Influence of cold plasma and nano zinc oxide seed treatment on the growth development of seedless watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai]

Abstract

Watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai] is one of the important crop belonging to the family Cucurbitaceae. Globally, it is considered as one of the most consumed cucurbit fruits. Triploid (seedless) watermelon has gained significant market share due to consumer preference, accounting for about 60 per cent of fresh cut and whole watermelon products in retail stores. The major problems with growing seedless watermelons are poor, inconsistent germination and the high seed cost. Triploid watermelon, like its tetraploid parent, has a hard and thick seed coat, which could serve as a physical barrier to radicle protrusion and contribute to seedcoat adherence to the cotyledons after emergence. Cold plasma (CP) treatment of seeds is a new approach that is being proposed to improve the germination and survival of seedlings. Plasma, the fourth state of matter, is a partially or fully ionized gas consisting of neutral and excited atoms, free radicals, negative and positive ions and UV photons. It increases water permeability through surface coat etching and stimulation of seed germination and seedling growth. Similarly, seed treatment with nanoparticles is an emerging technology which positively influences the seed metabolism and signaling pathways. Seed treatment with nano zinc oxide (nZnO) effectively improves seed germination, seedling vigour, yield and quality of the crop. Due to their plant growth-enhancing properties, nZnO has emerged as the most widely produced, utilised and studied nano zinc formulations in recent years. In this study, the optimum voltage and treatment time for two methods of CP exposure (direct CP and seed-in-water CP) were standardised. With respect to characteristics like germination percentage, germination velocity index, length of shoot and root, fresh and dry weight of shoot and root, seedling vigour index 1 and 2, direct CP at 20 kV for 30 min and seed-in-water CP at 25 kV for 15 min were positively influenced. Among these two methods, direct CP was found to be superior, hence this method was followed for further experiments. Seed treatment with nZnO at 20 ppm exhibited the highest germination percentage, germination velocity index, shoot and root length, fresh weight of shoot and root, dry weight of shoot and root, seedling vigour index 1and 2. The individual seed treatments with CP and nZnO were compared with their combination treatment (CP+nZnO) to ascertain any complementary effect. However, the individual CP treatment was found to be superior to combination and nZnO seed treatment with respect to seedling growth characteristics like shoot and root length, fresh weight of shoot and root, dry weight of shoot and root. The same treatment showed an increase in the biochemical characteristics of the seedlings like chlorophyll a and b, carotenoid content, catalase content, DPPH free radical scavenging activity, peroxidase content and phenol content. In the field evaluation, the plants raised from CP treated seeds showed greater vigour and earliness of the crop which was evident from the observations, viz. more number of primary branches, highest internodal length, lowest nodal position for first female flower, less number of days for first female flower opening and for first harvest. The growth characters like number of primary branches, internodal length, days to first female flower opening and node number of the first female flower of the plants under CP+nZnO treatment were found to be on par with CP treatment. The fruits of plants raised from CP treated seeds were harvested 58.3 days after pollination as well as enhanced the yield and quality attributes like TSS, lycopene, beta carotene, citrulline content, potassium and sodium content compared to the untreated control plants. The mode of action of CP is by increasing the porosity of the seeds by creating micropores and channels on the seed surface which was confirmed by scanning electron microscopy (SEM) imaging. The present study provides insights into an innovative method of seed treatment by exposing the seeds to CP resulting in enhanced seed germination, growth, yield and quality in triploid watermelons. The study also explores the effects of nZnO and the combined effect of CP and nZnO on seed germination and plant growth. Upon evaluating the morphological and biochemical traits of the seedlings, it was found that seeds treated with CP were better than those treated with nZnO alone or in combination with CP. Hence CP treatment could be used as a viable technique in alleviating the low germination problem in seedless watermelon, which is a major bottleneck in seedless watermelon cultivation as well as improving the yield and quality.