A "Point of No Return" in the Cell Cycle of Chlorella

In a Chlorella culture growing synchronously at pH 6.3 undera 12 hr light/12 hr dark regime, DNA replication occurs betweenthe 8th and the 12th hour of the cycle, the main period of proteinand chlorophyll synthesis occurring between the 4th and 12thhour of the cycle. When the culture is transferred to alkalinepH at any time up to the 8 hr of the cycle, autospore releaseis prevented, and the pattern of synthesis of DNA, protein andchlorophyll is altered. However, when the culture is transferredto alkaline conditions after the 8th hour of the cycle, thepattern follows that of a culture growing at pH 6.3 with respectto cell number and volume, as well as protein, chlorophyll andDNA contents. Thus, a transition point seems to occur afterthe 8 hr of the cycle. The existence of such a point was alsodemonstrated by reciprocal experiments in which Chlorella wascultured at an alkaline pH and transferred to pH 6.3 at varioustimes in the cell cycle. ¹ Present address: Applied Research Institute, Ben-Gurion Universityof the Negev, P.O. Box 1025, Beer-Sheva 84110, Israel. (Received October 2, 1981; Accepted January 20, 1982)     

Alkalinity-induced Aggregation in Chlorella vulgaris II. Changes in the Cell Wall during the Cell Cycle

In a Chlorella culture growing synchronously at pH 6.3 undera 12 hr light-12 hr dark regime, autospores are released duringthe dark period of the cycle by mechanical rupture of the cellwall. When the culture is made in an alkaline medium (pH 9.5)during the first 8 hr of the light period, the release of autosporesis prevented, and at the end of the cycle, the cells are enlarged.It was found that the diurnal fluctuations in cellulase specificactivity and in polysaccharide content, which normally takeplace at pH 6.3, do not occur at alkaline pH. The concentrationof polysaccharides, as measured by the periodic acid-Schiffreagent, and cellulase activity are almost doubled. Peak levelsare reached at the 8th hour of the cell cycle rather than theusual 4th hour and persist during the second part of the cellcycle, whereas they normally decrease. It is suggested thatunder alkaline conditions, the diurnal increase in cellulaseactivity is amplified and prolonged, resulting in increasedflexibility of the mother cell wall. As a result, the cell wallis stretched, rather than ruptured, during the development ofdaughter autospores, which are thus prevented from being released. ¹ Present address: Research & Development Authority, Ben-GurionUniversity of the Negev, P.O. Box 1025, Beer-Sheva 84110, Israel. (Received June 16, 1981; Accepted October 22, 1981)     

Effect of Sodium Chloride on Fruit Ripening of the Nonripening Tomato Mutants nor and rin

Tomato (Lycopersicon esculentum Mill) plants of the nonripening mutant nor, the ripening-inhibited mutant rin, and the normal cultivar `Rutgers' were grown in nutrient solution supplemented with 3 grams per liter NaCl from the time of anthesis. In plants treated with NaCl, all the ripening parameters of the fruits of the nor mutant increased, but those of the rin mutant did not. The ripening of the fruits of the NaCl-treated nor plants was characterized by the development of a red color and taste, increased pectolytic activity, and increased evolution of CO₂ and ethylene. These changes do not normally take place in nor under control conditions. The values of these ripening parameters in nor were lower than those of the normal Rutgers fruits. In addition, both in nor and rin and in the normal variety, exposure of the plants to NaCl shortened the developmental period of the fruit, decreased the fruit size, and increased the concentrations of total soluble solids, Na⁺, Cl⁻, reducing sugars, and titratable acids in the fruit. The role of NaCl in overcoming the inability of nor to ripen is discussed.     

Alkalinity-induced aggregation in Chlorella vulgaris I. Changes in cell volume and cell-wall structure

In Chlorella vulgaris cell aggregation, the clustering of singlecells into groups is induced by an alkaline pH (9.5). The processof alkalinity-induced aggregation may be divided into two stages:the first stage (0–24 hr after exposure to the alkalinepH) is characterized by enhanced precipitation of cells fromthe medium, as well as by a seven fold increase in cell volume.The second stage (24–120 hr) is associated with a furtherincrease in the extent of cell precipitation in the culture,which seems to result from the aggregation of clusters of enlargedcells. Electron micrographs reveal the existence, at this phase,of a number of autospores in the cells within a modified multi-layeredmother cell wall. The pectin content of cells at this stageis twice that of control cells grown at pH 6.3. In addition,the relative content of the different pectin fractions is modifiedas a result of the exposure to alkalinity. It is suggested thatthe aggregates result from the repeated failure of the cellsto detach from their original mother cell walls, thus formingclusters which represent several generations of cells. ¹Present address: Division of Food Storage, ARO Beit Dagan,P.O.B. 6, Israel. (Received September 3, 1979; )