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.     

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.     

Glycinebetaine increases chilling tolerance and reduces chilling-induced lipid peroxidation in Zea mays L.

Chilling tolerance was increased in suspension‐cultured cells and seedlings of maize (Zea mays L. cv ‘Black Mexican Sweet’) grown in media containing glycinebetaine (GB). A triphenyl tetrazolium chloride (TTC) reduction test indicated that after a 7 d chilling period at 4 °C, cells treated with 1 mm GB at 26 °C for 1 d had a survival rate (30%) that was twice as high as that of untreated controls. The addition of 2·5 m M GB to the culture medium resulted in maximum chilling tolerance (40%). The results of a cell regrowth assay were consistent with viability determined by the TTC method. In suspension‐cultured cells supplemented with various concentrations of GB, accumulation of GB in the cells was proportional to the GB concentration in the medium and was saturated at a concentration of 240 μ mol (g DW) ^{? 1}. The degree of increased chilling tolerance was positively correlated with the level of GB accumulated in the cells. The increased chilling tolerance was time‐dependent; i.e. it was first observed 3 h after treatment and reached a plateau after 14 h. Feeding seedlings with 2·5 m M GB through the roots also improved their chilling tolerance, as evidenced by the prevention of chlorosis after chilling for 3 d at 4 °C/2 °C. Lipid peroxidation, as expressed by the production of malondialdehyde, was significantly reduced in GB‐treated cells compared with the untreated controls during chilling. These results suggest that increased chilling tolerance may be due, in part, to the reduction of lipid peroxidation of the cell membranes in the presence of GB.     

Response to Chilling of Zea mays, Tripsacum dactyloides and their Hybrid

Maize (Zea mays ssp. mays) and eastern gamagrass (Tripsacum dactyloides) are known for their susceptibility to chilling injuries. Their hybrid (Z. mays × T. dactyloides) showed higher tolerance to low temperatures (–2 °C) in the field than its parents. Exposure to 5 °C for 2 or 3 d reduced the variable to maximal chlorophyll fluorescence ratio (F_V/F_M), an indicator of the maximum photochemical efficiency of the photosystem 2, and the variable to minimal fluorescence ratio (F_V/F₀) more in maize and eastern gamagrass than in hybrid plants. Chlorophyll contents for rewarming plants (25 °C for 3 d) were lower than before chilling in both parents while values for hybrid plants were similar. Electrolyte leakage was higher in chilled than control plants but it did not show significant differences among genotypes. Our data suggest that hybrid plants have higher capacity to recover from chilling injury in controlled conditions than their parents.     

Low growth temperature-induced changes to pigment composition and photosynthesis in Zea mays genotypes differing in chilling sensitivity

The effects on pigment composition and photosynthesis of low temperature during growth were examined in the third leaf of three chilling-tolerant and three chilling-sensitive genotypes of Zea mays L. The plants were grown under a controlled environment at 24 or 14 °C at a photon flux density (PFD) of 200 or 600 μ mol m^–2 s^–1. At 24 °C, the two classes of genotypes showed little differences in their photosynthetic activity and their composition of pigments. At 14 °C, photosynthetic activity was considerably reduced but the chilling-tolerant genotypes displayed higher photosynthetic rates than the chilling-sensitive ones. Plants grown at 14 °C showed a reduced chlorophyll (Chl) a + b content and a reduced Chl a / b ratio but an increased ratio of total carotenoids to Chl a + b . These changes in pigment composition in plants grown at low temperature were generally more pronounced in the chilling-sensitive genotypes than in the tolerant ones, particularly at high PFD. Furthermore, at 14 °C, all the genotypes showed increased ratios of lutein, neoxanthin and xanthophyll-cycle carotenoids to Chl a + b but a reduced ratio of β -carotene to Chl a + b , especially at high PFD. At 14 °C, the chilling-tolerant genotypes, when compared with the sensitive ones, were characterized by higher contents of β -carotene and neoxanthin, a lower content of xanthophyll-cycle carotenoids, a lower ratio of xanthophylls to β -carotene, and less of their xanthophyll-cycle carotenoid pool in the form of zeaxanthin. These differences between the two classes of genotypes were more pronounced at high PFD than at low PFD. The results are discussed in terms of the relationship that may exist in maize between pigment composition and the capacity to form an efficient photosynthetic apparatus at low growth temperature.     

