The growth patterns of the first-order lateral roots of young oak (Quercus robur L.) tree seedlings were studied and related to variation in apical diameter. Seven seedlings were grown in root observation boxes and monitored daily for 6 wk. Apical diameters were measured daily on a sample of 48 lateral roots. A principal component analysis performed on the data set for 479 lateral roots showed that growth patterns were characterized by two major features. First, they could be described by a general growth potential (first factorial, 80% of the total variability) which was related to growth rate and growth duration. Second, but much less important (second factorial axis, 15% of the total variability), there was a distinction between early-growing roots (growing fast just after emergence and stopping early), and late-growing roots (having a low growth rate at emergence and a higher growth rate later). This growth pattern typology was related to the time-dependent variation in apical diameter. On almost all the roots, the apical diameter tended to decrease after emergence. On short roots (with a low growth potential) the initial apical diameter was low and its decrease was steep. Conversely, long roots had a greater initial diameter and tended to maintain it at a higher level. The so-called late-growing roots had a large apical diameter but a low growth rate in the early stages, possibly because of the between root competition for growth factors. The interest of the apical diameter as a morphological marker of sink strength, and the interactive relationships between growth rate, apical diameter and sucrose supply are discussed.
Keywords: Root system; apical diameter; competition; growth; sink strength..