Differential response of radish plants to supplemental ultraviolet-B radiation under varying NPK levels: chlorophyll fluorescence, gas exchange and antioxidants

Current and projected increases in ultraviolet‐B (UV‐B; 280–315 nm) radiation may alter crop growth and yield by modifying the physiological and biochemical functions. This study was conducted to assess the possibility of alleviating the negative effects of supplemental UV‐B (sUV‐B; 7.2 kJ m^?2 day^?1; 280–315 nm) on radish (Raphanus sativus var Pusa Himani) by modifying soil nitrogen (N), phosphorus (P) and potassium (K) levels. The N, P and K treatments were recommended dose of N, P and K, 1.5 times recommended dose of N, P and K, 1.5 times recommended dose of N and 1.5 times recommended dose of K. Plants showed variations in their response to UV‐B radiation under varying soil NPK levels. The minimum damaging effects of sUV‐B on photosynthesis rate and stomatal conductance coupled with minimum reduction in chlorophyll content were recorded for plants grown at recommended dose of NPK. Flavonoids increased under sUV‐B except in plants grown at 1.5 times recommended dose of N. Lipid peroxidation (LPO) also increased in response to sUV‐B at all NPK levels with maximum at 1.5 times recommended dose of K and minimum at recommended dose of NPK. This study revealed that sUV‐B radiation negatively affected the radish plants by reducing the photosynthetic efficiency and increasing LPO. The plants grown at 1.5 times recommended dose of NPK/N/K could not enhance antioxidative potential to the extent as recorded at recommended dose of NPK and hence showed more sensitivity to sUV‐B.     

Differential response of leaf gas exchange to enhanced ultraviolet-B (UV-B) radiation in three species of herbaceous climbingplants

We measured diurnal changes in photosynthetic rate, transpiration rate, stomatal conductance and water use efficiency in three species of herbaceous climbing plants (Luffa cylindrica, Trichosanthes kirilowii and Dioscorea opposita) exposed to two intensities of UV-B radiation: 3.0 μw cm^?2 (R1) and 8.0 μw cm^?2 UV-B (R2) radiation under ambient growth conditions. Responses differed per species and per treatment. In Luffa all values increased compared to the Control in both treatments, except for stomatal conductance in R2. In Trichosanthes photosynthetic rates and water use efficiency increased, while the transpiration rates decreased under both treatments, and stomatal conductance was lower in R1. In Dioscorea photosynthetic rates and water use efficiency decreased under both treatments, while the transpiration rates and stomatal conductance increased. The results suggested that to some extent increased UV-B radiation was beneficial to the growth of L. cylindrica and T. kirilowii, but detrimental to D. opposita.     

Differential response of leaf gas exchange to enhanced ultraviolet-B (UV-B) radiation in three species of herbaceous climbingplants

We measured diurnal changes in photosynthetic rate, transpiration rate, stomatal conductance and water use efficiency in three species of herbaceous climbing plants (Luffa cylindrica, Trichosanthes kirilowii and Dioscorea opposita) exposed to two intensities of UV-B radiation: 3.0 μw cm^?2 (R1) and 8.0 μw cm^?2 UV-B (R2) radiation under ambient growth conditions. Responses differed per species and per treatment. In Luffa all values increased compared to the Control in both treatments, except for stomatal conductance in R2. In Trichosanthes photosynthetic rates and water use efficiency increased, while the transpiration rates decreased under both treatments, and stomatal conductance was lower in R1. In Dioscorea photosynthetic rates and water use efficiency decreased under both treatments, while the transpiration rates and stomatal conductance increased. The results suggested that to some extent increased UV-B radiation was beneficial to the growth of L. cylindrica and T. kirilowii, but detrimental to D. opposita.     

