The proportion of self-pollinated Hass fruit increases at greater distance from another cultivar Wiebke Kämper (Griffith University), Steven Ogbourne (University of the Sunshine Coast), David Hawkes (Australian Genome Research Facility), Stephen Trueman (Griffith University)
Avocado flowers are female when they first open and male when they open a second time. In theory, flowers of Type A cultivars such as Hass open as female in the morning, close in the late morning, and re-open as male on the afternoon of the next day. Flowers of Type B cultivars such as Shepard open as female in the afternoon, close in the late afternoon, and reopen as male on the following morning. Orchards are established with a mixture of Type A and Type B cultivars to maximise the chance that pollen from male-stage flowers is available for pollination of female-stage flowers. However, the timing of the female and male stages may change depending on climatic conditions such as cool temperatures during flowering. There can often be significant overlap between female and male stages within the same cultivar. Avocado orchards are typically established with each cultivar planted in a wide block, rather than different cultivars being interplanted within the same row or in alternating rows. Wide blocks simplify farm management and may reduce the costs of pest control, disease control and fruit harvesting. However, planting each cultivar in a wide block might reduce the chance that female-stage flowers are pollinated successfully. Avocado fruit can be produced from selfpollination (by pollen from the same cultivar) or crosspollination (by pollen from a different cultivar). Therefore, planting each cultivar in a wide block might also increase the chance that most fruit result from self-pollination. In other crops, such as macadamia, self-pollinated fruit are often smaller and of lower quality than cross-pollinated fruit. We know comparatively little about the effect of selfpollination on avocado fruit. Here, we assessed the contributions of self-pollination and cross-pollination to avocado fruit production in two Queensland orchards. We also determined whether fruit mass, flesh mass, mineral nutrient concentrations and fatty acid composition differed between self-pollinated and cross-pollinated fruit.
Methods We harvested Hass fruit from 32 trees in two orchards near Childers, Queensland. The first orchard contained blocks of Hass and Shepard that were 26 rows wide, with row spacing
TALKING AVOCADOS AUTUMN 2021
of 10-11 m. The trees were 13-years-old. The second orchard contained blocks of Hass, Shepard, Lamb Hass and Wurtz that were 6-22 rows wide, with row spacing of 10m. Trees were 18-21-years-old. The 32 harvested Hass trees were located along eight transects, with each transect consisting of four individual trees at: (a) 1 row, (b) 2 rows, (c) 3 rows, and (d) 11–14 rows from another cultivar. The last sampling point represented the middle of a Hass block. Six fruit per tree were stored at 4°C before being ripened at room temperature for 10-12 days. We measured total fruit mass and flesh mass of each fruit. We analysed mineral nutrient concentrations in the flesh using inductively coupled plasma – atomic emission spectroscopy. We analysed fatty acid composition by gas chromatography – mass spectrometry. We determined the pollen parent of each fruit by extracting DNA from the seed and performing MassARRAY genotyping using methods that we developed for avocado.
Results A total of 52% of Hass fruit arose from self-pollination and 48% of Hass fruit arose from cross-pollination. Almost all (95%) of the cross-pollinated fruit were pollinated by Shepard. The percentage of self-pollinated fruit increased with increasing distance from another cultivar (Figure 1). Approximately 37% of Hass fruit were self-pollinated and 63% were cross-pollinated in the row next to another cultivar. In contrast, 75% of fruit were self-pollinated and 25% were crosspollinated in the middle of the Hass blocks, i.e. at 11–14 rows from another cultivar. Hass fruit that were self-pollinated by Hass did not differ significantly in total fruit mass or flesh mass from Hass fruit that were pollinated by Shepard (Table 1). The flesh of cross-pollinated fruit had 10% higher calcium and 11% lower phosphorus concentration than self-pollinated fruit (Table 2). Other mineral nutrient concentrations did not differ significantly (data not presented). Self-pollinated and cross-pollinated fruit did not differ significantly in the relative contributions of palmitic, palmitoleic, stearic, oleic, elaidic or linoleic acid to their total fatty acid composition (Table 2).
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