Winter hardiness in Lolium multiflorum Lam. III. Selection for improved cold tolerance and its effect on agronomic performance

Nine varieties of Italian ryegrass were subjected to two generations of selection for cold tolerance using a controlled environment test. Selection led to a significant overall improvement in average tiller survival from 39·8 to 47·7%, even though the selection intensity achieved was not more than 20%. Although individual varieties varied in initial tolerance from 31·1 to 71·4%, the cold room tests were not sufficiently sensitive to attach significance to differences in response to selection which varied from 0·3 to 17·6%. Agronomic assessment of spaced plants indicated that the winter and spring growth potential of all nine varieties was unaffected by selection for cold tolerance. Although some of the selected populations showed a significant decline in their total yields and persistency, others showed no significant change in these characteristics. The plant breeding implications of these results are discussed.     

Winter hardiness in Lolium multiflorum Lam. I. The effect of nitrogen fertilizer and autumn cutting managements in the field

Four Italian ryegrass varieties of different origins and winter hardiness were grown as spaced plants at a lowland and an upland site and subjected to ten managements comprising two nitrogen levels and five autumn cutting treatments. Tiller mortality after the winter of 1971–72 was measured in March and subsequent spring recovery from winter damage in April. Although the winter was not severe, there was considerable winter kill, particularly at the upland site where the most severe treatment combinations killed up to 90% of the plants of the more susceptible varieties. In general, winter kill was increased by raising the level of nitrogen fertilizer and by late or frequent autumn defoliations, and significant interactions between locations and the two treatment factors emphasized the importance of the siting of the experiment. The varieties difFered in their overall winter hardiness, with Bb 1430 and RvP suffering less tiller mortality than S22 and Grasslands Paroa in all the treatment combinations, but significant interactions of varieties with locations and cutting treatments revealed that the magnitude of the difference between varieties varied with location and cutting management. Regression analysis of these interactions, however, showed that the tiller mortality of all four varieties increased linearly with the increasing stresses applied by the location × management combinations. There was no differential variety recovery from winter kill, the spring yields being entirely related to the level of tiller mortality experienced. These results are discussed in relation to the breeding of winter hardy varieties, the choice of variety and autumn management practice.     

Response of herbage regrowth and water-soluble carbohydrate concentration of ryegrass species to defoliation practices when grown in a Mediterranean environment

Three experiments were conducted to determine the association between leaf number per tiller at defoliation, water‐soluble carbohydrate (WSC) concentration and herbage mass of juvenile ryegrass plants when grown in a Mediterranean environment. Seedlings of ryegrass were grown in nursery pots arranged side‐by‐side and located outside in the open‐air to simulate a mini‐sward in Experiments 1 and 2, and a mixture of annual ryegrass and subterranean clover (Trifolium subterraneum L.) was grown in a small plot field study in Experiment 3. Swards were defoliated mechanically with the onset of defoliation commencing within 28 d of germination. Frequency of defoliation ranged from one to nine leaves per tiller, whilst defoliation height ranged from 30 mm of pseudostem height that removed all leaf laminae in Experiment 1, to 50 mm of pseudostem height with some leaf laminae remaining post‐defoliation in Experiments 2 and 3. A positive relationship between herbage mass of ryegrass, WSC concentration and leaf number per tiller at defoliation was demonstrated in all experiments. In Experiment 1, the herbage mass of leaf, pseudostem and roots of tillers defoliated at one leaf per tiller was reduced to 0·10, 0·09 and 0·06 of those tillers defoliated less frequently at six leaves per tiller. However, the reduction in herbage mass from frequent defoliation was less severe in Experiment 2 and coincided with a 0·20 reduction in WSC concentration of pseudostem compared with 0·80 measured during Experiment 1. In Experiment 3, the highest harvested herbage mass of ryegrass occurred when defoliation was nine leaves per tiller. Although the harvested herbage from this sward contained senescent herbage, the in vitro dry‐matter digestibility of the harvested herbage did not differ significantly compared with the remaining treatments that had been defoliated more frequently. Leaf numbers of newly germinated ryegrass tillers in a Mediterranean environment were positively associated with WSC concentration of pseudostem and herbage mass. A minimum period of two to three leaf appearances was required to restore WSC concentrations to levels measured prior to defoliation thereby avoiding a significant reduction in herbage mass. However, maximum herbage mass of a mixed sward containing ryegrass and subterranean clover was achieved when defoliation was delayed to nine leaves per tiller.     

