Princeton Writing Program, Princeton University, Princeton, NJ, 08544, USA.
Department of Ecology, Evolution & Natural Resources, Rutgers University, New Brunswick, NJ, 08901, USA.
Am J Bot. 2020 Feb;107(2):339-349. doi: 10.1002/ajb2.1427. Epub 2020 Feb 21.
Plant sex is usually fixed, but in rare cases, sex expression is flexible and may be influenced by environmental factors. Theory links female sex expression to better health, but manipulative work involving the experimental change of health via injury is limited, particularly in sexually plastic species. A better understanding of mechanisms influencing shifts in sex is essential to our understanding of life history theory regarding trade-offs in sex allocation and differential mortality.
We investigated the relationship between physiological stress and sex expression in sexually plastic striped maple trees (Acer pensylvanicum) by inflicting damage of various intensities (crown pruning, defoliation, and hydraulic restriction). We then monitored the sex expression of injured and control individuals for 2 years to assess the extent to which injury may cue changes in sex expression.
We found that severe damage such as full defoliation or severe pruning increased odds of changing sex to female and decreased odds of changing to male. In fact, no pruned male trees flowered male 2 years later, while all males in the control group flowered partially or fully male. After full defoliation, trees had 4.5 times higher odds of flowering female. Not all injury is equal; less-severe physical trauma did not affect the frequency of sex change to femaleness.
This work demonstrates that physical trauma in striped maple appears to exhibit a threshold effect in which only the most stressful of physiological cues instigate changes in sex expression, a phenomenon previously unknown, and that damage stress is strongly correlated with switching to femaleness. These findings have implications for population sex ratios and sustainability within an increasing stressful climate regime.
植物的性别通常是固定的，但在极少数情况下，性别表达是灵活的，可能受到环境因素的影响。理论将雌性性别表达与更好的健康联系起来，但通过伤害来操纵健康的实验性改变的工作是有限的，尤其是在具有性别可塑性的物种中。更好地了解影响性别转变的机制对于我们理解关于性别分配和死亡率差异的生活史理论中的权衡至关重要。
我们通过对具有性别可塑性的条纹枫（Acer pensylvanicum）进行不同强度的伤害（树冠修剪、去叶和水力限制），研究了生理压力与性别表达之间的关系。然后，我们监测了受伤和对照个体的性别表达，持续 2 年，以评估伤害可能在多大程度上提示性别表达的变化。
我们发现，严重的伤害，如完全去叶或严重修剪，增加了变为雌性的可能性，并降低了变为雄性的可能性。事实上，没有修剪的雄性树在两年后没有开花雄性，而对照组中的所有雄性树都部分或完全开花雄性。完全去叶后，树木开花雌性的可能性增加了 4.5 倍。并非所有伤害都是平等的；较轻的身体创伤不会影响向雌性转变的频率。
这项工作表明，条纹枫的物理创伤似乎表现出一种阈值效应，只有最具压力的生理线索才能引发性别表达的变化，这是以前未知的现象，而且伤害压力与向雌性转变强烈相关。这些发现对种群性别比例和在不断增加的压力气候条件下的可持续性具有重要意义。
