Post Pollination Pollen Tubes Embryonic and Seedling Development

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As in other angiosperms, following the transfer of pollen to stigma, the pollen germinates and a pollen tube grows through the stigma, style, and locule to the micropyle of the ovule where it penetrates the synergid cells (e.g. Raven et al., 1999; Kuo and den Hartog, Chapter 3). The pollen tube facilitates the transfer of sperm cells that fertilize the egg cell and the endosperm nucleus. As indicated above, post-pollination processes are complicated in seagrasses by the complete submergence of reproductive organs in the marine environment (McConchie and Knox, 1989a). Consequently, pollen and stigmas have a waterproof adhesive that

Fig. 6. Pollen tube growth in the Zostera marina under (A) bright field and (B) epifluorescence using alanine blue dye. This composite representation includes images from the style at right (note the abscission scar, where the stigmas were lost) to the rear of the locule at left. Note the location of the uniovulate ovary in the second left-most image and the penetration of the egg apparatus indicated by the bright fluorescence near the micropyle (scale bar = 100 |m; modified from Ackerman, 1993).

Fig. 6. Pollen tube growth in the Zostera marina under (A) bright field and (B) epifluorescence using alanine blue dye. This composite representation includes images from the style at right (note the abscission scar, where the stigmas were lost) to the rear of the locule at left. Note the location of the uniovulate ovary in the second left-most image and the penetration of the egg apparatus indicated by the bright fluorescence near the micropyle (scale bar = 100 |m; modified from Ackerman, 1993).

operates on contact and serves to maintain the contact between pollen and stigma for some time (Pettitt, 1984). It is relevant to note that pollen longevity is on the order of a day in Zostera and Posidonia (de Cock, 1980; Smith and Walker, 2002). This is important as pollen germination requires several hours of contact with stigmas (Ackerman, 1993). This may appear to contradict reports of precocious pollen germination (e.g. Clavaud, 1878), but germination appears to require the presence of stigma-borne chemicals ("stigmatic" or "germination" factors) in the water or growth medium (de Cock, 1978; McConchie and Knox, 1989b). Pollen germination is evident as protuberances or beads on the filiform pollen (Hofmeister, 1852) that eventually grow between the swollen stigmatic cells (Clavaud, 1878; McConchie and Knox, 1989b; Ackerman, 1993) into the carpel-late flower. In the case of Z. marina, pollen growth was continuous through the style, but lacked orientation at the entrance of the locule (second-third image of Fig. 6) and then again near the rear of the locule opposite the ovary (not shown). This lack of orientation and the observation of multiple pollen tubes within a single carpel speak to the possibility of pollen-tube competition and potential of an incompatibility system in seagrasses (Ackerman, 1993; c.f. McConchie and Knox, 1989b). Regardless, the time required for pollen tubes to reach the ovary was between 7 and 11 h in Z. marina (Hofmeister, 1852; Johansen, 1940; Ackerman, 1993).

The embryonic and seedling development of sea-grasses is also an area of interest. Mature seeds lack endosperm, which is resorbed as the embryo matures (Table 2; Tomlinson, 1982). The endosperm develops via helobian endosperm development in the Hydrocharitaceae and via nuclear endosperm development in the Zosteraceae, which reinforces the concept of polyphylly of the seagrasses (Tomlinson, 1982). Embryonic development leads to an enlarged hypocotyl, which may be straight in Thalassia and curved in Zostera (Tomlinson, 1982). These features may serve to orient and stabilize the plants in the sediments (Cook, 1987). Seedling development has been best described in Z. marina (Hofmeister, 1852; Rosenberg, 1901b; Taylor, 1957a,b; Churchill, 1983), although data exist for Enhalus (Kausik, 1940), Cymodocea (Bornet, 1864), Halophila (Balfour, 1879; ecological characteristics in Zakaria et al., 1999), and Posidonia (Balestri et al., 1998), and it is an area of renewed research as it relates directly to the recruitment of new individuals to a population.

As in other aquatic plants the dispersal of seagrass diaspores (fruit, seed) is not well described (Van der Pijl, 1972; Hay et al., 2000), even though the Greek Botanist Theophrastus noted the occurrence of the "sea oak", which was the free-floating fruit of Posidonia oceanica (L.) Delile (Cavolini, 1792 translated by Konig, 1805). Fortunately, the recognition of the importance and relevance of seagrass diaspore dispersal phenomena has received more attention of late (Orth et al., Chapter 5). There is a great deal of diversity in the morphology and dispersal ecology of seagrass diaspores although a number of patterns are evident (Table 2). For example, seven seagrass genera have dormant seeds, and Halodule, Cymod-ocea, and Halophila release their seeds at or under the sediment (geocarpy), which may facilitate recovery from disturbances in Halodule (Table 2; Inglis, 2000). At least two genera (Amphibolis and Tha-lassodendron) have viviparous seedlings that germinate on the female plant and disperse following their detachment at a larger size, much like mangroves (den Hartog, 1970; Fig. 15, Chapter 3). Interestingly, Amphibolis has a comb-like orientation and grappling device on the seedling that apparently aids in seedling establishment (den Hartog, 1970). Vivipary and vegetative propagation of reproductive shoots has also been noted in other genera (Zostera and Heterozostera), but this appears to be an occasional/rare event that may serve in the dispersal phase of the detached reproductive material (e.g. Addy, 1947; Cambridge et al., 1983). A number of genera (Phyllospadix and Zostera) release relatively small, negatively buoyant diaspores that would likely disperse —1-10 (< 100) m horizontally under normal conditions (Okubo et al., 2002). It is interesting to note that the bifid, barbed fruits of Phyllospadix require regions with branched turf forming algae (with cylindrical thalli) onto which they recruit (Turner, 1983). However, as indicated above for Z. marina, there are opportunities for detached fruiting material to disperse over great distances (see below) and for seeds to float on bubbles, which can extend dispersal distances to ~10-100 m (Churchill et al., 1985). Floatation of buoyant diaspores (Enhalus, Thalas-sia, and Posidonia) and detached infructescences and reproductive material extend dispersal distance to -100 m-10 km (e.g. Harwell and Orth, 2002; Lacap et al., 2002; Orth et al., Chapter 5). It has long been speculated that animal mediated dispersal in the guts of birds, sea turtles, and fish could lead to very large dispersal distances to -100 m-1000

km, especially in migrating birds (e.g. Baldwin and Lovvorn, 1994).

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