Comparison of chlorophyll fluorescence with several other techniques used to assess chilling sensitivity in plants

Chilling tolerance in eight taxa was estimated from field and controlled environment observations and compared to tolerance estimated using a variety of techniques. The controlled environment observations suggested that the eight taxa could be ranked from chilling tolerant to chilling sensitive in the order: pea ( Pisum sativum L. cv. Greenfeast), Passiflora edulis Sims., Passiflora ligularis Juss. and Passiflora quadrangularis L., pepino ( Solanum muricatum L.) cultivars Comeraya, Suma, Miski, and mungbean [ Vigna radiata (L.) R. Wilcz]; although the relationship between the passionfruit as a group and the pepinos was unclear.
The change in the variable component of chlorophyll fluorescence (F_R) with time near 0°C in darkness was the most reliable method of ranking the plants according to relative chilling tolerance. It was also sufficiently sensitive to discriminate clearly between the closely related pepino cultivars. The Passiflora species and pea were not susceptible to short term reductions in F_R, with or without a 20 min exposure to light. Exposure to light at temperatures near 0°C emphasised the reductions in F_R in the more sensitive species. Pea was the only species capable of recovering a measurable F_R after a 60 min exposure to white light.
Measurement of electrolyte leakage and ethylene evolution from leaf disks after a low temperature treatment could allow discrimination between closely related varieties, but not between genera. Catalase activity was reduced in all taxa in response to low temperature. However, both initial catalase levels and relative response to dark treatment at 20°C enabled the ranking of plants within the closely related subgroups according to susceptibility to chill injury.
No one method clearly distinguished chilling sensitivity over all taxa.     

Chilling injury and recovery in detached and attached leaves measured by chlorophyll fluorescence

Smillie, R. M., Nott, R., Hetherington, S. E. and Öyustt, G. 1987. Chilling injury and recovery in detached and attached leaves measured by chlorophyll fluorescence Chilling injury was compared in detached and attached leaves chilled at 0 or 0.5°C by measuring the decrease in induced chlorophyll fluorescence in vivo. The fluorescence parameter measured was F_R, the maximal rate of rise of induced chlorophyll fluorescence emission after irradiating dark-adapted leaves. The plants used were bean, Phaseolus vulgaris L. cv. Pioneer, and maize, Zea mays L. cvs hybrid GH 390 and Northern Belle. Leaves were detached and placed on wet paper and covered with thin polyethylene film to prevent water loss during chilling. Leaves left attached on plants were treated similarly. When chilled in this way at 100% relative humidity, the chilling-induced decrease in F_R was the same in detached and attached leaves. For the attached leaves, the same result was obtained whether just a single leaf was chilled or the whole plant. Expression of chilling injury was greatest in fully turgid leaves and comparisons can be invalid unless the water status of the detached and attached leaves are the same. Problems arising from diurnal fluctuations in water potential of plants grown in a glasshouse were circumvented by placing leaves on the wet filter paper under polyethylene film prior to chilling, which allowed high water potentials to be regained, or mist sprays in the glasshouse were employed. Determinations of the time course for changes in F_R of maize (cv. Northern Belle) during chilling at 0°C showed that F_R decreased exponentially, at the same rate (time to 50% decrease in F_R was 9.3 h) in detached and attached leaves. Chilling injury was largely reversible for the first 20 h of chilling stress as both detached and attached leaves recovered their pre-chilling values of F_R after a further 20 h at 20°C in darkness. Leaves chilled for 48 h showed partial recovery, while those chilled for 72 h did not recover. Recovery was impeded by light. Inability to recover from chilling as indicated by measurements of F_R was paralleled by the incidence of visible symptoms of injury. It is concluded that detached and attached leaves behave similarly during chilling and short-term recovery, provided a similarity in treatments is rigorously maintained.     

