The traits included
the epidermal structure (papillae, trichomes, and guard cells) (Figure 2), internal structure (cell type differentiation,
cell components of the vascular bundle, and density of cytoplasm) (Figure 3), and cellular components
(chloroplasts, starch, and lignified cell walls) (Figure 4). Although the transitions of all traits do not occur simultaneously,
our analyses indicated that most of the transitions occur in the restricted part of the P4 leaf along
the proximal-distal axis. In addition, the transition was shown to be determined independently of leaf blade and
leaf sheath differentiation. Therefore, our analysis suggested that the P4 leaf primordium is the optimal stage of
leaf development to uncover the genetic mechanism underlying developmental and physiological transitions
during leaf development