铜胁迫对玉米幼苗生长、叶绿素荧光参数和抗氧化酶活性的影响

本文研究了铜(Cu)胁迫下玉米(Zeamays)幼苗生长、叶绿素含量、叶绿素荧光参数和抗氧化酶活性的变化。研究结果表明,5￣20μmol.L-1Cu处理10天明显抑制玉米幼苗根系生长,并减少玉米幼苗的干物重,以及增加玉米幼苗地上部和根系含Cu量;玉米幼苗吸收的Cu大部分积累在根系,在地上部分布较少。Cu处理还降低玉米叶片的叶绿素含量和Fv/Fm、ETR、qP和qy值。在10天的Cu处理期间,根系中SOD、POD、CAT和GR活性呈现先上升后下降的趋势。而叶片中的SOD、POD、CAT和GR活性在处理前期不受Cu胁迫的显著影响,处理后期则因Cu胁迫而增强。实验表明抗氧化酶在抵御过量Cu引起的氧化胁迫中发挥了一定的作用。     

Environmental effects on the early stages of tassel morphogenesis in maize (Zea mays L.)

The effects of various environmental conditions on the initiation of tassel branches (NTB) and spikelet‐pairs (NSP) were examined in the stress‐sensitive maize inbred F53. Chilling induced the most important effect, with a dramatic decrease in both NTB and NSP, provided it was applied at the end of the vegetative phase and start of the floral transition phase. The primary cause of chilling‐induced abortion of the tassel branches could be oxidative stress in the leaves, since lowering light irradiance during chilling greatly reduced the effect of cold. The comparison of inbreds F53 and F2 revealed that both genotypes exhibited a similar period of cold sensitivity at the floral transition phase, although F2 was considered from field observations as a stress‐insensitive genotype (at least for tassel development). However, our results also showed a chilling acclimation response in inbred F2 but not in inbred F53. The similarities with the work by Lejeune & Bernier (1996 Plant, Cell and Environment 19, 217–224.) concerning the effect of chilling on ear initiation in the sensitive inbred, B22, are emphasized.     

Depolymerization of cortical microtubules is not a primary cause of chilling injury in corn (Zea mays L. cv Black Mexican Sweet) suspension culture cells

Cold-induced depolymerization of cortical microtubules were examined in suspension culture cells of corn (Zea mays L. cv Black Mexican Sweet) at various stages of chilling. In an attempt to determine whether microtubule depolymerization contributes to chilling injury, experiments were carried out with and without abscisic acid (ABA) pretreatment, since ABA reduces the severity of chilling injury in these cells. Microtubule depolymerization was detectable after 1 h at 4°C and became more extensive as the chilling was prolonged. There was little chilling injury after 1 d at 4°C in either ABA-treated or non-ABA-treated cells. After 3 d at 4°C, there was about 26% injury for ABA-treated and 40% injury for non-ABA-treated cells, as evaluated by 2,3,5-triphenyl-tetrazolium chloride reduction and by regrowth. After 1d at 4°C, less than 10% of cells retained full arrays of microtubules in both ABA-treated and non-ABA-treated cells, the remainder having either partial arrays or no microtubules. After 3d at 4°C, about 90% of cells showed complete or almost complete depolymerization of microtubules in both ABA-treated and non-ABA-treated cells. ABA did not stabilize the cortical microtubules against cold-induced depolymerization. In about 66% of ABA-treated cells and 57% of non-ABA-treated cells that had been held at 4°C for 3d, repolymerization of cortical microtubules occurred after 3h at 28°C. These results argue against the hypothesis that depolymerization of cortical microtubules is a primary cause of chilling injury.     

Leaf photosynthetic parameters of two maize (Zea mays) cultivars in response to various patterns of chilling temperatures and photon flux densities

Chilling‐induced photosynthetic impairment was examined in leaves of maize (Zea mays L.) seedlings of two cultivars, one adapted to western Europe and one adapted to Mexican highlands. Three experiments were performed in a controlled environment. The effects of chilling night temperatures, of chilling at high light intensity and of variable chilling day temperatures on photosynthetic parameters, were evaluated. Chilling in the dark period resulted in stomatal limitation of net photosynthesis. Chilling at moderate to high light intensities caused chilling‐dependent photoinhibition of CO₂ uptake. Photobleached maize leaves did not resume normal photosynthetic function. Maize cv. Batan 8686 from the highlands of Mexico was less susceptible to photosynthetic damage than maize cv. Bastion adapted for cultivation in W. Europe, when exposed to chilling night temperatures, or to mild chilling photoinhibitory conditions.     

