Degradation and resynthesis of chlorophyll during increased oxidative stress and prolonged darkness differ between annual and perennial flax (Linum L.)

Among plants, there is considerable variation in lifespan: annuals live less than one year, whereas perennials live for several years, with the longest-living perennial having survived 43,600 years. As proposed by the Disposable Soma Theory, this lifespan variation among plants likely reflects differential investment of limited energy and nutrient resources, with perennials investing more energy and nutrients into biomolecular maintenance compared to annuals in order to ensure persistence over multiple seasons. Such differential investment may be particularly important during periods of exogenous stress, which are known to accelerate biomolecular damage. The present study evaluated this hypothesis using annual and perennial flax (Linum L.) subjected to two exogenous stressors—increased oxidative stress (i.e., foliar H2O2 spraying) and complete prolonged darkness. As chlorophyll has been shown to exhibit degradation in response to changes in environmental conditions, we utilized changes in chlorophyll levels during and after periods of exogenous stress to evaluate our hypotheses. We predicted that i) perennials would exhibit a slower rate of chlorophyll degradation during exposure to exogenous stressors compared to annuals, and ii) perennials would exhibit a faster rate of chlorophyll resynthesis following such exposure compared to annuals. Chlorophyll levels before, during, and after exposure to both exogenous stressors were measured in two separate trails, once using image colour analysis and once using spectrophotometry. While chlorophyll degradation rates in response to oxidative stress did not differ between annuals and perennials, contrary to our predictions, chlorophyll resynthesis rates following such exposure were significantly higher in perennials, as predicted. When plants were subjected to complete prolonged darkness, chlorophyll degradation rates were significantly lower in perennials than annuals, as predicted; however, when plants were subsequently reintroduced to natural photoperiod, chlorophyll resynthesis rates did not consistently differ between annuals and perennials, though they tended to be higher in the latter, as predicted. Overall, our study illuminates that evolutionary transitions between life history strategies in plants have been accompanied by physiological modifications to chlorophyll dynamics that permit perennial species to better maintain chlorophyll levels—and thus photosynthetic energy acquisition—in the face of exogenous stressors, which likely underlies their capacity to survive for multiple growing seasons. Future studies should explore whether other key biomolecules (e.g., proteins, DNA) are also better maintained in perennial plants, especially in the face of exogenous stress.