Tracheary elements from calli of Japanese horse chestnut (Aesculus turbinata) form perforation-like structures.

MAIN CONCLUSION: Calli derived from young leaves of Aesculus turbinata contained tracheary elements with large pores that resembled perforations of vessel elements. The differentiation of tracheary elements in vitro provides a useful system for detailed analysis of xylem cell differentiation. To examine the mechanism of formation of cell wall structures, new differentiation systems are required that allows us to induce highly organized structures, such as perforations. In this study, we developed such a system in which we were able to induce formation of tracheary elements with perforations, using calli of a hardwood, Aesculus turbinata. Young leaves of A. turbinata were placed on modified MS medium that contained 5 µM 2,4-dichlorophenoxyacetic acid (2,4-D) and 5 µM benzyladenine (BA). Tracheary elements were induced in calli derived from young leaves of A. turbinata. Some tracheary elements formed broad areas of secondary wall with typical features of secondary xylem. Other tracheary elements formed spiral thickenings, which are typical features of vessel elements in secondary xylem of A. turbinata. Approximately 10% of tracheary elements formed large pores that resembled perforations of vessel elements and various types of the perforation plate were observed. Addition of NAA and brassinolide to the induction medium enhanced the differentiation of tracheary elements in calli of A. turbinata. Newly induced tracheary elements also formed typical features of secondary xylem such as perforations of the vessel elements. Our model system might be useful in efforts to understand the mechanisms of formation of highly organized structures in tracheary elements in secondary xylem.

Seasonal changes of δD and δ18O in tree-ring cellulose of Quercus crispula suggest a change in post-photosynthetic processes during earlywood growth.

Leaf photosynthetic and post-photosynthetic processes modulate the isotope ratios of tree-ring cellulose. Post-photosynthetic processes, such as the remobilization of stored starch in early spring, are important to understanding the mechanisms of xylem formation in tree stems; however, untangling the isotope ratio signals of photosynthetic and post-photosynthetic processes imprinted on tree rings is difficult. Portions of carbon-bound hydrogen and oxygen atoms are exchanged with medium water during post-photosynthetic processes. We investigated the δD and δ18O values of tree-ring cellulose using Quercus crispula Blume trees in two different habitats to evaluate seasonal changes in the exchange rate (f-value) of hydrogen or oxygen with medium water, and examined the associations of the post-photosynthetic processes. Theoretically, if the f-value is constant, δD and δ18O would be positively correlated due to meteorological factors, while variation in the f-value will create a discrepancy and weak correlation between δD and δ18O due to the exchange of carbon-bound hydrogen and oxygen with medium water. The values of δD decreased drastically from earlywood to latewood, while those of δ18O increased to a peak and then decreased toward the latewood. The estimated seasonal f-value was high at the beginning of earlywood and decreased toward the latewood. The post-photosynthetic processes associated with changes in the f-value were the remobilization of stored starch and triose cycling during cellulose synthesis because of the shortage of photo-assimilates in early spring. Although we did not evaluate relevant physiological parameters, the seasonal pattern of δD and δ18O in tree-ring cellulose of Q. crispula was clear, suggesting that the dual isotope (δD and δ18O) approach can be used to reveal the resource allocation mechanisms underlying seasonal xylem formation.

Localized cooling of stems induces latewood formation and cambial dormancy during seasons of active cambium in conifers.