Role of light in changes in free amino acid and polyamine contents at chilling temperature in maize (Zea mays)

The effect of irradiance on changes in photosynthesis, free amino acids and polyamines was investigated. Two-week-old maize ( Zea mays L.) plants were chilled at 5°C in the light (250 μmol m⁻² s⁻¹ PAR) or dark. The chlorophyll fluorescence ratio, F_v/F_m, decreased in the light by ca 50% but did not change in the dark. Similarly to the F_v/F_m, there was no change in the transpiration rate or the stomatal conductance in the dark, while these parameters decreased by ca 55% in the light. The net photosynthesis rate declined in both cases, but to a far greater extent in the light (73%) than in the dark (40%). The intercellular CO₂ partial pressure increased by ca 50% in all cases. The free amino acid contents increased compared to the control during the cold treatment. In most cases this increase was more pronounced in the light than in the dark. There was a continuous increase in the putrescine level, which was more pronounced in the light than in the dark. The spermidine content increased one and a half times after one day in the light but decreased by 70% in the dark compared to the control values. From the second day a 50% decline in the spermidine content was observed in the light and an 80% decline in the dark. These results suggest that light has an influence not only on the photosynthetic processes during chilling stress but also on other stress markers such as polyamines and free amino acids.     

A study of the impaired growth of roots of Zea mays seedlings at low oxygen concentrations

Abstract. Seedlings of Zea mays L. were grown in the dark at 27°C. Four-day-old seedlings were then exposed for 3 days to solutions equilibrated with gas mixtures to give O₂ concentrations between 0.02 and 0.25 mol m^?3. Root growth was impaired just as severely at 0.06 as 0.02 mol O₂ m^?3 while growth at 0.16 mol O₂ m^?3 was about the same as in solutions in equilibrium with air (0.25 mol O₂ m^?3). Growth of young seedlings at low O₂ concentrations was inhibited to the same extent in nutrient solution and 0.5 ml m^?3 CaCl₂, showing that the adverse effect of O₂ deficits on growth was not due to less uptake of inorganic nutrients. Furthermore, at low O₂ concentrations neither exposure of the shoots to a relative humidity of 100% (26.0 g H₂O m^?3) nor excision of the entire shoot enhanced root growth relative to that in plants with shoots at a relative humidity of 50% (13.0 g H₂O m^?3). Therefore, for these seedlings growing in the dark, impairment of root growth at low O₂ concentrations was not a consequence of water deficits in the shoot or of other shoot-root interactions. Total soluble sugars and amino acid concentrations were generally greater at low (0.02–0.06 mol O₂m^?3) than at high O₂ concentrations (0.16–0.25 mol O₂ m ^?3). This applied specifically to the root apices (0–2 mm) and expanding (2–15 mm) tissue except in some experiments where sugar concentrations in expanding tissue were slightly greater at high than at low O₂ concentrations. Critical O₂ pressures for respiration of excised root segments were approximately 0.117 and 0.065 mol O₂ m^?3 in the expanding and expanded zones of the roots, respectively. In contrast, the critical O₂ pressure exceeded 0.20 mol O₂ m^?3 in the apex, suggesting that O₂ supply for metabolic processes is most likely to be sub-optimal in this zone. Our results show clearly that the adverse effects of low O₂ concentrations are unlikely to be a consequence of substrate shortage for either respiration or synthesis of macromolecules; low rates of ATP regeneration in growing root tissues are the logical cause for impaired growth in young seedlings while they are being sustained by seed reserves.     

Effects of low temperature on lateral diffusion in plasma membranes on maize (Zea mays L.) root cortex protoplasts: relevance to chilling sensitivity

The effects of temperature on the lateral diffusion of fluorescent phospholipids, sterols and proteins in the plasma membranes of maize root cortex protoplasts were monitored using fluorescence photobleaching recovery (FPR). Diffusion parameters were measured in two cultivars of maize having different chilling tolerance. Hydrodynamic theory predicts that the diffusion coefficient, D, should increase with increasing temperature. In the more chilling-tolerant cultivar, however, D for all three probes was nearly insensitive to temperature. In the more chilling-sensitive cultivar, D was also insensitive to temperature over the range from 12 to 21°C, but D for the lipid probes tended to be higher and more variable at lower temperatures. The proportion of probe molecules free to diffuse in the membrane was less than 1 for all probes, and increased significantly with increasing temperature for the protein probe. These results, taken together, support the concept that the plasma membrane contains domains having differing diffusional characteristics. Temperature effects on membrane diffusion are moderated by the existence of these domains to limit significant changes. The observed tendency for higher diffusion coefficients at low temperatures in the chilling-sensitive cultivar may correlate to morphological changes observed with protoplasts of that cultivar at low temperatures.     

