The combination of a novel pollen counting methodology and light microscopy reveal pollen abnormalities

ISHS Secretariat
The combination of a novel pollen counting methodology and light microscopy reveal pollen abnormalities

I am Michelle Stanton. A short time ago, I completed the requirements to receive an MSc (Agric) in Horticultural Sciences in the Department of Plant and Soil Sciences at the University of Pretoria, South Africa, under the supervision of Professor Elsa S. du Toit. My dissertation explored pollination as it related to the low-yielding nature of avocado trees. Within the study, my team developed a novel pollen counting methodology. Light microscopy revealed pollen abnormalities in cold stressed avocado (Persea americana Mill.) flowers cultivated in subtropical climates. This under-researched facet of cold stress affected pollen health and development in avocado trees cultivated in subtropical regions. I used the novel pollen counting method to counteract the problem of inconsistent cutting depths encountered with semithin sectioning preparations for light microscopy. Anthers were prepared for light microscopy using a series of ethanol dehydrations and subsequent resin embedding. Problems with the anthers were displayed as absent and/or empty compartments. While many pollen grains appeared to be healthy, some were malformed, and a few were observed as empty shells. P. americana ‘Fuerte’ anthers appeared to be more adept at maintaining pollen health during cold, while P. americana ‘Hass’ performed better during warm periods. The influence of a warm-front just prior to anther collection in one of the sampling years may have improved the overall pollen health. This suggested that pollen health depends on sufficiently warm temperatures late in the flower development process if the pollen is to mature properly. A slight difference in the amount of pollen was detected in the anthers of flowers sampled in the male and female phase. This suggests that the overnight closure of these dichogamous flowers is physiologically important and potentially sensitive to cold temperatures. Furthermore, analysis of the temperature trends for the sampling area showed that the avocado trees were regularly exposed to sub -15°C temperatures during the period of meiosis, a critical level in tropical plants. These findings suggested that avocados grown in subtropical and temperate areas, which experience low temperatures during and after the period of meiosis, may result in less pollen and a higher degree of abnormalities being produced. This further compounded the problem of low pollination in commercial avocado orchards, and fruit yield was reduced. Throughout the past three years, I have gained valuable knowledge and indispensable experience, for which I am very grateful. It is an honour and blessing to have this work acknowledged by the International Society for Horticultural Sciences. Thank you!

Michelle Stanton won the ISHS Young Minds Award for the best oral presentation at the I International Symposium on Reproductive Biology of Fruit Tree Species, which was held virtually in France in November 2021.

Michelle Stanton, Department of Plant and Soil Sciences, University of Pretoria, Private Bag X20, Hatfield, Pretoria, 0028, South Africa, e-mail:

The article is available in Chronica Horticulturae

pollen counting
light microscopy
pollen abnormalities
Young Minds Award Winners