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

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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.