An Interaction between the Effects of Kinetin and Gibberellin in Retarding Leaf Senescence

The relation hetween the effects of kinetin and gibberellin on retardation of leaf senescence was studied. Leaf discs were incubated for five days in several hormone concentrations, the chlorophyll was extracted and its amount estimated spectrophotometrically. Investigation of leaves from actively growing plants of Taraxacum megalorrhizon and Tropaeolum majus showed that an interaction existed between the effect of both hormones. Leaf age, light intensity and day length had a marked effect on the degree of response to the hormonal treatments, hut the change in response effected by these conditions remained similar for both hormones. Possible interpretations of the interaction observed between the effects of kinetin and gibberellin are discussed.     

Leaf water content and hormone effects on ribonuclease activity

In barley (Hordeum vulgare) leaves in which the water balance was not hampered, kinetin and abscisic acid effected the well documented decrease and increase, respectively, in RNase activity. When the plants were exposed to water shortage, leaf-water saturation deficit increased steadily, with kinetin enhancing and abscisic acid retarding the rise. Under drought, the pattern of hormonal effects was inverted, with kinetin enhancing RNase activity over and above the activity assayed in abscisic acid-treated leaves. A very close relationship between RNase activity and water saturation deficit was found and significantly, it was maintained irrespective of the hormonal treatment, which in itself markedly modified leaf—water saturation deficit. The inverted effects of kinetin and of abscisic acid on RNase activity under conditions of water shortage were interpreted as resulting primarily from the effects of these hormones on leaf-water. It is suggested that under conditions of increased water deficiency in the plant, cell-water supersedes hormonal regulation in effecting RNase activity.     

Abscisic Acid and cytokinin contents of leaves in relation to salinity and relative humidity

The question is raised whether the hormonal modifications in a plant exposed to osmotic root stress result directly from the decrease in water potential of the root environment or from disturbances of the plant's water balance.     

Effect of temperature on yield and night biomass loss in Spirulina platensis grown outdoors in tubular photobioreactors

Outdoor experiments carried out in Florence, Italy (latitude 43.8° N, longitude 11.3° E), using tubular photobioreactors have shown that in summer the average net productivity of a Spirulina platensis culture grown at the optimal temperature of 35 °C was superior by 23% to that observed in a culture grown at 25 °C. The rates of night biomass loss were higher in the culture grown at 25 °C (average 7.6% of total dry weight) than in the one grown at 35 °C (average 5%). Night biomass loss depended on the temperature and light irradiance at which the cultures were grown, since these factors influenced the biomass composition. A net increase in carbohydrate synthesis occurred when the culture was grown at a low biomass concentration under high light irradiance or at the suboptimal temperature of 25 °C. Excess carbohydrate synthesized during the day was only partially utilized for night protein synthesis.     

Apparent heterogeneity in the response of quiescent swiss 3T3 cells to serum growth factors: implications for the transition probability model and parallels with "cellular senescence" and "competence"

When subconfluent, Swiss 3T3 cells made quiescent by serum deprivation are stimulated with low concentrations of serum (ca. 1%), only a proportion of them (roughly 50%) enter S phase despite daily replacement with fresh, low-serum medium. The cells that fail to enter S phase are not incapable of doing so, since most of them initiate DNA synthesis after transfer to 10% serum. It would appear that individual cells vary in their growth factor requirements. Using time-lapse cinemicroscopy a few of the cells that respond to low serum were seen to give rise to several generations of progeny, while the majority of cells failed to divide at all, or divided once at most. Despite this, differences between cells in growth factor requirements do not seem to be heritable in the long term, since attempts to enrich for responding cells by prolonged culture in 1% serum have been unsuccessful. Rather, it would appear that the capacity to respond to low serum is an unstable property lost after a few generations in low serum. The loss of responsiveness shows parallels with "cellular senescence" and could conceivably result from decay of the platelet-derived growth factor-induced state of "competence." But regardless of why some cells respond to low serum while others do not, it is clear that the kinetics of entry into S phase after serum stimulation of quiescent 3T3 cells are not strictly first-order, since the labelling index plateaus after roughly 3 days at values substantially below 100%. As such, the kinetics, though not contradicting the transition probability model, cannot be taken to support it as was previously thought.     

Molecular evolution of rodent insulins

Several trees of amino acid sequences of rodent insulins were derived with the maximum-parsimony procedure. Possible orthologous and paralogous relationships were investigated. Except for a recent gene duplication in the ancestor of rat and mouse, there are no strong arguments for other paralogous relationships. Therefore, a tree in agreement with other biological data is the most reasonable one. According to this tree, the capacity to form zinc-binding hexamers was lost once in the ancestor of the hystricomorph rodents, followed by moderately increased evolutionary rates in the lineages to African porcupine and chinchilla but highly increased rates in at least three independent lines to other taxa of this suborder: guinea pig, cuis, and Octodontoidea (coypu and casiragua).      

Molecular characterization and chromosomal mapping of melanoma growth stimulatory activity, a growth factor structurally related to beta-thromboglobulin.

Melanoma growth stimulatory activity (MGSA) is a mitogenic polypeptide secreted by Hs294T human melanoma cells. Comparison of the N-terminal sequences of the 13 and 16 kd MGSA species with the cDNA sequence revealed that the mature form of human MGSA is maximally 73 amino acids long. Expression of the cDNA in mammalian cells results in the secretion of this peptide with mitogenic activity. MGSA is structurally related to the platelet-derived beta-thromboglobulin and to several other polypeptides. These factors may constitute a family of growth factors. MGSA mRNA was detected in a variety of cell types. The level of MGSA mRNA in melanoma cells is strongly elevated by treatment with MGSA. MGSA is the gene product of a recently detected gene gro. The gene was mapped to chromosome 4 (region q13----q21). This same region also contains genes for two of the structurally related factors, for c-kit, a receptor for an as yet unidentified ligand, and for 'piebald trait', an inherited skin pigmentation disorder.     

PHYSIOLOGICAL CHARACTERISTICS OF SPIRULINA PLATENSIS (CYANOBACTERIA) CULTURED AT ULTRAHIGH CELL DENSITIES1

Photosynthetic activity and growth physiology of Spirulina platensis (Nordstedt) Geitler cultures maintained at ultrahigh cell densities (i.e. above 100 mg chlorophyll-L^?1) in a newly designed photobioreactor were investigated. Nitrogen (NaNO₃) in standard Zarouk medium was characterized as a major nutrient-limiting factor in such cultures. The effect of ultrahigh cell density on photoinhibition of photosynthesis, as reflected by chlorophyll fluorescence and photosynthetic oxygen evolution, was studied: elevating the population density may arrest photoinhibition induced by high photon flux density, as well as low temperature. The relationship between incident irradiance and oxygen production rate was linear in situ for cultures at the optimal cell density, indicating that light limitation rather than light saturation or photoinhibition is the dominant condition outdoors in cultures of ultrahigh cell densities. In contrast with other reports, the extent of biomass loss at night due mainly to dark respiration was found to be relatively small when cell density was optimal, exerting only a minor effect on overall net productivity. Measurements of oxygen consumption at night revealed low rates of respiration, which may be explained by the low value of the volumetric mass transfer coefficient (K_La) of oxygen. Hence, reduced oxygen tension may play a role in preventing full expression of the respiratory potential in ultrahigh cell density cultures in which photoadaptive strategy may explain cell composition. Ultrahigh cell densities optimized with respect to the intensity of the light source, the length of the light path, and the extent of stirring represent the key for obtaining high output rates of cell mass and some natural products.