BACKGROUND AND AIMS: In temperate regions, trees undergo annual cycles of cambial growth, with periods of cambial activity and dormancy. Environmental factors might regulate the cambial growth, as well as the development of cambial derivatives. We investigated the effects of low temperature by localized cooling on cambial activity and latewood formation in two conifers, Chamaecyparis obtusa and Cryptomeria japonica. METHODS: A plastic rubber tube that contained cooled water was wrapped around a 30-cm-wide portion of the main stem of Chamaecyparis obtusa and Cryptomeria japonica trees during seasons of active cambium. Small blocks were collected from both cooled and non-cooled control portions of the stems for sequential observations of cambial activity and for anatomical measurements of cell morphology by light microscopy and image analysis. KEY RESULTS: The effect of localized cooling was first observed on differentiating tracheids. Tracheids narrow in diameter and with significantly decreased cambial activity were evident 5 weeks after the start of cooling in these stems. Eight weeks after the start of cooling, tracheids with clearly diminished diameters and thickened cell walls were observed in these stems. Thus, localized low temperature induced narrow diameters and obvious thickening of secondary cell walls of tracheids, which were identified as latewood tracheids. Two months after the cessation of cooling, a false annual ring was observed and cambium became active again and produced new tracheids. In Cryptomeria japonica, cambial activity ceased earlier in locally cooled portions of stems than in non-cooled stems, indicating that the cambium had entered dormancy sooner in the cooled stems. CONCLUSIONS: Artificial cooling of stems induced latewood formation and cessation of cambial activity, indicating that cambium and its derivatives can respond directly to changes in temperature. A decrease in the temperature of the stem is a critical factor in the control of cambial activity and xylem differentiation in trees.

Geographic variation in shoot traits and branching intensity in relation to leaf size in Fagus crenata: A common garden experiment.

PREMISE OF THE STUDY: Differences in leaf size are expected to be coordinated with various shoot traits and branching intensity because these relationships will influence light capture efficiency, water use, and biomechanics. Previous studies have mainly focused on interspecific patterns of these trait relationships, but not on intraspecific patterns at the geographic scale. We investigated intraspecific variation in shoot traits and branching intensity of Fagus crenata in Japan. METHODS: Allometric relationships between the traits of current-year shoots and branching intensity per branch unit of 1-m length on the main axis (BI) and its coordination with latitude were investigated using trees from 10 provenances in a common garden. KEY RESULTS: Individual trees originating from lower latitudes have smaller leaves with greater leaf mass per area and nitrogen content per area, greater Huber value (stem cross-sectional area per total leaf area [ATL]) of current-year shoots, and greater BI. Notably, the slope of the log-log relationship between BI and ATL was close to -1.0 across the trees from different source sites, implying that branching in this species occurs to control leaf area. CONCLUSIONS: Shoot traits and branching intensity were apparently coordinated with leaf size to control leaf area deployment in this species. Such patterns probably reflect differences in competition for hydraulic conductance among nearby shoots within crowns, as a consequence of different meteorological conditions across the source sites.

The effects of localized heating and disbudding on cambial reactivation and formation of earlywood vessels in seedlings of the deciduous ring-porous hardwood, Quercus serrata.

BACKGROUND AND AIMS: The networks of vessel elements play a vital role in the transport of water from roots to leaves, and the continuous formation of earlywood vessels is crucial for the growth of ring-porous hardwoods. The differentiation of earlywood vessels is controlled by external and internal factors. The present study was designed to identify the limiting factors in the induction of cambial reactivation and the differentiation of earlywood vessels, using localized heating and disbudding of dormant stems of seedlings of a deciduous ring-porous hardwood, Quercus serrata. METHODS: Localized heating was achieved by wrapping an electric heating ribbon around stems. Disbudding involved removal of all buds. Three treatments were initiated on 1 February 2012, namely heating, disbudding and a combination of heating and disbudding, with untreated dormant stems as controls. Cambial reactivation and differentiation of vessel elements were monitored by light and polarized-light microscopy, and the growth of buds was followed. KEY RESULTS: Cambial reactivation and differentiation of vessel elements occurred sooner in heated seedlings than in non-heated seedlings before bud break. The combination of heating and disbudding of seedlings also resulted in earlier cambial reactivation and differentiation of first vessel elements than in non-heated seedlings. A few narrow vessel elements were formed during heating after disbudding, while many large earlywood vessel elements were formed in heated seedlings with buds. CONCLUSIONS: The results suggested that, in seedlings of the deciduous ring-porous hardwood Quercus serrata, elevated temperature was a direct trigger for cambial reactivation and differentiation of first vessel elements. Bud growth was not essential for cambial reactivation and differentiation of first vessel elements, but might be important for the continuous formation of wide vessel elements.

In vitro induction of secondary xylem-like tracheary elements in calli of hybrid poplar (Populus sieboldii × P. grandidentata).

