II (66.79). In this study Gamma irradiation proved to be an effective method for controlling pulse beetle infestation during storage in grain cowpea. However the treatment with higher doses 400 Gy and 500 Gy affected the germination parameters negatively and produced some abnormalities in the progeny. Thus, the gamma irradiation at 200Gy and 300 Gy can be recommended for safe storage of grain cowpea seeds. Chitosan at 5% @ 5 ml 50g-1 exhibited higher values for seed germination parameters and showed no pulse beetle infestation till the end of the storage period of six months. Chitosan treatment at 5% @ 5 ml 50g-1 can be recommended for safe storage of grain cowpea seeds. Both gamma irradiation and chitosan seed coating maintained the longevity of seeds during storage and were effective in controlling the storage pests. Seed coating with chitosan had an additional advantage of substantial improvement of seed germination parameters. Gamma irradiation and chitosan seed coating are eco-friendly methods in enhancing the storage life of grain cowpea. Since gamma irradiation requires special facilities for seed treatment, chitosan seed coating will be a better technology for small scale farmers.