In the last decade, many studies have been conducted on intra-annual wood formation in both conifer and broadleaf species, but none, as far as is known, concerned xylogenesis in roots. This investigation described and compared cell production and differentiation between stem and roots in two boreal forest species during three growing seasons. The results revealed similar xylem phenologies in 2004–2005, but delayed endings of cell differentiation in roots compared with the stem in 2006. The production of xylem was concentrated in the stem early in the season, while most cell divisions occurred 1 month later in roots.
Cambial reactivation is assumed to be promoted by auxin, produced in the younger shoots and exported basipetally along the stem to induce the production of xylem (Larson, 1969
; Aloni, 2001
) and regulate xylem development (Tuominen et al., 1997
; Uggla et al., 1998
). Following the basipetal movement of the auxin, periclinal divisions in the cambium should begin close to buds, spread downwards toward branches and stem and finally occur in the roots (Larson, 1969
; Denne, 1979
; Lachaud et al., 1999
). However, in this study no pattern in the direction of the progression of the onset of cambial activity was observed between stem and roots in the two studied species, suggesting that the auxin required for cambial resumption could already be available in the dormant tissues before spring (Little and Wareing, 1981
; Sundberg et al., 1991
The hypotheses on the direction of cessation of cambial activity and xylem production through the different plant tissues are still contradictory (Zimmermann and Brown, 1971
; Riding and Little, 1986
; Vaganov et al., 2006
). In the present investigation, the ending of xylem differentiation occurred in similar periods in 2004–2005, but in 2006 a longer period of xylem formation was observed in roots than in the stem. This longer differentiation period in the developing root tissues corresponded to a higher number of xylem cells produced, particularly in P. mariana
. The ending of latewood differentiation is related to cell production in vascular cambium; the higher the number of cells produced along a radial row, the longer the overall period of tree ring formation becomes (Gričar et al., 2005
, Rossi et al., 2008a
). Incorporation of wall constituents in latewood cells is an energy-expensive process for a tree and wall thickening and lignification can require up to 40 d for a xylem tracheid (Rossi et al., 2006b
). The greatest number of cells produced by roots in 2006 led to a higher accumulation of developing cells in differentiation and delayed the conclusion of xylem maturation.
The onset of xylem production occurred early in the season and terminated at the end of August, at temperatures still favourable for growth. This could indicate a different physiological mechanism involved in controlling xylem phenology at the beginning and ending of growth, with effects of both internal and external factors on cambial activity. Thermal sums are known to precisely estimate the beginning of growth (Schmitt et al., 2004
; Seo et al., 2008
) and Rossi et al. (2007)
assessed a threshold temperature limiting spring carbon allocation in xylem of Alpine treeline trees, which suggests an important influence of temperature on cambial reactivation in these environments. On the contrary, the cessation of cell production is unrelated to the annual temperature trend (S. Rossi et al.
, 2008). Rossi et al. (2006b)
observed that the growth rate in the stem culminated during the summer solstice, when the photoperiod was maximum. However, this work reports a different growth pattern in roots, with delays of up to 1 month for the culmination of growth in the soil. At the end of cell division, high amounts of abscisic acid were detected in stems and associated with the termination of cell production (Lachaud, 1989
), but proof that abscisic acid directly induces cambial dormancy is still lacking.
In the boreal forest, temperature is the main factor driving xylogenesis during spring (Antonova and Stasova, 1997
; Vaganov et al., 1999
; Deslauriers and Morin, 2005
). Vascular cambium is a sink for non-structural carbohydrates, and cambial activity requires a continuous supply of energy in the form of sucrose, extracted from the storage tissues for the first formed cells, or produced by photosynthesis (Hansen and Beck, 1994
; Oribe et al., 2003
). Temperature allows the metabolism of carbon allocation to be accomplished (Körner, 1998
). Although cell differentiation was estimated to begin in the same period at both sample locations, stem and roots concentrated most xylem production in two different periods of the year, with earlier tracheid accumulation observed in the stem. In spring, air temperature increases sharply in May–June, although the soil temperature remains close to zero during snowmelt and is observed to increase at least 2 weeks later than the air (Rossi et al., 2007
). This delayed temperature increase in the soil could explain the different patterns of xylem growth observed between stem and roots.
Further investigations should be interesting to follow the reactivation of the cambial zone during the spring. In the present study, sampling only started at the beginning of May and approximately within half of the observations, the number of cambial cells exceeded the minimum number counted at the end of the previous year. As reported in the literature, physiological processes occurred in the cambial zone before cell division is observed (Frankenstein et al., 2005
). However, the present method did not allow the observation of structural modifications in the cambial zone in order to identify the onset of cell production according to Frankenstein et al. (2005)
In conclusion, the xylem of stem and roots in A. balsamea and P. mariana showed similar periods of cell differentiation during 2004–2006, but with different intra-annual growth dynamics. This behaviour could be related to internal and external factors such as the amount of cells produced by the vascular cambium and the different patterns of air and soil temperature in spring.