Effects of Hydropriming and Explant Origin on in vitro Culture and Frequency of Tetraploids in Small Watermelons

In vitro culture has the potential to develop micropropagation, genetic transformation, and somaclonal variation in a variety of crops, while hydropriming may promote germination efficiency and seedling growth. In this study, we investigated the effects of hydropriming and explant origin on in vitro culture and the frequency of tetraploids in a small diploid watermelon. Hydropriming statistically reduced germination initiation time, and the time taken for 50% germination. It also enhanced the germination index, and the number of uniform seedlings with enlarged cotyledons increased by 15.5%. Correspondingly, the proximal cotyledons and cotyledonary nodes of hydroprimed seeds produced significantly more explant-producing shoots (24.4% and 17.2%, respectively). The highest level of shoot initiation was seen in hydroprimed cotyledonary nodes (94.7 ± 5.8%), followed by non-primed cotyledonary nodes (77.5 plus-minus 6%), hydroprimed cotyledons (72.4 plus-minus 3.9%), and non-primed cotyledons (48 plus-minus 3.9%). Shoots were not formed from either hydroprimed or non-primed hypocotyls; however, the number of shoots initiated from cotyledonary nodes and cotyledon explants was insignificant, suggesting that both cotyledons and cotyledonary nodes are good sources for the in vitro culture of watermelon. When attempting to produce plants with higher ploidy, choosing explant sources favoring tetraploidy should be considered: in our study, 10.5% of tetraploid regenerants were identified entirely from cotyledon explants. It is possible that cotyledon tissue cells undergo ploidy level changes during culture differentiation. Alternatively, some variations may have already been present in the donor plant tissues. Studying the cytological nature of regenerated plants and the mechanisms leading to tissue culture variation can be useful for inducing future somaclonal variations for crop improvement. Morphological changes in tetraploid regenerants indicated that the altered autopolyploidy trait is genotype-dependent.