Ambient levels of UV-B in Hawaii combined with nutrient deficiency decrease photosynthesis in near-isogenic maize lines varying in leaf flavonoids: Flavonoids decrease photoinhibition in plants exposed to UV-B

Near-isogenic lines of maize varying in their genes for flavonoid biosynthesis were utilized to examine the effects of foliar flavonoids and nutrient deficiency on maximum net photosynthetic rate (P _N) and chlorophyll (Chl) fluorescence (F_v/F_m) in response to ultraviolet-B (UV-B) radiation. Plants with deficient (30 to 70 % lower N, K, Mn, Fe, and Zn) and sufficient nutrients were exposed to four irradiation regimes: (1) no UV-B with solar photosynthetically active radiation (PAR), (2) two day shift to ambient artificial UV-B, 8.2–9.5 kJ m⁻² d⁻¹ (21–25 mmol m⁻² d⁻¹); (3) continuous ambient artificial UV-B; (4) continuous solar UV-B in Hawaii 12–18 kJ m⁻² d⁻¹ (32–47 mmol m⁻² d⁻¹). The natural ratio of UVB: PAR (0.25–0.40) was maintained in the UV-B treatments. In the adequately fertilized plants, lines b and lc had higher contents of flavonoids and anthocyanins than did lines hi27 and dta. UV-B induced the accumulation of foliar flavonoids in lines hi27 and b, but not in the low flavonoid line dta or in the high flavonoid line lc. In plants grown on deficient relative to adequate nutrients, flavonoid and anthocyanin contents decreased by 30–40 and 40–50 %, respectively, and Chl a and Chl b contents decreased by 30 and 70 %, respectively. The UV-B treatments did not significantly affect P _N and F_v/F_m in plants grown on sufficient nutrients, except in the low flavonoid lines dta and hi27 in which P _N and F_v/F_m decreased by ∼15 %. P _N, F_v/F_m, and stomatal conductance decreased markedly (20–30 %) in all lines exposed to UV-B when grown on low nutrients. The decrease in F_v/F_m was 10 % less in higher flavonoid lines b and lc. The photosynthetic apparatus of maize readily tolerated ambient UV-B in the tropics when plants were adequately fertilized. In contrast, ambient UV-B combined with nutrient deficiency significantly reduced photosynthesis in this C₄ plant. Nutrient deficiency increased the susceptibility of maize to UV-B-induced photoinhibition in part by decreasing the contents of photoprotective compounds.     

Contribution of phenolic compounds to the UV-B screening capacity of developing barley primary leaves in relation to DNA damage and repair under elevated UV-B levels

Epidermally located UV-absorbing hydroxycinnamic acid conjugates and flavonoid glycosides are known to be efficient UV-B protectants in higher plants, although important biological molecules are not always fully protected. However, repair mechanisms also exist, such as repair of damaged DNA by photolyases. To distinguish between the relative importance of the phenolic compounds and of DNA repair, developing primary leaves of two barley lines, mutant ant 30-310, deficient in flavonoids, and its parent line Ca 33787, were grown under relatively high visible light (650-700 micromol m(-2) s(-1) max for 6 h in a 13 h photoperiod) and supplemented with (+ UV-B) or without (-UV-B) 12 kJ m(-2) UV-B(BE) for 6 h daily. UV-B screening capacity of the leaf phenolics was determined at 315 nm during leaf development and compared with thymine dimers (TD) accumulation, as an indicator of UV-B-induced DNA damage and potential subsequent repair. The degree of damage was related to the phenolic contents of the leaves. UV-B screening capacity was increased ca. 4-fold in the parent line (+ UV-B), mainly due to UV-induced flavonoid (saponarin, lutonarin) accumulation in epidermal and subepidermal mesophyll tissue, relative to the flavonoid-deficient mutant. Nevertheless, in the parent line an 8-fold increase in TD levels occurred over the growth period of 18 days, whereas the mutant accumulated additional DNA damage, with 6- to 9-fold higher TD amounts. Surprisingly, under the high UV-B irradiation, growth and development of the primary leaves in both lines were only slightly reduced.     

