The Maize Leaf

The leaf is the most conspicuous organ of the repeating shoot segment. A typical elite inbred, such as W23, has approximately 20 leaves. Because leaves are founded sequentially within the meristem over weeks of time, the older, more juvenile leaves arise earlier and in a more basal position than the younger, more adult leaves. Two numbers designate any particular leaf: the leaf number (L) counting the nonleaf coleoptile as zero, and the plastochron number (P), counting the established but predivision meristematic founder cells as zero. By this convention, L10, P3 is the tenth leaf above the coleoptile of the plant at an approximately 4-mm primordial developmental stage, when the leaf is the third leaf from the meristem. Such a leaf might be called a young, adult leaf. L2, P15 is a juvenile, old leaf. A plastochron is actually a unit of time—the time between the initiation of one leaf and the next leaf from the meristem, as if it were a constant; thus, P4 means four units of time have passed since being founded in the meristem. The actual developmental meaning of any particular L, P designation is different in different genotypes and at different stages of shoot development. Even within any inbred line, each leaf is unique, and the plastochron of each may be different. (See Sylvester et al. 1990, and references therein.) Therefore we use both L and P designations when they apply and accurate descriptions of the genotype and growth conditions whenever a leaf is mentioned.