Iron Superoxide Dismutase Protects against Chilling Damage in the Cyanobacterium Synechococcus species PCC7942

A strain of Synechococcus sp. PCC7942 lacking functional Fe superoxide dismutase (SOD), designated sodB⁻, was characterized by its growth rate, photosynthetic pigments, inhibition of photosynthetic electron transport activity, and total SOD activity at 0°C, 10°C, 17°C, and 27°C in moderate light. At 27°C, the sodB⁻ and wild-type strains had similar growth rates, chlorophyll and carotenoid contents, and cyclic photosynthetic electron transport activity. The sodB⁻ strain was more sensitive to chilling stress at 17°C than the wild type, indicating a role for FeSOD in protection against photooxidative damage during moderate chilling in light. However, both the wild-type and sodB⁻ strains exhibited similar chilling damage at 0°C and 10°C, indicating that the FeSOD does not provide protection against severe chilling stress in light. Total SOD activity was lower in the sodB⁻ strain than in the wild type at 17°C and 27°C. Total SOD activity decreased with decreasing temperature in both strains but more so in the wild type. Total SOD activity was equal in the two strains when assayed at 0°C.     

A Comparison of the Effects of Chilling on Thylakoid Electron Transfer in Pea (Pisum sativum L.) and Cucumber (Cucumis sativus L.)

Experiments comparing the photosynthetic responses of a chilling-resistant species (Pisum sativum L. cv Alaska) and a chilling-sensitive species (Cucumis sativus L. cv Ashley) have shown that cucumber photosynthesis is adversely affected by chilling temperatures in the light, while pea photosynthesis is not inhibited by chilling in the light. To further investigate the site of the differential response of these two species to chilling stress, thylakoid membranes were isolated under various conditions and rates of photosynthetic electron transfer were determined. Preliminary experiments revealed that the integrity of cucumber thylakoids from 25°C-grown plants was affected by the isolation temperature; cucumber thylakoids isolated at 5°C in 400 millimolar NaCl were uncoupled, while thylakoids isolated at room temperature in 400 millimolar NaCl were coupled, as determined by addition of gramicidin. The concentration of NaCl in the homogenization buffer was found to be a critical factor in the uncoupling of cucumber thylakoids at 5°C. In contrast, pea thylakoid membranes were not influenced by isolation temperatures or NaCl concentrations. In a second set of experiments, thylakoid membranes were isolated from pea and cucumber plants at successive intervals during a whole-plant light period chilling stress (5°C). During wholeplant chilling, thylakoids isolated from cucumber plants chilled in the light were uncoupled even when the membranes were isolated at warm temperatures. Pea thylakoids were not uncoupled by the whole-plant chilling treatment. The difference in integrity of thylakoid membrane coupling following chilling in the light demonstrates a fundamental difference in photosynthetic function between these two species that may have some bearing on why pea is a chilling-resistant plant and cucumber is a chilling-sensitive plant.     

Effects of chilling on the biochemical and functional properties of thylakoid membranes

The mechanism of chilling resistance was investigated in 4-week-old plants of the chilling-sensitive cultivated tomato, Lycopersicon esculentum Mill. cv H722, and rooted cuttings of its chilling-resistant wild relative, L. hirsutum Humb. and Bonpl., which were chilled for 3 days at 2°C with a 14-hour photoperiod and light intensity of 250 micromoles per square meter per second. This chilling stress reduced the chlorophyll fluorescence ratio, stomatal conductance, and dry matter accumulation more in the sensitive L. esculentum than in the resistant L. hirsutum. Photosynthetic CO₂ uptake at the end of the chilling treatment was reduced more in the resistant L. hirsutum than in L. esculentum, but recovered at a faster rate when the plants were returned to 25°C. The reduction of the spin trap, Tiron, by isolated thylakoids at 750 micromoles per square meter per second light intensity was taken as a relative indication of the tendency for the thylakoids to produce activated oxygen. Thylakoids isolated from the resistant L. hirsutum with or without chilling treatment were essentially similar, whereas those from chilled leaves of L. esculentum reduced more Tiron than the nonchilled controls. Whole chain photosynthetic electron transport was measured on thylakoids isolated from chilled and control leaves of the two species at a range of assay temperatures from 5 to 25°C. In both species, electron transport of the thylakoids from chilled leaves was lower than the controls when measured at 25°C, and electron transport declined as the assay temperature was reduced. However, the temperature sensitivity of thylakoids from chilled L. esculentum was altered such that at all temperatures below 20°C, the rate of electron transport exceeded the control values. In contrast, the thylakoids from chilled L. hirsutum maintained their temperature sensitivity, and the electron transport rates were proportionately reduced at all temperatures. This sublethal chilling stress caused no significant changes in thylakoid galactolipid, phospholipid, or protein levels in either species. Nonchilled thylakoid membranes from L. hirsutum had fourfold higher levels of the fatty acid 16:1, than those from L. esculentum. Chilling caused retailoring of the acyl chains in L. hirsutum but not in L. esculentum. The chilling resistance of L. hirsutum may be related to an ability to reduce the potential for free radical production by close regulation of electron transport within the chloroplast.     