Effects of elevated CO2 on leaf gas exchange in beech and oak at two levels of nutrient supply: consequences for sensitivity to drought in beech

Beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) were grown from seed for two whole seasons at two CO₂ concentrations (ambient and ambient + 250 μmol mol^?1) with two levels of soil nutrient supply. Measurements of net leaf photosynthetic rate (A) and stomatal conductance (g_s) of well-watered plants were taken over both seasons; a drought treatment was applied in the middle of the second growing season to a separate sample of beech drawn from the same population. The net leaf photosynthetic rate of well-watered plants was stimulated in elevated CO₂ by an average of 75% in beech and 33% in oak; the effect continued through both growing seasons at both nutrient levels. There were no interactive effects of CO₂ concentration and nutrient level on A or g_s in beech or oak. Stomatal conductance was reduced in elevated CO₂ by an average of 34% in oak, but in beech there were no significant reductions in g_s except under cloudy conditions (–22% in elevated CO₂). During drought, there was no effect of CO₂ concentration on g_s in beech grown with high nutrients, but for beech grown with low nutrients, g_s was significantly higher in elevated CO₂, causing more rapid soil drying. With high nutrient supply, soil drying was more rapid at elevated CO₂ due to increased leaf area. It appears that beech may substantially increase whole-plant water consumption in elevated CO₂, especially under conditions of high temperature and irradiance when damage due to high evaporative demand is most likely to occur, thereby putting itself at risk during periods of drought.     

Competition and sensitivity of wheat and wild oat exposed to enhanced UV-B radiation at different densities under field conditions

The influence of enhanced UV-B radiation (approximating a 15% ozone layer reduction) on competitive interaction between spring wheat (Triticum aestivum) and wild oat (Avena fatua) was examined in the field. The density-dependent mortality of both wheat and wild oat did not exhibit a significant difference between control and UV-B treatment conditions. A relatively high degree of competitive stress enhanced the effects of UV-B stress on biomass reduction. The relative competitive status of wheat in terms of total biomass increased under UV-B enhancement while it decreased when based upon grain production. Shifts in competitive balance occurred with significant changes in total biomass, especially when plants grew at higher densities in monocultures and mixtures. The sensitivity of wild oat to intensification of UV-B radiation at higher densities in mixtures was greater than that at lower densities. At all densities examined, wheat grown in mixture was significantly less sensitive to UV-B radiation than that in monoculture, and just the opposite for wild oat. The density of monocultures did not alter the response index (RI) of wheat and wild oat to enhanced UV-B radiation.     

The sensitivity of soil respiration to soil temperature,moisture, and carbon supply at the global scale

Soil respiration (Rs) is a major pathway by which fixed carbon in the biosphere is returned to the atmosphere, yet there are limits to our ability to predict respiration rates using environmental drivers at the global scale. While temperature, moisture, carbon supply, and other site characteristics are known to regulate soil respiration rates at plot scales within certain biomes, quantitative frameworks for evaluating the relative importance of these factors across different biomes and at the global scale require tests of the relationships between field estimates and global climatic data. This study evaluates the factors driving Rs at the global scale by linking global datasets of soil moisture, soil temperature, primary productivity, and soil carbon estimates with observations of annual Rs from the Global Soil Respiration Database (SRDB). We find that calibrating models with parabolic soil moisture functions can improve predictive power over similar models with asymptotic functions of mean annual precipitation. Soil temperature is comparable with previously reported air temperature observations used in predicting Rs and is the dominant driver of Rs in global models; however, within certain biomes soil moisture and soil carbon emerge as dominant predictors of Rs. We identify regions where typical temperature‐driven responses are further mediated by soil moisture, precipitation, and carbon supply and regions in which environmental controls on high Rs values are difficult to ascertain due to limited field data. Because soil moisture integrates temperature and precipitation dynamics, it can more directly constrain the heterotrophic component of Rs, but global‐scale models tend to smooth its spatial heterogeneity by aggregating factors that increase moisture variability within and across biomes. We compare statistical and mechanistic models that provide independent estimates of global Rs ranging from 83 to 108 Pg yr^?1, but also highlight regions of uncertainty where more observations are required or environmental controls are hard to constrain.     

