Chilling-tolerant Genotypes Research Articles

Reactions of anthocyanin rich in maize genotypes to low temperature treatments according to photosynthesis, gas exchange properties, and bio-active compounds

Low temperatures during the early growing stages limit the productivity of maize considerably. Investigating responses of different coloured corns (Zea mays L.) to chilling may reveal alternative genotypes which can be preferred under early sowing conditions of water-shortage farming areas. The aim of this study is determining whether the color factor affects the tolerance to chilling in maize and which properties are effective on the low temperature tolerance. We exposed corns with different colours (white, yellow, red, purple) to different temperatures [8°C, 12°C, 16°C, 25°C (control)] and analysed the effects of temperature on morphological, physiological, bio-active properties and stress indicators. Using 14-day old seedlings, we noted that purple corn had the highest seedling length, seedling weight, chlorophyll content, stomatal conductance, chlorophyll B and total phenolic content in the 8°C and followed by white (photosynthesis rate, chlorophyll fluorescence, chlorophyll A and carotenoids), yellow (transpiration rate, sub stomatal CO2, and total antioxidant activity) and red corns (water use efficiency, total anthocyanin content and proline). On the other hand, white corn maintained its superiority in other treatments, receiving the highest values in 9 of 17 characteristics at 12°C, in 8 of 17 at 16°C and in 10 of 17 in the control. Performance of purple corn in the 8°C was the most remarkable one in all genotypes and treatments. Based on our results, it has been concluded that white and purple corns are more chilling tolerant genotypes and may be alternative for early sowing conditions in drought farming areas.

Comparative physiological and transcriptomic analyses reveal key regulatory networks and potential hub genes controlling peanut chilling tolerance.

The unclear molecular mechanism by which peanuts adapt to chilling stress limits progress in molecular breeding for peanut chilling tolerance. Here, the physiological and transcriptional differences between two genotypes with contrasting tolerance under chilling stress were compared. The inhibition of photosynthesis mainly caused by stomatal factors was a common response of peanut seedlings to chilling stress. Chilling-tolerant genotypes could inhibit the accumulation of ROS to adapt to chilling stress, and enhanced activities of CAT and APX were major causes of lower H2O2 content. The results of a conjoint analysis of physiological indices and the RNA-Seq database by WGCNA indicated that the genes in key modules were significantly enriched in pathways related to the oxidation-reduction process. Hub genes encoding RLK, CAT, MYC4, AOS, GST, PP2C, UPL5 and ZFP8 were likely to positively regulate peanut chilling tolerance, but hub genes encoding PAO, NAC2 and NAC72 were likely to negatively regulate peanut chilling tolerance.

Response of photosynthetic capacity and antioxidative system of chloroplast in two wucai (Brassica campestris L.) genotypes against chilling stress.

Chilling stress during the growing season could cause a series of changes in wucai (Brassica campestris L.). WS-1 (chilling-tolerant genotype) and Ta2 (chilling-sensitive genotype) were sampled in present study to explore the chilling tolerance mechanisms. Our results indicated that photosynthetic parameters exhibited lower level in Ta2 than in WS-1 under chilling stress. The rapid chlorophyll fluorescence dynamics curve showed that chilling resulted in a greater inactivation of photosystem II reaction center in Ta2. Reactive oxygen species and malondialdehyde content of chloroplast in Ta2 were higher than WS-1. The ascorbate-glutathione cycle in chloroplast of WS-1 played a more crucial role than Ta2, which was confirmed by higher activities of antioxidant enzymes including Ascorbate peroxidase, Glutathione reductase, Monodehydroascorbate reductase and Dehydroascorbate reductase and higher content of AsA and GSH. In addition, the ultrastructure of chloroplasts in Ta2 was more severely damaged. After low temperature stress, the shape of starch granules in Ta2 changed from elliptical to round and the volume became larger than that of WS-1. The thylakoid structure of Ta2 also became dispersed from the original tight arrangement. Combined with our previous study under heat stress, WS-1 can tolerant both chilling stress and heat stress, which was partly due to a stable photosynthetic system and the higher active antioxidant system in plants, in comparison to Ta2.