Temperature response of plasma membranes in tuber-bearing solanum species

Permeability coefficients (Kp) of nonelectroytes were determined at several temperatures for nonacclimated and acclimated plasma membranes of the frost sensitive Solanum tuberosum and the frost resistant Solanum commersonii. In nonacclimated membranes, Kp were equal at 25°C for the two species. Kp decreased with decreased temperature in both species; however, the decrease was much greater in the frost sensitive S. tuberosum than in frost resistant S. commersonii.

Kp increased with cold acclimation. After acclimation the temperature sensitivity of Kp or activation energy (Ea) for S. tuberosum was similar to that of S. commersonii; however, Kp for S. tuberosum were lower than those for S. commersonii at all temperatures.

The differences in Kp and activation energy indicate plasma membrane differences between the species before acclimation. After acclimation there was still a difference between the plasma membranes of the two species as well as a difference between the nonacclimated and acclimated membranes of the same species.

     

Control of Continuous Irradiation Injury on Potatoes with Daily Temperature Cycling

Two controlled-environment experiments were conducted to determine the effects of temperature fluctuations under continuous irradiation on growth and tuberization of two potato (Solanum tuberosum L.) cultivars, Kennebec and Superior. These cultivars had exhibited chlorotic and stunted growth under continuous irradiation and constant temperatures. The plants were grown for 4 weeks in the first experiment and for 6 weeks in the second experiment. Each experiment was conducted under continuous irradiation of 400 micromoles per square meter per second of photosynthetic photon flux and included two temperature treatments: constant 18°C and fluctuating 22°C/14°C on a 12-hour cycle. A common vapor pressure deficit of 0.62 kilopascal was maintained at all temperatures. Plants under constant 18°C were stunted and had chlorotic and abscised leaves and essentially no tuber formation. Plants grown under the fluctuating temperature treatment developed normally, were developing tubers, and had a fivefold or greater total dry weight as compared with those under the constant temperature. These results suggest that a thermoperiod can allow normal plant growth and tuberization in potato cultivars that are unable to develop effectively under continuous irradiation.     

Photoinactivation of Catalase Occurs under Both High- and Low-Temperature Stress Conditions and Accompanies Photoinhibition of Photosystem II

