Identification of pre-zygotic reproductive and morphological barriers limiting controlled crossed seed production of triploid Acacia mearnsii

A study of the reproductive biology of black wattle (Acacia mearnsii de Wild) was conducted, specifically with regard to the compatibility of the diploid x tetraploid cross to produce a triploid variety, together with identifying ways of increasing triploid seed production. Fluorescent microscopy was used to determine pollination and fertilisation rates and to identify any pre-zygotic barriers at the stigma, style and ovary. Both diploid and tetraploid families were tested as maternal parents to establish if this was imperative to producing a triploid. Morphological measurements were documented in order to determine any incompatibilities in the cross to produce a triploid. The in vivo results showed that successful fertilisation of the ovary was possible whether one used a diploid or tetraploid maternal parent. When the maternal parent was a tetraploid, however, the pollination rate (polyads adhering to the stigma) and ovary fertilisation rates were significantly (p<0.05) greater. Morphological measurements and observations also revealed that tetraploid floral parts were significantly (p<0.05) larger than the diploids. The morphological size differences between the diploid and tetraploid polyads and pistils did not appear to influence the fertilisation of the ovaries and thus did not pose any identifiable barrier in the cross to produce a triploid. When considering the results from the cross to produce a triploid (2n x 4n or 4n x 2n), the diploid polyads were significantly (p<0.05) more vigorous and suitable in fertilising the tetraploid ovaries as opposed to the reverse. Possible pre-zygotic barriers at the stigma, style or ovary were investigated and the only area that could be identified limiting seed production was within the ovary. (C) 2011 SAAB. Published by Elsevier B.V. All rights reserved.