Chilling sensitivity of the frost-tolerant potato Solanum commersonii

Frost tolerance has been reported in the shoots of wild, tuberiferous potato species such as Solanum commersonii when the plants are grown in either field or controlled conditions. However, these plants can survive as underground tubers and avoid unfavorable environmental conditions altogether. As such, leaf growth and photosynthesis at low temperature may not be required for survival of the plants. In order to determine the temperature sensitivity of S. commersonii shoots, we examined leaf growth, development and photosynthesis in plants raised at 20/16°C (day/night). 12/9°C and 5/2°C. S. commersonii leaves grown at 5°C exhibited a marked decrease in leaf area and in total chlorophyll (Chl) content per leaf area when compared with leaves grown at 20°C. Furthermore, leaves grown at 5°C did not exhibit the expected decrease in either water content or susceptibility to low-temperature-induced photoinhibition that normally characterizes cold acclimation in frost-tolerant plants. Measurements of CO₂-saturated O₂ evolution showed that the photosynthetic apparatus of 5°C plants was functional, even though the efficiency of photosystem II photochemistry was reduced by growth at 5°C. A decrease in the resolution of the M-peak in the slow transients for Chl a fluorescence in leaves grown at 12 and 5°C and in all leaves exposed to high light at 5°C indicated that low temperature significantly affected processes on the reducing side of Q_A, the primary quinone electron acceptor in photosystem II. Thus S. commarsonii exhibits the characteristics of a plant that is limited by chilling temperatures. Although S. commersonii can tolerate light frosts, its sensitivity to chilling temperatures may result in shoot dieback in winter in its native habitat. The plants may avoid both chilling and freezing temperatures by overwintering as underground tubers.     

Photoinhibition at chilling temperatures and effects of freezing stress on cold acclimated spinach leaves in the field. A fluorescence study

The role of high light stress in a natural environment was studied on spinach plants ( Spinacia oleracea L. cv. Wolter) grown in the field during the winter season. Fluorescence induction (at 293 K and 77 K) of leaves was used to characterize the stress effects. Night frost with minimum temperatures between – 1.5°C and –7.5°C (i.e. above the'frost killing point'at ca. –11.5°C) led to impaired photosynthesis. This was seen as increased initial fluorescence (F_o), decreased ratio of variable to maximum fluorescence (F_V/F_M) and lowered rates of O₂ evolution. The freezing injury was reversible within several frostless days. Exposure to high light (about 900 mol m_–2 s_–1) at chilling temperatures in the field caused photoinhibition, manifested as decreased variable fluorescence (F_V) and F_V/F_M ratio without changes in F_O. The photoinhibitory fluorescence quenching was not stronger after frost than after frostless nights; synergism between light stress and preceding freezing stress was not observed. Fluorescence induction signals at 77 K showed that F_V of photosystems I and II decreased to the same extent, indicating increased thermal deactivation of excited chlorophyll. Photoinhibition was fully reversible at +4°C within 1 h in low light, but only partially in moderate light. Preceding night frosts did not affect the recovery. The photoinhibition observed here is regarded as a protective system of thermal dissipation of excess light energy.     

Nitrate reductase activity, nitrogenase activity and photosynthesis of black alder exposed to chilling temperatures

Actinorhizal ( Frankia -nodulated) black alder [ Alnus glutinosa (L.) Gaertn.] seedlings fertilized with 0.36 m M nitrate (low nitrate fertilizer treatment) or 7.14 m M nitrate (high nitrate fertilizer treatment) and acclimated in a growth chamber for 2 weeks were exposed to 2.5 h of night-time chilling temperatures of −1 to 4°C. Cold treatment decreased nitrogenase activity (acetylene reduction activity) 33% for low nitrate fertilized plants and 41% for high nitrate fertilized plants. Recovery of nitrogenase activity occurred within 7 days after chilling treatment. In contrast, in vivo nitrate reductase (NR) activities of leaves and fine roots increased immediately after chilling then decreased as nitrogenase activities recovered. Fine roots of alder seedlings exhibited NR activities proportional to the amounts of nitrate in the rooting medium. In contrast, the NR activities of leaves were independent of substrate and tissue nitrate levels and corresponded to nitrogenase activity in the root nodules. In a separate experiment, net photosynthesis (PS) of similarly treated black alder seedlings was measured before and after chilling treatments. Net PS declined in response to chilling by 17% for plants receiving low nitrate fertilizer and 19% for plants receiving high nitrate fertilizer. After chilling, stomatal conductance (g_s) decreased by 39% and internal CO₂ concentration (c_i) decreased by 5% in plants receiving the high nitrate fertilizer, whereas plants receiving the low nitrate fertilizer showed no change in g_s and a 13% increase in c_i. Results indicate that chilling stimulates stomatal closure only at the high nitrate level and that interference with biochemical functions is probably the major impact of chilling on PS.     

