Flowering and seed production in Eucalyptus nitens

This project examined silvicultural and environment factors which affect precocious and abundant flowering in trees of the economically important plantation species Eucalyptus nitens with the aim of optimising seed production. Two separate studies were undertaken to examine how flowering and seed quality were affected by the macroenvironment. The first studied an altitudinal gradient which would span the range where operational seed orchards might be located. Flowering abundance and seed production was greatest on sites where growth rate was highest. Seed weight, germination success and germination rate decreased as site altitude increased. The second study examined the effects of water availability. Flowering abundance was highest in trees experiencing water stress, whilst seed quality remained unaffected by parental water status. Overall, the maternal tree had a greater influence on seed quality traits than the environmental effects studied. On the microenvironmental scale, the effect of tree spacing on flower abundance and capsule survival was studied at two sites where trees were 5 and 13 years old. As the spacing between trees increased so too did reproductive yield, not only per tree but also per hectare. Furthermore, it appears that as trees mature, tree density needs to be decreased to maintain the maximum reproductive yield per hectare. To overcome the strong genetic control of flowering precocity a hormone manipulation approach was tested. The gibberellin biosynthesis inhibitors paclobutrazol, chlormequat chloride and prohexadione were applied to seedlings and their relative effects compared. These treatments reduced both growth rate and endogenous levels of GA1 to varying degrees, with paclobutrazol the most effective. However, none of the treatments promoted precocious flowering. Further environmental and/or chemical manipulation would be required to induce precocious flowering in E. nitens seedlings. The application of nitrogen fertiliser to juvenile trees stimulated precocious and abundant flowering. This was due in part to accelerated growth rate but nitrogen also acted independently of growth rate. In contrast, phosphorus had no effect on growth or reproductive output but did cause trees to undertake vegetative phase change earlier. Nitrogen fertiliser combined with hormone manipulation with paclobutrazol was applied to juvenile and mature plantation grown trees to promote precocious and abundant flowering. There was an additive effect in combining the treatments in promoting both precocious and abundant flowering. The production of pedigree seed traditionally required three visits to the mother tree to carry out controlled pollination. To improve the efficiency of this process, a number of novel controlled pollination procedures were tested on both E. nitens and E. globulus. A new single visit pollination protocol for E. globulus was successfully developed, whilst the techniques applied to E. nitens yielded no advantage over the traditional method. This new protocol for E. globulus is expected to reduce the cost of pedigreed seed production by more than half. This thesis identifies a number of beneficial practices which will improve the productivity and economic performance of E. nitens seed orchards.