Physiological basis of chilling tolerance and early-season growth in miscanthus.

The high productivity of Miscanthus × giganteus has been at least partly ascribed to its high chilling tolerance compared with related C4 crops, allowing for a longer productive growing season in temperate climates. However, the chilling tolerance of M. × giganteus has been predominantly studied under controlled environmental conditions. The understanding of the underlying mechanisms contributing to chilling tolerance in the field and their variation in different miscanthus genotypes is largely unexplored. Five miscanthus genotypes with different sensitivities to chilling were grown in the field and scored for a comprehensive set of physiological traits throughout the spring season. Chlorophyll fluorescence was measured as an indication of photosynthesis, and leaf samples were analysed for biochemical traits related to photosynthetic activity (chlorophyll content and pyruvate, Pi dikinase activity), redox homeostasis (malondialdehyde, glutathione and ascorbate contents, and catalase activity) and water-soluble carbohydrate content. Chilling-tolerant genotypes were characterized by higher levels of malondialdehyde, raffinose and sucrose, and higher catalase activity, while the chilling-sensitive genotypes were characterized by higher concentrations of glucose and fructose, and higher pyruvate, Pi dikinase activity later in the growing season. On the early sampling dates, the biochemical responses of M. × giganteus were similar to those of the chilling-tolerant genotypes, but later in the season they became more similar to those of the chilling-sensitive genotypes. The overall physiological response of chilling-tolerant genotypes was distinguishable from that of chilling-sensitive genotypes, while M. × giganteus was intermediate between the two. There appears to be a trade-off between high and efficient photosynthesis and chilling stress tolerance. Miscanthus × giganteus is able to overcome this trade-off and, while it is more similar to the chilling-sensitive genotypes in early spring, its photosynthetic capacity is similar to that of the chilling-tolerant genotypes later on.

Variable content and distribution of arabinogalactan proteins in banana (Musa spp.) under low temperature stress.

Information on the spatial distribution of arabinogalactan proteins (AGPs) in plant organs and tissues during plant reactions to low temperature (LT) is limited. In this study, the extracellular distribution of AGPs in banana leaves and roots, and their changes under LT stress were investigated in two genotypes differing in chilling tolerance, by immuno-techniques using 17 monoclonal antibodies against different AGP epitopes. Changes in total classical AGPs in banana leaves were also tested. The results showed that AGP epitopes recognized by JIM4, JIM14, JIM16, and CCRC-M32 antibodies were primarily distributed in leaf veins, while those recognized by JIM8, JIM13, JIM15, and PN16.4B4 antibodies exhibited predominant sclerenchymal localization. Epitopes recognized by LM2, LM14, and MAC207 antibodies were distributed in both epidermal and mesophyll cells. Both genotypes accumulated classical AGPs in leaves under LT treatment, and the chilling tolerant genotype contained higher classical AGPs at each temperature treatment. The abundance of JIM4 and JIM16 epitopes in the chilling-sensitive genotype decreased slightly after LT treatment, and this trend was opposite for the tolerant one. LT induced accumulation of LM2- and LM14-immunoreactive AGPs in the tolerant genotype compared to the sensitive one, especially in phloem and mesophyll cells. These epitopes thus might play important roles in banana LT tolerance. Different AGP components also showed differential distribution patterns in banana roots. In general, banana roots started to accumulate AGPs under LT treatment earlier than leaves. The levels of AGPs recognized by MAC207 and JIM13 antibodies in the control roots of the tolerant genotype were higher than in the chilling sensitive one. Furthermore, the chilling tolerant genotype showed high immuno-reactivity against JIM13 antibody. These results indicate that several AGPs are likely involved in banana tolerance to chilling injury.

