Abscisic‐acid‐induced turion formation in Spirodela polyrrhiza L. II. Ultrastructure of the turion; a stereological analysis

Abstract

The ultrastructural features of the abscisicacid-induced turion of Spirodeta polyrrhiza are briefly described and a comparison between turion and vegetative frond tissue was tnade by stereological analysis. The turion is characterized by its small size, renifonn shape, and dark-brown coloration; the mesophyll is undifferentiated and totally laeking the substantial aerenchyma development found in the vegetative frond. The turion cells have a stnaller vacuole and a denser cytoplastn than the cells of the vegetative frond. Stereological analysis showed that the tissues differed quantitatively only in three main respects; air space formation, vacuolation, and starch and cell wall material accumulation. During development, it is suggested that the cells of the turion, while reaching the satne final size as the vegetative frond cells, accumulate numerous starch grains, thick cell walls, and large deposits of tannins and anthocyanin pigment at the expense of the vacuolar expansion characteristic of the nortnal maturity progratnme. Certain features of the turion ultrastructure indicate a differential cell sensitivity to ABA. fCev-

Abscisic‐acid‐induced turion formation in Spirodela polyrrhiza L III. Specific changes in protein synthesis and translatable RNA during turion development

“…The developmental process leading to turion formation was associated with a large accumulation of stareh (Smart & Trewavas, 1983b) and anthoeyanin pigment (Smart, 1981), both of which appeared to be largely confined to the ABA-sensitive tissue (i.e. the developing turion) and a drastic reduction of nucleic acid and protein synthesis.…”

“…the developing turion) and a drastic reduction of nucleic acid and protein synthesis. The development of the vegetative Spirodela frond involves the processes of cell division, expansion, differentiation and separation (Smart & Trewavas, 1983b). Since ABA induced the formation of turions with eoneomitant inhibition of growth (Smart & Trewavas, 1983a), it is conceivable that the mode of action of ABA might involve interference of DNA replication concerned in cell division and/or DNA-dependent RNA synthesis, since differentiation is believed to involve regulation of gene expression.…”

“…After step 2 vegetative frond cells can no longer switch to the turion developmental programme, and after step 5 turion cells can no longer switeh to the vegetative IVond programme. All the developmental steps were determined as diseussed in Smart & Trewavas (1983a), and the cell-type diagrams drawn on the figure serve as a reminder of the ultrastruetural details of the mature vegetative frond and the turion de.seribed in Smart & Trewavas (1983b). to be selective, as shown initially by a preferential decline in the level of soluble protein, and later by investigation of the in vivo pattern of proteins synthesized each day after ABA application.…”

“…la). Ultrastructural analysis indicated that the main period of cell division is probably completed by the time a vegetative frond is 2 mm long (Smart & Trewavas, 1983b), However, the vegetative frond continues to accumulate DNA until maturity, mainly due to the initiation of new areas of meristematic activity in the reproductive pockets, although contributions from polyploidy, gene amplification or stimulation of chloroplast replication during development, cannot be ruled out. The increase in DNA content shown by the developing vegetative frond would not be expected in the developing turion, since the latter has only quiescent primordia within its meristematic pockets.…”

Abscisic‐acid‐induced turion formation in Spirodela polyrrhiza L. IV. Comparative ion flux characteristics of the turion and the vegetative frond and the effect of ABA during early turion development

“…In terms of turion formation, the response concentration range of ABA is narrow, having a clearly defined threshold, optimum and upper limit (Smart & Trewavas, 1983a). In addition there is a sensitivity window to ABA in the developmental life of frond cells within which ABA initiates a new programme of events leading to turion cell formation (Smart & Trewavas, 1983b). The developmental process leading to the formation of the ABA-induced turion is accompanied by a specific repression of nucleic acid and protein synthesis (Smart & Trewavas, 1984).…”