The chilling sensitivity of root ammonium influx in a cultivated and wild tomato

A chilling episode of a few hours damaged root ammonium absorption in a cultivated tomato ( Lycopersicon esculentum cv. T-5), but not in a wild congener from high altitudes ( Lycopersicon hirsutum LA1778). In the cultivar, ammonium influx was strongly temperature dependent and showed the residual effects of chilling, whereas ammonium efflux was nearly temperature invariant and showed no persistent effects. A 2 h exposure to 5 °C significantly depressed subsequent ammonium absorption at 20 °C, and about 12 h at 20 °C was required for recovery. For both the cultivated and wild species, rerooted cuttings were slightly less sensitive to chilling than seedlings. The relative inhibition (mean ± SE) of ammonium absorption before and after chilling was 58·4 ± 2·5% for the cultivated species and 29·0 ± 9·1% for the wild species. The F₁ hybrid between the species showed a relative inhibition of 52·4 ± 3·6%, suggesting that chilling sensitivity may be dominant. In a backcross of the hybrid to L. esculentum , the phenotypic distribution of the relative inhibition of ammonium absorption indicated that this trait is segregating.     

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.     

Introduction of the cDNA for shape Arabidopsis glycerol-3-phosphate acyltransferase (GPAT) confers unsaturation of fatty acids and chilling tolerance of photosynthesis on rice

The chilling sensitivity of several plant species is closely correlated with the levels of unsaturation of fatty acids in the phosphatidylglycerol (PG) of chloroplast membranes. Plants with a high proportion of unsaturated fatty acids, such as Arabidopsis thaliana, are resistant to chilling, whereas species like squash with only a low proportion are rather sensitive to chilling. The glycerol-3-phosphate O-acyltransferase (GPAT) enzyme of chloroplasts plays an important role in determining the levels of PG fatty acid desaturation.A cDNA for oleate-selective GPAT of Arabidopsis under the control of a maize Ubiquitin promoter was introduced into rice (Oryza sativa L.) using the Agrobacterium-mediated gene transfer method. The levels of unsaturated fatty acids in the phosphatidylglycerol of transformed rice leaves were found to be 28% higher than that of untransformed controls. The net photosynthetic rate of leaves of transformed rice plants was 20% higher than that of the wild type at 17°C. Thus, introduction of cDNA for the Arabidopsis GPAT causes greater unsaturation of fatty acids and confers chilling tolerance of photosynthesis on rice.     

RFLP mapping of QTLs conferring cold tolerance during seed germination in an interspecific cross of tomato

Most cultivars of tomato, Lycopersicon esculentum, are sensitive to low (chilling) temperatures (0–15 °C) during seed germination; however, genetic sources of cold (chilling) tolerance have been identified within the related wild species. The purpose of this study was to identify quantitative trait loci (QTLs) that contribute to cold tolerance during germination in tomato using a backcross population of an interspecific cross between a cold-sensitive tomato line (NC84173, recurrent parent) and a L. pimpinellifolium accession (LA722) that germinates rapidly under low temperatures. A total of 119 BC1 individuals were genotyped for 151 restriction fragment length polymorphism (RFLP) markers and a genetic linkage map was constructed. The parental lines and 119 BC1S1 families (self-pollinated progeny of the BC1 individuals) were evaluated for germination at a low temperature (11±0.5 °C). Germination was scored visually as radicle protrusion at 8 h intervals for 28 consecutive days. Germination response was analyzed by the survival analysis and the times to 25, 50 and 75% germination were calculated. In addition, a germination index (GI) was calculated as the weighted mean of the time from imbibition to germination for each family/line. Two QTL mapping techniques, interval mapping (using MAPMAKER/QTL) and single-point analysis (using QGENE), were used to identify QTLs. The results of both methods were similar and two chromosomal locations (3–5 putative QTLs) with significant effects on low temperature germination were identified. The L. pimpinellifolium accession had favorable QTL alleles on chromosomes 1 and NC84173 had favorable QTL alleles on chromosome 4. The percentage of phenotypic variation explained (PVE) by individual QTLs ranged from 11.9% to 33.4%. Multilocus analysis indicated that the cumulative action of all significant QTLs accounted for 43.8% of the total phenotypic variance. Digenic epistatic interactions were evident between two of the QTL-linked markers and two unlinked markers. Transgressive phenotypes were observed in the direction of cold sensitivity. The results indicate that low temperature germination of tomato seed can be improved by marker-assisted selection.     

