Wiebke Kämper

Background: Climate change increases the risk of late spring frosts, which damage flowers and developing fruitlets in tree crops. Ethephon, a plant growth regulator, may delay flowering and potentially reduce frost damage. Methods: We investigated the effects of ethephon on flowering time and fruit set in two sweet cherry (Bellise and Kordia) and two apple cultivars (Elstar and Kanzi). Treatments included two ethephon concentrations (500 and 750 ppm) applied either in early or late autumn. Results: Ethephon significantly delayed flowering in cherry cultivar Kordia, but not cultivar Bellise, with a lower proportion of Kordia floral buds in bloom in two of four treatments compared to controls. Specifically, on 9 April 88% of floral buds on control trees were in bloom, compared to 56% and 63% on trees treated with high ethephon in early autumn and low ethephon in late autumn, respectively. No significant flowering delay was observed in the apple cultivars. Initial and final fruit set were generally unaffected, although Kordia showed a reduction in final fruit set from 16 to 6% after the late-high treatment. Conclusions: Our results indicate that ethephon can effectively delay bloom without major yield loss, offering a practical tool to mitigate spring frost damage under climate change. However, its effectiveness was crop- and cultivar-dependent.

Pollen limitation occurs when insufficient pollen or the wrong pollen genotype reaches the stigma. The pollination efficiency of flower visitors to crops has been investigated, but the genotypes of crop pollen carried have rarely been identified. We developed a method that detects SNPs in the pollen carried by single bees, using a customized single allele base extension reaction (SABER) with MassARRAY to distinguish genotypes that contribute only a small fraction to a mixed-genotype pollen sample. We used this method to identify the cultivars of pollen carried by honeybees at increasing distances from a cross-pollen source in two multi-cultivar macadamia orchards, one with wide single-cultivar blocks and one with narrow single-cultivar blocks. We found that many honeybees carried exclusively self-pollen. Only 30–53% of honeybees carried cross-pollen, representing the maximum that potentially contributes to crop production in self-incompatible crops. Distance from a cross-pollen source or the orchard design did not significantly affect the percentage of honeybees carrying cross-pollen. This study demonstrates significant potential to increase the effectiveness of honeybees as pollinators. Orchards can be re-designed to interplant cross-pollen sources and maximize the number of honeybees contributing to crop production. Improving pollination effectiveness will help to alleviate the growing shortfall in the supply of beehives required for crop pollination.

Pollen limitation occurs when insufficient pollen or the wrong pollen genotype reaches the stigma, resulting in failure to fertilise ovules and trigger seed development. The pollination efficiency of flower visitors to crops has been investigated previously, assessing their on-flower behaviour or amount of pollen deposited during a single flower visit. However, the genotypes of crop pollen carried by flower visitors have rarely been identified even though many crops depend on, or benefit from, cross-pollination for seed production. Here, we developed a genotyping method that can detect both high-abundance and low-abundance single nucleotide polymorphisms (SNPs) in the DNA of pollen carried by a single bee. This method uses a customised single allele base extension reaction (SABER) with MassARRAY detection to distinguish genotypes that contribute only a small fraction to a mixed-genotype pollen sample. We used this method to identify the cultivars of pollen carried by honeybees at increasing distances from a cross-pollen source. We assessed the percentage of honeybees that carried cross-pollen in two different multi-cultivar macadamia orchards, one where each single-cultivar block was 5 rows wide and another where each single-cultivar block was more than 40 rows wide. This percentage of honeybees represents the maximum that potentially contribute to cross-pollination and seed production in self-incompatible crops, such as macadamia. Many honeybees carried exclusively self-pollen, with only 30–53 % of honeybees carrying cross-pollen. Distance from a cross-pollen source did not significantly affect the percentage of honeybees carrying cross-pollen in either orchard. This study demonstrates significant potential to increase the effectiveness of honeybees as pollinators. Orchards can be re-designed to interplant cross-pollen sources and maximise the number of honeybees contributing to crop production. Improving pollination effectiveness will help to alleviate the growing shortfall in the supply of beehives required for crop pollination.

