|Basic avocado flowers|
|ML Arpaia |
University of California
Fetscher University of California
|R. Hofshi |
Del Rey Avocado
Avocado productivity is affected by many factors and involves several steps. The critical first step, however, is the pollination process which leads to fertilization and fruit set. Fertilization occurs when pollen is deposited on a receptive stigma. This poster outlines some of the considerations for avocado pollination.
Avocado flowering is unusual in many ways. The mature tree can produce over a million flowers during the flowering period. Flowers appear in panicles of several tens or hundreds of flowers. There are two different types of avocado inflorescences: determinate and indeterminate (see Figure 1). In a given inflorescence the tip of the flower-bearing stem will terminate in a flower. Indeterminate inflorescences, which tend to be more common in our environment, terminate in a vegetative bud.
Figure 1 The avocado flower has both functional male and female organs. The male plant organ, which produces pollen, consists of anthers and stamens. The female plant organ consists of the stigma (which receives the pollen), the style and the ovary.
The avocado exhibits a type of flowering known as “modern dicography”. A separate flower will be open for 2 days, however the timing of the male and female phases are separate. When the flower first opens it is in the female phase and the stigma is receptive to pollen. At the end of the female phase, which lasts 2 to 4 hours, the flower will close. On the second day the same flower opens again in the male phase and sheds its pollen. See images to illustrate the female and male phase flowers.
Figure 3. Diagrammatic representation of female (top) and male (bottom) flowers in avocado. Note that in the female spectra the petals and the male part of the flower (stamens) are reflected downwards and descend. In the male phase the stamens are erect. (Taken from McGregor, SE 1976. The Fertilization of Cultivated Insect Crops, Crop Manual No. 496) Figure # 3 The avocado is also unusual in that the timing of the male and female phases differs between cultivars. There are two types of flowering, referred to as “A” and “B” flower types. “A” varieties open as females on the morning of the first day. The flower closes in late morning or early afternoon. The flower will remain closed until the afternoon of the second day when it opens as a male. “B” varieties open as females in the afternoon of the first day, close in the late afternoon and open again in the male phase the following morning. See figure 4 for a diagrammatic representation.
Since there are hundreds of flowers on an avocado tree at any time the actual situation in the field as shown here (Figure 5). Arrows indicate pollen movement between complementary flower types.
Figure # 4 Figure # 5 What are the practical dimensions?
Table 1 lists some of the common cultivars and their flower type. The flowering behavior of the avocado is thought to promote cross-pollination, as the male and female phases of a single flower appear at different times. It is believed that transplanting complementary types of flowers can enhance fruit set and therefore yield by making pollen available.
|Table 1. Avocado cultivars and flowering types.|
|“A” Varieties||“B” Varieties|
|Note: Varieties in italics are from the UC Player and are under evaluation.|
Unfortunately, most of the currently available “B” varieties are classified as “greenskins”, which return less to growers. Two ‘B’ cultivars currently being evaluated by the UC breeding program are ‘Nobel’ and ‘Marvel’. These varieties produce a black ‘Hass-type’ fruit.’Nobel’ and ‘Marvel’
The variety that provides pollen to the female phase flower is called a “pollinizer” variety (Figure 6). There is much debate as to whether planting pollinizer varieties is worthwhile. Figure 7 shows data from a trial established in 1998. Fruit based on ‘Hass’ trees planted at varying distances from ‘B’ flower cultivars are shown. Measurements were affected by the presence of a pollinizer, pollinizer variety, and distance from the pollinizer. Note: ‘Hass’ trees planted in 1998 and pollinizer varieties planted in 1999. ‘SirPrize’ and ‘Fuerte’ trees did not flower in 2000. Figure # 6 Figure # 7
The separation in time of the male and female phases has led most observers to believe that a carrier or “pollinator” is required to carry pollen from one flower to another. The European bee is the usual pollinator. Evidence from the pollinizer project suggests that pollinator spatial placement may be critical due to beekeeping foraging behavior, since most bees tend to forage within a relatively short radius of 1 to 4 trees.
We have been monitoring avocado bee activity over the last year and have compared the effectiveness of two bee races (Italian and New World Carniolan). We monitor the percentage of bees that visit avocado flowers during the day. Figure 8. Show a bee visiting a male phase “Ettinger” flower. We used the presence of perseitol, a sugar unique to avocados, to test bee visitation. The mid-day drop approximates the time when the female and male flower phases are in transition (Figure 9).
Interesting facts have emerged from this work. During this year we are collaborating with other researchers in Florida and Israel. We hope to further elucidate the mechanism by which pollen is transferred from male to female flowers in order to understand the role of the bee. This information will help growers make informed decisions about pollination. Figure #8