Severe photoinactivation of catalase (EC 1.11.1.6) and a decline of variable fluorescence (F_v), indicating photoinhibition of photosynthesis, were observed as rapid and specific symptoms in leaves exposed to a high heat-shock temperature of 40°C as well as in leaves exposed to low chilling temperatures in white light of only moderately high photosynthetic photon flux density of 520 μE m⁻² s⁻¹. Other parameters, such as peroxidase (EC 1.11.1.7), glycolate oxidase (EC 1.1.3.1), glutathione reductase (EC 1.6.4.2), or the chlorophyll content, were hardly affected under these conditions. At a compatible temperature of 22°C, the applied light intensity did not induce severe photoinactivations. In darkness, exposures to high or low temperatures did not affect catalase levels. Also, decline of F_v in light was not related to temperature sensitivity in darkness. The effective low-temperature ranges inducing photoinactivation of catalase differed significantly for chilling-tolerant and chilling-sensitive plants. In leaves of rye (Secale cereale L.) and pea (Pisum sativum L.), photoinactivation occurred only below 15°C, whereas inactivation occurred at 15°C in cucumber (Cucumis sativus L.) and maize (Zea mays L.). The behavior of F_v was similar, but the difference between chilling-sensitive and chilling-tolerant plants was less striking. Whereas the catalase polypeptide, although photoinactivated, was not cleaved at 0 to 4°C, the D1 protein of photosystem II was greatly degraded during the low-temperature treatment of rye leaves in light. Rye leaves did not exhibit symptoms of any major general photodamage, even when they were totally depleted of catalase after photoinactivation at 0 to 4°C, and catalase recovered rapidly at normal temperature. In cucumber leaves, the decline of catalase after exposures to bright light at 0 to 4°C was accompanied by bleaching of chlorophyll, and the recovery observed at 25°C was slow and required several days. Similar to the D1 protein of photosystem II, catalase differs greatly from other proteins by its inactivation and high turnover in light. Inasmuch as catalase and D1 protein levels depend on continuous repair synthesis, preferential and rapid declines are generally to be expected in light whenever translation is suppressed by stress actions, such as heat or chilling, and recovery will reflect the repair capacity of the plants.     

Relation between pigment content and photosynthetic characteristics in a blue-green algae

1. The blue-green alga Anacystis nidulans was cultured under steady state conditions at 25 and 39°C. and under several different light intensities to give five different types of cells. 2. Cells were submitted to pigment analysis based upon acetone extracts and aqueous extracts obtained by sonic disintegration. The different cell types show a threefold range of chlorophyll content and a fourfold range of phycocyanin content with only minor changes in the chlorophyll/phycocyanin ratio. Cells of highest pigment content were estimated to contain 2.8 per cent chlorophyll a and 24 per cent phycocyanin, the latter on a total chromoproteid basis. 3. Light intensity curves of photosynthesis were obtained for each of the cell types at 25 and at 39°C. The slopes of the light-limited regions of the curves are approximately linear functions of chlorophyll and phycocyanin contents. Maximum light-saturated rates of photosynthesis at 25 and 39° show no simple relation to pigment content.     

Effect of temperature conditioning on chilling injury of cucumber cotyledons: possible role of abscisic Acid and heat shock proteins

Endogenous abscisic acid levels and induced heat shock proteins were measured in tissue exposed for 6 hours to temperatures that reduced their subsequent chilling sensitivity. One-centimeter discs excised from fully expanded cotyledons of 11-day-old seedlings of cucumber (Cucumis sativus L., cv Poinsett 76) were exposed to 12.5 or 37°C for 6 hours followed by 4 days at 2.5 or 12.5°C. Ion leakage, a qualitative indicator of chilling injury, increased after 2 to 3 day exposure to 2.5°C, but not to 12.5°C, a nonchilling temperature. Exposure to 37°C before chilling significantly reduced the rate of ion leakage by about 60% compared to tissue exposed to 12.5°C before chilling, but slightly increased leakage compared to tissue exposed to 12.5 or 37°C and held at the nonchilling temperature of 12.5°C. There was no relationship between abscisic acid content following exposure to 12.5 or 37°C and chilling tolerance. Five heat shock proteins, with apparent molecular mass of 25, 38, 50, 70, and 80 kilodaltons, were induced by exposure to 37 or 42°C for 6 hours, and their appearance coincided with increased chilling resistance. Heat shock treatments reduced the synthesis of three proteins with apparent molecular mass of 14, 17, and 43 kilodaltons. Induction of heat shock proteins could be a possible cause of reduced chilling injury in tissue exposed to 37 or 42°C.     

