Leaf development, leaf area distribution, growth and productivity (dry matter of leaves, roots, crowns; and sugar yield) of sugar beet (Beta vulgaris L. cv. Monoscope) plants were studied during the 1983 and 1984 growing seasons on an experimental field plantation. Photosynthetic capacity (P(c); photosynthetic photon flux density saturated net photosynthetic rate) was measured at regular time intervals in 1984 and 1985. Using these data, the performance of a model (SUBEMO) simulating growth, yield and sugar accumulation was evaluated. At the end of the growing season, viz. October, about 50 leaves were developed by the sugar beet plant, yielding maximal leaf area index values of around 4 to 5. P(c) changed considerably during the growing season showing significantly lower values before appearance of leaf number 20 (around mid-July), increasing later in the season and slightly decreasing near the end of the growing season (mid-October). P(c) of mature leaves was significantly and positively correlated with total leaf dry matter of the sugar beet plants at the end of the growing season. Comparisons between simulated and measured dry matter production of leaves and beets (roots + crowns) confirmed the need to integrate seasonal trends in P(c) in the simulation model.