Differential effects of chilling on the activity of C4 enzymes in two ecotypes of Echinochloa crus-galli from sites of contrasting climates

Five-week-old plants of Echinochloa crusgalli (L.) Beauv. from Mississippi and from Québec grown under controlled conditions were subjected to dark chilling for 10 h at 5°C or light chilling treatments for 14 h at 7°C under hight light (1 000 μmol m⁻² s⁻¹). The activities of four C₄ enzymes of Québec plants, measured 4 h after the completion of the cold treatment, were not affected by the chilling treatment in the dark. The activities of pyruvate, P_i dikinase (PPDK; EC 2.7.9.1) and NADP⁺-malic enzyme (NADP⁺-ME; EC 1.1.1.40), were significantly reduced in dark-chilled Mississippi plants. Chilling under high light conditions elicited significant levels of reduction in the activities of the four enzymes from both ecotypes but the reductions were significantly less severe for Québec plants. The recovery of activities of phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) and PPDK for both ecotypes was completed within 36 to 60 hours following the chilling treatment, but NADP⁺-malate dehydro-genase (NADP⁺-MDH; EC 1.1.1.82) and NADP⁺-ME activities of chilled Mississippi plants remained below that of control plants at the end of the 5-day monitoring period. PPDK was inactivated in vitro at 0 and 10°C and the rates of cold inactivation were significantly higher for PPDK extracted from Mississippi plants. The activity of PEPC of Mississippi extracts was slightly, but significantly reduced by a 60 min treatment at 0°C.     

Chilling injury in coleus as influenced by photosynthetically active radiation, temperature and abscisic acid pretreatment. I. Morphological and physiological responses

Abstract. Coleus blumei Benth. (PI No. 354190), a green-leafed cultivar, was exposed to 5°C for 48 or 72 h after pretreatment for 48 h at two levels of photosynthetically active radiation (PAR) (8 and 320 μmol s⁻¹ m⁻²), two temperatures (13 and 20°C), and two abscisic acid (ABA) levels (0 and 200 g m⁻³ of the racemic mixture). Plants given low PAR for only 48 h prior to chilling treatment (48 or 72 h at 5°C) showed increased protection against chilling injury while those given high PAR were severely injured. The former plants were darker green, contained greater concentrations of chlorophyll- a , chlorophyll- b , total chlorophyll and anthocyanin and generally had a lower abscission rate than the latter plants. There were no differences, however, in chlorophyll- a/b ratio among plants grown at the two PAR levels, two temperatures or two ABA concentrations. Temperature and ABA pretreatment and number of hours at 5°C had no significant effect on chilling injury as measured by leaf chlorosis, but generally had a significant effect on leaf abcission, especially at 3 and 7 d after returning the plants to the greenhouse. Enclosing intact plants or excised shoots in plastic bags to maintain 100% relative humidity during 72 h chilling treatment failed to provide protection against chilling injury. These findings indicate that the protective effects of low PAR applied prior to chilling treatment may be as important or more important than that applied during chilling. They also indicate the importance of making careful measurements of PAR levels when conducting studies on chilling injury.     

