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theses

Plant sex is expressed via flowers and is usually a fixed characteristic present throughout a plant’s reproductive lifespan. In rare cases, sex expression is flexible and may be influenced by size or environmental factors. A better understanding of the mechanisms that influence shifts in sex is essential to our understanding of life history theory regarding trade-offs in sex allocation, sex ratios, and differential mortality. My objective is to elucidate how complex environmental signals and individual stress affect labile sex determination in plants, using striped maple (Acer pensylvanicum; Sapindaceae) as a study system. Striped maple is often considered an example of environmental sex determination (ESD), although doubt has been raised about the extent of its plasticity and whether it should truly be classified has exhibiting environmental sex determination. The timing of the changes in sex expression and the correlations with environmental cues remain unexplored.

During the course of my research I investigated sex ratios and mortality, the patterns of sex expression and flowering, the correlates of sex expression, and the impact of trauma on sex expression in striped maple. I found that populations were highly male-skewed and female mortality greatly exceeded male mortality. During any two-year period, approximately two-thirds of trees will not change sex; however over half of trees changed sex during the 2014-2017 time period. Of trees that changed sex expression, 25% changed sex at least twice. Even the relatively small percentage of trees that had both male and female flowers in consecutive years could change their percentage of female flowering by up to 95%. In contrast to general theory predicting femaleness for trees at larger sizes and in better condition, I found that femaleness in this species did not depend on size and correlated instead with reduced health. When assessing the relationship between sex expression and non-structural carbohydrates (NSC, stored sugar resources) I found that changes from male to female correlated with higher NSC concentration than trees remaining male. Larger trees did not have larger NSC concentrations. Furthermore, female trees that were dying did not draw down NSCs as they approached death. In manipulative experiments I found that severe damage such as full defoliation or pruning increased odds of changing to female, while less severe physical trauma did not have an effect. Striped maple trees have the potential to change sex expression within three weeks of flowering, with the excising of branches being the instigating cue for sex change in that branch. This work demonstrates that striped maple does exhibit ESD and responds to cues in manners and at timescales previously unknown.

ETD_9073

doi:10.7282/T3TH8RBB

Ph.D.

Includes bibliographical references

by Jennifer Blake-Mahmud

ETD doctoral

The author owns the copyright to this work.