Expression of a bacterial ice nucleation gene in plants

We have introduced an ice nucleation gene (inaZ) from Pseudomonas syringae pv. syringae into Nicotiana tabacum, a freezing-sensitive species, and Solanum commersonii, a freezing-tolerant species. Transformants of both species showed increased ice nucleation activity over untransformed controls. The concentration of ice nuclei detected at −10.5°C in 15 different primary transformants of S. commersonii varied by over 1000-fold, and the most active transformant contained over 100 ice nuclei/mg of tissue. The temperature of the warmest freezing event in plant samples of small mass was increased from approximately −12°C in the untransformed controls to −4°C in inaZ-expressing transformants. The threshold nucleation temperature of samples from transformed plants did not increase appreciably with the mass of the sample. The most abundant protein detected in transgenic plants using immunological probes specific to the inaZ protein exhibited a higher mobility on sodium dodecyl sulfate polyacrylamide gels than the inaZ protein from bacterial sources. However, some protein with a similar mobility to the inaZ protein could be detected. Although the warmest ice nucleation temperature detected in transgenic plants is lower than that conferred by this gene in P. syringae (−2°C), our results demonstrate that the ice nucleation gene of P. syringae can be expressed in plant cells to produce functional ice nuclei.     

Relative sensitivity of photosynthesis and respiration to freeze-thaw stress in herbaceous species : importance of realistic freeze-thaw protocols

The relative effect of a freeze-thaw cycle on photosynthesis, respiration, and ion leakage of potato leaf tissue was examined in two potato species, Solanum acaule Bitt. and Solanum commersonii Dun. Photosynthesis was found to be much more sensitive to freezing stress than was respiration, and demonstrated more than a 60% inhibition before any impairment of respiratory function was observed. Photosynthesis showed a slight to moderate inhibition when only 5 to 10% of the total electrolytes had leaked from the tissue (reversible injury). This was in contrast to respiration which showed no impairment until temperatures at which about 50% ion leakage (irreversible injury) had occurred. The influence of freeze-thaw protocol was further examined in S. acaule and S. commersonii, in order to explore discrepancies in the literature as to the relative sensitivities of photosynthesis and respiration. As bath cooling rates increased from 1°C/hour to about 3 or 6°C/hour, there was a dramatic increase in the level of damage to all measured cellular functions. The initiation of ice formation in deeply supercooled tissue caused even greater damage. As the cooling rates used in stress treatments increased, the differential sensitivity between photosynthesis and respiration nearly disappeared. Examination of agriculturally relevant, climatological data from an 11 year period confirmed that air cooling rates in the freezing range do not exceed 2°C/hour. It was demonstrated, in the studies presented here, that simply increasing the actual cooling rate from 1.0 to 2.9°C/hour, in frozen tissue from paired leaflet halves, meant the difference between cell survival and cell death.     

Temperature-induced fluorescence changes : a screening method for frost tolerance of potato (solanum sp.)

Field-grown tuber-bearing potatoes were screened for frost tolerance in a late stage of development. Three different clones of Solanum tuberosum L. and two interspecific crosses between clones of S. tuberosum and the wild potato species S. demissum Lindl. were studied. Two different methods were used. (a) Temperature-induced fluorescence changes of intact leaves were measured in freeze-thaw cycles between 20°C and −10°C. The variable fluorescence pattern was characterized in relation to frost tolerance. (b) Controlled freezings of plants in a climate chamber with successively increased low temperature stress, of 1 to 2 hours duration during the dark period. Freezing damages were classified visually.     

Chilling sensitivity of cucumber cotyledon protoplasts and seedlings

Cucumber (Cucumis sativus L.) seedlings are more sensitive to chilling stress when transferred to low temperature from the night cycle than from the day cycle. However, greater damage occurs when chilling is carried out in light than in dark. Freshly isolated protoplasts are extremely sensitive to damage when chilled at 4°C in light, but suffer significantly less injury when chilled in dark. If freshly isolated protoplasts are pre-chill conditioned at 27°C in either light or dark for a few hours prior to exposure to various chilling stresses, subsequent chilling damage is markedly reduced. Damage to chilled protoplasts also is reduced if cultures are placed in dark instead of light immediately following removal from low temperature. Experiments utilizing the cell wall synthesis inhibitor, dichlorobenzonitrile, showed that cell wall regeneration during the pre-chill conditioning period at 27°C does not appear to be associated with the enhanced chilling tolerance observed in these cultures. The results obtained in this investigation suggest that the physiological properties of cucumber cotyledon protoplasts accurately reflect those of intact seedlings, and hence provide a good system for studies into the mechanism of chilling damage in plants.     

