Temporal Changes in the Shape and Mechanical Parameters of the Hypocotyl of Cabbage Plug Seedlings

Lodged seedlings unfit as transplanters and harvesters are often grown in cabbage cultures. These seedlings cause transplanting and harvesting losses and reduce working efficiency. Why such lodging of the seedlings occurs is unknown. It is thought that the seedling’s own weight, the growing environment such as rain and wind after transplanting, and the accuracy of the seedling transplanter are possible causes. We therefore focused on the hypocotyl of the plug seedling given how soft it is and how susceptible it may be to damage from the weight of the cabbage. Additionally, the flexural rigidity of the hypocotyl—a mechanical characteristic indicating pliability—was measured and was considered in relation to parameters such as the length and diameter of the hypocotyl and the fresh weight of the seedling. Given that the hypocotyl is relatively soft and deflection may be large, numerical calculations with a large deflection equation were used to measure flexural rigidity. The results showed that the hypocotyl diameter and the seedling fresh weight in all cultivars increased more slowly later in the seedling stage, with the flexural rigidity of the hypocotyl increasing exponentially. It was inferred that Young’s modulus increases in proportion to the number of days that elapse after the day of sowing, since the increase in flexural rigidity was greater than what would be proportional to the hypocotyl diameter in theory. Moreover, it was suggested that the cabbage plug seedling would not be easily lodged given its weight as the seedling grows.