Relationship between iron chlorosis and alkalinity in Zea mays

Mengel, K. and Geurtzen, G. 1988. Relationship between iron chlorosis and alkalinity in Zea mays . - Physiol. Plant. 72: 460–465.
Maize ( Zea mays L. cv. Anjou 21) grown in nutrient solution with Fe-EDTA and with nitrate as the sole nitrogen source showed typical Fe-chlorosis symptoms after a growth period of 14–21 days. Alkalinity in roots, stems and leaves of the chlorotic plants was high. Transferring the chlorotic plants from the nitrate-containing nutrient solution to a solution of (NH₄)₂SO₄ resulted in a regreening of leaves within 2–3 days which was associated with a decrease in solution pH, a decrease in alkalinity of plant parts, a translocation of Fe from roots to tops and a release of Fe into the outer solution. Similar effects were obtained when Fe chlorotic plants were transferred to a dilute HO solution with pH 3.5.
Spraying chlorotic leaves with indoleacetic acid or with fusicoccin led also to a regreening of leaves without having a major effect on leaf alkalinity.
Interpretation of the experimental results is based on the assumption that nitrate as sole N source leads to a high pH level in the apoplast resulting in the precipitation of Fe compounds, probably Fe oxide hydrate. Ammonium nutrition has the reverse effect since it lowers the apoplast pH and this can result in the dissolution of Fe compounds. Application of indoleacetic acid as well as fusicoccin supposedly stimulates the proton pumps in the plasmalemma of the leaf tissue. The resulting decrease in apoplast leaf pH in the microenvironment also leads to a dissolution of Fe compounds in the apoplast and thus promotes the uptake of Fe by the symplasm.     

Changes in fatty acids of leaf polar lipids during chilling and post-chilling rewarming of Zea mays genotypes differing in response to chilling

Changes in fatty acids of leaf polar lipids: monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG) in maize seedlings of chiling-sensitive (CS) CM 7 and Co 151 lines and chilling-tolerant (CT) S 215 and EP 1 lines upon chilling for either 4 or 6 days in the dark and after rewarming for 4 days at original growth conditions were studied. The content of free fatty acids (FFA) in control leaves as well as alterations in the proportion of major fatty acids, unsaturation ratio (UR), double bond index (DBI) and changes in the proportion of heigh-temperature melting of both phosphatidylglycerol (htm-PG) and sulfoquinovosylglycerol (htm-SQDG) after chilling and rewarming of seedlings were estimated. FFA content in intact leaves was 2–3-fold higher in the chilling susceptible CM 7 line than in the other three inbreeds studied. After chilling for 6 days the level of FFA increased only in CM 7 and S 215 lines by about 30 %. Upon rewarming seedlings chilled for 6 days the level of FFA increased about two-fold in CS Co 151 line and CT EP 1 line and decreased in CS CM 7 line. Limited accumulation of FFAs during chilling and post-chilling rewarming of maize seedlings, did not correspond to the extent of polar lipid breakdown (Kaniuga et al. 1999b) probably due to the contribution of active oxidative systems to the peroxidation of fatty acids under these conditions. During rewarming seedlings chilled for 6 days major changes were observed in decrease of 18:3 and an increase of 16:0 in all four polar lipids studied with more pronounced changes in CS than CT lines. Similarly, in CS inbreeds a decrease in UR of fatty acids in MGDG, DGDG and SQDG after post-chilling rewarming was greater than in CT lines. Proportion of htm-fraction in both PG and SQDG increased after post-chilling rewarming in all four inbreeds, however to a lesser extent in CT than CS lines. A similar pattern of changes in DBI in CS and CT maize seedlings was observed in glycolipid and combine lipid classes. More extensive degradation of polar lipids in CS than CT maize inbreeds following galactolipase action in chloroplasts (Kaniuga et al. 1998) provides FFAs for initiation of peroxidation by LOX which is manifested by decrease of UR and DBI. This sequence of reactions during chilling and post-chilling rewarming appears to be a main route of fatty acids peroxidation responsible for secondary events involved in chilling injury. In addition, the extent of these changes differentiates CS and CT inbreeds. Contribution of esterified fatty acids in thylakoid lipids to direct peroxidation, may be of minor importance.     

