Plant CO₂ responses in the long term: plants from CO₂ springs in Florida and tombs in Egypt

Summary

SUMMARY

Seeds were collected from populations of Boehmeria cylindrica growing in naturally enriched concentrations of CO₂. In the controlled environment plants grown from populations which are normally exposed to enriched atmospheres of CO₂ (400 to 575 ppmv) show a greater CO₂-sensitivity of growth than plants from ambient (370 ppmv) sites. Observed differences in height growth and dry weight partitioning could be explained in terms of the different rates of plant growth under the imposed treatments, rather than in terms of population differences in growth partitioning. Comparisons of stomatal density-CO₂ responses for species from different climates and over the different time scales of centuries and millennia indicate a similar decline in density with increasing CO₂ concentrations, with no evidence of any changes in the CO₂ sensitivity. However, the clear species-specific nature of the stomatal density response suggests it is unlikely to be observed ubiquitously in plants growing by high-CO₂ springs. Nevertheless, there is a close similarity between the stomatal density responses shown by plants growing under CO₂ enrichment adjacent to springs, and the stomatal density changes recorded from fossil leaves, indicating one valuable use of high-CO₂ springs in biological research.

INTRODUCTION

Measurements of the atmospheric concentration of CO₂, typically from air locked up in permanent ice caps, provide time courses for a continuously varying CO₂ concentration over the last 160,000 years (Barnola et al., 1987). Estimates of CO₂ by indirect means also show variations of CO₂ concentration over millions of years (Hays, Imbrie & Shackleton, 1976; Berner, 1990).

An interesting consequence of these variations is the potential for a CO₂-controlled selection in higher plants.