Studies on fertility and crossability of species in the genus Leucadendron

[摘要] ENGLISH ABSTRACT:The exomorphology and size of Leucadendron pollen was examined using a scanningelectron and light microscope respectively. Pollen was found to have a consistenttriangular shape with three apertures. Pollen grain size however, show differencebetween species, sections and subsections on Leucadendron. Pollen of all speciesexamined had a woven appearance like the intertwined threads of a fabric withorbicules present on the surface of L. chamelaea, L. elimense subsp. elimense and L.galpinii. Pollen viability was successfully assessed using a solidified agar mediumcontaining 2g agar and 109 sucrose. Pollen germination for all species was found tobe above 55% viability. A diallellayout of crosses has demonstrated conclusivelythat fecundity differs when crossing between species of the genus Leucadendron.Artificial hand pollination was applied successfully on Leucadendron and showedrepeatedly that seed set following intraspecific crosses between the male and femaleinflorescence of the same species gave the same high rate of seed set as found innature. However, seed numbers declined sharply when crossing between species ofthe section 'Leucadendron'. Seed set following crosses between species of differentsections or sub-sections was the lowest and in most cross combinations there was noseed harvested or no seed germination. The diallellayout was useful in identifyingincompatible species and for locating possible incompatibility barriers to interspecificseed development. The morphology of the stigma was examined with a scanningelectron microscope. Stigma appearance of all species had a consistent round to ovalshape, except for L. rubrum, which had an elongated shape. Stigma surfaces of allspecies were densely covered with a large number of unicellular papilar cells on theswollen base. The aniline blue staining technique, together with the fluorescentmicroscope technique was used to follow the growth of the pollen tube following compatible and incompatible cross combinations. Pollen on the stigmas ofcompatible and incompatible species examined showed signs of germination. Pollentubes grew between the papilla cells in all directions and only the most vigorous onesreached the upper part of the style. From the upper region of the style, yellow greentubes grew cohesively in the middle of the style towards the ovule. In compatiblecombinations a not more than 4 tubes reached the ovule region, but was difficult toobserve when they entered the micropyle for fertilization. In incompatible species alarge number of abnormalities occurred beyond the upper region of the style.

[发布机构] Stellenbosch University