Comparative study of direct and indirect evaluations of frost tolerance in barley

The aim of this paper was to compare different field, laboratory and physiological methods for the evaluation of frost tolerance in barley; as well as to show both the possibilities and limitations of these individual methods and approaches. The tolerances of 39 barley cultivars and breeding lines were evaluated by four direct methods (based on the exposure of plants to frost) and three indirect (based on diagnostic traits and markers). The direct methods included the evaluation of: (1) field survival after five winters 1999–2004; (2) winter survival in a provocation pot test under natural conditions; (3) lethal temperature (LT50) of plants taken from a field in winter; and (4) LT50 of plants grown and hardened in a growth chamber. The indirect methods were based upon: (1) endogenous levels of abscisic acid (ABA); (2) water content (WC); (3) osmotic potential (OP) of the plants cultivated and hardened in growth chambers. All four direct methods correlated well with one another, and resulted in similar sequences of frost tolerance of the tested barley cultivars and lines. While it was only possible to distinguish individual barley cultivars based on field survival after the 2002/2003 winter, by using the provocation method it was possible to distinguish between barley cultivars after all of the winters. Moreover, the average winter survival of barley from multi-year pot tests was in correlation with the minimal LT50, evaluated in plants hardened in both the field or in growth chambers. From indirect indicators, the levels of ABA increased slightly, while WC and OP clearly decreased in all cultivars during cold hardening. No correlation was observed between LT50 and the ABA content. A significant correlation between LT50 and WC, and between LT50 and OP was found, but only after 1 or more weeks of cold acclimation of the plants had been completed, since both the duration and the rate of decline of these parameters differed in individual cultivars during the cold hardening. The LT50-values of barley cultivars not only accurately characterized their field tolerance to frost, but also were indicative of changes associated with the induction of frost tolerance in plants under the given conditions, when comparing the direct and indirect methods.     

Interaction between water-soluble carbohydrate reserves and defoliation severity on the regrowth of perennial ryegrass (Lolium perenne L.)-dominant swards

A field-study was undertaken in Hamilton, New Zealand to determine if there was an interaction between water-soluble carbohydrate (WSC) reserve content and defoliation severity on the regrowth of perennial ryegrass-dominant swards during winter. Perennial ryegrass plants with either low or high WSC content were obtained by varying the defoliation frequency. At the third defoliation at the one-leaf stage and at the first defoliation at the three-leaf stage (harvest H₁), swards were mown with a rotary lawnmower to residual stubble heights of 20, 40 or 60 mm. All swards were then allowed to regrow to the three-leaf stage before again defoliating to their treatment residual stubble heights (H₂). Frequently defoliated plants contained proportionately between 0·37 and 0·48 less WSC in the stubble after defoliation, depending on the severity of defoliation. There was no interaction between WSC content and defoliation severity for herbage regrowth between harvests H₁ and H₂. Herbage regrowth was lower from swards containing low WSC plants compared with high WSC plants (2279 vs. 2007 kg DM ha⁻¹). Furthermore, swards defoliated to 20 or 40 mm had greater herbage regrowth compared with those defoliated to 60 mm (2266, 2249 and 1914 kg DM ha⁻¹ for swards defoliated to residual stubble heights of 20, 40 and 60 mm, respectively). Regrowth of perennial ryegrass was positively correlated with post-defoliation stubble WSC content within defoliation severity treatment, implying that WSC contributed to the defoliation frequency-derived difference in herbage yield. However, the effect of defoliation severity on herbage regrowth was not associated with post-defoliation stubble WSC content.     