Effect of paclobutrazol and chilling on leaf membrane lipids in cucumber seedlings

The effects of paclobutrazol on the leaf membrane lipid composition of seedlings of cucumber ( Cucumis sativus L. cv. Victory) subjected to chilling temperatures were assessed. At a non-injurious temperature (12.5°C), there was no difference in the polar lipid fatty acid composition or in the glycolipid, phospholipid or free sterol content of leaves from treated vs untreated seedlings, regardless of whether paclobutrazol was administered 1 or 7 days prior to analysis. In the latter case (7 days pretreatment), there were clear effects of the bioregulator on plant growth and morphology as well as on leaf chlorophyll content. At an injurious chilling temperature (5°C), desaturation of leaf polar lipid fatty acids was markedly reduced in both treated and untreated seedlings. Chilling at 5°C resulted in losses of fresh weight and membrane lipids in leaves of both groups of plants. These losses were either reversible or irreversible, depending upon the duration of chilling and of pretreatment with paclobutrazol. Seedlings pretreated with 10 μg ml⁻¹ paclobutrazol generally sustained less chilling injury than untreated controls, as judged by the extent of wilting, necrosis and desiccation. This correlated with reduced losses of leaf fresh Weight and membrane lipids.     

Photosynthesis of Coffea arabica after chilling

Net photosynthetic CO₂ exchange of 1-year-old plants of Coffea arabica L. was studied after the above-ground parts had been exposed once or repeatedly to night temperatures in the chilling range. Chill-reduced rates of CO₂ uptake (measured at 24°C and at natural CO, level) were observed after a 12 h night exposure to about 6°C. After exposure to 4°C, activity was reduced to less than half of that of the controls, and after exposure to 0.5°C the leaves suffered visible necrotic injury and were no longer able to take up Co₂ If the leaves were not lethally injured, net photosynthesis recovered completely within 2 to 6 days. About 25% of chill-induced reduction of CO₂ uptake was due to reduced stomatal aperture and 75% to impairment of carboxylation efficiency.
Chilling on successive nights at 4–6°C reduced CO, uptake progressively on each day following treatment. After 10 nights, activity was decreased to less than 10% of initial performance. Conditioning at temperatures slightly above the chilling level (e.g. 15/I2°C) for 2 weeks led to almost complete impairment of photosynthetic activity without additional chilling stress instead of improving chilling tolerance.     

Reversal by light of deleterious effects of chilling on oxygen evolution, manganese and free fatty acid content in tomato thylakoids is not accompanied by restoration of the original membrane conformation

Free fatty acids (FFA) generated in thylakoids upon chilling of tomato leaves at 0°C for a few days result in release of functionally active Mn and inactivation of O₂ evolution. Chilling does not lead to a decrease in the extrinsic 16, 23 and 33 kDa polypeptides. Upon illumination of chilled leaves both Mn content and O₂ evolution in thylakoids are restored and FFA content is reduced to the level of the control. Photoactivation of O₂ evolution in chilled leaves does not change the ratio of unsaturated/saturated FFA. Constant Arrhenius activation energy (Ea) for O₂ evolution by thylakoids isolated from control leaves was found, whereas it increased at temperatures below 8.0 and 10.5°C in thylakoids from cold-treated and photoactivated leaves, respectively. This indicates that restoration of O₂ evolution as well as of FFA and Mn contents is not accompanied by a complete reversal of membrance conformation.     

Chilling syndrome in light-exposed maize leaves and its easing by low doses of DCMU

When maize plants ( Zea mays L. line ps-lye) were subjected to chilling (at 8 ± 2°C, 80% relative humidity for 24 h under illumination by 80 W m² between 400–700 nm), the leaves were wilted and photosynthetic membranes were permanently damaged. This was shown by the swelling of grana thylakoids and a deerease in the charging capacity of the electron transport chain. Water loss and photosynthetic dysfunction were connected in the process of a chilling-induced increase of stomatal aperture. Chilling injury could be eased to a considerable extent by a mild treatment with DCMU preventing stomatal opening, wilting, and the irreversible loss of CO₂ fixation capacity.     