Somaclonal variation and chilling tolerance improvement in rice: Changes in fatty acid composition

The relationship between chilling tolerance of six rice cultivars – Facagro 57, Facagro 76, Fujisaka 5, Kirundo 3, Kirundo 9 and IR64 -and the fatty acid composition in total lipids, phospholipids, galactolipids and neutral lipids from leaves was studied. Higher double bond index and proportions of linolenic acid in the phospholipid and galactolipid classes were related to cultivar chilling tolerance, but this was not so for the total lipids nor the neutral lipid class. The somaclonal families derived from Facagro 76, Kirundo 3 and Kirundo 9 that showed enhanced chilling tolerance as compared to their original parental cultivar were analyzed for fatty acid composition in phospholipids and galactolipids from leaves. Altered proportions in fatty acid composition in phospholipids, galactolipids or both were found in the somaclonal families derived from Facagro 76 and Kirundo 9, but not from Kirundo 3. These changes most usually resulted in higher double bond index and higher proportions in linoleic and linolenic acids which were related either to lower ratio of C16 to C18 fatty acids or to higher unsaturation in the C18 fatty acid fraction. Different mechanisms thus seem to be implicated in the altered fatty acid composition of somaclones, which may be related to the chilling tolerance improvement of some somaclonal families.     

Hardening trumps acclimation in improving cold tolerance of Drosophila melanogaster larvae

Abstract Chill‐susceptible insects are able to improve their survival of acute cold exposure over both the short term (i.e. hardening at a relatively severe temperature) and longer term (i.e. acclimation responses at milder temperatures over a longer time frame). However, the mechanistic overlap of these responses is not clear. Four larval stages of four different strains of Drosophila melanogaster are used to test whether low temperature acclimation (10 °C for 48 h) improves the acute cold tolerance (LT₉₀, ～2 h) of larvae, and whether acclimated larvae still show hardening responses after brief exposures to nonlethal cold or heat, or a combination of the two. Acclimation results in increased cold tolerance in three of four strains, with variation among instars. However, if acclimation is followed by hardening pre‐treatments, there is no improvement in acute cold survival. It is concluded that short‐term thermal responses (e.g. hardening) may be of more ecological relevance to short‐lived life stages such as larvae, and that the mechanisms of low temperature hardening and acclimation in D. melanogaster may be antagonistic, rather than complementary.     

Metabolism of hydrogen peroxide between diapause and non‐diapause eggs of the silkworm,Bombyx Mori during chilling at 5°C

When diapause and non‐diapause eggs of the same bivoltine strain of Bombyx mori were chilled at 5°C for more than 30 days, the hatchability of diapause eggs increased while that of non‐diapause eggs decreased, respectively. To investigate the relationship between effects of chilling on the hatchability and the metabolism of hydrogen peroxide (H₂O₂), content of H₂O₂ and activities of superoxide dismutase (SOD), xanthine oxidase (XO), and catalase (CAT) between diapause and non‐diapause eggs were determined during the chilling at 5°C. The significant enhancement of H₂O₂ occurred prior to the quick increase of the hatchability in diapause eggs and coincided with the quick decline of the hatchability in non‐diapause eggs, respectively. Diapause eggs contained significantly higher H₂O₂ and XO activity and lower CAT activity compared to non‐diapause eggs. Our results showed that there were significant differences in the metabolism of H₂O₂ between diapause and non‐diapause eggs during chilling and that significant enhancement of H₂O₂ may be involved in the diapause termination of diapause eggs and the cell damage of non‐diapause eggs. © 2010 Wiley Periodicals, Inc.     

Physiological girdling of pine trees via phloem chilling: proof of concept

Quantifying below-ground carbon (C) allocation is particularly difficult as methods usually disturb the root-mycorrhizal-soil continuum. We reduced C allocation below ground of loblolly pine trees by: (1) physically girdling trees and (2) physiologically girdling pine trees by chilling the phloem. Chilling reduced cambium temperatures by approximately 18 degrees C. Both methods rapidly reduced soil CO2 efflux, and after approximately 10 days decreased net photosynthesis (P(n)), the latter indicating feedback inhibition. Chilling decreased soil-soluble C, indicating that decreased soil CO2 efflux may have been mediated by a decrease in root C exudation that was rapidly respired by microbes. These effects were only observed in late summer/early autumn when above-ground growth was minimal, and not in the spring when above-ground growth was rapid. All of the effects were rapidly reversed when chilling was ceased. In fertilized plots, both chilling and physical girdling methods reduced soil CO2 efflux by approximately 8%. Physical girdling reduced soil CO2 efflux by 26% in non-fertilized plots. This work demonstrates that phloem chilling provides a non-destructive alternative to reducing the movement of recent photosynthate below the point of chilling to estimate C allocation below ground on large trees.