Evaluation of pollen viability, stigma receptivity, and the cross barrier between tropical and hardy water lily cultivars

Abstract

An improved germplasm combining the advantageous traits of tropical and hardy water lilies was assembled by cross-breeding. The pollen viability, stigma receptivity, and pollen tube behavior were assessed to avoid potential developmental problems resulting from cross-breeding. This is the investigation using the measurement of water lily pollen viability by culturing stigmatic secretions in vitro. Germination rates measured by this method were consistent with those found using 2,3,5 triphenyl-tetrazolium chloride staining. Pollen growth in vitro was the same on both the stigma and the style. Stigma morphology, stigma receptivity, and stigmatic secretions showed that the optimal time for pollination was in the cup phase with extension for one-day of the flat period. Three crosses in tropical water lilies all resulted in seeds (73.33%, 86.67%, and 100%) with the pollen tube growing normally in the style. Two out of three hardy water lily crosses produced seeds (16.67% and 65.38%); the third cross produced no seeds and little pollen germination, and it was found that the pollen tube was bent and the pollen was not able to enter the ovule smoothly. There were serious pre-fertilization barriers in the crosses between hardy and tropical water lilies, including low pollen germination, abnormal growth of the pollen tube in the style, showing bending at the apex or expansion in the middle, or the production of a callose, preventing the pollen tube from entering the ovule effectively. Overcoming the pre-fertilization barrier is an important way to improve the cross efficiency of tropical and hardy water lilies.