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PubMed1995

Acclimation of Arabidopsis thaliana to the light environment: regulation of chloroplast composition.

The regulation by light of the composition of the photosynthetic apparatus was investigated in Arabidopsis thaliana (L.) Heynh. cv. Landsberg erecta. When grown in high- and low-irradiance white light, wild-type plants and photomorphogenic mutants showed large differences in their maximum photosynthetic rate and chlorophyll a/b ratios; such changes were abolished by growth in red light. Photosystem I (PSI) and PSII levels were measured in wild-type plants grown under a range of light environments; the results indicate that regulation of photosystem stoichiometry involves the specific detection of blue light. Supplementing red growth lights with low levels of blue light led to large increases in PSII content, while further increases in blue irradiance had the opposite effect; this latter response was abolished by the hy4 mutation, which affects certain events controlled by a blue-light receptor. Mutants defective in the phytochrome photoreceptors retained regulation of photosystem stoichiometry. We discuss the results in terms of two separate responses controlled by blue-light receptors: a blue-high-fluence response which controls photosystem stoichiometry; and a blue-low-fluence response necessary for activation of such control. Variation in the irradiance of the red growth light revealed that the blue-high-fluence response is attenuated by red light; this may be evidence that photosystem stoichiometry is controlled not only by photoreceptors, but also by photosynthetic metabolism.

Acclimatization↗
PubMed1983

The fine structure of the gill epithelium of a fresh-water flea, Daphnia magna (Crustacea: Phyllopoda) and changes associated with acclimation to various salinities. I. Normal fine structure.

Two kinds of epithelial cells, dark and light types, are alternately arranged in the gill of Daphnia magna. The dark cell has numerous mitochondria and an elaborate tubular system containing two kinds of cytoplasmic tubules, small about 70 nm in diameter, and large about 130 nm in diameter. The former occur in bundles and seem to be smooth-surfaced endoplasmic reticulum. The latter, lined with a ridged surface coat and frequently open at the lateral and basal cell membrane, are regarded as extensions of the cell membrane. The atypical cell membrane of the dark cell is modified by repeated subunits of a cytoplasmic coat on the inner leaflet of the unit membrane. The light cell exhibits a high degree of basal infoldings of the cell membrane, which represent a magnification of the surface area of the cell. Large mitochondria between the infoldings often come into intimate association with the infolded cell membrane to form a regular array of parallel mitochondria interposed with the double cell membranes. The results suggest that at least the dark epithelial cells play an important role in the osmoregulation of this animal.

Animals↗
PubMed1985

Myocardial capillarity in acclimation to hypoxia.

Capillarity, O2 diffusion distances and fiber cross-sectional growth were measured in the hearts of guinea pigs exposed early during growth to hypobaric hypoxia (PB = 430 torr, PO2 = 90 torr). Twelve 5-week old males were maintained in a hypobaric chamber for 4-14 weeks. Their hearts were perfusion-fixed via the aorta with a 2.5% glutaraldehyde, 1% formaldehyde buffered solution; blocks were cut from left (LV) and right (RV) ventricles, post-fixed in OsO4, dehydrated and embedded in Spurr medium. Blocks were cut transversely to fiber orientation, 0.5 micron thick, stained with Toluidine Blue, and photographed at 400 X. Number and location of capillaries and fiber cross-sectional areas (FCSA) were scored from these photographs and from those of normoxic controls. Growth rates were similar for control and hypoxic guinea pigs. As animals grew, LV and RV weights increased linearly with body weight. Hypoxic guinea pigs had LV weights similar to controls but the RV showed varying degrees of hypertrophy. Control and hypoxic guinea pigs showed similar linear increases in FCSA with ventricular weight, suggesting that hypertrophy was due to increased FCSA. Capillary density (CD) decreased and capillary-to-fiber ratio (C:F) increased with FCSA, and O2 diffusion distances lengthened in LV and RV of animals in both groups. CD and C:F were higher and O2 diffusion distances were shorter in most hypoxic animals compared to controls. When RV hypertrophy was large (RV greater than 0.7 g) and failure imminent, CD, C:F and O2 diffusion distances were similar to controls suggesting that in these hearts oxygenation was impaired.

Acclimatization↗
PubMed1989

Membrane acclimation by unicellular organisms in response to temperature change.

Unicellular organisms possess a wide variety of molecular mechanisms for altering the lipid composition (and thereby the physical properties) of their membranes in response to changes in environmental temperature. These are discussed with a view to establish which of the mechanisms are of more importance to bacteria, algae, and protozoa in coping with extremes of temperature.

Animals↗
PubMed2001

Acclimative changes in root epidermal cell fate in response to Fe and P deficiency: a specific role for auxin?

