Growth cessation and bud dormancy in relation to ABA level in seedlings and coppice shoots of Betula pubescem as affected hy a short photoperiod, water stress and chilling Paivi Rinne, Annikki Saarelainen and Olavi Junttila Rinne, P., Saarelainen. A. and Junttila, O. 1994. Grouth cessation and bud dormancy in relation to ABA level in seedlings and coppice shoots of Betula pubescens as affected by a short photoperiod, water stress and chilling. -Physiol. Plant. 90; 451^58.Seedlings and coppice shoots of Betula pube.scens Ehrh. « ere grown under controlled conditions designed to simulate the annual growth cycle, and a water stress was introduced during the short day iSD). .Alleviation of bud domiancy after increasing periods at chilling temperatures was tested under long day (LDl conditions. .Abscisic acid (ABA) was analysed in leaf and bud samples by gas chromatography-mass spectrometrs using l-EJjJ.ABA as the internal standard. Elongation growth of coppice shools uas faster than that of .seedlings under both LD and SD conditions, while the final growth cessation occurred in a similar manner and was not affected by water stress, v\hich significantly reduced growth rate in both plant types. Bud domianex gradually decreased with increasing length of chilling, starting from the basal paits of the plant axis. Water stress did not retard budburst. but rather improved it in the chilled coppice shoots and in the non-chi!led and partially chilled seedlings. Water content of buds was higher in coppice shoots than in seedlings, but after exposure to SD, it gradually decreased to 45^;? in both plant types and was not affected by water stress or chilling. The .ABA le\el in both lea\es and buds increased during SD treatment and was enhanced by water stress. No clear differences in bud .AB.A level were found between the seedlings and coppice shoots under SD conditions, although coppice shoots had less AB.A during the preceding LD conditions. There was, in general, no clear effect of chilling on bud AB,A level, Budburst in chilled, single-node cuttings was inhibited by external AB.A treatment, which raised the internal ABA levels 10 to 1.50 times above normal. The observed con elation between ABA level and water content in buds during induction of dortnancy under SD and water stress conditions indicates a possible role ibr ABA in the regulation of dormancy.

Seedlings and coppice shoots of Betula pubescens Ehrh. were grown under controlled conditions designed to simulate the annual growth cycle, and a water stress was introduced during the short day (SD). Alleviation of hud dormancy after increasing periods at chilling temperatures was tested under long day (LD) conditions. Abscisic acid (ABA) was analysed in leaf and bud samples by gas chromatography‐mass spectrometry using [2H4]ABA as the internal standard. Elongation growth of coppice shoots was faster than that of seedlings under both LD and SD conditions, while the final growth cessation occurred in a similar manner and was not affected by water stress, which significantly reduced growth rate in both plant types. Bud dormancy gradually decreased with increasing length of chilling, starting from the basal parts of the plant axis. Water stress did not retard hudhurst. but rather improved it in the chilled coppice shoots and in the non‐chilled and partially chilled seedlings. Water content of buds was higher in coppice shoots than in seedlings, but after exposure to SD. it gradually decreased to 45% in both plant types and was not affected by water stress or chilling. The ABA level in both leaves and buds increased during SD treatment and was” enhanced by water stress. No clear differences in bud ABA level were found between the seedlings and coppice shoots under SD conditions, although coppice shoots had less ABA during the preceding LD conditions. There was, in general, no clear effect of chilling on bud ABA level. Budbursl in chilled, single‐node cuttings was inhibited by external ABA treatment, which raised the internal ABA levels 10 to 150 times above normal. The observed correlation between ABA level and water content in buds during induction of dormancy under SD and water stress conditions indicates a possible role for ABA in the regulation of dormancy.

Six-year-old cloned Betula pubescens Ehrh. trees, grown outdoors at 65 degrees 01' N, were cut on six dates during the growing season to study coppice shoot development in relation to root-produced cytokinin-like compounds. Bleeding sap was collected over timed intervals for two days after cutting, and endogenous cytokinin-like compounds were measured by ELISA assay in HPLC-purified fractions of xylem sap. Initiation and development of coppice shoots on the clonally propagated plants were comparable to those in seedlings. Coppice shoot initiation was affected by the time of cutting, diminishing significantly after June. Of the cytokinin-like compounds detected in the xylem sap, zeatin riboside-like (ZR) compounds were present in the highest concentrations, and the concentrations of dihydrozeatin riboside-like (DHZR) and isopentenyladenoside-like (IPA) compounds were approximately one third and one eighth of the ZR concentrations, respectively. The concentration of cytokinin-like compounds was positively correlated with xylem sap flow rate. The export of cytokinin-like compounds, especially DHZR- and ZR-types, was positively correlated with the initiation and elongation rate of coppice shoots, the number of lateral branches, and the radial growth of the more slowly growing coppice shoots. The export of cytokinin-like compounds collected immediately after cutting may represent the basal value for each tree. This value is probably affected by the size and activity of the root system and may be a relevant estimate for predicting the success of coppicing.

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