Wheat catalase expressed in transgenic rice can improve tolerance against low temperature stress

We examined whether the expression of wheat catalase (EC 1.11.1.6) cDNA in transgenic rice ( Oryza sativa L.) could enhance tolerance against low temperature injury. Transgenic rice plants expressing wheat CAT protein showed an increase of activities in leaves at 25°C, 2- to 5-fold that in non-transgenic rice. At 5°C, catalase activities were about 4–15 times higher than those in non-transgenic rice were. A comparison of damage observed in leaves as they withered due to chilling at 5°C showed that transgenic rice displayed an increased capability to resist low temperature stress. The exposure of these plants to low temperature at 5°C for 8 days resulted in decreased catalase activities in leaves at 25°C, but the transgenic plants indicated 4 times higher residual catalase activities than those of non-transgenic ones. The concentration of H₂O₂ in leaves was kept lower in transgenic rice than that of the control plants during the 8 days chilling. These results suggest that the improved tolerance against low temperature stress in genetically engineered rice plants be attributed to the effective detoxification of H₂O₂ by the enhanced catalase activities.     

Chilling-induced leaf abscission of Ixora coccinea plants. III. Enhancement by high light via increased oxidative processes

The role of increased oxidation induced by successive stresses of chilling and high light in the induction of leaf abscission was studied in Ixora coccinea plants in relation to auxin metabolism and oxidative processes. Exposure of plants following dark chilling (7°C for 3 days) to high light (500–700 μmol m⁻² s⁻¹ photosynthetically active radiation) for 5 h at 20–25°C enhanced chilling-induced leaf abscission. This abscission was inhibited by pretreatment with the antioxidant butylated hydroxyanisole, α -naphthaleneacetic acid or the ethylene action inhibitor, 1-methylcyclopropene. The oxidative processes initiated during the low light period following the dark chilling period, such as indoleacetic acid (IAA) decarboxylation and lipid peroxidation, were further enhanced by subsequent exposure to high light. Photoinhibition, expressed by the reduction of the chlorophyll fluorescence parameter Fv/Fm, was evident following exposure to high light, irrespective of the temperature of the pretreatment, but this reduction persisted only in chilled plants. This suggests that oxidative processes generated during and after the chilling period might have inhibited the recovery from photoinhibition. The chilling stress under darkness induced a 60% reduction in superoxide dismutase (SOD) activity and significant increases (130–600%) in the activities of several other antioxidative enzymes. These data suggest that the chilling-induced reduction in SOD activity may well be responsible for the increase in the oxidative stress induced by the subsequent light treatment, as expressed by the increased enzymatic activities. Taken together, this study provides further support for the involvement of oxidative processes in the events occurring in tissues exposed to sequential chilling and light stresses, leading to reduction in free IAA content in the abscission zone and to leaf abscission.     

Genotype-specific differences in chilling tolerance of maize in relation to chilling-induced changes in water status and abscisic acid accumulation

Four inbred maize lines differing in chilling tolerance were used to study changes in water status and abscisic acid (ABA) levels before, during and after a chilling period. Seedlings were raised in fertilized soil at 24/22°C (day/night), 70% relative humidity. and a 12-h photoperiod with 200 μmol m⁻² s⁻¹ from fluorescent tubes. At an age of 2 weeks the plants were conditioned at 14/12°C for 4 days and then chilled for 5 days at 5/3°C. The other conditions (relative humidity, quantum flux, photoperiod) were unchanged. After the chilling period the plants were transferred to the original conditions for recovery. The third leaves were used to study changes in leaf necrosis, ion efflux, transpiration, water status and ABA accumulation. Pronounced differences in chilling tolerance between the 4 lines as estimated by necrotic leaf areas, ion efflux and whole plant survival were observed. Conditioning significantly increased tolerance against chilling at 5/3°C in all genotypes. The genotypes with low chilling tolerance had lower water and osmotic potentials than the more tolerant genotypes during a chilling period at 5/3°C. These differences were related to higher transpiration rates and lower diffusive resistance values of the more susceptible lines. During chilling stress at 5/3°C ABA levels were quadrupled. Only a small rise was measurable during conditioning at 14/12°C. However, conditioning enhanced the rise of ABA during subsequent chilling. ABA accumulation in the two lines with a higher chilling tolerance was triggered at a higher leaf water potential and reached higher levels than in the less tolerant lines. We conclude that chilling tolerance in maize is related to the ability for fast and pronounced formation of ABA as a protective agent against chilling injury.     

