Plant Growth and Water Use With Limited Water Supply in High CO₂ Concentrations. II. Plant Dry Weight, Partitioning and Water Use Efficiency

Abstract

Plants of 18 species were grown from seed in two glasshouses, one with normal and one with twice the present atmospheric CO₂ concentration. Plants were grown singly with a diminishing soil moisture content and were harvested either after the initial period of rapid growth and water use or after subsequent prolonged soil drying. Plant dry weight (DW) was increased substantially by high CO₂ in all but two species (cotton and maize) at the first harvest (average increase of 65%, range from 26 to 132%). Over the whole period, increases in DW with high CO₂ (average of 53%) were associated with increases in water use efficiency (WUE) of between 40 and 80% (average of 67% for all species). Cowpea and sunflower plants grown through two additional soil moisture drying cycles showed 26 and 21% increases, respectively, in the effect of high CO₂ on WUE. Approximate calculations indicated that high CO₂ increased the apparent efficiency of use of intercepted radiation by 11-49% (average of 27%) in C₃ species but in C₄ species there was no effect. The effect of high CO₂ on the partitioning of DW varied between species and between harvests. During the first period, 14 species showed increased specific leaf weight of 10-40% in high CO₂ and 16 species showed no effect of high CO₂ on the root : shoot ratio. Over the whole experiment, eight species had lower leaf to total plant weight ratio in high CO₂ attributable to increased non-leaf aerial tissues.