Organogenesis of the Staminate and Pistillate Inflorescences of Pop and Dent Corns: Relationship to Leaf Stages

Until recently, popcorn and dent corn (Zea mays L.) were considered analogous in their seasonal developmental patterns. Differences in the rates of seasonal floral development may be important in the relative susceptibility of the respective types of corn to stress. Although several researchers have considered that a relationship exists between the number of leaves emerged on a corn plant and inflorescence development, no intensive attempts to investigate floral development relative to leaf stage have been reported. This is an essential criterion if numeric staging systems are to be developed for use with regression techniques to model corn floral development relative to stress and consequent grain production. The objectives of this pilot study were to relate floral development to external leaf stage, and to compare the rate of floral development between a popcorn and its assumed analogue, a dent corn. Plants of two greenhouse-grown corn cultivars, ‘Iopop 12’ and ‘B73 ✕ Mo17’, were sampled at intervals from emergence of the third leaf to that of the 13th leaf. The shoot apex and uppermost pistillate inflorescence were excised and examined fresh in a scanning electron microscope (SEM) or fixed and prepared for SEM examination using traditional techniques. Staminate and pistillate spikelet development in B73 ✕ Mo17 and in Iopop 12 followed a similar sequence; but, relative to leaf stages, Iopop 12 tended to lag one to two emerged-leaf stages behind B73 ✕ Mo17. With respect to emerged-leaf stages, B73 ✕ Mo17 and Iopop 12, respectively, developed as follows: tassel initiation at fourth vs. fifth, ear initiation at sixth vs. eighth, and pistillate silk formation at 11th vs. 13th. Staminate and pistillate spikelet organogenesis followed a similar sequence both within and between cultivars, with initiation of two bisexual florets and subsequent abortion of appropriate floral organs to form unisexual florets. One floret developed in each pistillate spikelet, whereas two developed in each taminate spikelet. Elongation of the apical dome to 0.4 mm was considered to be the most definitive indicator of tassel and ear initiation. The appearance of the ligule above the auricles of the previous leaf was considered the most definitive indicator of leaf-blade emergence.