Chilling and frost tolerance in Miscanthus and Saccharum genotypes bred for cool temperate climates

Miscanthus hybrids are leading candidates for bioenergy feedstocks in mid to high latitudes of North America and Eurasia, due to high productivity associated with the C4 photosynthetic pathway and their tolerance of cooler conditions. However, as C4 plants, they may lack tolerance of chilling conditions (0-10 °C) and frost, particularly when compared with candidate C3 crops at high latitudes. In higher latitudes, cold tolerance is particularly important if the feedstock is to utilize fully the long, early-season days of May and June. Here, leaf gas exchange and fluorescence are used to assess chilling tolerance of photosynthesis in five Miscanthus hybrids bred for cold tolerance, a complex Saccharum hybrid (energycane), and an upland sugarcane variety with some chilling tolerance. The chilling treatment consisted of transferring warm-grown plants (25/20 °C day/night growth temperatures) to chilling (12/5 °C) conditions for 1 week, followed by assessing recovery after return to warm temperatures. Chilling tolerance was also evaluated in outdoor, spring-grown Miscanthus genotypes before and after a cold front that was punctuated by a frost event. Miscanthus×giganteus was found to be the most chilling-tolerant genotype based on its ability to maintain a high net CO2 assimilation rate (A) during chilling, and recover A to a greater degree following a return to warm conditions. This was associated with increasing its capacity for short-term dark-reversible photoprotective processes (ΦREG) and the proportion of open photosystem II reaction centres (qL) while minimizing photoinactivation (ΦNF). Similarly, in the field, M.×giganteus exhibited a significantly greater A and pre-dawn F v/F m after the cold front compared with the other chilling-sensitive Miscanthus hybrids.

Candidate gene expression profiling in two contrasting tomato cultivars under chilling stress

Tomato (Solanum lycopersicum Mill.) is sensitive to chilling stress during all stages of plant development. Genetic variation for chilling tolerance exists between cultivated tomato and its related wild species, but intra-specific variation has not been thoroughly investigated so far. Seedlings of 63 tomato accessions were evaluated under low temperature and two contrasting cultivars were identified for the trait: Albenga and San Marzano, the former being more chillingtolerant. To clarify the molecular mechanisms of chilling tolerance in tomato, changes in candidate gene expressions in the two tomato genotypes were analysed, using quantitative RT-PCR. Candidate genes were chosen among those known to be induced by chilling and/or with putative roles in CBF/DREB and ROS-mediated pathways. Results show that besides a CBF regulon, whose function is conserved, ROS and C2H2-type zinc finger protein-mediated cold signalling pathways were also involved in chilling tolerance. Under the chilling stress, the up-regulation of respective transcripts was consistently higher in the chilling-tolerant genotype than in the chilling-sensitive ones.

Comparative Transcriptome Profiling of Chilling Stress Responsiveness in Two Contrasting Rice Genotypes

Rice is sensitive to chilling stress, especially at the seedling stage. To elucidate the molecular genetic mechanisms of chilling tolerance in rice, comprehensive gene expressions of two rice genotypes (chilling-tolerant LTH and chilling-sensitive IR29) with contrasting responses to chilling stress were comparatively analyzed. Results revealed a differential constitutive gene expression prior to stress and distinct global transcription reprogramming between the two rice genotypes under time-series chilling stress and subsequent recovery conditions. A set of genes with higher basal expression were identified in chilling-tolerant LTH compared with chilling-sensitive IR29, indicating their possible role in intrinsic tolerance to chilling stress. Under chilling stress, the major effect on gene expression was up-regulation in the chilling- tolerant genotype and strong repression in chilling-sensitive genotype. Early responses to chilling stress in both genotypes featured commonly up-regulated genes related to transcription regulation and signal transduction, while functional categories for late phase chilling regulated genes were diverse with a wide range of functional adaptations to continuous stress. Following the cessation of chilling treatments, there was quick and efficient reversion of gene expression in the chilling-tolerant genotype, while the chilling-sensitive genotype displayed considerably slower recovering capacity at the transcriptional level. In addition, the detection of differentially-regulated TF genes and enriched cis-elements demonstrated that multiple regulatory pathways, including CBF and MYBS3 regulons, were involved in chilling stress tolerance. A number of the chilling-regulated genes identified in this study were co-localized onto previously fine-mapped cold-tolerance-related QTLs, providing candidates for gene cloning and elucidation of molecular mechanisms responsible for chilling tolerance in rice.

