Trade-offs among growth, reproduction and defence in dioecious juniper

Plants often face challenges in sharing nutrient resources among the competing demands of growth, reproduction, and chemical defence. This is particularly evident in dioecious species due to the difference in reproductive investment between males and females. A dioecious female plant invests more in reproduction than a male because, despite both males and females producing reproductive organs with gametes, a female must also produce fruits with seeds and zygotes inside. Previous studies focussing on resource allocation trade-offs in dioecious species have paid close attention to deciduous phenolic producing species, such as the Salix and Populus genera, whilst dioecious terpen-producing evergreens, such as junipers, have been underrepresented.

In their new study published in AoBP, Markó et al. provide a unique insight into growth-defence resource allocation conflicts in a less studied and long-lived dioecious conifer species, the common juniper (Juniperus communis). The common juniper is an evergreen, dioecious conifer shrub, widely distributed in the Northern Hemisphere occurring across various environmental gradients. In junipers, similarly to other pines, the chemical defence is primarily based on terpenoids, represented mostly by essential oils. The essential oil yield can be a good proxy to describe the actual degree of chemical defence.

Markó et al. tested correlations between the essential oil yield with other individual-specific traits (e.g. sex, age), the presence of the growth-defence trade-off, and the differential growth and survival patterns between males and females through an extensive field survey in three natural populations in the Kiskunság National Park, Hungary. They observed sex-specific differences in growth and essential oil accumulation due to conflict between growth and chemical defence, resulting in male-biased populations.

The authors conclude that dioecious plants can respond adaptively to environmental challenges via flexible growth-defence mechanisms. The study revealed a contrasting and unique essential oil accumulation driven by the complex allocation trade-off mechanisms within individuals, which could be a flexible and adaptive defence response against the increasing biotic and abiotic environmental stresses exacerbated under global climate change.