Sugar beets plants start from tiny seeds and grow to be large plants. where does the bulk of their mass come from?

Sugar beet
Sugar beet is a plant whose root contains a high concentration of sucrose and which is grown commercially for sugar production. In plant breeding it is known as the Altissima cultivar group of the common beet (Beta vulgaris).[1] Together with other beet cultivars, such as beetroot and chard, it belongs to the subspecies Beta vulgaris subsp. vulgaris. Its closest wild relative is the Sea beet (Beta vulgaris subsp. maritima) I hope this is maybe helpful

Covalent bond is the correct answer