Oxygen-mediated cold-acclimation in cucumber (Cucumis sativus) seedlings

Cold acclimation of etiolated cucumber seedlings, consisting of cooling at 12°C for 48 h followed by a warming period at 25°C, led to tolerance to subsequent chilling at 2°C. Tolerance, as evidenced by freedom from chilling injury and continued growth, developed during the warming period in a time-course manner for 12 h but decreased with prolonged warming. A similar increase and subsequent decrease was also observed in the content of palmitic, linoleic and linolenic acids in total lipid fraction from cucumber hypocotyl tissue. During the warming period supra-ambient oxygen stimulated, whereas subambient oxygen inhibited, the increase in fatty acid content as well as development of chilling tolerance. A strong correlation between oxygen-mediated changes in fatty acid content and associated development of cold tolerance suggests that both these processes are interrelated. Cold acclimation, but not cold stress, led to an increase followed by a decrease in CO₂ evolution suggesting that a respiratory upsurge is yet another feature of cold acclimation in cucumbers.     

Temperature and chemical shocks induce chilling tolerance in germinating Cucumis sativus (cv. Poinsett 76) seeds

Roots of 24-h-old germinated cucumber ( Cucumis sativus cv. Poinsett 76) seeds were subjected to thermal and chemical stresses, equilibrated at 25°C for 2 h and chilled at 2.5°C for 96 h. The germinated seeds were then held at 25°C for 72 h after they were chilled and the elongation of the primary root was used as a measure of chilling tolerance. Control roots elongated from an initial length of 0.2 cm to a final length of 6.3 cm at the end of 72 h. while chilled roots elongated to a final length of only 0.4 to 0.6 cm. Exposure to 0.4 M ethanol for 4 h or to 40°C for 1 h induced substantial chilling tolerance and the roots had a final length of 4.1 and 3.1 cm. respectively. Exposure to 7.5°C for 3 h conferred less chilling tolerance (elongation to 1.4 cm). while exposure to other chemicals (i.e. aqueous solutions of Ca(NO₃)₂, mannitol. methanol and NaCl) produced less, though still significant increases in chilling tolerance. A more severe chilling treatment of 144 h at 2.5°C was required to consistently induce elevated rates of ion leakage. Only the heat and the ethanol shock treatments significantly reduced chilling-induced ion leakage. Inclusion of the protein synthesis inhibitor cycloheximide negated the protective effects of these shock treatments. It appears that de novo protein synthesis is required for induction of chilling tolerance by a variety of chemical and thermal shock treatments.     

Photoinhibition, xanthophyll cycle and in vivo chlorophyll fluorescence quenching of chilling-tolerant Oxyria digyna and chilling-sensitive Zea mays

The relationship between susceptibility to photoinhibition, zeaxanthin formation and chlorophyll fluorescence quenching at suboptimal temperatures was studied in chilling-sensitive maize and in non-acclimated and cold-acclimated Oxyria digyna , a chilling-tolerant plant of arctic and alpine habitats. In maize, zeaxanthin formation was strongly suppressed by chilling. Zeaxanthin formed during preillumination at 20°C did not protect maize leaves from photoinhibition during a subsequent high-light, low-temperature treatment, as judged from the ratios of variable to maximal fluorescence, F_v/F_m. However, such preillumination significantly increased non-photochemical quenching (q_N) at low temperatures, mainly due to an enhancement of the fast-relaxing q_N component (i.e., of energy-dependent quenching. q_E). In O. digyna , cold-acclimation resulted in an increased zeaxanthin formation in the temperature range of 2.5–20°C. Cold-acclimation substantially decreased the susceptibility towards photoinhibition at 4°C, but q_N remained nearly unchanged between 2 and 38°C, as compared to control plants. Effects of cold acclimation on photosynthesis, photochemical quenching and quantum efficiency of photosystem II were small and indicated a slight amelioration only of the function of the photosynthetic apparatus at suboptimal temperatures (2–20°Ct. I) is concluded, that the xanthophyll cycle is strongly influenced by cold acclimation, while effects on the photosynthetic carbon assimilation only play a minor role in O. digyna.     