Temperature-sensitivity of D1 protein metabolism in isolated Zea mays chloroplasts

The effects of low temperature on the synthesis and stability of the 32 kDa D1 protein of photosystem II were investigated in chloroplasts isolated from maize (Zea mays cv. LG11) leaves. The synthesis of D1 by intact chloroplasts in vitro was strongly dependent on temperature; the Q₁₀ for the initial rate of incorporation of [³⁵S]-methionine into D1 was ca. 2.6 over the range 13–25°C. The synthesis of other thylakoid polypeptides exhibited a similar temperature dependence, whilst synthesis of stromal proteins was considerably less temperature-dependent, with the exception of two polypeptides of ca. 56 and 59.5 kDa. The stability of newly-synthesized D1 in the thylakoid membranes was dependent both on the temperature at which the plants were grown and on the temperature during the pulse-labelling period when the protein was synthesized. In chloroplasts isolated from maize leaves grown at 25°C, D1 that was synthesized and assembled at 25 °C in vitro was rapidly degraded during the chase period. At lower chase temperatures the protein was more stable. When chloroplasts from 25°C-grown leaves were pulse-labelled at 13°C, the stability of D1 was markedly enhanced at all temperatures during the chase period. This effect was even more pronounced in chloroplasts isolated from plants grown at 14°C. The implications of these results are discussed with regard to the ability of maize to recover from photoinhibitory damage at low temperatures.     

Membrane stabilization by abscisic acid under cold aids proline in alleviating chilling injury in maize (Zea mays L.) cultured cells

Previous studies of maize suspension‐cultured cells showed that abscisic acid (ABA) treatment at warm temperatures improved the tolerance of cells to subsequent chilling. In the present study, it is shown that both ABA‐treated and untreated maize cells accumulated proline in response to chilling. However, ABA‐treated cells displayed less lipid peroxidation during chilling, and thus, unlike untreated cells, were able to retain the accumulated proline intracellularly. Proline application experiments indicate that an intracellular proline level higher than 2 µmole (g FW)^?1 prior to chilling was needed to meaningfully reduce chilling‐enhanced lipid peroxidation and significantly improve chilling tolerance. The results suggest that total proline accumulation in ABA‐treated as well as untreated cells during chilling was enough to potentially improve chilling tolerance, but proline leakage rendered the control cells unable to benefit from the endogenous synthesis of proline in relation to the alleviation of chilling injury. Proline participated in chilling tolerance improvement in ABA‐treated maize cells, as evidenced by: (1) the inhibition of proline accumulation by l ‐methionine‐d , l ‐sulphoximine (MSO), an inhibitor of glutamine synthetase, reduced ABA‐improved chilling tolerance, and (2) the addition of glutamine into the medium prevented the MSO‐induced reduction in chilling tolerance. The revised relationship between proline accumulation and membrane stability at cold is discussed in the light of these current findings.     

Galactolipase activity but not the level of high-melting-point phosphatidylglycerol is related to chilling tolerance in differentially sensitive Zea mays inbred lines

Galactolipase activity, the level of high-melting-point phosphatidylglycerol (HMP-PG) as well as degradation of lipids during chilling and rewarming were studied in seedlings of maize inbred lines with different chilling responses. In aged chloroplasts of chilling-sensitive (CS) lines, galactolipase activity was considerably higher than that determined in aged chloroplasts isolated from chilling-tolerant (CT) ones. Chilling of seedlings at 5 °C for 6 days induced neither loss of chlorophyll content nor visible changes in the leaves, while a slight decline in total acyl lipid content by about 15.5% and 12.5% in CS and CT lines, respectively, was observed. Among total acyl lipids, only monogalactosyldiacylglycerol (MGDG) levels were decreased significantly upon chilling. Following return to the original growth conditions for 4 days, visible chilling injury in seedlings as well as essential differences in the decrease in total acyl lipids by about 53% and 20% in CS and CT lines, respectively, were found. These changes were accompanied by more extensive degradation of MGDG, digalactosyldiacylglycerol and phosphatidylglycerol in CS than in CT lines. As the levels of HMP-PG in fresh leaves were the same in all four lines of maize, it seems that galactolipase activity and not the level of HMP-PG is related to chilling response in maize. Received: 4 July 1997 / Revision received: 18 September 1997 / Accepted: 3 March 1998     