Involvement of abscisic Acid in potato cold acclimation

Upon exposure to 2°C day/night (D/N), leaves of Solanum commersonii (Sc) began acclimating on the 4th day from a −5°C (killing temperature) hardy level to −12°C by the 15th day. Leaves of S. tuberosum L. (St) cv `Red Pontiac' typically failed to acclimate and were always killed at −3°C. Leaves of control (20/15°C, D/N) and treated plants (2°C, D/N) of St showed similar levels of free abscisic acid (ABA) during a 15-day sampling period. In treated Sc plants, however, free ABA contents increased 3-fold on the 4th day and then declined to their initial level thereafter. The increase was not observed in leaves of Sc control plants.

Treated St plants showed a slightly higher content of leaf soluble protein than controls. In Sc, leaves of controls maintained relatively constant soluble proteins, but leaves of treated plants showed a distinct increase. This significant increase was initiated on the 4th day, peaked on the 5th day, and remained at a high level throughout the 15-day sampling period.

Exogenously applied ABA induced frost hardiness in leaves of Sc plants whether plants were grown under a 20°C or 2°C temperature regime. When cycloheximide was added to the medium of stem-cultured plants at the beginning of 2°C acclimation, or at the beginning of the ABA treatment in the 20°C regime, it completely inhibited the development of frost hardiness. However, when cycloheximide was added to plants on the 5th day during 2°C acclimation, the induction of frost hardiness was not inhibited. The role of ABA in triggering protein synthesis needed to induce frost hardiness is discussed.

     

The Influence of Dark Adaptation Temperature on the Reappearance of Variable Fluorescence following Illumination

The effect of chilling temperatures (5°C) on chlorophyll fluorescence transients was used to study chilling-induced inhibition of photosynthesis in plant species with differing chilling sensitivities. A Brancker SF-20 fluorometer was used to measure induced fluorescence transients from both attached and detached leaves of chilling-sensitive cucumber (Cucumis sativus L. cv Ashley) and chilling-resistant pea (Pisum sativum L. cv Alaska). The rate of reappearance of the variable component of fluorescence (F_v), following a period of illumination at 25°C, was dependent on the temperature at which the leaf was allowed to dark adapt in chilling-sensitive cucumber, but not in chilling-resistant pea. In cucumber, dark adaptation at 25°C following illumination resulted in a much faster return of F_v than dark adaptation at 5°C following illumination. However, F_v reappearance during the dark adaptation period in chilling-resistant pea was temperature independent. The difference in the temperature response of F_v following illumination correlated with temperature sensitivity of these two species. The process responsible for the difference in F_v may represent a site of chilling sensitivity in the photosynthetic apparatus.     

The Solanum commersonii Genome Sequence Provides Insights into Adaptation to Stress Conditions and Genome Evolution of Wild Potato Relatives

Here, we report the draft genome sequence of Solanum commersonii, which consists of ∼830 megabases with an N50 of 44,303 bp anchored to 12 chromosomes, using the potato (Solanum tuberosum) genome sequence as a reference. Compared with potato, S. commersonii shows a striking reduction in heterozygosity (1.5% versus 53 to 59%), and differences in genome sizes were mainly due to variations in intergenic sequence length. Gene annotation by ab initio prediction supported by RNA-seq data produced a catalog of 1703 predicted microRNAs, 18,882 long noncoding RNAs of which 20% are shown to target cold-responsive genes, and 39,290 protein-coding genes with a significant repertoire of nonredundant nucleotide binding site-encoding genes and 126 cold-related genes that are lacking in S. tuberosum. Phylogenetic analyses indicate that domesticated potato and S. commersonii lineages diverged ∼2.3 million years ago. Three duplication periods corresponding to genome enrichment for particular gene families related to response to salt stress, water transport, growth, and defense response were discovered. The draft genome sequence of S. commersonii substantially increases our understanding of the domesticated germplasm, facilitating translation of acquired knowledge into advances in crop stability in light of global climate and environmental changes.     