The importance of the ratio UV-B/photosynthetic active radiation (PAR) during leaf development as determining factor of plant sensitivity to increased UV-B irradiance: effects on growth, gas exchange and pigmentation of bean plants (Phaseolus vulgaris cv. Label)

To evaluate the effect of different naturally occurring irradiation conditions on the sensitivity of bean (Phaseolus vulgaris cv. Label) to increased UV-B levels, plants were grown under six different light treatments. In the control series (at ambient levels of UV-B), UV-B and visible light were decreased in parallel, resulting in three different total irradiation treatments with the same UV-B/PAR ratio. A second series with a 15% increase in UV-B irradiation at each PAR level was used to investigate the effect of UV-B under the varying total irradiance levels. The different total irradiance levels resulted in large differences in total dry weight, specific leaf weight, photosynthesis-light response and pigment concentrations. Nevertheless, the 15% increase in UV-B resulted in equal reductions in total dry weight (from 24.5 to 34.3%) and effective photosynthesis for all light levels. The accumulation of protective pigments in the primary bean leaves was strongly correlated to the total irradiance level (200% increase from the lowest to the highest light level), but was not influenced by increasing UV-B levels. As the UV-B/PAR ratio outside increases with decreasing total irradiance (when induced by cloud cover) this implies that low radiation levels are potentially dangerous to some plants, even though the UV-B levels may seem negligible.     

Surface morphology, leaf and cuticle thickness of four dwarf shrubs from a sub-Arctic heath following long-term exposure to enhanced levels of UV-B

In 1991 a field experiment was established in subarctic heathland at Abisko (68°35'N, 18°82'E), northern Sweden, to investigate the effects of enhanced UV-B (280–315 nm) radiation, simulating 15% ozone depletion, on plants in their natural environment. Leaves of the four dominant dwarf shrubs, the deciduous Vaccinium myrtillus L. and V. uliginosum L. and the evergreen V. vitis-idaea L. and Empetrum hermaphroditum Hagerup were examined after 7 years of UV-B treatment. SEM and ESEM were used to visualize surface features and to determine trichome density. Multiphoton laser scanning microscopy showed that UV-B absorbing compounds were localized in the trichomes of all species. Trichomes varied in size, number and distribution between the species. Enhanced UV-B reduced adaxial trichome density significantly (by approximately 25%) in only one species, V. uliginosum . This effect could be of importance for the UV-B absorbing potential of the adaxial epidermis of V. uliginosum . Epicuticular wax structures were found only on the abaxial surface of V. uliginosum and were unaffected by enhanced UV-B. The cuticular surfaces of all other species were smooth and featureless. Leaf thickness, adaxial and abaxial cuticle thickness varied between the species although there was no apparent effect of enhanced UV-B. It is concluded that long-term enhancement of UV-B has an effect on adaxial trichome density in V. uliginosum , but that there is no general effect on leaf morphology of the other species.     

Responses of gas exchange,cellular membrane integrity,and antioxidant enzymes activities of salinity-stressed winter wheat to ozone pollution