Carbohydrate levels are important regulators of the growth and yield of tree crops. Current methods for measuring foliar carbohydrate concentrations are time consuming and laborious, but rapid imaging technologies have emerged with the potential to improve the effectiveness of tree nutrient management. Carbohydrate concentrations were predicted using hyperspectral imaging (400–1000 nm) of leaves of the evergreen tree crops, avocado, and macadamia. Models were developed using partial least squares regression (PLSR) and artificial neural network (ANN) algorithms to predict carbohydrate concentrations. PLSR models had R² values of 0.51, 0.82, 0.86, and 0.85, and ANN models had R² values of 0.83, 0.83, 0.78, and 0.86, in predicting starch, sucrose, glucose, and fructose concentrations, respectively, in avocado leaves. PLSR models had R² values of 0.60, 0.64, 0.91, and 0.95, and ANN models had R² values of 0.67, 0.82, 0.98, and 0.98, in predicting the same concentrations, respectively, in macadamia leaves. ANN only outperformed PLSR when predicting starch concentrations in avocado leaves and sucrose concentrations in macadamia leaves. Performance differences were possibly associated with nonlinear relationships between carbohydrate concentrations and reflectance values. This study demonstrates that PLSR and ANN models perform well in predicting carbohydrate concentrations in evergreen tree-crop leaves.

Yield in many crops is affected by abscission during the early stages of fruitlet development. The reasons for fruitlet abscission are often unclear but they may include genetic factors because, in some crops, self-pollinated fruitlets are more likely to abscise than cross-pollinated fruitlets. Pollen parentage can also affect final fruit size and fruit quality. Here, we aimed to understand the effects of pollen parentage on fruitlet retention and nut quality in orchards of macadamia (Macadamia integrifolia Maiden & Betche). We identified the pollen parent of macadamia ‘cultivar ‘816’ embryos by analysing single nucleotide polymorphisms (SNPs) in their DNA using customised MassARRAY and Single Allele Base Extension Reaction (SABER) methods. This allowed us to determine the proportions of self-fertilised and cross-fertilised progeny during premature fruit drop at 6 weeks and 10 weeks after peak anthesis, as well as at nut maturity. We determined how pollen parentage affected nut-in-shell (NIS) mass, kernel mass, kernel recovery, and oil concentration. Macadamia trees retained cross-fertilised fruitlets rather than self-fertilised fruitlets. The percentage of progeny that were cross-fertilised increased from 6% at 6 weeks after peak anthesis to 97% at nut maturity, with each tree producing on average 22 self-fertilised nuts and 881 cross-fertilised nuts. Three of the four cross-pollen parents provided fruit with significantly higher NIS mass, kernel mass, or kernel recovery than the few remaining self-fertilised fruit. Fruit that were cross-fertilised by ‘842’, ‘A4’, or ‘A203’ had 16–29% higher NIS mass and 24–44% higher kernel mass than self-fertilised fruit. Nuts that were cross-fertilised by ‘A4’ or ‘A203’ also had 5% or 6% higher kernel recovery, worth approximately $US460–540 more per ton for growers than self-fertilised nuts. The highly selective abscission of self-fertilised fruitlets and the lower nut quality of self-fertilised fruit highlight the critical importance of cross-pollination for macadamia productivity.

