Function and dynamics of auxin and carbohydrates during earlywood/latewoodtransition in Scots pine

In temperate regions the annual pattern of wood development is characterize d 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 divi sion declines. To gain new insight into the regulation of this process, mic ro-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 (Pinu s 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 re cent derivatives. Thus, initiation of latewood formation and cessation of c ambial cell division is not a consequence of decreased IAA concentrations i n dividing and expanding cells. Rather, IAA most likely has a role in defin ing the altered developmental pattern associated with latewood formation. C arbohydrates and enzyme activities showed distinctive radial distribution p atterns. Suc peaked in the phloem and decreased sharply to low levels acros s the cambial zone, whereas fructose and glucose reached their highest leve ls in the maturing tracheids. Suc synthase was the dominating Suc cleaving enzyme with a peak in the secondary M;all-forming tracheids and in the phlo em. Soluble acid invertase peaked in dividing and expanding cells. Suc-phos phate synthase had its highest activities in the phloem. Activities of cell wall bound invertase were low. The absence of major seasonal variations in dicates that carbohydrate availability is not a trigger for latewood initia tion. However, steep concentration gradients of the sugars suggest a role f or sugar signaling in vascular development.