Photosynthetic Apparatus Responses to Short‐term low‐temperature Flooding May Contribute to Freezing Tolerance Changes in Forage Grasses

Winter conditions are subjected to rapid climate changes. Increased precipitation and snow melting during warmer winters may result in low‐temperature flooding. These factors probably affect plant overwintering strategies. This study investigated the relationships between the photosynthetic adjustment and freezing tolerance following a short‐term low‐temperature flooding in two forage grasses (Lolium perenne and Festuca pratensis). The effect of flooding on the photosynthetic apparatus acclimation to cold was determined using chlorophyll a fluorescence measurements. Freezing tolerance was estimated using an electrolyte leakage test. It has been shown that genotypes activating non‐photochemical mechanisms of photosynthetic acclimation to cold during low‐temperature flooding may show increased freezing tolerance. Freezing tolerance reduction observed in Festuca pratensis was probably connected with decreased photochemical activity and photoinhibition of photosynthesis. It can be concluded that photosynthetic apparatus response to low‐temperature flooding may contribute to changes in the freezing tolerance. The direction of the changes is associated with different photosynthetic apparatus performance.     

Associations of PCR markers with freezing tolerance and photosynthetic acclimation to cold in winter barley

Associations of PCR markers with freezing tolerance and acclimation of photosynthetic apparatus to cold were tested on 28 winter barley cultivars and advanced breeding forms to select alleles for practical application in marker assisted selection (MAS). We found significant associations between freezing tolerance evaluated with field-laboratory method (FLM) and markers located on 5H chromosome in region of gene Fr-H2 (bin9-10: Xbmag812, Xmwg2230) and region of gene Fr-H1 (bin11: Xmwg514, HvBM5, Xmwg644). Additionally, significant associations with photochemical quenching of chlorophyll a fluorescence (q_P) were found for PCR markers Xpsr115 and Xmwg2062. In our study variation in the promoter region of Vrn-H1 (HvBM5) was directly connected with freezing tolerance of plants partially de-acclimated in the field. The results obtained here showed that different loci of freezing tolerance may play role in variable selection pressure of winter conditions.     

冬前低温条件下冬小麦品种间叶绿素荧光参数的比较

为比较抗寒性不同的冬麦品种低温和不同光强作用下叶绿素荧光参数的变化,以3个抗寒性不同的冬麦品种为材料,分别在低温驯化期、低温驯化结束期和封冻期取样,室内暗处理后给予6个不同的光强,测定不同光强下的光合电子传递速率(ETR)、光化学猝灭系数(qP)和非光化学猝灭系数(qN)。结果表明,低温、高光强下抗寒品种东农冬麦1号的ETR、qP和qN显著高于其他2个品种,济麦22的这几个叶绿素荧光参数最低。封冻前抗寒中间型东农705品系的热耗散保持较高水平,但进入封冻期后其热耗散能力迅速降低。     

Use of chlorophyll fluorescence to evaluate the cold acclimation and freezing tolerance of winter and spring oats

The efficiency of the excitation capture by open Photosystem II (PSII) reaction centres was measured by the F_v/F_m ratios in a collection of winter and spring oats in order to assess the effects of hardening and freezing on the functionality of PSII and also the suitability of a chlorophyll fluorescence‐based method to screen oat cultivars for frost tolerance. A significant reversible decrease in F_v/F_m was found in all genotypes during acclimation to low, non‐freezing temperatures. F_v/F_m analysis appears to be an attractive test for the evaluation of frost tolerance in oats, being rapid, non‐invasive and capable of monitoring a trait related to a crucial stage in the acquisition of frost tolerance. It is more sensitive and precise than other standard methods and highly correlated with field‐evaluated frost damage. The measurements made during recovery 1 or 2 days after stress when the visual symptoms are not yet expressed, were especially advantageous because of the large variability in genotype response. The r‐values (close to 0.8) were reduced due to the non‐standard behaviour of the winter cultivar ‘Aintree’. The cold acclimation response of this genotype has been analysed in detail and the limits of artificial freezing tests are discussed.     