Different photosynthetic responses to night chilling among twelve populations of Jatropha curcas

Jatropha curcas, one of the most important energy plant resources, is vulnerable to chilling. To evaluate the effects of chilling on photosynthesis of J. curcas and intraspecific differences in chilling tolerance, seedlings of twelve populations were treated with the temperature of 4-6°C for five consecutive nights with normal environmental temperature during the day. Night chilling treatment decreased light-saturated photosynthetic rate (Pmax) significantly for all populations. Stomatal limitation could not explain the decreased Pmax because intracellular CO2 concentration was not significantly reduced by night chilling in all populations (with only one exception). The decreased soluble-protein content, which may be related to the increased malondialdehyde (MDA) content, contributed to the decreased Pmax. The increased MDA content indicated that oxidative stress occurred after night chilling, which was associated with the larger decrease in Pmax compared with the decrease in actual photochemical efficiency of photosystem II, and the slight increase in thermal dissipation of excessive energy. After five-day recovery, MDA (with two exceptions) and Pmax still did not recover to the levels as those before night chilling treatment for all populations, indicating that J. curcas was vulnerable to chilling. Chilling tolerance was significantly different among populations. Populations originating from high elevations had greater chilling-tolerant abilities than populations originating from low elevations, showing a local adaptation to environmental temperatures of origins. Our study shed light on the possibility to find or breed chilling-tolerant genotypes of J. curcas.

Chilling tolerance in maize: agronomic and physiological approaches

Maize is a C4 plant species with higher temperature optima than C3 plant species. Growth and productivity of maize are severely constrained by chilling stress. Here, we review the effects of chilling stress on growth, phenology, water and nutrient relations, anatomy, and photosynthesis in maize. Several management strategies to cope with chilling stress are also proposed. In maize, chilling stress is known to reduce leaf size, stem extension and root proliferation, disturb plant water relations, and impede nutrient uptake. Chilling stress in maize is a complex phenomenon with physiological and biochemical responses at both cellular and whole-organ level. CO2 assimilation by leaves is reduced mainly due to membrane damage, photoinhibition, and disturbed activity of various enzymes. Enhanced metabolite flux through the photorespiratory pathway increases the oxidative load on tissues as both processes generate reactive oxygen species (ROS). Injury caused by ROS to macromolecules under chilling stress is one of the major deterrents to growth. Low-molecular-weight osmolytes, including glycinebetaine, proline, and organic acids, are crucial in sustaining cellular function under chilling stress. Plant growth substances such as salicylic acid, gibberellic acid, and abscisic acid modulate the response of maize to chilling stress. Polyamines and several enzymes act as antioxidants and reduce the adverse effects of chilling stress. Chilling tolerance in maize can be managed through the development and selection of chilling-tolerant genotypes by breeding and genomic approaches. Agronomic approaches such as exogenous application of growth hormones and osmoprotectants to seeds or plants, and early vigour, can also aid in chilling tolerance.

Role of thermal dissipation in the photoprotection in cucumber plants after exposure to a chill stress

Experiments were carried out to investigate the changes in CO2 assimilation, photon allocation, and photosynthetic electron flux in leaves of cucumber (Cucumis sativus L.) plants after chilling stress. Chilling significantly decreased CO2 assimilation, the energy flux via linear electron transport (J PS2) and non-constitutive thermal dissipation (J NPQ) but increased fluorescence and constitutive thermal dissipation (J f,D) in chilling-sensitive genotype Jinyan No. 4. In contrast, chilling had little effects on J NPQ and J f,D although CO2 assimilation and J PS2 were inhibited in chilling-tolerant genotype Jinchun No. 3. In parallel with the reduction in J PS2, electron flux to oxygenation and carboxylation by ribulose-1,5-bisphosphate carboxylase/oxygenase all significantly decreased while electron flux to O2 significantly increased, especially in chilling-sensitive genotype. Thermal and fluorescence dissipation were the main energy dissipation pathways whilst water-water cycle was an important electron sink when photosynthetic carbon reduction was suppressed after chilling. Chilling sensitivity of the photosynthetic apparatus was related to the operation of different photoprotection mechanisms.