Seedling growth, mitochondrial characteristics, and alternative respiratory capacity of corn genotypes differing in cold tolerance

Four maize (Zea mays L.) inbreds representing genetic differences in seedling cold tolerance were used to determine the effect of growth temperatures on dry weight accumulation and mitochondrial properties, especially the alternative oxidase capacity. Seedlings were grown in darkness at 30°C (constant), 14°C (constant), and 15°C for 16 hours and 8°C for 8 hours. Inbreds B73 and B49 were characterized as cold tolerant while G50 and G84 were cold sensitive. Shoot growth rate of cold-sensitive inbreds in the lower temperatures was slower relative to the tolerant inbreds. Mesocotyl tissue was particularly sensitive to low temperatures during growth after germination. There were no significant differences in relative rates of mitochondrial respiration in the cold-tolerant compared to cold-sensitive inbreds measured at 25°C. Mitochondria from all seedlings grown at all temperatures had the ability to phosphorylate as indicated by the observation of respiratory control. This result indicated that differences in low temperature growth were probably not related to mitochondrial function at low temperatures. Alternative oxidase capacity was higher in mitochondria from seedlings of all inbreds grown at 14°C compared to 30°C. Capacities in seedlings of 14°C-grown B73 and G50 were higher than in B49 and G84. Capacities in seedlings grown for 16 hours at 15°C and 8 hours at 8°C were similar to those from 14°C-grown except in G50 which was lower and similar to those grown at 30°C. Mesocotyl tissue was the most responsive tissue to low growth temperature. Coleoptile plus leaf tissue responded similarly but contained lower capacities. Antibody probing of western blots of mitochondrial proteins confirmed that differences in alternative oxidase capacities were due to differences in levels of the alternative oxidase protein. Male sterile lines of B73 were also grown under the three different temperature regimes. These lines grew equally as well as the normal B73 at all temperatures and the response of alternative oxidase capacity and protein to low growth temperature was similar to normal B73.     

Involvement of Abscisic Acid in Regulating Water Status in Phaseolus vulgaris L. during Chilling

During the first hours of chilling, bean (Phaseolus vulgaris L., cv Mondragone) seedlings suffer severe water stress and wilt without any significant increase in leaf abscisic acid (ABA) content (P. Vernieri, A. Pardossi, F. Tognoni [1991] Aust J Plant Physiol 18: 25-35). Plants regain turgor after 30 to 40 h. We hypothesized that inability to rapidly synthesize ABA at low temperatures contributes to chilling-induced water stress and that turgor recovery after 30 to 40 h is mediated by changes in endogenous ABA content. Entire bean seedlings were subjected to long-term (up to 6 d) chilling (3°C, 0.2-0.4 kPa vapor pressure deficit, 100 μmol·m⁻²·s⁻¹ photosynthetic photon flux density, continuous fluorescent light). During the first 24 h, stomata remained open, and plants rapidly wilted as leaf transpiration exceeded root water absorption. During this phase, ABA did not accumulate in leaves or in roots. After 24 h, ABA content increased in both tissues, leaf diffusion resistance increased, and plants rehydrated and regained turgor. No osmotic adjustment was associated with turgor recovery. Following turgor recovery, stomata remained closed, and ABA levels in both roots and leaves were elevated compared with controls. The application of ABA (0.1 mm) to the root system of the plants throughout exposure to 3°C prevented the chilling-induced water stress. Excised leaves fed 0.1 mm ABA via the transpiration stream had greater leaf diffusion resistance at 20 and 3°C compared with non-ABA fed controls, but the amount of ABA needed to elicit a given degree of stomatal closure was higher at 3°C compared with 20°C. These findings suggest that endogenous ABA may play a role in ameliorating plant water status during chilling.     