Chlorophyll Fluorescence‐Based Studies of Frost Damage and the Tolerance for Cold‐Induced Photoinhibition in Freezing Tolerance Analysis of Triticale (×Triticosecale Wittmack)

Freezing tolerance of 60 breeding lines of winter hexaploid triticale (×Triticosecale Wittmack) was studied in the field‐laboratory experiment. The experiment was repeated over three winters. The survey was also carried out in plants grown and cold‐acclimated in the laboratory. In both the experiments, plant survival analysis and chlorophyll fluorescence‐based studies on energy flows in photosystem II (PSII) (JIP‐test) after freezing of detached leaves were performed. In the laboratory experiment, the temperature of 50 % electrolyte leakage from leaves and the resistance against cold‐induced photoinhibition were additionally investigated. In the case of plants’ cold‐acclimated in the field, determination of the freezing tolerance of PSII gave similar results as the determination of freezing tolerance of whole plants. Both traits were strongly affected by genotype–environmental interactions, but these effects were less visible for PSII characteristics. In the laboratory experiments, a strong correlation between freezing tolerance and the tolerance for cold‐induced photoinhibition of photosynthesis was observed. The possibility of the use of chlorophyll fluorescence‐based techniques for the determination of freezing tolerance in triticale is discussed.     

Relationship between Frost Tolerance and Cold-Induced Resistance of Spring Barley, Meadow Fescue and Winter Oilseed Rape to Fungal Pathogens

Plants exposed to one stress factor may become more tolerant to another. Cold is the most often documented factor inducing plant resistance to pathogens. The aim of this work was to investigate whether resistance of spring barley and meadow fescue to Bipolaris sorokiniana and resistance of winter oilseed rape to Phoma lingam induced at 5 °C for 2, 4 or 6 weeks are associated with frost tolerance, water potential and soluble carbohydrate content. Cold‐acclimated plants of each species showed increased resistance to the studied pathogens. Barley, fescue and rape plants demonstrated higher frost tolerance after hardening, but only in the case of fescue a correlation between resistance to frost and resistance to B. sorokiniana was found. A significant decrease in the water potential of leaf cells was observed in cold‐acclimated barley and fescue. In these two species, water potential greatly affected resistance to B. sorokiniana. However, only in barley did accumulation of fructose, glucose and sucrose correlate as well with changes in water potential as with cold‐induced resistance to the pathogen. In the case of hardened rape, no correlation between the studied parameters was found. The results obtained indicated that the temperature of 5 °C used during cold acclimation was not favourable for hardening of this plant species.     

Increasing pre‐acclimation temperature reduces the freezing tolerance of winter‐type faba bean (Vicia faba L.)

Autumn‐sown winter‐type faba bean (Vicia faba L.) has been shown to have a yield advantage over spring sowing. Still, adoption of this overwintered pulse crop remains limited in temperate locations, due to inadequate winter hardiness. This research sought to understand how the prevailing temperature during emergence and seedling development, that is pre‐acclimation, influences freezing tolerance. Seedlings grown under a controlled “warm” 17/12°C (day/night) pre‐acclimation environment were initially less freezing tolerant than those grown under a “cold” 12/5°C temperature treatment. Stem and particularly root tissues were primarily responsible for slower cold acclimation, and there was a genotype specific response of above‐ground tissues to pre‐acclimation treatment. Both above and below‐ground tissues should be tested across a range of pre‐acclimation temperatures when screening faba bean germplasm for freezing tolerance.     

