| Summary: | In Malaysia, dragon fruits are graded and priced according to their fruit weight.
As dragon fruit is a type of berry, its fruit weight depends on the number of
developing seeds. In order to obtain better quality and yield in term of fruit weight,
a study was conducted on the floral structure and pollen load on fruit
development of red-fleshed dragon fruit (Hylocereus polyrhizus). The first
objective was to determine the pollination events of H. polyrhizus. Light and
scanning electron microscopy (SEM) were used to examine pollen and stigma
morphology and pollen-stigma interaction of red-fleshed dragon fruit at 0, 1, 2, 3
and 4 day after pollination (DAP). Fluorescence microscopy was used to detect
pollen tube growth stained with aniline blue in the style at 0, 2 and 4 DAP. The
flowers of H. polyrhizus bloomed at night in an upright position with a spatial
different between the anthers and the stigma suggesting cross-pollination. The
pollen grains were categorized as large with polar axis (P) to equatorial diameter
(E) ratio of 0.97, oblate spheroidal-shaped and trizonocolpate with the exine
covered with echinates. The stigma had multiple lobes (n = 28 ± 3) and belonged
to the dry type. The style is long (27 ± 2.54 cm) and solid with one hollow canal.
Pollen grains germinated within 2 h after landing on the receptive stigma and
pollen tubes grow within the transmitting tract located beneath the hollow canal
of the style to the ovary. Starch granules within the style tissue depleted as DAP
progressed. Findings indicated that a few pollen tubes had started to arrive in
the ovary cavity after 2 DAP. Successful fertilization of male gametes (from
pollen tube) with female gamete (ovule) led to fruit set. The second objective was
to evaluate the changes in cellular structure using light microscopy and SEM
during fruit development of H. polyrhizus at every 5 DAP. Starch pattern during
fruit development was examined using iodine-potassium iodide (I2/KI). After
successful fertilization, microscopic studies revealed that fruit development
occurred in two distinct period of growth of cell division and cell expansion
especially in the pulp region. Initially, parenchyma cells were small and rigid but loosen as DAP progressed. The early phase of cell division occurred in the first
20 DAP followed by cell enlargement thereafter. These cellular structure
changes occurred concomitantly with the decrease of starch granules and colour
change in the pulp. Starch granules accumulated within the pulp parenchyma
cells and hydrolysed to sugar once the pulp turned red at 25 DAP as indicated
by iodine stain clearing. For the third objective, effects of different pollen load
size on H. polyrhizus fruits in term of fruit size and postharvest qualities were
studied. Freshly dehisced pollen grains collected during anthesis were weighed
at 0.001, 0.05, 0.10, 0.15 and 0.20 g (representing pollen load of 0.01, 0.5, 1.0,
1.5 and 2.0 x, respectively). The usual pollen load applied by the grower was
used as control (C). Fruits were harvested at every 5 DAP until day 35. Results
indicated that pollen load of 0.01 x produced the smallest fruit in term of fruit
weight, length and diameter, followed by 0.5 x. Fruits of other treatments
produced similar fruit size as control. There were no significant differences in
peel and pulp colour, respiration rate and chemical characteristic among
treatments except for soluble solids concentration (SSC). Smaller fruit tends to
yield higher SSC. A positive correlation between seed number and fruit weight
was determined (r = 0.931, P ≤ 0.001). In conclusion, flowers of H. polyrhizus
are nocturnal bloomers with large pollen grains that are covered with echinates
suggested zoophilous pollination. Fruits of H. polyrhizus are considered as fast
crop as it took 30 days suggested by cellular structure and starch pattern change
during fruit development. Findings indicated that fruit weight of H. polyrhizus
could be manipulated by the amount of pollen grains as the pulp derived from
the funiculli of developing seed.
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