This study focused on the internal conductance (g(i)) along the plant profile of Ethiopian mustard under two light conditions: (i) light from the top only (I1); (ii) light from the top integrated by supplementary lateral light along the whole plant profile (I2). Lateral light strongly increased the productivity (e.g. +104% of seed oil) and net photosynthesis (A). The latter appeared more driven by g(i) (r=0.78**) than by stomatal conductance (g(s)) (r=0.51*). Importantly, irradiance also considerably shortened the time from leaf appearance to senescence, which means that corresponding leaves in I1 and I2 had different ages. Therefore, since leaf age and irradiance have counteracting effects on g(i), I1 sometimes showed higher g(i) values than I2. With respect to irradiance, leaf age had clearly higher effects on g(i), which radically declined from the top to the basal leaves, even under constant light conditions. The internal conductance caused a significant drawdown of CO(2) from the sub-stomatal cavity (C(i)) to the site of carboxylation (C(c)) that, in turn, led to a substantial underestimation of V(cmax) calculated using the A/C(i) model. Again, the trends of g(i) and g(s) were not consistent along the plant profile, and so the ratio between stomatal and internal limitations to A changed from top to bottom leaves, accordingly. This study suggests that g(i) may be a valuable trait for increasing photosynthetic capacity and productivity; nonetheless, it suggests caution in selecting leaves for high g(i), as the latter can considerably change along the plant profile due to leaf age and irradiance effects.