Inheritance of tolerance to zinc deficiency in barley

Barley (Hordeum vulgare L.) is often grown on alkaline zinc (Zn)‐deficient soils where reductions in yield and grain quality are frequently reported. Currently, the use of Zn‐based fertilizer along with Zn‐deficiency‐tolerant genotypes is considered the most thorough approach for cropping the Zn‐deficient soils; however, developing or breeding genotypes with higher Zn efficiency requires a good understanding of the inheritance of tolerance to Zn deficiency. This study was conducted to determine genetic control of this trait in barley. Two parental cultivars ('Skiff, moderately tolerant; and ‘Forrest’, sensitive), 185 F₂ plants, and 48 F₂‐derived F₃ families from this cross were screened to determine inheritance of tolerance to Zn deficiency using a visual score of deficiency symptoms. The segregation ratios observed indicated that greater tolerance to Zn deficiency in ‘Skiff compared with ‘Forrest’ at the seedling stage is controlled by a single gene with no dominance. The results also indicate that visual scores are useful for genetic analysis of tolerance to Zn deficiency.     

Temperature Before Cold Acclimation Affects Cold Tolerance and Photoacclimation in Timothy (Phleum pratense L.), Perennial Ryegrass (Lolium perenne L.) and Red Clover (Trifolium pratense L.)

The expected temperature rise in late summer/early autumn can change the conditions for acclimation and affect the winter survival of perennial crops. This study examined the effect of the temperature just before the onset of cold acclimation (pre‐acclimation) on freezing tolerance of timothy (Phleum pratense L.), perennial ryegrass (Lolium perenne L.) and red clover (Trifolium pratense L.) populations (both cultivars and breeding populations) adapted to either northern or southern parts of Norway. Using phytotron experiments, we studied whether increasing pre‐acclimation temperature delays growth cessation, affects photoacclimation and reduces freezing tolerance. Furthermore, we assessed whether these effects were related to the latitudinal adaptation of the plant material. The results showed that a rise in pre‐acclimation temperature decreased both cold acclimation capacity and photoacclimation in these species. This affected the freezing tolerance, which was reduced significantly more in northern‐adapted population of timothy and perennial ryegrass compared with southern‐adapted populations. Red clover was less affected by temperature changes than the grasses.     

Studies for assessing the influence of hardening on cold tolerance of barley genotypes

Summary Frost tolerance of 30 barley (Hordeum vulgare L.) cultivars have been field evaluated in North Italy during the 1990/1991 winter season that was characterized by exceptionally low temperatures without snow cover. The results showed a significant correlation between cold injury and grain yield loss (r=0.61^**). Five cultivars chosen for their varying degree of frost tolerance were further evaluated using laboratory tests. Measurements of survival rate and membrane damage were used to assess the influence of hardening on frost resistance. The reliability of the tests is shown by the high correlation to the field data. For both the laboratory temperature regimes and field conditions, the tested cultivars showed the same order of classification. The effect of a rise in temperature at the end of the hardening treatment on frost tolerance is also reported. The laboratory tests here proposed can be integrated in a breeding programme for improving frost tolerance in barley.     

Variation in resistance to different winter stress factors within a full-sib family of perennial ryegrass

Wintering ability in the field and resistance to different winter-stress factors under controlled environmental conditions were studied in a full-sib family of perennial ryegrass (Lolium perenne L.). Significant variation in tolerance to freezing and ice encasement, resistance to pink snow mould (Microdochium nivale) and also in winter survival and spring growth were found between the different genotypes. No strong correlations were found between the resistances to the different stress factors. These results indicate that resistance to different winter-stress factors is controlled by separate genes in perennial ryegrass. A low but significant positive correlation was found between spring growth of plants in the field after the first winter and both freezing tolerance and M. nivale resistance measured in controlled environments. Cold hardening seemed to influence freezing tolerance and M. nivale resistance differently in the different genotypes, since no distinct correlation in tolerance to freezing or resistance to M. nivale was found between unhardened and hardened plants. Tolerance or resistance to most of the winter stress factors measured was positively correlated with plant size. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cold tolerance during juvenile development in sorghum: a comparative analysis by genomewide association and linkage mapping