A sand-culture experiment was conducted in open-top chambers which were constructed in a greenhouse to investigate the responses of salt-stressed wheat (Triticum aestivum L.) to O₃. Plant seeding of JN17 (a popular winter wheat cultivar) was grown in saltless (−S) and saline (+S, 100 mM NaCl) conditions combined with charcoal-filtered air (CF, < 5 ppb O₃) and elevated O₃ (+O₃, 80 ± 5 ppb, 8 h day⁻¹) for 30 d. O₃ significantly reduced net photosynthetic rate (PN), stomatal conductance, chlorophyll contents and plant biomass in -S treatment, but no considerable differences were noted in those parameters between +O₃+S and CF+S treatments. O₃-induced loss in cellular membrane integrity was significant in -S plants, but not in +S plants evidenced by significant elevations being measured in electrolyte leakage (EL) and malondialdehyde (MDA) content in -S plants, but not in +S plants. Both O₃ and salinity increased proline content and stimulated antioxidant enzymes activities. Soluble protein increased by salinity but decreased by O₃. Abscisic acid (ABA) was significantly elevated by O₃ in -S plants but not in +S plants. The results of this study suggested that the specificity of different agricultural environments should be considered in order to develop reliable prediction models on O₃ damage to wheat plants.     

Leaf gas exchange, chlorophyll fluorescence and growth responses of Genipa americana seedlings to soil flooding

Effects of soil flooding on photosynthesis and growth of Genipa americana L. seedlings, a neotropical fruit-tree species used in gallery forest restoration programs, were studied under glasshouse conditions. Despite the high survival rate and wide distribution in flood-prone habitats of the neotropics, previous studies demonstrated that growth of G. americana is reduced under soil flooding. Using leaf gas exchange and chlorophyll fluorescence measurements, we tested the hypothesis that stomatal limitation of photosynthesis is the main factor that reduces carbon uptake and growth rates of G. americana seedlings. Throughout a 63-day flooding period, the survival rates were 100%. The maximum values of the net photosynthetic rate (A) and stomatal conductance to water vapor (g_s) of control seedlings were 9.86 μmol CO₂ m⁻² s⁻¹ and 0.525 mol H₂O m⁻² s⁻¹, respectively. The earliest effects of flooding were significant decreases in g_s and A, development of hypertrophied lenticels and decrease in the dry weight of roots. A strong effect of the leaf-to-air vapor pressure deficit (LAVPD) on g_s and A were observed that was enhanced under flooded conditions. Between 14 and 63 days after flooding, significant reductions in g_s (31.7% of control) and A (52.9% of control) were observed followed by significant increments in non-photochemical quenching (q_N) (187.5% of control). During the same period, there were no differences among treatments for the ratio between variable to initial fluorescence (F_v/F₀), the maximum quantum efficiency of the photosystem II (F_v/F_m) and photochemical quenching (q_P), indicating that there was no damage to the photosynthetic apparatus. Based on the results, we conclude that decreases in stomatal opening and stomatal limitation of photosynthesis, followed by decrease in individual leaf area are the main causes of reductions in carbon uptake and whole plant biomass of flooded seedlings.     

Relevance of drug uptake,cellular distribution and cell membrane fluidity to the enhanced sensitivity of Down's syndrome fibroblasts to anticancer antibiotic-mitoxantrone

Sensitivity of human fibroblasts derived from Down's syndrome (DS) individuals (S-240, T-158, T-74, T-164) and normal donors (S-126, WA-1) to anticancer antibiotic-mitoxantrone (1,4-dihydroxy-5,8-bis((2-((2-hydroxy-ethyl)amino)ethyl)amino)-9,10-anthracenedione dihydrochloride; MIT) and its relationship to the transport rate, cellular distribution and interaction with cell membrane were studied. The survival assay showed that MIT was more toxic to trisomic fibroblast lines than to normal cells. Studies of transport kinetics indicated that the amount of drug taken up and extruded by DS cells was diminished, compared to control cells. In contrast, the cellular level of MIT associated with DNA was greater in trisomic than in normal cells. The fluorescence anisotropy measurements of TMA-DPH and 12-AS demonstrated that the fluidity of the polar region of the outer lipid monolayer of DS cell membrane was decreased in comparison with normal cells. MIT treatment decreased fluidity of the inner hydrophobic region of plasma membrane, but only slightly influenced the fluidity of the outer surface of the cell membrane. Finally, we concluded that lowered membrane fluidity, diminished amount of MIT extruded by cells and the enhanced level of the drug associated with DNA could be responsible for the enhanced sensitivity of DS fibroblasts to the MIT treatment.     

