DULBERGER, RIVKA. Department of Plant Sciences, Tel Aviv University, Tel Aviv, 69978 Israel. - Variation of stigma characters in relation to their function.
Receptive surfaces of angiosperm stigmas have been classified by
cyto-physiological and histological criteria. The main categories
distinguished are wet or dry, with the receptive surface papillate or
smooth in each category. In most families or even orders examined, the
characteristics of each category were found to be uniform. The
criteria used in this classification are mainly significant for
processes initiated once the pollen grains have landed on the stigma.
Morphological stigma characters, on the other hand, such as shape,
size, positioning, orientation, and distribution of the receptive area
are likely to be relevant for capturing of pollen grains. In the last
four decades most studies on stigmas were related to their
interactions with pollen grains and to incompatibility, while studies
of pollination paid little attention to stigma morphology. The present
work attempts to assess the variation of morphological stigma traits
at various levels of taxonomic hierarchy. Is variation in these traits
similar to the variation in characters significant for pollen-stigma
interactions? A preliminary survey of the literature revealed
dissimilar patterns of diversity for characters involved in the two
functions of the stigma. Numerous examples were encountered in which
details of stigma morphology relevant for pollination were among
diagnostic criteria used for distinguishing between sub-families,
tribes, genera, sections, and species. Diversity of stigma morphology
among taxa of the same rank is mostly associated with uniformly wet or
dry and papillate or non-papillate receptive areas. Properties of the
receptive area functioning in pollen-stigma interactions appear to be
more constant than properties involved in stigma moulding in
accordance to requirements of pollen acquisition. Diversification of
morphological properties frequently occurs in taxa with specialized
pollination mechanisms, e.g., secondary pollen presentation or
buzz-pollination. It may also serve as an isolation mechanism between
sympatric species.
Key words: pollination, stigma characters, taxonomy