New root growth of Douglas-fir seedlings at low carbon dioxide concentration

New root growth of conifer nursery seedlings is dependent on light, but whether this is necessary only for photosynthesis, or also has some other root growth promoting effect is unknown. This question was investigated using one-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings grown at two nurseries from the same seed lot and taken from cold storage in March and April. New root production was examined in two 10-day root growth capacity (RGC) experiments. Seedlings were subjected to one of four treatments: (1) control, (2) exclusion of light, (3) low CO(2) concentration, and (4) girdling to separate the phloem connection between shoot and root. Shoots were enclosed in Plexiglas cuvettes and supplied with scrubbed air to reduce the CO(2) concentration to 11 to 22 microl l(-1) in the light. The closed system used in Experiment 1 was opened to the atmosphere briefly each day to restore O(2) to ambient, but this was unnecessary for the open system used in Experiment 2. In Experiment 1, new root production was affected by treatments in the order: control > low CO(2) concentration > dark > girdling. Greater new root production in the low CO(2) treatment compared with the dark treatment was attributed to brief increases in CO(2) to ambient concentrations when O(2) was restored each day. In Experiment 2, new root production at the low CO(2) concentration in the light was essentially the same as in darkness, but only 17% of the control value. Thus light appeared to play no part in new root production other than permitting photosynthesis. Limited production of new roots occurred in the absence of photosynthesis, which was further reduced by girdling, presumably because, after girdling, only root system reserves could be used.