The productivity of many tree crops is limited by low yield, partly due to high rates of fruitlet abscission during early fruit development. Early studies suggested that cross-pollinated fruitlets may be selectively retained during fruit development, although paternity testing of fruitlets to test this hypothesis was technically challenging. We used MassARRAY genotyping to determine the effects of pollen parentage on fruitlet retention and fruit quality of Hass avocado. We identified the paternity of abscised and retained fruitlets at 6 and 10 weeks, and mature fruit at 36 weeks, after peak anthesis. We measured the embryo mass, pericarp mass, total mass and nutrient concentrations of fruitlets, and the seed mass, flesh mass, total mass, diameter, length, nutrient concentrations and fatty-acid composition of mature fruit. The percentages of progeny on the tree that were cross-fertilised increased from 4.6% at 6 weeks after peak anthesis to 10.7% at fruit maturity. Only 1.0% of freshly abscised fruitlets on the ground at 10 weeks after peak anthesis were cross-pollinated even though 6.5% of retained fruitlets on the tree were cross-pollinated. At this stage, cross-pollinated fruitlets had similar nutrient concentrations to self-pollinated fruitlets, but they had higher total contents of P, K, Al, Ca, Fe, Mn and Zn due to having greater fruitlet mass. At maturity, cross-pollinated fruit were 6% heavier and had 2% greater diameter than self-pollinated fruit, without significant differences in flesh nutrient concentrations or fatty acid composition. The results demonstrate that Hass avocado trees selectively retain cross-pollinated fruitlets, which are larger than self-pollinated fruitlets and ultimately produce larger mature fruit. Avocado growers can increase fruit size and yield by improving the opportunities for cross-pollination, possibly by closely interplanting type A and type B cultivars and introducing more beehives into orchards.

Global fruit and nut yields are affected by shortfalls in pollinator populations, and pollen limitation is most prevalent among tropical, bee-pollinated and self-incompatible plants. Macadamia is a subtropical, bee-pollinated crop in which some cultivars have been found to be highly outcrossing. We aimed to determine the extent of outcrossing and its effects on nut quality across a wide range of international macadamia cultivars in three countries. We sampled fruit from 19 macadamia cultivars across 23 sites in Australia, Brazil and South Africa. We used genotype-by-sequencing and MassARRAY methods to assign paternity to individual fruit and we assessed pollen-parent effects on nut quality. Macadamia was highly outcrossing, producing 80–100% of fruit by cross-pollination, at 17 of the 23 sites. Mixed mating (41–72% outcrossing) was identified at five sites, and low outcrossing (10%) was identified in one cultivar at one site where it was isolated from other flowering macadamia trees. Outcrossed fruit often had significantly better quality than selfed fruit, with 1.61–3.39 g higher nut-in-shell mass, 0.53–1.55 g higher kernel mass, 3.3–6.4% higher kernel recovery, and 3.0–3.5% higher oil concentration. The differences in kernel recovery equated to differences in value of USD 433–841 per ton of nut-in-shell at prices of USD 3000 per ton. In summary, macadamia cultivars were mostly highly outcrossing, and outcrossed nuts often had higher quality than selfed nuts. Growers should consider interplanting different cultivars more closely and distributing bee hives more widely to maximise cross-pollination, produce high yields, and optimise nut quality.

Pollination is essential for the reproductive output of crops. Anthropogenic disturbance and global pollinator decline limit pollination success, reducing the quantity or quality of pollen. Relationships between the abundance of flower visitors and fruit production are often poorly understood. We aimed to (1) identify and quantify flower visitors in a mango orchard; (2) assess how much of the crop resulted from self- versus cross-pollination at increasing distances from a cross-pollen source in large, single-cultivar blocks of the cultivar Kensington Pride or the cultivar Calypso; and (3) determine how pollen parentage affected the size, colour, flavour attributes, and nutritional quality of fruit. Mango flowers were mostly visited by rhiniid flies and coccinellid beetles. Approximately 30% of the fruit were the result of cross-pollination, with the percentage significantly decreasing with an increasing distance from a cross-pollen source in the cultivar Calypso. Self-pollinated Calypso fruit were slightly larger and heavier, with higher acid and total polyphenol concentrations than cross-pollinated fruit. Our results showed higher-than-expected levels of cross-fertilisation among fruit, although self-pollen was likely more abundant than cross-pollen in the large orchard blocks. Our results suggest the preferential cross-fertilisation of flowers or the preferential retention of cross-fertilised fruitlets, both representing strategies for circumventing inbreeding depression. Growers should establish vegetated habitats to support pollinator populations and interplant cultivars more closely to maximise cross-pollen transfer.