Pollination rate and pollen tube growth of avocado, in relation to yield

Shoval, S. (1987) Unpublished MSc Thesis, The Hebrew University of Jerusalem, Israel

Low fertility is a severe problem in avocado orchards in Israel and many other countries. The enormous number of flowers which the avocado tree carries in relation to the almost nil fruit set, makes it very difficult to study the comparison between pollination rates and yield levels. This is probably one of the main reasons for the lack of information in the literature about pollination, pollen germination and pollen-tube growth rates under orchard conditions. Many researchers have, nevertheless, discussed the possibility of these processes being important limiting factors in avocado fertility.

The purpose of our work was to study this problem by determining pollination, pollen germination and pollen-tube growth rates in different orchards in relation to yield.

We first studied pollen germination rates and pollen-tube growth in the orchard, when flowers were pollinated at different stages of anthesis. We found that a lot of pollen germination already occurred at the end of the female stage, although generally at lower rates than that of pollination.

Pollen which reached the flower stigma during the male stage, was found to germinate readily, although pollen tubes usually stopped growing either in the stigma, or at the upper part of the style. Pollination; at the male stage which resulted in pollen-tubes reaching the ovary, occurred in only one instance. In a few cases, however, pollen-tubes reached the nucellus one day after pollination. Although, a longer period of time was usually required for the pollen-tubes to reach this destination.

We sampled flowers at various stages of their life-cycles and determined the status of their pollination and fertilization. We came to the conclusion that sampling at the male stage opening was the only practical way to determine "effective pollination" rates in avocado orchards as the rate of flowers with pollen-tubes reaching the ovary was found to be the most reliable index for "effective pollination" rate at this stage.

The effect of the pollen quantity placed on the stigma, on germination rates and pollen-tube growth was examined in detached flowers. We found that pollen-tube growth was faster, when pollen quantity was higher. Nevertheless in some flowers pollen-tubes succeeded reaching the ovary when pollinated with only a few pollen grains. It is possible that there is an in-vivo "population effect" in avocado pollen germination and growth. However, the difference may be mainly because of differences in the potential of pollen grains to reach the ovary.

Hand pollination of different cultivars at the male stage was performed under phytotron and orchard conditions. This usually resulted in pollen germination and pollen-tube penetration into the upper part of the style in most of the pollinated flowers. In the phytotron, a small number of pollen-tubes reached the ovary in ‘Fuerte’ and ‘Ettinger’ flowers. It seems that an optimal temperatures regime in the phytotron of 17°C at night and 22°C during daytime, or relatively cool weather in the orchards, causing minimal delay in the male stage, kept the gynoecium viable.

We have never encountered pollen-tube penetration into the nucellus after pollination at the male stage. We therefore doubt whether pollination at the male stage, in the cultivars and under the conditions we have examined results in fertilization.

During the citrus flowering period, there was a considerable decrease in bee activity in the avocado orchards and as a result, there was an evident decrease in the pollination rates. We used ‘Ettinger’ and ‘Fuerte’ flowers from different orchards as samples during the 1985 and l986 flowering seasons. A high correlation between the pollen-tube growth rate and yield, was found in ‘Ettinger’ in two flowering seasons - 1985 and 1986, whereas in ‘Fuerte’, a high correlation was found only in the 1986 season. In our opinion, pollination and fertilization were important limiting factors in tree productivity, in the orchards and during the seasons in which we worked.