Comparative morphology of in vivo and in vitro laticiferous cells and potential use of in vitro laticifers in early selection of rubber tree clones

Abstract

The laticifer density in callus of young tissue is positively correlated with the rubber yields of adult trees and can serve as an early selection marker to accelerate rubber tree breeding. The rubber tree (Hevea brasiliensis Muell. Arg.) is a perennial crop with a breeding cycle of approximately 30 years in the conventional breeding practices. The lack of suitable parameters linking the juvenile and adult trees has made the early selection attempts inefficient. We performed comparative morphology of laticifer cells in callus induced from young shoots and the primary and secondary laticifers in the trunk of adult trees, and investigated their relationships to rubber yields. Results indicate that the secondary laticifers have smooth lateral walls and perforated end walls and belong to articulated anastomosing laticifers. The primary laticifers are unbranched, non-articulated, and anastomosing, and have bumpy lateral walls when matured. The callus-derived laticifers are morphologically similar to the secondary laticifers with smooth lateral walls and the existence of end walls, and similar relative cell wall thickness. The laticifer density in callus and the laticifer ring number in bark are both genotype-dependent, and they are both highly correlated with the rubber yields with correlation coefficients R = 0.846 (P < 0.002) and R = 0.899 (P < 0.002) for the callus and bark, respectively. Since callus can be induced from tissues of young seedlings in cross-pollinated populations, the laticifer density in callus can serve as an early selection marker of seedlings for high rubber yields and may be used to shorten the rubber tree breeding cycle.