Reproductive biology of a tropical Acacia Hybrid (Acacia mangium Willd. x A. auriculiformis A. Cunn. ex Benth.)
Date
2018-07-26
Authors
Sornsathapornkul, Prasert
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Abstract
The Acacia hybrid (Acacia mangium x A. auriculiformis, Leguminosae: Mimosoideae)
has created considerable interest for plantations because of its adaptability and growth
performance when compared to the parental species. This study concentrated on sexual
reproduction, and seed and seedling quality using light and electron microscopy, histochemistry,
and seed and seedling tests.
Two peak flowering periods in the hybrid appear to coincide with high rainfall and
temperature, whereas two fruit-maturation periods occur during a windy dry season. The
hybrid is andromonoecious. A floral spike consists of about 150 loosely arranged flowers.
Flowers are cream colored and fragrant and have no floral nectaries. The pistil has a solid style
with a smooth, wet stigma and amphitropous ovules with immature integuments at pollination.
The flowers are weakly protogynous. Anthesis is complete at 0500-0600 h but peak female
receptivity begins at 0200-0300 h and is completed that day. The stigmatic exudate is of the
lipophilic type and is secreted from the stigmatic cells by a holocrine mechanism. Pollen is the
main floral reward for the insect pollinators. There are several floral characteristics which
facilitate pollen transfer from anthers to the stigmas. Apis mellifera and Ceratina sp. are the
most effective pollinators. They are the most common insect visitors and carry a heavy load of
hybrid polyads. However, their behavior in foraging for pollen in the same tree may promote
self-pollination. The 16-pollen polyads have the highest viability at anthesis (over 80%) but
lose viability within 3 days. In vivo pollen germination occurs within a few hours and pollen
tubes grow up to 16 μm/min, reaching the ovarian chamber 7 to 8 hr after pollination. In vivo
pollen tube growth is supported by the stylar secretion that may be stimulated by pollination
and an ovarian secretion which is independent of pollination. Abnormalities of pollen tube
growth were observed and probably result from self-pollination. There is no evidence of
pollen-tube competition and pollen tube penetration of the ovules appears to occur randomly.
Fertilization in the hybrid occurs within 3 days after pollination. One of the two
synergids is the site of pollen tube penetration and its degeneration is triggered by the pollen
tube penetration of the nucellus. Endoplasmic reticulum is likely involved in the polar nuclear
fusion but not in the fusion of sperm nuclei with the egg and polar nuclei. Because no sperm-cytoplasmic
fusion occurs during karyogamy, the hybrid, therefore, possesses maternal
cytoplasmic inheritance. The hybrid zygote is metabolically inactive and has a two-month
dormant period due to delays in embryo nutrition. Proembryo cell divisions are of the
Trifolium variation of the Onagrad type without formation of a suspensor. Endosperm
formation is of the nuclear type. The breakdown of stored products, abundant in the central
cell and nucellus, provides nourishment to the developing endosperm through many nutrient
pathways. The endosperm then becomes the main nutrient source for the embryo.
Carbohydrates, lipids and proteins are the main seed storage products.
The hybrid has very low reproductive success (0.0054). Low fruit set in the hybrid (2%)
was attributed primarily to insufficient pollination (65% of total) and early fruit abortion (33% of
total). Low seed set (24%) is mainly caused by failure of pollen tube penetration of the ovules
(over 70%). The seed treatment of soaking seeds in boiling water for 1 min gives high
germination percentages (over 80%) and is practical. The F2 hybrid seedlings possess features
intermediate between the parental species. At 3 months, the F2 seedlings have a high survival
rate (90%) and their height and diameter growths vary significantly among parental trees but
are superior to those of the parental species.
Description
Keywords
Legumes, Breeding, Plants, Reproduction, Trees, Breeding, Tropics, Seeds