Dependence on Cross-Pollination in Macadamia and Challenges for Orchard Management

Cross-pollination is essential for the reproduction of self-incompatible plants and it improves the yield and fruit quality of some self-compatible plants. Cross-pollination of clonal horticultural crops is achieved only when pollen from one cultivar reaches the stigma of another cultivar. We quantified self- and cross-fertilisation rates in macadamia nuts (Macadamia integrifolia, M. tetraphylla and hybrids) that were harvested at different distances from a cross-pollen source in two different plantation designs (mixed blocks of multiple cultivars vs. pure blocks of a single cultivar). We also estimated the distance of effective pollen movement, evaluated how different pollen parents affect nut quality, and assessed whether distance from a cross-pollen source affected the number of nuts harvested. We found that almost all nuts resulted from cross-pollination in both plantation designs, with almost all nuts being cross-pollinated even at 23 rows from another cultivar. However, most pollen did not travel far, and nearby cultivars were usually the pollen parent. Cross-pollination increased nut mass and changed the nutritional quality of the nut. The number of nuts harvested decreased with increasing distance from a cross-pollen source in large pure blocks, but not in mixed blocks. Dependence on cross-pollination coupled with low distances of pollen movement may cause suboptimal fruit set. Our results suggest that fruit set in macadamia plantations comprised of large single-cultivar blocks is limited and that plantations can benefit from closer interplanting of different cultivars

A benefit to providing information? Flower size cues, plant attractiveness, and plant visit length

In many plant species, flower size is correlated with the production of floral rewards such as nectar and pollen and, therefore, provides information to pollinators about flower quality. However, how relationships between flower size and rewards influence plant fitness is not well understood. In particular, it is unclear whether indicating to pollinators which flowers are unrewarding harms or benefits plants. We used a laboratory system with artificial flowers to examine bumblebees’ (Bombus impatiens) responses to plants that had flower size as an informative cue (with large flowers rewarding and small flowers unrewarding) as compared with “deceptive” plants that had a mixture of rewarding and unrewarding large flowers and plants with only large, rewarding flowers. Bees had previously foraged in a context in which only large flowers provided rewards. Small flowers were visited less often than large flowers. In comparing plants with different numbers of flowers, we found that small flowers, although they added less to a plant’s attractiveness than large flowers, did increase a plant’s attractiveness if present in sufficient number. Furthermore, plants with informative cues received substantially fewer flower visits per plant visit in comparison with deceptive plants, even when the plants with informative cues had a larger number of flowers. Cues identifying unrewarding flowers could, therefore, reduce rates of within-plant pollen movement, increasing the plant’s fitness gains per flower visit. Their contribution to whole-plant attractiveness and avoidance of inbreeding could help explain why many plants produce small, relatively unrewarding flowers even though pollinators avoid visiting them.

Corrigendum to: Wild seedlings of a tree endemic on granite outcrops show no evidence of inbreeding depression

Plant communities on granite outcrops are characterised by high biodiversity and endemism, and strong population genetic structuring. The floristic diversity and structure of inselberg flora receives much attention in international conservation research, yet our understanding of how recruitment influences population genetic structure of plants endemic to granite outcrops is limited. To determine the immediate genetic consequences of recruitment and to quantify mating and dispersal patterns by parentage analysis, I conducted an opportunistic study following a wildfire in a stand of the lignotuberous tree Eucalyptus caesia Benth. (Myrtaceae) at Boyagin Nature Reserve. The entire adult stand (n=180) plus seedlings (n=115) were genotyped with 14 microsatellite loci. Compared with adults, seedlings had low heterozygosity and a high inbreeding co-efficient, but similar levels of allelic diversity. There was no evidence of post-germination selection against homozygous seedlings over the 20 month survey period. Seedlings that resulted from cross-pollination were more heterozygous than seedlings resulting from selfing, but could not be distinguished based on height measurements or survivorship. Parentage analysis in CERVUS revealed mostly limited seed dispersal, assuming that candidate parents closest to seedlings were the maternal parent. By comparison, pollen movement was more extensive, yet still restricted to plants within the stand. Recruitment has increased the population size, but appears insufficient to enhance genetic diversity in the Boyagin stand of E. caesia. Nonetheless, E. caesia appears adept at persisting as extremely small populations.

Wild seedlings of a tree endemic on granite outcrops show no evidence of inbreeding depression

Plant communities on granite outcrops are characterised by high biodiversity and endemism, and strong population genetic structuring. The floristic diversity and structure of inselberg flora receives much attention in international conservation research, yet our understanding of how recruitment influences population genetic structure of plants endemic to granite outcrops is limited. To determine the immediate genetic consequences of recruitment and to quantify mating and dispersal patterns by parentage analysis, I conducted an opportunistic study following a wildfire in a stand of the lignotuberous tree Eucalyptus caesia Benth. (Myrtaceae) at Boyagin Nature Reserve. The entire adult stand (n = 180) plus seedlings (n = 115) were genotyped with 14 microsatellite loci. Compared with adults, seedlings had low heterozygosity and a high inbreeding co-efficient, but similar levels of allelic diversity. There was no evidence of post-germination selection against homozygous seedlings over the 20 month survey period. Seedlings that resulted from cross-pollination were more heterozygous than seedlings resulting from selfing, but could not be distinguished based on height measurements or survivorship. Parentage analysis in CERVUS revealed mostly limited seed dispersal, assuming that candidate parents closest to seedlings were the maternal parent. By comparison, pollen movement was more extensive, yet still restricted to plants within the stand. Recruitment has increased the population size, but appears insufficient to enhance genetic diversity in the Boyagin stand of E. caesia. Nonetheless, E. caesia appears adept at persisting as extremely small populations.