Carbon dioxide enrichment does not reduce leaf longevity or alter accumulation of carbon reserves in the woodland spring ephemeral Erythronium americanum

Abstract

Background and aims: Woodland spring ephemerals exhibit a relatively short epigeous growth period prior to canopy closure. However, it has been suggested that leaf senescence is induced by a reduction in the carbohydrate sink demand, rather than by changes in light availability. To ascertain whether a potentially higher net carbon (C) assimilation rate could shorten leaf lifespan due to an accelerated rate of storage, Erythronium americanum plants were grown under ambient (400 ppm) and elevated (1100 ppm) CO2 concentrations.

Methods: During this growth-chamber experiment, plant biomass, bulb starch concentration and cell size, leaf phenology, gas exchange rates and nutrient concentrations were monitored.

Key results: Plants grown at 1100 ppm CO2 had greater net C assimilation rates than those grown at 400 ppm CO2. However, plant size, final bulb mass, bulb filling rate and timing of leaf senescence did not differ.

Conclusions: Erythronium americanum fixed more C under elevated than under ambient CO2 conditions, but produced plants of similar size. The similar bulb growth rates under both CO2 concentrations suggest that the bulb filling rate is dependant on bulb cell elongation rate, rather than on C availability. Elevated CO2 stimulated leaf and bulb respiratory rates; this might reduce feed-back inhibition of photosynthesis and avoid inducing premature leaf senescence.

Fig. 1.

Changes in Erythronium americanum bulb biomass during the epigeous growth period, in plants grown at ambient and elevated [CO₂] in (A) 2004 and (B) 2005. Means ± s.e. are shown (n = 7 in 2004 and n = 6 in 2005). The arrows indicate the onset of leaf yellowing.

Fig. 2.

Changes in Erythronium americanum total leaf biomass (A, D), total root biomass (C), and total leaf area (B, E) from complete leaf unfolding (day 4) up to the onset of leaf yellowing, for plants grown at ambient and elevated [CO₂] in 2004 (A–C) and 2005 (D, E). Means ± s.e. are shown (n = 7 in 2004 and n = 6 in 2005).

Fig. 3.

Changes in the net photosynthetic rate of Erythronium americanum expressed per leaf area from day 4 after leaf unfolding to the onset of leaf yellowing in (A) 2004 and (B) during the first 2 weeks after leaf unfolding in 2005, for plants grown at ambient and elevated [CO₂]. Means ± s.e. are shown (n = 1–12).

Fig. 4.

Changes in starch concentration (A, C) and starch content (B, D) of Erythronium americanum bulbs during the epigeous growth period. Data are from plants grown at ambient and elevated [CO₂] in (A, B) 2004 and (C, D) 2005. Means ± s.e. are shown (n = 7 in 2004 and n = 6 in 2005). The arrows indicate the onset of leaf yellowing.

Fig. 5.

Changes in Erythronium americanum foliar N concentration (A, D), foliar N content (B, E) and bulb N content (C) for plants grown at ambient and elevated [CO₂] in (A–C) 2004 and (D, E) 2005. Means ± s.e. (n = 4) are shown from leaf unfolding to half leaf senescence (harvests 1–5), except for bulb N content where data from completely senesced plants are also presented. The arrows indicate the onset of leaf yellowing.