In temperate regions the annual pattern of wood development is characterized by the formation of radially narrow and thick walled latewood cells. This takes place at the later part of the growing season when cambial cell division declines. To gain new insight into the regulation of this process, micro-analytical techniques were used to visualize the distribution of indole-3-acetic acid (IAA), soluble carbohydrates, and activities of sucrose (Suc)-metabolizing enzymes across the cambial region tissues in Scots pine (Pinus sylvestris). The total amount of IAA in the cambial region did not change with latewood initiation. But its radial distribution pattern was altered, resulting in an increased concentration in the cambial meristem and its recent derivatives. Thus, initiation of latewood formation and cessation of cambial cell division is not a consequence of decreased IAA concentrations in dividing and expanding cells. Rather, IAA most likely has a role in defining the altered developmental pattern associated with latewood formation. Carbohydrates and enzyme activities showed distinctive radial distribution patterns. Suc peaked in the phloem and decreased sharply to low levels across the cambial zone, whereas fructose and glucose reached their highest levels in the maturing tracheids. Suc synthase was the dominating Suc cleaving enzyme with a peak in the secondary wall-forming tracheids and in the phloem. Soluble acid invertase peaked in dividing and expanding cells. Suc-phosphate synthase had its highest activities in the phloem. Activities of cell wall bound invertase were low. The absence of major seasonal variations indicates that carbohydrate availability is not a trigger for latewood initiation. However, steep concentration gradients of the sugars suggest a role for sugar signaling in vascular development.The annual transition from earlywood to latewood formation is a conspicuous developmental switch in temperate region trees. Latewood is induced during the later part of the growing season, when cell division activity in the cambial meristem declines. It involves a reduction in radial expansion and an increase in wall thickening of the cambial derivatives. Thus, earlywood is characterized by large-diameter and thin-walled tracheids, whereas latewood is composed of narrow diameter tracheids with thick cell walls. The induction of latewood cells and cambial dormancy offers a natural system by which we can gain new insight into the regulation of the basic growth processes of cell division and cell morphology.Early investigators concluded that the initiation of latewood formation was induced by shortening of the photoperiod, and was associated with cessation of apical and needle growth at a time when current year needles had become net exporters of photosynthetic assimilate (Richardson and Dinwoodie, 1960; Larson, 1964; Gordon and Larson, 1968). It was also observed that exogenous IAA could cause cambia that were forming narrow-diameter latewood tracheids to revert to forming large-diameter earlywood tracheids...

Summary. The distributions of reserve carbohydrates and of three dominant heartwood extractives were determined in the trunkwood of Robinia pseudoacacia L. The trees were cut at different times of the year (September, November, January, and April). With the exception of the tree felled in January, all trunks exhibited highest contents of nonstructural storage carbohydrates (glucose, fructose, sucrose, and starch) in the youngest, outermost sapwood zone. With increasing depth of the trunk, the levels of carbohydrates decreased. At the sapwood-heartwood transition zone, only trace amounts of nonstructural carbohydrates were present. The heartwood itself contained no storage material. The wood zones of different ages of the trees cut in September, November, and January exhibited glucose/fructose ratios of approximately 1. In April, however, there was a shift to glucose. In the youngest sapwood the amounts of soluble sugars were higher in the early-than in the latewood. Older zones of the sapwood and the sapwood-heartwood transition zone showed the opposite behaviour. Three main wood extractives of Robinia were characterized and quantified: the flavanonol dihydrorobinetin (DHR), the flavonol robinetin (ROB) and a hydroxycinnamic acid derivative (HCA). Only DHR was present -in very low amounts -in the younger sapwood of all trunks investigated, Higher amounts (>1 ~tmol/g dry weight) of this compound and the HCA were present in the sapwood-heartwood transition zone. DHR augmented within the heartwood up to a more or less constant level. HCA increased towards the heartwood and decreased again in the inner heartwood parts. ROB appeared in the innermost parts of the sapwood-heartwood transition zone and reached maximum values in older parts of the heartwood. The results indicate that starch is hydrolyzed at the sapwood-heartwood boundary and thus represents a pri-* Dedicated to Prof. Meinhart H. Zenk oll the occasion of his 60th birthdayCorrespondence to: E. Magel mary major source of hydroxycinnamic acid and flavonoid synthesis.

SummaryEctomycorrhizas are mutalistic symbiotic associations formed between fine roots of higher plants, mostly trees, and a wide range of soil ascomycetes and basidiomycetes. It is commonly accepted that there is mutual benefit to the partners, due to the exchange of plant-derived carbohydrates for amino acids and nutrients supplied by the fungus. While the major concepts of mycorrhizal functioning (exchange of nutrients and metabolites) were proposed in the 1960s, their verification at the molecular level started approximately 10 years ago. This review covers concepts at the molecular level concerned with the fungal carbohydrate supply in symbiosis. We discuss: strategies used by host plants to compensate (and perhaps restrict) carbohydrate drain to the fungal partner; fungal mechanisms that generate strong monosaccharide sinks in colonized plant roots (the formation of a strong carbohydrate sink is a prerequisite for efficient fungal carbohydrate support by the plant partner); and the impact of apoplastic hexose concentration on the regulation of fungal metabolism in symbiosis, since monosaccharides not only serve as nutrients but also as a signal that regulates gene expression.

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