Seed size effects on plant establishment under low atmospheric CO(2), with implications for seed size evolution

BACKGROUND AND AIMS: Low atmospheric CO(2) concentration depresses photosynthesis and resource use efficiency, and therefore can inhibit phases of the life cycle such as seedling establishment. Seed reserves can compensate for photosynthetic inhibition by accelerating seedling growth. We therefore hypothesize that seedlings arising from large seeds show less inhibition from low atmospheric CO(2) than young plants from small seeds. Seed size effects on seedling responses to low CO(2) may also be enhanced in warm environments, due to greater photorespiration at high temperature. METHODS: Phaseolus and Vigna seeds differing in mass by over two orders of magnitude were planted and grown for 14 d in growth chambers with CO(2) concentrations of 370, 180 or 100 ppm, in thermal regimes of 25 °C/19 °C, 30 °C/24 °C or 35 °C/29 °C (day/night). We measured leaf area expansion, shoot growth and mortality of the seedlings arising from the variously sized seeds at 14 days after planting (14 DAP). KEY RESULTS: Relative to small-seeded plants, large-seeded genotypes produced greater leaf area and shoot mass at 14 DAP across the range of CO(2) treatments in the 25 °C/19 °C and 30 °C/24 °C regimes, and at 100 ppm in the 35 °C/29 °C treatment. The proportional decline in leaf area and seed mass with CO(2) reduction was generally greater for seedlings arising from small than from large seeds. Reductions in leaf area due to CO(2) reduction increased in the warmer temperature treatments. In the 35 °C/19 °C treatment at 100 ppm CO(2), seedling mortality was greater in small- than in large-seeded genotypes, and the small-seeded genotypes were unable to exit the seedling stage by the end of the experiment. CONCLUSIONS: The results support a hypothesis that seedlings from large seeds grow and establish better than seedlings from small seeds in warm, low CO(2) environments. During low CO(2) episodes in Earth’s history, such as the past 30 million years, large seeds may have been favoured by natural selection in warm environments. With the recent rise in atmospheric CO(2) due to human activities, trade-offs between seed size and number may already be affected, such that seed size today may be non-optimal in their natural habitats.