Alteration of oxidative and carbohydrate metabolism under abiotic stress in two rice ( Oryza sativa L.) genotypes contrasting in chilling tolerance

Abiotic stress is a major limiting factor in crop production. Physiological comparisons between contrasting abiotic stress-tolerant genotypes will improve understanding of stress-tolerant mechanisms. Rice seedlings (S3 stage) of a chilling-tolerant (CT) genotype (CT6748-8-CA-17) and a chilling-sensitive (CS) genotype (INIAP12) were subjected to abiotic stresses including chilling (13/12 degrees C), salt (100mM NaCl), and osmotic (200mM mannitol). Measures of physiological response to the stresses included changes in stress-related sugars, oxidative products and protective enzymes, parameters that could be used as possible markers for selection of improved tolerant varieties. Metabolite analyses showed that the two genotypes responded differently to different stresses. Genotype survival under chilling-stress was as expected, however, CT was more sensitive to salt stress than the CS genotype. The CT genotype was able to maintain membrane integrity better than CS, perhaps by reduction of lipid peroxidation via increased levels of antioxidant enzymes during chilling stress. This genotype accumulated sugars in response to stress, but the accumulation was usually less than in the CS genotype. Chill-stressed CT accumulated galactose and raffinose whereas these saccharides declined in CS. On the other hand, the tolerance mechanism in the more salt- and water-deficit-tolerant CS may be associated with accumulation of osmoprotectants such as glucose, trehalose and mannitol.

Genotypic Variation of Rubisco Expression, Photosynthetic Electron Flow and Antioxidant Metabolism in the Chloroplasts of Chill-exposed Cucumber Plants

Genetic improvement of agronomic crops is necessary to cope with chilling stress. To identify the physiological factors responsible for this genotypic difference in chill-induced inhibition of photosynthesis, leaf CO2 assimilation, the electron flux in the chloroplast and the antioxidant metabolism in isolated chloroplasts were examined in two genotypes of cucumber (Cucumis sativus) plants with distinct chilling tolerance. Cucumber plants were exposed to 100 micromol m(-2) s(-1) at 9/7 degrees C (day/night) for 10 d and were then returned to optimal conditions for 2 d. Chilling resulted in more significant reductions in rbcL and rbcS transcripts, ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) content and initial Rubisco activity, leading to higher electron flux to O2 in the chilling-sensitive genotype than in the chilling-tolerant genotype. The chilling-tolerant genotype showed lower H2O2 contents in the chloroplasts by maintaining higher H2O2-scavenging activity in the chloroplasts than in the chilling-sensitive genotype. H2O2 accumulation in chloroplast was negatively correlated with the initial Rubisco activity and photosynthetic rate.

The Inheritance of Chilling Tolerance in Tomato (Lycopersicon spp.)

During the past 25 years, chilling tolerance of the cultivated (chilling-sensitive) tomato Lycopersicon esculentum and its wild, chilling-tolerant relatives L. peruvianum and L. hirsutum (and, less intensively studied, L. chilense) has been the object of several investigations. The final aim of these studies can be seen in the increase in chilling tolerance of the cultivated genotypes. In this review, we will focus on low-temperature effects on photosynthesis and the inheritance of these traits to the offspring of various breeding attempts. While crossing L. peruvianum (male symbol) to L. esculentum (female symbol) so far has brought the most detailed insight with respect to physiological questions, for practical purposes, e.g., the readily cross ability, crossing programmes with L. hirsutum as pollen donor at present seem to be a promising way to achieve higher chilling-tolerant genotypes of the cultivated tomato. This perspective is due to the progress that has been made with respect to the genetic basis of chilling tolerance of Lycopersicon spp. over the past five years.

