Fig. 2.

Theoretical functions of temporal evolution of organ extensibility, sink strength and seed assimilate contribution. (A) Organ extensibility is defined as maximal at organ initiation with a gradual decrease to zero at the end of organ growth. This definition is identical to the one in the theoretical model (Coussement et al., 2018a) and agrees with experimental observations (e.g. Nonami and Boyer, 1990b for soybean). Radial internode growth is defined so that (secondary) radial stem growth continues throughout plant development. (B) Scale factor for the sink function of the plant organ gradually increases to a maximal value and decreases towards organ maturity (eqn 3 with fixed ${\mathrm{P}}_{\mathrm{m}\mathrm{a}\mathrm{x}}^{\mathrm{o}\mathrm{r}\mathrm{g}\mathrm{a}\mathrm{n}}$). As a result, the timing of the highest sink strength overlaps with the timing of the highest relative growth rate. Sink strength of the radial internodes remains constant after reaching its highest value to maintain sugar supply for radial stem growth (eqn 5). Note that the true sink function of the plant organ also depends on the temporal evolution of ${\mathrm{P}}_{\mathrm{m}\mathrm{a}\mathrm{x}}^{\mathrm{o}\mathrm{r}\mathrm{g}\mathrm{a}\mathrm{n}}$, which is linked to volumetric extensibility. (C) Seed assimilate contribution totals to ${\mathrm{C}}_{\mathrm{s}\mathrm{e}\mathrm{e}\mathrm{d}}^{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}$, which is the total area under the curve between 0 and t_e (eqn 6).