ALBERT

Notes: Changes in protein synthesis in cambial region cells were monitored in 1-year-old cuttings of Scots pine (Pinus sylvestris L.) collected in November, when the cambium was dormant, and subjected to environmental conditions that promoted or inhibited cambial growth. The proteins were labelled in vivo with L-[35S]-methionine and separated using 2-dimensional polyacrylamide gel electrophoresis. In budded cuttings cultured under environmental conditions favoring cambial reactivation, there was a reproducible quantitative change in 55 proteins (33 induced and 22 repressed), a less certain increase or decrease in 40 proteins, and no apparent change in about 150 proteins. Under the same conditions, 8 proteins were induced and 6 others were repressed in debudded cuttings treated apically with 1 mg indole-3-acetic acid (IAA) in 1 g lanolin, in which cambial reactivation occurred, compared with debudded cuttings treated with plain lanolin in which the cambium did not reactivate. Three of the proteins induced in the IAA-reated cuttings only appeared after cambial cell division and derivative differentiation actually began, and the same proteins were found in budded cuttings after their cambium had become reactivated. In contrast, protein expression in cuttings exposed to environmental conditions that prevented cambial reactivation was similar at the beginning and end of the experimental period. These results indicate that the cambium was in the quiescence stage of dormancy at the start of the experiment, that quiescent cambial region cells can synthesize proteins as soon as exposed to environmental conditions favoring reactivation, and that only 3 of the approximately 250 proteins detected were specifically involved in cambial growth

Notes: The relationship between from hardiness and growth potential, and their dependence on temperature and photoperiod, was investigated in the one-year-old cambium of balsam fir [Abies balsamea (L.) Mill.]. Six-year-old trees were exposed for 9 weeks to either the natural environment or one of 4 controlled environments in the fall (18 September-18 November), spring (12 April–14 June) and summer (19 July – 19 September). The 4 controlled environments were (1) WS, warm temperature (24/20°C in day/night) + short day (8 h). (2) WL. warm temperature (24/20°C) + long day (8 h + 1 h night break), (3) CS. cold temperature (9/5°C) + short day (8 h) and (4) CL, cold temperature (9/5°C) + long day (8 h + 1 h night break). At the beginning and end of each exposure, cambial activity was measured by recording the number of xylem, cambium and phloem cells, frost hardiness was estimated from the cambium's ability to survive freezing to –40°C, and cambial growth potential was deduced from the duration of the cell cycle and the production of xylem, cambium and phloem cells in cuttings cultured for 4 weeks with exogenous indole-3-acetic acid (IAA) under environmental conditions favourable for cambial activity. In the natural environment, frost hardening began in September and was completed in November, while dehardening occurred when the cambium reactivated. CL, CS, and to a lesser extent WS, promoted hardening in the summer and fall, but did not prevent dehardening in the spring. The cambial growth potential in the natural environment declined from a maximum in April to a low level in June, reached a minimum in September, then increased to a high level in November. This potential was promoted by CL and CS on all dates by WL in the summer and fall. The ratio of xylem to phloem induced by IAA treatment was greatest in June and least in September in cuttings from trees exposed to the natural environment, and was increased by CL and CS in the fall. The cambium in intact branches of trees protected from chilling during the fall and winter resumed cell cycling after less than 9 weeks of dormancy, but produced mostly or only phloem in the subsequent growing period. It is concluded that the frost hardiness of the cambium, the IAA-induced cycling of cambial cells, and IAA-induced xylem to phloem ratio vary independently with season, temperature and photoperiod, and that the periodicity of these processes is regulated endogenously.

Notes: Previously, we showed that the size of the nuclear genome, measured cytophotometrically in Feulgen-stained fusiform cambial cells of Abies balsamea (L.) Mill., oscillates annually between a maximum in spring and a minimum in late summer, the labile, extra DNA being synthesized during the fall. To determine it the oscillation is induced by the concomitant seasonal changes in temperature and photoperiod, genome size was measured in cambial cells obtained from one-year-old branches of 6-year-old potted trees at the beginning and end of 9 weeks of exposure during the fall, spring and summer to either the natural environment or one of 4 controlled environments, viz. (1) WS, warm temperature (24/20°C in day/night) and short photoperiod (8 h). (2) WL, warm temperature (24/20°C) and long photoperiod (8 h + 1 h night break), (3) CS, cold temperature (9/5°C) and short photoperiod (8 h). and (4) CL, cold temperature (9/5°C) and long photoperiod (8 h + 1 h night break). Overall, genome size (2C) varied between 20 and 34 pg. In the fall, when the cambium was initially dormant, the genome size increased in the natural environment, did not change under short days (WS and CS), and decreased under long days (WL and CL). The cambium reactivated in both WS and WL conditions. In the spring, while the cambium reactivated, the size of the genome decreased in the natural, WS and WL conditions, but not in the CS environment. In the CL conditions, the genome size started to decrease at the end of the 9-week exposure period. The decrease apparently occurred between prophase and telophase, which suggests that the extra DNA is extrachromosomal. In the summer, while the cambium ceased activity, the genome size did not change in the WS, WL and natural environments, whereas it decreased in the CS and CL conditions. The results indicate that increasing temperature and lengthening photoperiod in the spring induce the loss of the extra DNA. However, the environmental conditions that promote DNA synthesis in the fall remain unknown. Genome size varied independently of cambial growth potential and frost hardiness measured previously in the same experimental trees, indicating that the regulation of these processes does not directly involve the extra DNA. However, the finding that cambial cells cycled in the CS and CL environments only in the spring, when their genome size was large, suggests that the extra DNA is important for cambial growth at low temperatures.

Notes: The nuclear DNA content in ray cells from the 1-year-old vascular cambium of white ash (Fraxinus americana L.) trees was determined at intervals during the annual cycle of cambial activity and dormancy by using Feulgen microspectrophotometry. By 10 September, these cells had entered dormancy in G1 with a normal DNA distribution and a minimal average DNA content of 2.65 pg. The average amount of DNA increased to 3.51 pg by 30 November, remained at this elevated value until at least 30 March, when the cambium was still dormant, then declined to the minimum level on 1 May and 10 June, when the cells were mitotically active. The springtime decline appeared to occur both before and during cell division. Between 1 May and 10 June, the prophase (4C) and telophase (2C) DNA contents decreased significantly. The amount of nuclear DNA measured by microspectrophotometry was verified by using flow cytometry and image analysis. The results support the view that there is an annual oscillation in the nuclear genome size of shoot meristematic cells in tree species native to the northern temperate zone.