Growth of plants in different oxygen concentrations

Dwarf french beans (Phaseolus vulgaris var. Canadian Wonder) were grown in chambers at 25°C with the roots aerated at 20 per cent oxygen and tops variously maintained at: T1 O2 0.21; CO2 270×10−6: T2; O2 0.05, CO2, CO2 270×10−6: T3; O2 0.21; CO2 550×10−6. Experiment 1 (T1 and T2) lasted 2 weeks: Experiment 2 (T1 T2 and T3) only one week. Hourly estimates of CO2 uptake were made by gas analysis and weekly estimates of fresh weight, dry matter in tops and roots, and leaf area, by sampling. Light intensity was 80 W m−2 of photosynthetically active radiation. An attempt was made to explain the results in terms of a simple light absorption model such that dvdt=βI0(l-θ-fL) where dV/dt is the rate of CO2 uptake per plant, β is the photosynthetic efficiency, I0 is the incident light intensity, f is the fraction of incident light absorbed by unit leaf layer and L is the leaf area index. The analysis showed that β(T2) was at least double β(T1), whilst f(T2) was smaller than f(T1) at a given leaf area. The results also required that throughout the period of the experiment, fL(T1)=fL(T2) at any given time, i.e. the treatment with the larger leaf area (T2) has the smaller value of f, and therefore intercepts less light per unit leaf area. This could be advantageous for plant growth, but requires further experiments. The photosynthetic rates per unit leaf are about 40 per cent greater in T2 than T1. Over the relatively short period of the experiment the results are adequately described by U=btn, where U is the accumulated carbon dioxide uptake, b is related to the photosynthetic efficiency (different for the differing treatments), and n is a constant (similar for all treatments). This relationship with time is believed to be a relationship with accumulated radiation, for the light was constant throughout the experiments. Comparisons of carbon fixed (measured gas uptake) and dry matter accumulation (sampling) show great scatter with an average value of 0.43. The first week's results were generally smaller than this value and the second week's greater. Energy fixation as a fraction of photosynthetically active radiation on the ground area covered by the plants ranged from 3.5 to 10 per cent. The results from treatment T3 were similar to T2 suggesting that increasing CO2 concentration decreases the growth inhibition at 21 per cent O2.