EFFECT OF ENVIRONMENT ON THE EARLY STEPS OF EAR INITIATION IN MAIZE (ZEA-MAYS L)

The effects of environmental conditions on ear-shoot initiation have b een investigated in three inbred genotypes of Zea mays L, which are us ed for seed production, Scanning electron microscopy (SEM) and binocul ar examination during the vegetative phase showed that axillary merist ems are initiated at the same rate as the leaf primordia on the apical meristem, but with a delay of 5.6-7.0 plastochrons, depending on the genotype, Furthermore, the topmost axillary meristem is initiated on t he same day as the tassel, whatever the genotype, One of the inbreds ( B22) used in this study has been reported to exhibit, in field conditi ons, a reproductive failure affecting ear initiation, causing the topm ost ear to be replaced at maturity by a sterile, leaf-like, structure, Scanning electron microscopic study of the formation of the abnormal axillary buds indicated that ear failure resulted from the early colla pse of the axillary meristem followed by elongation of the prophyll or of the meristem itself, Using controlled environments, ear abortion w as mimicked by a chilling treatment (10 degrees C), given just before tassel initiation, Other factors, such as high irradiance and flooding , enhanced the abortive response, The critical stage for the main ear was just before the initiation of the topmost axillary meristem which also corresponded to tassel initiation, Chilling the plants before or after tassel initiation either induced an acclimation response or had no effect, The three inbreds showed differential responses to the stre ss treatment, indicating that a genetic factor is implicated as well, It is suggested that chilling causes a perturbation of apical dominanc e which, in the responsive genotypes, represses axillary meristem deve lopment, The use of a stress-sensitive inbred such as B22 as a model s ystem could yield some interesting clues to the mechanism of endogenou s control of ear initiation in maize.