THE SHOOT APICAL ONTOGENY OF THE PICEA ABIES SEEDLING. II. GROWTH RATES

Abstract

Shoot apices of greenhouse‐reared Picea abies seedlings undergo a sequence of age‐related developmental changes before closely approaching a steady‐state condition when about 140 days old. Apical dome diameter and height increase with seedling age. Thus, although the relative volume growth rate of the apical dome declines as the seedlings age, the increase in size of the mean apical dome itself is so large that the absolute volume of new tissue generated per day still increases several fold. Therefore, the observed decline in plastochron duration, decreases in relative growth rates per plastochron, increase in absolute volume growth per plastochron, and increase in tissue doubling time with increasing age are all mutually compatible. The apical dome of the mean 20‐day‐old plant produced about 110,000 μ3 of tissue per plastochron of 18.5 hr, or about 140,000μ3 per day. Relative volume growth rate was about 20 % per day. At that rate about 4.4 plastochrons or 82 hr would be needed to double a unit volume of apical dome tissue. In contrast, the apical dome of the mean 140‐day‐old plant produced about 220,000μ3 of tissue per plastochron of 5.7 hr, or about 940,000 μ3 per day. Relative volume growth rate was about 12 % per day; consequently, about 23 plastochrons or 130 hr would be needed to double a unit volume of apical dome tissue. Further analysis indicated that an internode, newly initiated by the mean 140‐day‐old plant, increased in volume at the rate of 7.46% per plastochron or 31.5% per day for the first 10 plastochrons. Methods are presented for estimating vertical, radial, and volume growth rates at any level of the shoot apex when steady‐state conditions prevail.