Improved cold tolerance during the juvenile phase is a major breeding goal to develop new sorghum cultivars suitable as an alternative energy crop in temperate regions. The objectives of this study were to identify marker‐trait associations for cold tolerance in a sorghum diversity panel fingerprinted with 2620 single nucleotide polymorphism (SNP) markers and to detect quantitative trait loci (QTL) in two F_2:3 populations. Traits of interest were dry matter growth rate (DMGR), leaf appearance rate (LAR), chlorophyll content (SPAD) and chlorophyll fluorescence (F_v′/F_m′ and Ф_PSII) in relation to temperature. The association panel comprised 194 genotypes, while the F_2:3 populations consisted of 80 and 92 genotypes. All populations were tested under controlled conditions in a minimum of four temperature regimes ranging from 9.4°C to 20.8°C. QTL were identified for means across environments and regression parameters describing temperature effects. Several marker‐trait associations for mean (m) DMGR, base temperature (T_b) of SPAD and Ф_PSII and temperature effect on LAR were validated by QTL detected in population 1 or 2. Promising QTL regions were found on chromosomes SBI‐01, SBI‐02, SBI‐03, SBI‐04, SBI‐06 and SBI‐09, among them genomic regions where candidate genes involved in the C‐repeat binding pathway or encoding cold‐shock proteins are located.     

Cold acclimation of winter and spring peas: carbon partitioning as affected by light intensity

Like most plants, pea (Pisum sativum L.) becomes tolerant to frost if it is first exposed to low non-freezing temperatures, a process known as cold acclimation. Cold acclimation is a complex process involving many physiological and metabolic changes. Two spring dry peas, two winter dry peas and one winter forage line were exposed to cold temperature in a controlled environment in two experiments, one using low light intensity and the other regular light intensity. Plants were harvested throughout the experiment and dry matter accumulation, water content, soluble and insoluble sugar concentrations were determined from shoot and root samples. Cold acclimation did not occur when temperatures were low if light intensity was low, even in winter peas. In contrast, with regular light intensity, the winter peas acquired more freezing tolerance than spring peas and a close relationship was found between the soluble sugar concentration of leaves just before the frost and the degree of freezing tolerance obtained by the different genotypes. Relationships between freezing tolerance and carbon partitioning between shoot and roots are discussed.     

Heritability estimates for winter hardiness in lentil under natural and controlled conditions

Heritability analysis for cold tolerance in lentil was conducted using parental, F₂ and F₃ populations at two locations in Balochistan, Pakistan and one controlled environment in Fort Collins, Colorado, USA. Populations of parental and F₂ families were grown over 2 years (1991‐92 and 1992‐93) at Quetta, Balochistan, Pakistan. In 1992‐93, parental and F₃ families were studied at Quetta, and Kalat, Balochistan, Pakistan. Evaluation for normality using non‐transformed and log‐transformed data failed. Attempts to use analysis of variance were then abandoned in favour of parent‐offspring regression for narrow‐sense heritability. Estimates of narrow‐sense heritability ranged from 0.31±0.06 to 0.71 ± 0.06 under field conditions. Under controlled conditions, the estimated heritability was maximized at 1.00 ± 0.17 using 6‐ to 8‐week‐old lentils. Significant transgressive segregants were found in five of the six populations in the F₃ generation. Transgressive segregants appeared in the controlled F₃ generation but were not observed in field environments. This indicates that cold tolerance is under additive gene control and is environmentally sensitive in gene expression.     

Bread and Durum Wheat under Heat Stress: A Comparative Study on the Photosynthetic Performance

The photosynthetic responses to heat stress, during grain filling, in four genotypes of Triticum aestivum L. (Sever and Golia) and Triticum turgidum subsp. durum (Acalou and TE 9306), chosen according to its genetic background diversity, were investigated. All wheat genotypes (excepting Golia) showed synergistic trends implicating the internal CO₂ concentration, net photosynthesis and stomatal conductance. Additionally, the modifications of net photosynthesis were associated with changes in stomatal control. Chlorophyll a fluorescence parameters (minimal fluorescence, maximal and variable fluorescence, intrinsic efficiency of PSII in darkness, non‐photochemical quenching, photochemical quenching and energy‐dependent chlorophyll fluorescence quenching) further pointed heat protective mechanisms, implicating F_v/F_m stabilization (i.e. maintaining the efficiency of PS II) and electron transport rate preservation. It is concluded that, comparatively to bread wheat, the photosynthetic performance of durum wheat is more tolerant to heat stress, as stomatal conductance and transpiration are less affected.     