The Role of Aquaporins and Membrane Damage in Chilling and Hydrogen Peroxide Induced Changes in the Hydraulic Conductance of Maize Roots

When chilling-sensitive plants are chilled, root hydraulic conductance (L(o)) declines precipitously; L(o) also declines in chilling-tolerant plants, but it subsequently recovers, whereas in chilling-sensitive plants it does not. As a result, the chilling-sensitive plants dry out and may die. Using a chilling-sensitive and a chilling-tolerant maize genotype we investigated the effect of chilling on L(o), and its relationship to osmotic water permeability of isolated root cortex protoplasts, aquaporin gene expression, aquaporin abundance, and aquaporin phosphorylation, hydrogen peroxide (H(2)O(2)) accumulation in the roots and electrolyte leakage from the roots. Because chilling can cause H(2)O(2) accumulation we also determined the effects of a short H(2)O(2) treatment of the roots and examined the same parameters. We conclude from these studies that the recovery of L(o) during chilling in the chilling-tolerant genotype is made possible by avoiding or repairing membrane damage and by a greater abundance and/or activity of aquaporins. The same changes in aquaporins take place in the chilling-sensitive genotype, but we postulate that membrane damage prevents the L(o) recovery. It appears that the aquaporin response is necessary but not sufficient to respond to chilling injury. The plant must also be able to avoid the oxidative damage that accompanies chilling.

Reduction of dark chilling stress in N-fixing soybean by nitrate as indicated by chlorophyll a fluorescence kinetics.

Sub-optimal night temperatures below 15 degrees C (dark chilling) frequently reduce soybean [Glycine max (L.) Merrill] production. Nitrate application is known to alleviate some of the negative effects of low root zone temperatures, probably by counteracting the inhibition caused by decreased symbiotic nitrogen fixation (SNF). Under field conditions, however, dark chilling is frequently not accompanied by low root zone temperatures. The possibility that nitrate might increase dark-chilling tolerance under these conditions is still largely unexplored. In addition to quantifying vegetative development by means of the plastochron index, O-J-I-P (O-I(1)-I(2)-P) chlorophyll a fluorescence transients were recorded in soybean genotypes of contrasting chilling tolerance during and following exposure to dark chilling in the absence of low root zone temperatures. Plants, inoculated with the N(2)-fixing bacteria, Bradyrhizobium japonicum, were grown with and without nitrate supplementation. The recorded O-J-I-P chlorophyll a fluorescence transients were analysed by the so-called JIP-test which translates stress-induced alterations in these transients to changes in biophysical parameters that quantifies the energy flow through photosystem II (PSII). One of these parameters, the performance index (PI(ABS)), combines the three main functional steps (light energy absorption, excitation energy trapping, and conversion of excitation energy to electron transport) of photosynthetic activity by a PSII reaction centre complex into a single multiparametric expression. By using the PI(ABS) we could convincingly show that nitrate supplementation considerably enhances dark-chilling tolerance and recovery capacity of plants in the absence of low root zone temperatures. This was especially true for the chilling-sensitive genotype ('Java 29'), suggesting that the response of SNF to dark chilling might be an important factor contributing towards genotypic differences in chilling tolerance. Our results corroborated previous reports about the superior chilling tolerance of 'Maple Arrow', a chilling-tolerant genotype. The results obtained indicated that the PI(ABS) is a far more sensitive indicator of dark-chilling stress than the maximum quantum yield of primary photochemistry (F(V)/F(M)).

In vitro salicylic acid inhibition of catalase activity in maize: differences between the isozymes and a possible role in the induction of chilling tolerance

It has been suggested that the inhibition of catalase (CAT) (EC 1.11.1.6) by salicylic acid (SA) plays a role in mediating stress responses. In this work, it is proposed that CAT-1 and CAT-2 isozymes of maize ( Zea mays L.) might differ in the nature of SA inhibition, as it was shown for 0 and 9 dpi (days postimbibition) scutellum preparations, containing mainly CAT-1 and CAT-2 isozymes, respectively. In the case of 9 dpi extract, only a weak, competitive inhibition of CAT activity was observed upon treatment with SA or several other phenolic compounds. On the other hand, CAT activity of the 0 dpi extract was inhibited to a significantly greater extent and in a non-competitive manner by SA and its analogues, except for p-hydroxybenzoic acid (pHBA), which showed the same kinetics of inhibition as for 9 dpi samples. All of the phenolic compounds but pHBA, were found to significantly increase chilling tolerance when added hydroponically to young maize seedlings. According to these results, CAT-1 might be a candidate for mediating the effect of SA on the induction of chilling tolerance in maize. Maize genotypes with varying degrees of chilling tolerance were compared: chilling-tolerant maize lines showed significant inhibition by SA, while chilling-sensitive lines were not uniform in this respect, as in the case of Mo17, catalase activity was hardly inhibited by SA, while Penjalinan showed the same rate of inhibition as the chilling-tolerant genotypes.