Zeaxanthin Synthesis, Energy Dissipation, and Photoprotection of Photosystem II at Chilling Temperatures

When leaves of a mangrove, Rhizophora mangle, were exposed to an excess of light at chilling temperatures, synthesis of zeaxanthin through violaxanthin de-epoxidation as well as nonphotochemical fluorescence quenching were markedly reduced. The results suggest a protective role of energy dissipation against the adverse effects of high light and chilling temperatures: leaves of R. mangle that had been preilluminated in 2% O₂, 0% CO₂ at low photon flux density and showed a high level of zeaxanthin, and leaves that had been kept in the dark and contained no zeaxanthin, were both exposed to high light and chilling temperatures (5°C leaf temperature) in air and then held under control conditions in low light in air at 25°C. Measurements of chlorophyll a fluorescence at room temperature showed that the photochemical efficiency of PSII and the yield of maximum fluorescence of the preilluminated leaf recovered completely within 1 to 3 hours under the control conditions. In contrast, the fluorescence responses of the predarkened leaf in high light at 5°C did not recover at all. During a dark/light transient in 2% O₂, 0% CO₂ in low light at 5°C, nonphotochemical fluorescence quenching increased linearly with an increase in the zeaxanthin content in leaves of R. mangle. In soybean (Glycine max) leaves, which contained a background level of zeaxanthin in the dark, a similar treatment with excess light induced a level of nonphotochemical fluorescence quenching that was not paralleled by an increase in the zeaxanthin content.     

Salt hardiness and dye reduction by potato tissue and mitochondrial fractions as influenced by previous storage of the tubers

Oxygen uptake and tetrazolium reduction occurred at higher rates in discs from potato tubers (Solanum tuberosum L.) stored at 0° than in discs from tubers stored at 12.8°. Tetrazolium reduction was at a higher rate in mitochondrial fractions from tubers stored at 0° than in mitochondrial fractions from tubers stored at 12.8°. These physiological activities were more resistant to hypertonic KCl treatments in tissue and mitochondrial fractions from tubers stored at 0° than in tissue and mitochondrial fractions from tubers stored at 12.8°. Inhibition of O₂ uptake and tetrazolium reduction progressively increased with increasing concentrations of KCl for tissue and mitochondrial fractions from tubers stored at 0 and 12.8°, but inhibition was more severe and occurred at lower concentrations of KCl for the material from tubers stored at 12.8°. Tissue from tubers stored at 0° was at the same time more sensitive to hypotonic solutions and more resistant to hypertonic solutions than corresponding tissue from tubers stored at 12.8°. Adaptive changes brought on in the tubers by the stress of cold storage were demonstrated in the discs and mitochondrial fractions prepared from cold-stored tubers.     

Photoinhibition at low temperature in chilling-sensitive and -resistant plants

Photoinhibition resulting from exposure at 7°C to a moderate photon flux density (300 micromoles per square meter per second, 400-700 nanometers) for 20 hours was measured in leaves of annual crops differing widely in chilling tolerance. The incidence of photoinhibition, determined as the decrease in the ratio of induced to total chlorophyll fluorescence emission at 693 nanometers (F_v/F_max) measured at 77 Kelvin, was not confined to chilling-sensitive species. The extent of photoinhibition in leaves of all chilling-resistant plants tested (barley [Hordeum vulgare L.], broad bean [Vicia faba L.], pea [Pisum sativum L.], and wheat [Triticum aestivum L.]) was about half of that measured in chilling-sensitive plants (bean [Phaseolus vulgaris L.], cucumber [Cucumis sativus L.], lablab [Lablab purpureus L.], maize [Zea mays L.], pearl millet [Pennisetum typhoides (Burm. f.) Stapf & Hubbard], pigeon pea [Cajanus cajun (L.) Millsp.], sesame [Sesamum indicum L.], sorghum [Sorghum bicolor L. Moench], and tomato [Lycopersicon esculentum Mill.]). Rice (Oryza sativa L.) leaves of the indica type were more susceptible to photoinhibition at 7°C than leaves of the japonica type. Photoinhibition was dependent both on temperature and light, increasing nonlinearly with decreasing temperature and linearly with increasing light intensity. In contrast to photoinhibition during chilling, large differences, up to 166-fold, were found in the relative susceptibility of the different species to chilling injury in the dark. It was concluded that chilling temperatures increased the likelihood of photoinhibition in leaves of both chilling-sensitive and -resistant plants. Further, while the photoinhibition during chilling generally occurred more rapidly in chilling-sensitive plants, this was not related directly to chilling sensitivity.