Biomass partition,leaf gas exchange and water relations of alfalfa and milkvetch seedlings in response to soil drying

This study compared physiological and growth responses to water stress of two legume species during the seedling stage. Potted alfalfa (Medicago sativa L. cv. Algonquin) and milkvetch (Astragalus adsurgens Pall. cv. Pengyang earlymaturing vetch) seedlings were grown under well-watered [soil water content (SWC) maintained at 14.92% daily] or water-stressed conditions (drying) for 15 days. Net photosynthetic rate (P _N), transpiration rate (E) and stomatal conductance (g _s) of both species decreased parabolically. When SWC decreased to 7.2% and 10.3%, g _s values for alfalfa and milkvetch were significantly different from those of the respective well-watered plants (p<0.05). When SWC decreased to 6.6% for alfalfa and 6.8% for milkvetch, leaf water potentials (ψ_L) were significantly different from those of the well-watered plants (p<0.05). Thus the difference between the SWC thresholds for a nonhydraulic root signal (nHRS) and a hydraulic root signal (HRS) were 0.6% and 3.5% for alfalfa and milkvetch, respectively. Milkvetch had a lower g _s than alfalfa for a given SWC (p<0.05). Although alfalfa seedlings had a higher dry mass (DM) and root:shoot ratio (R/S) than milkvetch in both treatments (p<0.05), we concluded that milkvetch seedlings had greater drought tolerance than alfalfa.     

The responses of spatial situation,surface structure characteristics of leaves and sensitivity of two local rice cultivars to enhanced UV-B radiation under terraced agricultural ecosystem

Under the situ terraced field experiments, effects of artificial UV-B radiation enhancement (0, 2.5, 5.0, 7.5 kJ m^?2) on spatial situation and surface structure of leaves and responses index of two local cultivars rice (Oryza sativa L.)—Yuelianggu and Baijiaolaojin in Yuanyang County, China in shooting stage were studied. The results showed that: (1) due to the enhanced UV-B radiation, leaf apex–base distance, leaf pedestal height, leaf rolling degree and wax content in leaves increased, while leaf apex–stem distance, distance between leaves and leaf angle decreased. The response index of growth was positive when UV-B levels were 2.5 and 5.0 kJ m^?2, which showed some adaptation. (2) The enhanced UV-B radiation resulted in smaller stomata with higher density and more papilla for both rice cultivars. (3) The enhanced UV-B radiation also leaded to larger silica cells and significantly increases the amount of papilla, spike and epidermal hair for both rice cultivars. (4) Yuelianggu cultivar showed an excellent adaptation on the aspect of spatial situation with UV-B radiation of 2.5 and 5.0 kJ m^?2, while Baijiaolaojin exhibited better adaptation respecting the surface structure of leaves when UV-B was 2.5 kJ m^?2. By changing spatial situation of leaves, structure and density of stomata, and non-stomatal structures (wax layer, silica cell, cork cell, papilla, spike and epidermal hair), two self-retention rice cultivars could adapt to the increased UV-B radiation. On the aspect of the response index, Baijiaolaojin showed better adaptation than Yuelianggu did when the UV-B was 2.5 kJ m^?2.     

