Key factors influencing forager distribution across macadamia orchards differ among species of managed bees

Combined analysis of multi-omics reveals the potential mechanism of flower color and aroma formation in Macadamia integrifolia

INTRODUCTION

Macadamia integrifolia Maiden & Betche is a domesticated high-value nut crop. The development of nut flower affects the fruit setting rate, yield and quality of nuts. Therefore, in this experiment, two varieties with different flower color, flowering time, flowering quantity and nut yield (single fruit weight) were selected as the research objects.

METHODS

Transcriptome (RNA-Seq) and metabolome (LC-MS/MS, GC-MS) analyses were performed to study the regulatory mechanisms of nut flower development, color and aroma.

RESULTS

The results indicated that plant hormone signal transduction, starch sucrose metabolism, phenylpropanoid metabolism, flavonoid biosynthesis, and anthocyanin biosynthesis pathways were related to nut flower development and flower color formation. In the early stage of flowering, most of the differentially expressed genes (DEGs) are involved in the IAA signal transduction pathway, while in the later stage, the brassinolide signal pathway is mainly involved. In starch and sugar metabolism, DEGs are mainly involved in regulating and hydrolyzing stored starch into small molecular sugars in flower tissues. In the phenylpropanoid biosynthesis pathway, DEGs are mainly related to the color and aroma (volatile organic compounds, VOCs) formation of nut flowers. Four color formation metabolites (anthocyanins) in nut flowers were found by LC-MS/MS detection. In addition, the VOCs showed no significant difference between red nut flowers (R) and white nut flowers (W), which was mainly reflected in the aroma formation stage (flowering time). And 12 common differentially accumulation metabolites (DAMs) were detected by GC-MS and LC-MS/MS. At the same time, the DEGs, AAT, LOX and PAL genes, were also identified to regulate key metabolite synthesis during nut flower development. These genes were further verified by qRT-PCR.

CONCLUSION

Our results provide insights to clarify the molecular mechanism of color and aroma formation during M. integrifolia flower development that pave the way for nut quality and yield breeding.

Biology, Genetic Diversity, and Conservation of Wild Bees in Tree Fruit Orchards

Different species of bees provide essential ecosystem services by pollinating various agricultural crops, including tree fruits. Many fruits and nuts depend on insect pollination, primarily by wild and managed bees. In different geographical regions where orchard crops are grown, fruit growers rely on wild bees in the farmscape and use orchard bees as alternative pollinators. Orchard crops such as apples, pears, plums, apricots, etc., are mass-flowering crops and attract many different bee species during their bloom period. Many bee species found in orchards emerge from overwintering as the fruit trees start flowering in spring, and the active duration of these bees aligns very closely with the blooming time of fruit trees. In addition, most of the bees in orchards are short-range foragers and tend to stay close to the fruit crops. However, the importance of orchard bee communities is not well understood, and many challenges in maintaining their populations remain. This comprehensive review paper summarizes the different types of bees commonly found in tree fruit orchards in the fruit-growing regions of the United States, their bio-ecology, and genetic diversity. Additionally, recommendations for the management of orchard bees, different strategies for protecting them from multiple stressors, and providing suitable on-farm nesting and floral resource habitats for propagation and conservation are discussed.

Pollen limitation and xenia effects in a cultivated mass-flowering tree, Macadamia integrifolia (Proteaceae)

BACKGROUND AND AIMS

Pollen limitation is most prevalent among bee-pollinated plants, self-incompatible plants and tropical plants. However, we have very little understanding of the extent to which pollen limitation affects fruit set in mass-flowering trees despite tree crops accounting for at least 600 million tons of the 9200 million tons of annual global food production.

METHODS

We determined the extent of pollen limitation in a bee-pollinated, partially self-incompatible, subtropical tree by hand cross-pollinating the majority of flowers on mass-flowering macadamia (Macadamia integrifolia) trees that produce about 200 000-400 000 flowers. We measured tree yield and kernel quality and estimated final fruit set. We genotyped individual kernels by MassARRAY to determine levels of outcrossing in orchards and assess paternity effects on nut quality.

KEY RESULTS

Macadamia trees were pollen-limited. Supplementary cross-pollination increased nut-in-shell yield, kernel yield and fruit set by as much as 97, 109 and 92 %, respectively. The extent of pollen limitation depended upon the proximity of experimental trees to trees of another cultivar because macadamia trees were highly outcrossing. Between 84 and 100 % of fruit arose from cross-pollination, even at 200 m (25 rows) from orchard blocks of another cultivar. Large variations in nut-in-shell mass, kernel mass, kernel recovery and kernel oil concentration were related to differences in fruit paternity, including between self-pollinated and cross-pollinated fruit, thus demonstrating pollen-parent effects on fruit quality (i.e. xenia).

CONCLUSIONS

This study is the first to demonstrate pollen limitation in a mass-flowering tree. Improved pollination led to increased kernel yield of 0.31-0.59 tons ha-1, which equates currently to higher farm-gate income of approximately $US3720-$US7080 ha-1. The heavy reliance of macadamia flowers on cross-pollination and the strong xenia effects on kernel mass demonstrate the high value that pollination services can provide to food production.