Analysis of Shoot Apical Growth of Picea sitchensis Seedlings

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Abstract
During the first 100 days after sowing (March-June) the following changes took place at the terminal shoot apices of Picea sitchensis seedlings: plastochrones (T) decreased from over 24 h to 4 h; apical domes enlarged from less than 0·20 mm to 0·45 mm diameter (D); the ‘projected’ area of tissues produced by the apical domes (i.e. viewed from above) increased in amount from less than 0·012 to 0·024 mm2 day-1; about 15 per cent of this tissue was re-invested in the apical domes, the rest was used to produce primordia; and the volume-doubling times of the apical dome tissues decreased from over 150 h to 50 h. After 100 days there was no further re-investment in the apical domes, but the domes did not decrease abruptly in size. Less tissue was produced per day, but the primordia were smaller so that the rate of primordia formation did not fall precipitously. Plastochrone ratios were inversely related to D, but the relationship between T and D depended on whether T was decreasing or increasing. Progenies which were known to be fast growing tended to build-up their apical domes rapidly (i.e. have large ‘re-investment ratios’) and to be capable of producing small primordia. These attributes can evidently be evaluated on seedlings and could help to lessen the cost of tree breeding progeny-test programmes.