Impact and causes of non-pollination-drop of walnut flowers (Juglans regia) in a semi-arid climate of Australia.

Emily Thomas

Research output: ThesisDoctoral Thesis

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In Australia, a majority (85 %) of walnut (Juglans regia) production occurs in growing regions with a semi-arid climate. The production of orchards in these growing regions is limited by non-pollination-drop (NPD). NPD is a premature fruit drop linked to unsuccessful pollination or fertilisation processes. Environmental conditions, including high temperature and low relative humidity, were previously observed as possible key factors in NPD incidence rates, however, the relationships between NPD and climatic variables, and physiological mechanisms were not well understood. This thesis has investigated the impact of semi-arid climatic conditions on pollen viability, flower receptivity and pollen availability to understand causes of NPD. The hypothesis is that hot dry weather conditions reduce pollen availability, pollen viability and pistillate flower receptivity and that climate is the primary driver of NPD under semi-arid climate growing conditions. To test this hypothesis, data was collected to describe NPD rates, bloom overlap, pollen viability and climate conditions in five walnut cultivars over two years. To assess the impact of climatic variables on pollen shedding, receptivity, pollen moisture content and viability of staminate inflorescences, pistillate inflorescences and pollen were subjected to four temperature treatments, four relative humidity treatments and six vapour pressure deficit treatments. Finally, to understand why moisture loss leads to a loss of pollen viability, the molecular mechanisms that drive pollen moisture and viability loss under environmental extremes were investigated.
NPD reduced walnut yield by up to 36 % and occurs when there is a lack of overlap between pollen shedding and receptivity and pollen viability is low. Pollen viability and bloom overlap are reduced under hot dry conditions during flowering, specifically temperatures greater than 35 °C, relative humidity lower than 20 % and vapour pressure deficit above 4 kPa. Later flowering cultivars are more affected by NPD because the timing of flowering is more likely to coincide with hot dry weather conditions which reduces pistillate flower receptivity and pollen viability. At an individual pistillate flower level, hot dry weather reduces the duration of optimal receptivity and stigmatic receptivity, limiting the window of opportunity for pollination to occur and the capacity of the pistillate flower to support pollen functions. In individual catkins pollen availability was not influenced by hot dry weather.
The viability of freshly shed pollen under semi-arid conditions is low (15 to 60 %) and hot dry weather during the dispersal period desiccates pollen causing further viability loss. Moisture loss appears to impair the integrity of the phospholipid membrane, compromising the ability of pollen grains to build osmotic pressure, an essential prerequisite to germination. Walnut pollen accumulates sucrose in response to early desiccation and this may be a physiochemical defence strategy to maintain pollen viability.
In semi-arid climate growing regions, hot dry conditions frequently occur during bloom reducing pistillate flower receptivity, pollen viability and the productivity of walnut trees.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Charles Sturt University
  • Steel, Christopher, Principal Supervisor
  • Lang, Michael, Co-Supervisor, External person
  • Simpson, Jacquelyn, Co-Supervisor, External person
  • Holzapfel, Bruno, Co-Supervisor, External person
Place of PublicationWagga Wagga, NSW
Publication statusPublished - 2021


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