Cytohistological Studies of the Apical Meristem of Amaranthus retroflexus Under Various Photoperiodic Regimes

Abstract

Plants of Amaranthus retroflexus L. (a quantitative short-day plant) were grown under conditions favorable to flowering and unfavorable for flowering. From the beginning of germination, plants were grown on daylengths of 8, 12, and 16 hr of light. Others were grown under natural summer day-lengths. In any of the light regimes an apex 6 days after the beginning of germination shows a zonation which is rather common for vegetative spices of many plants, namely, an axial or central zone, weak in pyroninophilia, surrounded by a flank meristem, strong in pyronino-philia (affinity of pyronin for ribonucleic acid, RNA). A 15-day-old plant, during the initiation of the fourth leaf, shows essentially the same zonation. The apex of a plant on an 8-hr photoperiod which has formed the fourth leaf may enter the reproductive phase following a brief transitional phase. Associated with this transition is a general increase in pyroninophilia of the axial zone resulting in the presence of a mantle of three or four layers of cells which stain uniformly. Throughout this period the total nucleolar volume (calculated in u3) of nucleoli increases, but more so in cells of the axial zone. These alterations mark the transition to the reproductive phase of the apex. If plants are grown under a 16-hr photoperiod, the apex bypasses the typical transformation to the reproductive phase at leaf 4 or 5 and passes to what may be termed an intermediate phase or condition. This is marked by differential responses of various regions of the axial zone. The cells of the tunica acquire a large nucleolar volume, but the cytoplasm remains weak in pyroninophilia. Pyroninophilia increases in cells of the central corpus, but total nucleolar volume remains less than that of cells in the tunica. The usual characteristics of the entire axial zone of a vegetative apex are no longer evident. The apical meristem nevertheless continues its vegetative development until about 14 to 20 leaves are formed. With passage to the reproductive phase (production of bracts) there is increased accentuation of pyroninophilia of central (corpus) and lateral cells forming a subtunica mantle. Associated with this increase is a marked increase in total nucleolar volumes of cells of the subtunica mantle in the axial position. Thus, a meristem of A. retroflexus, prevented temporarily from flowering by long photoperiods, alters its structure and cytochemistry. There is a special activity in the central axial cells usually characteristic of reproductive meristems. These facts of development are probably the bases of many errors, anomalies, and controversies regarding flowering in species exhibiting a similar mode of development.