Different root low temperature response of two maize genotypes differing in chilling sensitivity

Here we report on the root hydraulic properties of intact and excised root systems of two maize genotypes differing in chilling sensitivity (Z7, tolerant and Penjalinan, sensitive) subjected for 3 d to 5 °C. When root hydraulic conductance (L) was measured under a hydrostatic force using an excised root system in a pressure chamber, an initial decrease of L was observed in both genotypes. However, the value of L increased in the chilling tolerant genotype after 30 h at 5 °C; in the chilling sensitive Penjalinan genotype there was no such increase. Osmotic root hydraulic conductance was measured in excised root systems exuding under atmospheric pressure. We observed a progressive decline during the chilling treatment of the osmotic root hydraulic conductance in the chilling sensitive Penjalinan plants; however, after 54 h at 5 °C, the chilling tolerant Z7 plants had a significantly higher osmotic hydraulic conductance. Moreover, in the chilling tolerant plants we found an increase in the inhibition caused by HgCl 2 of the osmotic hydraulic conductance during the chilling treatment, indicating a possible increase in the contribution of aquaporins to root hydraulic conductance in the chilling tolerant Z7 plants during chilling treatment.

Effect of growing season on the photosynthetic apparatus and leaf antioxidative defenses in two maize genotypes of different chilling tolerance

The effect of the growing season on the photosynthetic apparatus and on leaf antioxidants was investigated in a chilling-tolerant genotype (Z7) and a chilling-sensitive genotype (Penjalinan) of maize ( Zea mays). Maize was sown in the field in Switzerland in 3 consecutive weeks, so plants sown early were exposed to chilling conditions, whereas those sown later developed under more favorable growth conditions. Measurements of the quantum efficiency of CO 2 fixation (Φ CO 2 ) and the photosystem II (Φ PSII) were made simultaneously on the third fully expanded leaves. The activity of scavenging enzymes and the content of pigments and antioxidants were also determined. Leaves that developed under chilling conditions showed typical chill-induced alterations, namely low photosynthetic capacity and efficiency, reduction in the pigment contents, and a decrease in catalase activity. These alterations were more pronounced in the chilling-sensitive than in the chilling-tolerant genotype. Determining the Φ PSII/Φ CO 2 ratio indicated that it was very unlikely that alternative electron sinks, such as the Mehler reaction, were activated to a significant extent in either of the genotypes. This was supported by the measurements of the activity of enzymes involved in the Mehler ascorbate peroxidase reaction. However, a comparison of the genotypes showed that chilling tolerance might be correlated with an increase in the α-tocopherol and glutathione contents as well as in the activity of glutathione reductase.

028 Selecting Sweetpotato Genotypes Tolerant to Chilling Stress

A procedure to determined selection of sweetpotato (Ipomoea batatas L. Lam.) genotypes tolerant to chilling injury was initiated by crossing two resistance lines and two susceptible lines. Experimental design on the F1 progeny was a completely randomized design (CRD) with two groups, tolerant and sensitive genotypes, and 90 lines in each group. Four plants per lines were selected and each plant represented a replication. The rating of plants according to the degree of chilling injury was recorded at 36 h after chilling temperature of 5 °C with 85% relative humidity. Significance of the analysis was based on the number of plants tolerant to chilling injury from both the resistant and the sensitive groups. Results from the statistical analysis based on visual rating of the F1 progeny plants for 36 h, indicated that higher populations of resistant plants could be produced when two resistant lines were crossed in a control pollination process. Evaluations to be continued are chlorophyll fluorescence, leakage of cell content, structural changes of the cell, and peroxidase content, before and after chilling stress. These assays will be used to further determine the similarities among the chilling-tolerant genotypes. Findings will aid in elucidating mechanism of chilling injury in fruit and vegetables.