Making a clean break: contrasting leaf abscission dynamics across temperate leaf habits

Leaf abscission is the process whereby plants actively shed leaves through physical detachment at the abscission zone (AZ). Leaf abscission is generally preceded by senescence, during which there is an active reclamation of leaf nutrients. The physiological regulation of leaf abscission remains poorly studied in trees, with a suite of environmental and endogenous signals believed to regulate the process. Here, we sought to characterize the role of water status, leaf gas exchange, senescence and the phytohormone abscisic acid (ABA) in regulating leaf abscission in temperate trees.

We developed a novel method to quantify AZ competency (AZC) and simultaneously measured leaf gas exchange, chlorophyll content, water potential, AZC and ABA levels from late summer until leaf death in four temperate tree species representing deciduous, brevi-deciduous, and marcescent leaf habits. We tested for associations between changes in key physiological traits and AZC in all species.

The two deciduous species showed contrasting physiological patterns leading to leaf abscission: one species degraded chlorophyll and ceased photosynthesis before complete AZC, while the other retained chlorophyll and continued photosynthesis until complete AZC. The brevi-deciduous species degraded most chlorophyll but developed AZC gradually over a longer period. The marcescent species' leaves fully senesced but did not develop AZC.

These findings demonstrate that leaf senescence and abscission are distinct and variably timed processes across temperate tree species. These results have implications for predicting future leaf lifespan as the climate changes, with the characterization of physiological diversity in the regulation of leaf abscission profoundly understudied.