PHYSIOLOGY OF GLOSSOPTERIS, A CIRCUMPOLAR PERMIAN SEED PLANT, AND IMPLICATIONS FOR THE TERRESTRIAL RECORD OF THE PERMIAN-TRIASSIC MASS EXTINCTION

The Permian-Triassic mass extinction is the largest in Earth history. A conspicuous casualty during this event was the woody, broad-leaved seed plant Glossopteris. Glossopteris plants grew throughout the Southern Hemisphere during the Permian Period, reaching their apex during Late Permian time at high paleolatitudes (≥80°S).

We built and employed fluid dynamics models of the hydraulic resistance of Glossopteris plants’ vascular anatomy to reconstruct its physiology. To constrain the model, we measured more than 10,000 individual xylem cells from Glossopteris roots, stems, branches, and leaves from South Africa, Australia, central Africa, and Antarctica. Using these observations, we calculated the hydraulic conductivity and environmental tolerances of individual organs of the plant, along with the maximal photosynthetic rate of the leaves.

Our results show that Glossopteris xylem is among the lowest-conductive water transport tissue to have evolved in vascular plants. Tracheids were narrow, thick-walled, and nonporous, compared with other seed plants’ xylem. In our dataset, rare large tracheids were observed in stems (large relative to gymnosperms; ≤55μm in diameter), and these are found within the earlywood of growth rings—a common adaptation for woody plants growing in temperate or polar environments. Maximal photosynthetic rates based on observations of leaf architecture are comparable with other gymnosperms.

Glossopteris’ physiology reveals key attributes that contributed to its extinction. This plant was adapted to strong seasonality and, at its extreme geographic range, polar darkness. Roots and leaves were adapted for a life cycle of rapid growth and photosynthetic productivity in the austral spring, and extreme resistance to frost damage in the winter, with the cost of hydraulic limits at high temperatures. Given current estimates of the duration and magnitude of climate warming at the P-T boundary, increased winter and summer temperatures would have generated severe dark respiration and limited summer photosynthetic productivity, respectively. These observations also help explain the differential survivorship of plants and invertebrates during the P-T extinction, and are consistent with long-term kill mechanisms related to solid Earth carbon dioxide fluxes and climate change.