Pollen Tube Growth and Fertilization in Tripped and Untripped Flowers of Alfalfa (Medicago sativa) Inbred Plants
E.V. Kvasova
Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, 630090, Novosibirsk, 90, Russia.
Tripping is an important process preceding fertilization in alfalfa producing rupture of the stigma surface membrane that prevents pollen tubes from penetration into style of untripped flowers. In field conditions, tripping and cross pollination of alfalfa plants are performed at the same time by the insect pollinators. Autotripping in alfalfa populations occurs very rarely. Application of inbreeding together with selection for autotripping character allows to increase quite rapidly both autotripping level per plant and the number of autotripped plants.
In closed (untripped) flowers of plants of alfalfa populations pollen germinates freely on stigma with intact membrane, and pollen tubes penetrate into ovaries. This was observed in our cytoembryological study. However, ovule fertilization in untripped plants of the population did not occur and, under isolation conditions without flower tripping, seeds did not set. The same was observed in the first three inbred generations: ovules did not fertilize in ovaries of closed flowers in the presence of pollen tubes. A few fertilized ovules in ovaries of untripped flowers first appeared in the fourth inbred generation. Fertilization in untripped flowers was more frequent in the subsequent inbred generations, and it was observed in all plants both of high and low levels of self-fertility after the ninth generation.
Under artificial self-pollination of the population plants a number of pollen tubes and their penetration deep into ovaries are mainly dependent on self-fertile level: the number of pollen tubes is considerably higher in ovaries of high self-fertile plants than in partly self-fertile and self-incompatible plants. Moreover, pollen tubes grow not in all ovaries (their number varies among plants from 26.3 to 85.6%), and the lower was the level of fertility, the smaller was the number of ovaries with pollen tubes. The pattern of pollen tube growth in untripped flowers was the same as in artificially opened (self-pollinated) flowers: the higher was self fertility, the larger was the number of ovaries with pollen tubes and the number of pollen tubes in ovary.
Inbreeding affected all characters and processes, including the pattern of pollen tube growth and fertilization. In plants of the population, there was a correlation between the number of ovaries with pollen tubes and self-fertility (r=0.79); in inbred plants, there was no such correlation (r=0.163-0.28). This means that active pollen tube growth cannot be an index of high self-fertility in alfalfa inbred plants. The following was commom to ovaries of untripped flowers of all the inbred plants of different inbred prolongations: 1) free growth of pollen tubes whose number is high enough to provide fertilization of all ovules in ovaries and 2) occurrence of fertilization although less frequent than in ovaries of autotripped flowers.
Prolonged inbreeding led to changes in autotripping time, the patterns of pollen tube growth and fertilization. As a result, we distinguished 4 types of plants differing in fertilization patterns: 1) fertilization in artificially tripped (self- or cross-pollinated) flowers in the absence of autotripping; 2) fertilization in autotripped flowers in the presence of high autotripping level; 3) fertilization in closed (untripped) flowers (rare) and 4) fertilization both in autotripped and in untripped flowers in the presence of opened and closed flowers in the same plants.