Tomatoes have been variously bred to produce higher yields, bigger fruit, and higher fruit sugar content. In the arena of plants bred for bigger fruit, Dutch cultivars consistently produce a higher fresh fruit weight than Japanese cultivars. Though both light use efficiency and photosynthetic rate play into this disparity, it has been unclear whether there are actual anatomical differences behind it as well. One study found that flow within the xylem — the vessels that move water and nutrients upward within the plant — was faster in Dutch cultivars than in Japanese.
In a new article published in Annals of Botany, lead authors Xiaohua Qi and Hirokazu Takahashi and colleagues attempted to answer this question by comparing the vasculature of the two types of cultivars at different points in their growth, as well as examining hormone profiles and gene expression in the hypocotyls of three- and four-week-old plants. To understand the source of key growth hormones, the researchers made reciprocal grafts in 17-day-old plants using the rootstock of one cultivar and the scion of the other, and then studied their hypocotyl anatomy after 18 days.
Anatomical study revealed that Dutch cultivars have a more developed secondary xylem than their Japanese counterparts, permitting more efficient transport of water, nutrients, and hormones to the shoots. Dutch cultivars also had higher levels of iP-type cytokinins, which are locally synthesized in the phloem and are important regulators of vascular development.
In the grafting experiment, the authors found that grafting did not affect growth or xylem anatomy in the scion, but did so in the rootstock, indicating that the hormones behind the faster development of the xylem were being produced in the shoot. “The accumulation of iP type cytokinins (which predominate in the leaves) rather than tZ type cytokinins (which predominate in the roots) implies that that shoot-derived cytokinins rather than root-derived cytokinins are involved in the difference in xylem development between the Dutch and Japanese cultivars,” write the authors.
They note that further experiments will be needed to determine whether the greater xylem development of the Dutch cultivars contributes directly to growth and fruit production.