Figure 3.

Effects of NPA on growth of young leaf primordia. A and C, Light micrographs of shoots containing five leaf primordia cultured for 14 d without NPA. Note that three new leaf primordia (6–8, a small protrusion on the right flank of the SAM) have developed in culture, and the first leaf primordium (1) was dissected to aid in tissue fixation. B and D, Equivalent shoots cultured in 30 μm NPA. Note that five total leaf primordia are present, and no new leaves are initiated. In addition, the NPA-treated fourth and fifth leaves (4 and 5 in B and D) are significantly smaller than corresponding leaves of shoots cultured without NPA (4 and 5 in A and B). Thus, NPA treatment induces the arrest of new leaf initiation and retards growth of existing young leaf primordia. Numbers refer to the leaf number (as opposed to the plastochron number) as described in Figure 2 and in “Materials and Methods.” Bars in C and D = 100 μm and A and B = 250 μm.