Effects of changes in growth temperature on photosynthesis and carotenoid composition in Zea mays leaves

The effects of changes in growth temperature on photosynthesis and carotenoid composition were examined in Zea mays L. leaves of different age and different developmental history. The plants were first grown at sub-optimal temperature (14°C) until the full development of the third leaf. At that time, the mature third leaf and the immature fourth leaf had a low chlorophyll (Chl) content, a low Chl a/b ratio, a high carotenoid/Chl a+b ratio, a high xanthophyll/β-carotene ratio, and about 80% of the xanthophyll cycle pool (violaxanthin [V] + antheraxanthin [A] + zeaxanthin [Z]) was in the form of zeaxanthin and antheraxanthin. When the temperature was increased from 14°C to 24°C for three days, increased Chl synthesis, accompanied by an increase in the Chl a/b ratio, took place. The ratios of lutein, neoxanthin, and V+A+Z to Chl a+b decreased markedly, whereas no significant changes appeared in the β-carotene/Chl a+b ratio. Furthermore, there was a sharp decrease in the xanthophyll/β-carotene ratio and most of zeaxanthin was converted to violaxanthin in the xanthophyll cycle. The third leaf and the tip segment of the fourth leaf, both expanded at 14°C, showed little difference in their pigment contents. However, the rate of CO₂ assimilation of the tip segment of the fourth leaf was nearly twice that of the third leaf on the third day at 24°C, while the photosynthetic activity was similar in both leaves before the transfer to 24°C. During the warm period at 24°C, new leaf tissue (basal segment of the fourth leaf and part of a fifth leaf) was formed. On the third day at 24°C, the pigment content of 24°C-grown leaf tissue did not differ much from that of 14°C-grown leaf tissue with the exception that the total carotenoid content was lower in the former as compared to the latter, mainly because of a lower V+A+Z content. The rate of CO₂ assimilation of 24°C-grown leaf tissue was comparable to that of the tip segment of the fourth leaf. Regardless of which leaf tissue is considered, reducing the temperature from 24°C to 14°C for 5 days slightly affected the pigment content, but violaxanthin was largely converted to zeaxanthin and antheraxanthin in the xanthophyll cycle. The results indicate that compared to old leaf tissue of mature leaves, physiologically younger leaf tissue of immature leaves is much more able to recover from depressions in the photosynthetic activity induced by growth at sub-optimal temperature when the plants experience optimal growth temperatures, but that factors other than the pigment content must determine this capability.     

The use of chlorophyll fluorescence as a screening method for cold tolerance in maize

Chlorosis in maize (Zea mays L.) is a common phenomenon in the 12 to 17°C temperature range. A newly developed chlorophyll-fluorescence technique was used to elucidate the underlying subcellular processes of resistance to chlorosis. Four populations were used that were developed by divergent mass selection for contrasting resistance to chlorosis in a cold-tolerant dent and a cold-tolerant flint population. Young plants from the four populations were kept for six days at 17/10, 15/10 and 13/10°C (day/night). After 1, 3 and 6 days various chlorophyll-fluorescence parameters were determined. The measurements were done on leaf 4. Differences were not uniform for all fluorescence properties. The resistant and susceptible populations of the two sets differed for the Q-quenching which is related to the electron transport rate in the chloroplast. For the E-quenching which is related to the Calvin cycle activity, the resistant dent differed significantly from the other three populations. The ratio Fm/Fo (related to the transfer of absorbed light-energy from antennae pigments to reaction centers in the chloroplast) was higher for the resistant dent population than for the susceptible one. The flint types did not differ for this property.Apparently, divergent mass selection for chlorosis resistance resulted in various changes at the subcellular level that are not necessarily comparable for flint and dent types.When after 6 days the temperature was raised from 13°C to 17°C, the fluorescence signals led to the conclusion that there was a full recovery of various processes after two days, except for the metabolic activity of the susceptible flint.