Present
3.4 Discussion
infrequent visitors toD. odorata. Several species ofbutterfly(Hyalites esebria esebria Hewitson, Cassionympha cassius Godart, Precis octavia sesamus Cramer, Vanessa cynthia cardui L., Anthene definitadefinita Butler and Belenoiszochalia zochalia Boisduval) were also observed to move from flower to flower within a patch.However,no pollen was found on their bodies and therefore their role as pollinators ofD. odorata is questionable. The importance of bees and hoverflies in the pollination of the closely related genus Senecio has been reported elsewhere (Gross and Wemer, 1983;Lawrence, 1985).
Pollen limitation
The results forthe three pollinationtreatments,at each site,were significantly different(Femcliffe: H=237.75, p<0.05;Dargle:H=63.48,p<0.05). At both study sites,the average number offruits set per capitulumwas higher for those receiving extra pollen than for control flowers on the same ramets (Fig.3.7). The hand-pollinated flowers (HP) therefore received more pollen and also more ofthe plant'sresources than did their respective controls.For the control flowers on experimental plants (EC) the fruit set per flower was similar to that of the control flowers on control plants (CC). The hand-pollinations therefore increased the amount of resources allocated to seeds at no expense to the other flowers. At Ferncliffe, relative to the controls, hand-pollination increased fruit set by 139 % (BC) and 173 % (CC) whilst fruit set between the two controls differed by only 14%. At Dargle fruit set for hand-pollinated flowers was 81 % greater than that of the experimental control and 145 % greater than the control flowers on control plants.The results of the open crosses can be used as measures of the maximum potential seed setofD. odorata at each site.
Self-incompatibility, the inability of a plant with functional gametes to produce selfed seeds, occurs widely amongst flowering plants and is known from a least 71 families. Itis one of the major outbreeding mechanisms in plants, having been recorded from 250 of the c. 600 genera studied. Incompatibility results from the inhibition of pollen tube growth (Brewbaker and Majumder, 1961; Richards, 1986). This inhibition commonly occurs at one of two sites: (1) on the stigma or soon after pollen tube germination and (2) in the pistil during the first few hours of pollen tube growth. Based on the site of reaction two major types of incompatibility systems are recognised in plants (Richards, 1986): plants having the sporophytic system normally show inhibition on the stigma, while plantsofthe gametophytic-type have pollen tube inhibition in the style or ovary.
Members ofthe Asteraceae mostly have sporophytic incompatibility systems (Crowe, 1954;
Brewbaker andMajumder, 1961; De Nettancourt, 1977;Lawrence, 1985; Richards, 1986; Abbot and Forbes, 1993). Several species of Senecio are self-incompatible (for example: Senecio squalidusL.,S.laurusUrv.,S.gregoriiF. Muell andS. odoratusHornem) and are likely to have asporophytic incompatibility system (Lawrence, 1985; Abbott and Forbes, 1993). Homomorphic sporophytic systems are characterised by a single multi-allelic locus and prezygotic mechanisms (De Nettancourt, 1977). In contrast, most ofthe complex genetic systems such as multifactorial incompatibilities and post-zygotic mechanisms are associated with gametophytic systems (De Nettancourt, 1977). In sporophytic systems crosses between individuals will result in 0-100 % seed set, depending on whether they share the same incompatibility alleles or not. Such a contrasting pattern expected in species with sporophytic systems is more easily detected than the complex pattern expected in gametophytic systems (Sobrevila, 1989).In addition, because the expected pattern is so well defined, it might be detectable even under field conditions, where environmental factors that affect seed set cannot be easily detected (Sobrevila, 1989). For this study, it was noted that many ofthecapitula which were hand-pollinated set no seed at all, while for others seed set was frequently above 70%.
Although the results of this study indicate that D. odorata possesses a strong sporophytic self- incompatibility system, it cannot be ruled out that under certain field conditions the effectiveness of this system will be weakened resulting in an increase in the level of self-compatibility.
Breakdown ofsporophytic incompatibility mechanisms can occur (Richards, 1986). Research has
shown that delayedpollination, high temperatures, high CO2concentrations and electrical stimuli can break incompatibility mechanisms (Richards, 1986).Roggen and van Dijk (1972) found that stigmatic abrasion with a wire brush also affected compatibility.They suggested that damage to the cuticle of the stigmatic papillae promoted penetration of selfed pollen tubes. This may account for the few seeds set in the self pollination trials of this study. Alternatively, as it is not known whether the pollination bags used completely excluded pollen, the few fruits produced by D. odorata from self pollination treatments may be due to inadvertent cross-pollinations or interference by small crawling insects e.g. thrips.
Delairea odorata does not conform to Baker's (1965) "ideal weed" model because it is self- incompatible. However, it is important to note that Baker (1965) based most of his analyses on agricultural weeds and while many of his "ideal weed" characters hold true for these plants they do always hold for invasive plants of natural habitats (Rejmanek, 1989; Cronk and Fuller, 1995;
Mack, 1996).For example, of the environmental weeds listed by Cronk and Fuller (1995), 13%
are dioecious and 11% monoecious (and the rest hermaphrodite). A similar level of dioecy is found in the nativeflora ofNew Zealand (12-13%) whichis a flora considered to have a high level ofdioecy (Cronk and Fuller,1995). A rather high percentage of invasive plants thus appear to be dioecious or monoecious - two mechanisms which promote cross-pollination and therefore outbreeding. Williamson and Fitter (1996) compared attributesofthe native and invasive species ofthe British flora to determine if any sets of characters were common to invading species.They found that most characters, particularly the "ideal weed" characters, failed to relate to invasion success.
For the pollen-limitation trials, the flowers that were given additional cross-pollen were able to produce nearly twice as many achenes per capitulum than those which were open pollinated (both EC and CC). These results indicate that seed set in D. odorata may be pollen limited. As previously stated, the occurrence of pollen limitation may occur in a number of ways: (1) low numbers ofpollinators (2) profuse flowering so that the pollinators become satiated (3) ineffective pollen transfer, and (4) restricted gene-flow which may have severe effects on seed fitness, particularly in self-incompatible clonal species, where a single clone may occupy a large patch (Goldingay and Whelan, 1990; Widen and Widen, 1990).
Pollen limitation of seed set innatural populations has been documented in several studies (for a review see Rathcke, 1983).Widen andWiden (1990) found that distance-dependent fecundity could have severe effects on seed fitness in clonal species occurring in large patches. In the present situation pollen limitation was most likely due to low frequency of visits by pollinators and/or high density oframets ofa single clone occupying a patch. However,seed-set for the open pollinated controls,particularly at Femcliffe, does show that patches are likely to comprise more than one genet since, despite the expected movements of pollinators between very near neighbours, some seed was set. Thus low seed set may indicate a general lack of pollinators.An isozyme analysis or genetic 'fingerprinting' would need to be carried out to confirm the genetic diversity of patches.