Drought enhances maize chilling tolerance. II. Photosynthetic traits and protective mechanisms against oxidative stress

In the present research we studied the photosynthetic traits and protective mechanisms against oxidative stress in two maize ( Zea mays L.) genotypes differing in chilling sensitivity (Z7, tolerant and Penjalinan, sensitive) subjected to 5°C for 5 days, with or without pretreatment by drought. The drought pretreatment decreased the symptoms of chilling injury in Penjalinan plants estimated as necrotic leaf area and maximum quantum yield of photosystem II. Furthermore, drought pretreatment diminished the level of lipid peroxidation caused by chilling in Penjalinan plants. After one day of recovery from chilling the Z7 and drought-pretreated Penjalinan plants showed higher net photosynthesis rates than the non-drought-pretreated Penjalinan plants, thereby decreasing the probability of generating reactive oxygen species. The greater net photosynthesis was correlated with the greater NADP-malate dehydrogenase activity. No differences in either the de-epoxidation state of the xanthophyll cycle or the antioxidant enzyme activities were found among the chilled groups of plants. However, a drastic decrease in ascorbate content was observed in chilled Penjalinan plants without drought pretreatment. As we found an increase of H₂O₂ content after drought pretreatment, we suggest its involvement as a signal in the drought-enhanced chilling tolerance of maize.     

Chilling injury and changes in adenosine triphosphate of cotton seedlings

Young Gossypium hirsutum L. seedlings chilled at 5° showed a continual decrease in ATP concentration with time of chilling. Chilled plants returned to optimum conditions were able to restore the initial ATP concentration when chilled only 1 day, but not when chilled 2 days. The decrease in ATP with chilling was prevented by hardening the seedlings at 15° for 2 days (14-hr-day-length) immediately before chilling. The ATP level of hardened plants was higher than of unhardened plants. When hardened plants were chilled at 5°, the ATP level increased in the leaves but decreased in the roots.     

The chilling injury induced by high root temperature in the leaves of rice seedlings

Root temperature is found to be a very important factor forleaves to alter the response and susceptibility to chillingstress. Severe visible damage was observed in the most activeleaves of seedlings of a japonica rice (Oryza sativa cv. Akitakomachi),e.g. the third leaf at the third-leaf stage, after the treatmentwhere only leaves but not roots were chilled (L/H). On the otherhand, no visible damage was observed after the treatment whereboth leaves and roots were chilled simultaneously (L/L). Thechilling injury induced by L/H, a novel type of chilling injury,required the light either during or after the chilling in orderto develop the visible symptoms such as leaf bleaching and tissuenecrosis. Chlorophyll fluorescence parameters measured aftervarious lengths of chilling treatments showed that significantchanges were induced before the visible injury. The effectivequantum yield and photochemical quenching of PSII dropped dramaticallywithin 24 h in both the presence and absence of a 12 h lightperiod. The maximal quantum yield and non-photochemical quenchingof PSII decreased significantly only in the presence of light.On the other hand, L/H chilling did not affect the functionof PSI, but caused a significant decrease in the electron availabilityfor PSI. These results suggest that the leaf chilling with highroot temperature destroys some component between PSII and PSIwithout the aid of light, which causes the over-reduction ofPSII in the light, and thereby the visible injury is inducedonly in the light.