The formation of tracheary elements was induced in calli derived from petioles of hybrid poplar (Populus sieboldii × P. grandidentata) after 10 days of culture on medium that lacked auxin but contained 1 µM brassinolide. Some differentiated cells formed broad regions of cell walls and bordered pits, which are typical features of tracheary elements of secondary xylem. Other differentiated cells resembled tracheary elements of primary xylem, with spiral or reticulate thickening of cell walls. The tracheary elements that developed in calli were formed within cell clusters. This induction system provides a new model for studies of the mechanism of differentiation of secondary xylem cells in vitro.

Common allometric response of open-grown leader shoots to tree height in co-occurring deciduous broadleaved trees.

BACKGROUND AND AIMS: Morphology of crown shoots changes with tree height. The height of forest trees is usually correlated with the light environment and this makes it difficult to separate the effects of tree size and of light conditions on the morphological plasticity of crown shoots. This paper addresses the tree-height dependence of shoot traits under full-light conditions where a tree crown is not shaded by other crowns. METHODS: Focus is given to relationships between tree height and top-shoot traits, which include the shoot's leaf-blades and non-leafy mass, its total leaf-blade area and the length and basal diameter of the shoot's stem. We examine the allometric characteristics of open-grown current-year leader shoots at the tops of forest tree crowns up to 24 m high and quantify their responses to tree height in 13 co-occurring deciduous hardwood species in a cool-temperate forest in northern Japan. KEY RESULTS: Dry mass allocated to leaf blades in a leader shoot increased with tree height in all 13 species. Specific leaf area decreased with tree height. Stem basal area was almost proportional to total leaf area in a leader shoot, where the proportionality constant did not depend on tree height, irrespective of species. Stem length for a given stem diameter decreased with tree height. CONCLUSIONS: In the 13 species observed, height-dependent changes in allometry of leader shoots were convergent. This finding suggests that there is a common functional constraint in tree-height development. Under full-light conditions, leader shoots of tall trees naturally experience more severe water stress than those of short trees. We hypothesize that the height dependence of shoot allometry detected reflects an integrated response to height-associated water stress, which contributes to successful crown expansion and height gain.

Seasonal changes in the temperature response of photosynthesis in canopy leaves of Quercus crispula in a cool-temperate forest.

Understanding seasonal changes in photosynthetic characteristics of canopy leaves is indispensable for modeling the carbon balance in forests. We studied seasonal changes in gas exchange characteristics that are related to the temperature dependence of photosynthesis in canopy leaves of Quercus crispula Blume, one of the most abundant species in cool-temperate forests in Japan. Photosynthetic rate and ribulose-1,5-bisphosphate (RuBP) carboxylation capacity (V(cmax)) at 20 degrees C increased from June to August and then decreased in September. The activation energy of V(cmax), a measure of the temperature dependence of V(cmax), was highest in summer, indicating that V(cmax) was most sensitive to leaf temperature at this time. The activation energy of V(cmax) was significantly correlated with growth temperature. Other parameters related to the temperature dependence of photosynthesis, such as intercellular CO(2) partial pressure and temperature dependence of RuBP regeneration capacity, showed no clear seasonal trend. It was suggested that leaf senescence affected the balance between carboxylation and regeneration of RuBP. The model simulation showed that photosynthetic rate and its optimal temperature were highest in summer.

Size dependency of photosynthetic water- and nitrogen-use efficiency and hydraulic limitation in Acer mono.

We examined open-grown Acer mono Maxim. trees of different sizes to test the hypotheses that (1) hydraulic limitation increases with tree size, thereby reducing photosynthesis, and (2) photosynthetic water- and nitrogen-use efficiencies change with tree size. Maximum net assimilation rate per unit dry mass was significantly lower in large trees than in small trees, whereas leaf nitrogen concentration increased with tree size. As a consequence, photosynthetic nitrogen-use efficiency decreased with increase in tree size. Photosynthetic water-use efficiency, however, increased with tree size, partly as a result of reduced stomatal conductance. Neither root-to-leaf hydraulic conductance nor minimum leaf water potential changed with tree size.