Relationship between Polyamines and Other Cold‐induced Response Mechanisms in Different Cereal Species

To find a connection between polyamines and various protective effectors involved in the development of cold tolerance, eight different cereal genotypes, including wheat, barley and oat species, were investigated during the acclimation phase to low temperature. Exposure to low temperature induced different changes in the levels of polyamines, and other signalling molecules, such as salicylic acid and abscisic acid, and of other protective compounds, namely flavonols, sugars and antioxidant enzyme activity, and in the lipid composition of certain membrane factions. The most remarkable differences were observed in the oat varieties compared to the other cereal genotypes, which was manifested in the lack of spermidine accumulation and of decrease in trans‐Δ³‐hexadecanoic acid content, in lower initial and not cold‐inducible abscisic acid content and guaiacol peroxidase activity after cold treatment. Correlation analysis revealed that spermidine shows strong positive relationship with flavonols, abscisic acid and ascorbate peroxidase, while was in negative relationship with trans‐Δ³‐hexadecanoic acid. These results suggest that spermidine may have a crucial role in the cold acclimation signalling processes in cereals.     

Assessing freeze tolerance in St. Augustinegrass: II. acclimation treatment effects

The adaptation of St. Augustinegrass (Stenotaphrum secundatum [Walt.] Kuntze) in the transitional climatic zone of the United States is limited due to a lack of sufficient freeze tolerance. Lab-based freeze testing provides plant breeders with a reliable method for evaluating freeze tolerance. However, in developing protocols for freeze testing it is important to account for the underlying mechanisms that affect freeze tolerance. The objective of this research was to evaluate the effects of cold acclimation and deacclimation on nine St. Augustinegrass genotypes ranging in their levels of freeze tolerance. Results indicated that accounting for all levels of acclimation provided excellent genotype separation across freezing temperatures (? 3 and ? 4 °C) and supports the hypothesis that the inclusion of acclimation response offers the best overall assessment of freeze tolerance. Genotypes with the highest known field winter survival had also the highest improved freeze tolerance upon cold-acclimation. Other genotypes did not respond to cold-acclimation which resulted in poor survival and recovery rates. Conversely, a significant loss of freeze tolerance was identified when plants were subjected to deacclimation events suggesting that St. Augustinegrass can be negatively affected by rapid temperature changes in the transitional climatic zone leading to increased sensitivity to winterkill. Overall, this study provides preliminary information regarding the complex relationships within and between mechanisms affecting freeze tolerance in St. Augustinegrass. Moreover, results presented here should aid in the development of protocols for selection of freeze tolerant breeding materials under controlled environmental conditions.     

The Yd2 gene and enhanced resistance to barley yellow dwarf virus (BYDV) in winter barley

A breeding programme was developed to obtain barley yellow dwarf virus (BYDV)-resistant winter genotypes using the Yd2 gene. The aim was to incorporate the Yd2 allele into the new high-yielding genotypes to release cultivars that allow barley cultivation in areas where BYDV is endemic. The resistant lines were developed using pedigree selection. An ICARDA resistant line (83RCBB130) carrying the Yd2 gene was crossed with three susceptible, high-yielding winter varieties and their F₁ lines were either selfed or backcrossed to the matching susceptible parent. The best lines selected from subsequent selfing generations were evaluated in replicated trials in the presence or absence of BYDV, starting from F₆ and BC₁F₅ to F₈ and BC₁F₇ generations. Four genotypes with superior agronomic traits and BYDV resistance were selected.