Root hair formation and the development of transfer cells in the rhizodermis was investigated in various existing auxin-related mutants of Arabidopsis thaliana and in the tomato mutant diageotropica. Wild-type Arabidopsis plants showed increased formation of root hairs when the seedlings were cultivated in Fe- or P-free medium. These extranumerary hairs were located in normal positions and in positions normally occupied by nonhair cells, e.g., over periclinal walls of underlying cortical cells. Defects in auxin transport or reduced auxin sensitivity inhibited the formation of root hairs in response to Fe deficiency completely but did only partly affect initiation and elongation of hairs in P-deficient roots. Application of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid or the auxin analog 2,4-dichlorophenoxyacetic acid did not rescue the phenotype of the auxin-resistant axr2 mutant under control and Fe-deficient conditions, indicating that functional AXR2 product is required for translating the Fe deficiency signal into the formation of extra hairs. The development of extra hairs in axr2 roots under P-replete conditions was not affected by auxin antagonists, suggesting that this process is independent of auxin signaling. In roots of tomato, growth under Fe-deficient conditions induced the formation of transfer cells in the root epidermis. Transfer cell frequency was enhanced by application of 2,4-dichlorophenoxyacetic acid but was not inhibited by the auxin transport inhibitor N-1-naphthylphthalamic acid. In the diageotropica mutant, which displays reduced sensitivity to auxin, transfer cells appeared to develop in both Fe-sufficient and Fe-deficient roots. Similar to the wild type, no reduction in transfer cell frequency was observed after application of the above auxin transport inhibitor. These data suggest that auxin has no primary function in inducing transfer cell development; the formation of transfer cells, however, appears to be affected by the hormonal balance of the plants.

Arabidopsis↗
PubMed1993

Seasonal acclimation of prairie deer mice.

Prairie deer mice responded to long nights by reducing their metabolic rates, core temperatures, thermal conductances and incremental metabolic responses to cold stimulus, while increasing their capacities for nonshivering thermogenesis. Some winter animals spontaneously entered daily torpor in the mornings and thereby further reduced their metabolic rates and core temperatures. Provision of exogenous melatonin (by subdermal implants) mimiced short photoperiod effects on metabolic rates and core temperatures of wild-caught, laboratory maintained animals. Provision of supplemental dietary tryptophan to laboratory animals conditioned to natural light cycles mimiced metabolic effects of long nights in summer animals, and further reduced metabolic rates of winter mice, but did not affect their core temperature levels. Newly caught, laboratory maintained deer mice responded to natural seasonal clues of short-photoperiod and increased dietary tryptophan by reducing their resting energy requirements through both lower metabolic and lower core temperature levels. Short photoperiod and seasonal change also promoted gonadal involution, and resulted in more socially tolerant huddling by mice with reduced core temperature. Reduced 24-hour LH excretion rates were also observed in winter animals which were exposed to seasonal light cycles at warm (25 degrees C) room temperatures. We propose that seasonal acclimatization involves pineal effects on sex hormone-influenced social behaviors and on resting metabolism. These effects serve to conserve resting energy expenditure and promote hypothermic insulation by wild prairie deer mice.

Acclimatization↗
PubMed1993

Changes in immunohistochemical distribution of cytochrome MC-P-448 in the rat liver during acclimation to cold.

Immunohistochemical localization of cytochrome MC-P-448 (form of cytochrome P-450 induced by methylcholanthrene) in rat hepatic lobule and the changes in their distribution pattern in response to cold exposure at 4 degrees C were investigated. The distribution of hepatocytes expressing immunoreactivity to cytochrome MC-P-448 was demonstrated with rabbit anti-MC-P-448 serum using a microphotomeasurment system P1 (Nikon). A duration of cold exposure for 1, 2, 3 or 4 weeks at 4 degrees C was applied to study the effect of cold adaptation of cytochrome MC-P-448. In control rats housed at 24 degrees C, hepatocytes showing high immunoreactivity to cytochrome MC-P-448 were located in the centrilobular areas of the hepatic lobules, whereas they disappeared markedly in the 4-week cold-exposed rats. In 1-week and 2-week cold-exposed rats, only a slight decrease in the expression of MC-P-448 positive hepatocytes was observed. These changes were clearly seen by visual inspection of the distribution topography as determined by a microphotomeasurement technique. In conclusion, cytochrome MC-P-448 forms which were predominantly located in centrilobular areas in the hepatic lobule decreased in 4-week cold-exposed rats. This was in contrast to our early report which showed an increasing tendency of cytochrome PB-P-450 forms in 4-week cold-exposed rats.

Acclimatization↗