Top and root growth and nutrient absorption of Prunus avium L. at two soil pH and P levels

One-year-old Prunus avium L. were grown under greenhouse conditions in a Countesswells soil in all combinations of 2 pH and 2 P levels. The soil, obtained from a long-term liming and fertilizer experiment, provided pH values throughout the experiment of 3.75–3.99 (pH 1) and 4.81–5.41 (pH 2). The P treatments had 0.43% acetic acid extractable P of 31–44 g g^-1 (P1) and 145–173 g g^-1 (P2). The trees were harvested 92 (H1), 134 (H2), and 168 (H3) days after initiation of growth.Top (leaf+new stem) dry weight was significantly increased for pH 2 and P2 at H2 and H3. P2 also increased leaf weight (H1), the weight of the original stem-root (H2 and H3), and root length but decreased root diameter at both soil pHs (H2 and H3). Total tree uptake of N, P, K, Ca, and Mg was also increased by pH-P combinations which had significantly greater dry matter production and root length. Total Mn uptake decreased at pH2. Root nutrient inflows (uM m^-1 day^-1) were increased for Ca at pH2 and for P at P2. Mn inflow decreased at pH2 and at pH1 P2 although the increased root length associated with the latter treatmen resulted in increased total tree Mn uptake. In general, high nutrient inflows occurred in all trees at H1 and in severely stunted trees at pH1 P1; both had larger than average root diameters.     

Freezing tolerance of citrus, spinach, and petunia leaf tissue : osmotic adjustment and sensitivity to freeze induced cellular dehydration

Seasonal variations in freezing tolerance, water content, water and osmotic potential, and levels of soluble sugars of leaves of field-grown Valencia orange (Citrus sinensis) trees were studied to determine the ability of citrus trees to cold acclimate under natural conditions. Controlled environmental studies of young potted citrus trees, spinach (Spinacia pleracea), and petunia (Petunia hybrids) were carried out to study the water relations during cold acclimation under less variable conditions. During the coolest weeks of the winter, leaf water content and osmotic potential of field-grown trees decreased about 20 to 25%, while soluble sugars increased by 100%. At the same time, freezing tolerance increased from lethal temperature for 50% (LT₅₀) of −2.8 to −3.8°C. In contrast, citrus leaves cold acclimated at a constant 10°C in growth chambers were freezing tolerant to about −6°C. The calculated freezing induced cellular dehydration at the LT₅₀ remained relatively constant for field-grown leaves throughout the year, but increased for leaves of plants cold acclimated at 10°C in a controlled environment. Spinach leaves cold acclimated at 5°C tolerated increased cellular dehydration compared to nonacclimated leaves. Cold acclimated petunia leaves increased in freezing tolerance by decreasing osmotic potential, but had no capacity to change cellular dehydration sensitivity. The result suggest that two cold acclimation mechanisms are involved in both citrus and spinach leaves and only one in petunia leaves. The common mechanism in all three species tested was a minor increase in tolerance (about −1°C) resulting from low temperature induced osmotic adjustment, and the second in citrus and spinach was a noncolligative mechanism that increased the cellular resistance to freeze hydration.     

Differential sensitivity of muntjac lymphocyte chromosomes to mitomycin C,bromodeoxyuridine and hydroxylamine at different cell-cylce stages

Quantitative and qualitative analyses were made of aberrations induced by 3 hitherto well-known mutagens, mitomycin C (MC), 5-bromodeoxyuridine (BUdR and hydroxylamine hydrocholride (HA), in muntjac chromosomes, during different stages of the cell cycle. The sensitivity ro MC was increased in G₁, reached its maximum in early S and was considerably decreased in late S and G₂ stage treated cells. BUdR induced maximal aberrations when given during the synthetic phase and the cells in G₁ and G₂ were least affected. The sensitivity of the cells to HA in terms of induced chromosomal aberrations increased as they moved through the cell cycle, i.e. more damage was observed in cells treated in late S and G₂ stages than in those treated at G₁ and early S stages. While there were defined patterns of cell-cylce stage-dependent sensitivity for all 3 chemicals, the chromosomal sites being preferentially affected by each were found to be specific and invariant at different stages. Thus, it is presumed that the functional state of such “preferred sites” at one or other stage of the cell cycle is the factor responsible for the stage-dependent sensitivity of a cell towards these chemicals.