甘蓝型油菜抗寒性的研究——Ⅰ.渗透调节与抗寒性的关系

在越冬期间,两个抗寒性不同的甘蓝型油莱品种叶片抗寒性的测定结果表明:抗寒锻炼期间,甘蓝型油菜抗寒能力逐渐提高,渗透势随抗寒力的增强而降低,可溶性糖等溶质含量增加,总含水量、自由水含量下降,束缚水含量上升,束缚水/自由水比值增大。元月初,上述物质的变化达到峰值,两个品种间的差异亦达最大值。渗透势、游离脯氨酸含量、总含水量及束缚水/自由水比值可以作为甘蓝型油菜品种抗寒性鉴定的生理生化指标。     

Priority for allocation of water-soluble carbohydrate reserves during regrowth of Lolium perenne

A glasshouse study was undertaken to determine the priority within the perennial ryegrass (Lolium perenne L.) plant for leaf and root growth and daughter tiller initiation after defoliation, in relation to various levels of water-soluble carbohydrate (WSC) reserves at defoliation. Individual plants were arranged in mini-swards, and underwent varying defoliation frequencies and ambient temperatures before defoliation, and harvest heights at defoliation, to obtain a gradient of WSC content at H₁, the date when all plants were defoliated. Defoliation interval consisted of defoliating either three times at the one new leaf tiller^–1 stage (1-leaf stage) of regrowth, or once only at the 3-leaf stage, up to H₁, while night temperature in the week prior to H₁ was altered from 15°C to either 8 or 20°C. At H₁, plants were defoliated to a stubble height of either 20 or 50 mm. Plants were subsequently destructively harvested at days 4, 6, 8, 12, 18 and 27. Leaf and root extension and tiller dynamics were also measured. On a regrowth timescale, tiller initiation was most sensitive, root regrowth moderately sensitive, and leaf regrowth relatively insensitive to a decrease in WSC. The time of daughter tiller initiation also coincided with replenishment of stubble WSC levels. In contrast to this sequence of regrowth events following defoliation, the quantitative effects on growth were different, with elongation and survival of roots most affected by reduced WSC levels. A 30-fold difference in stubble WSC at H₁ between high and low WSC plants (1·52 vs. 0·05 mg tiller^–1) produced only a 4-fold increase in leaf dry matter (DM) after 27 d (2·2 vs. 0·6 g plant^?1), while tiller number plant^?1 increased 6-fold (138 vs. 23% increase in tiller number from H₁). Root elongation rate was 59 times higher in the high than in the low WSC plants (1·18 vs. 0·02 mm d^?1). From a pasture management perspective, the study confirms that defoliation, coinciding with the 3-leaf stage of regrowth and around a stubble height of 50 mm, optimizes persistence and productivity of perennial ryegrass. By allowing more rapid replenishment of WSC reserves, this optimal defoliation strategy enables a greater proportion of WSC to be allocated to maintain a more active root system, and promotes tillering, compared with more frequent and close defoliation.     

Patterns of leaf and root regrowth, and allocation of water-soluble carbohydrate reserves following defoliation of plants of prairie grass (Bromus willdenowii Kunth.)

This study utilized leaf stage‐based defoliation intervals to describe the concentrations and contents of water‐soluble carbohydrate (WSC) and nitrogen (N) in stubble and root reserves and their effect on the regrowth of prairie grass (Bromus willdenowii Kunth.) plants. The priority sequence for allocation of WSC reserves during the regrowth period was also investigated. There were substantially higher concentrations of WSC and N in the stubble compared with the roots following defoliation, confirming the stubble as the primary site for energy storage, with roots playing a lesser role. However, high R² values for the relationships between WSC concentration in roots and regrowth variables suggested that plants of prairie grass were reliant on WSC reserves from the roots in addition to the stubble to meet the energy requirements of plants until adequate photosynthetic tissue had been produced. The sequence of priority for allocation of WSC reserves followed the order of leaf growth, root growth and tillering during the regrowth period. Although WSC reserves were identified as the primary contributor to plant regrowth following defoliation, there was also a strong relationship between stubble N concentration and regrowth variables.     

Leaf number as a criterion for determining defoliation time for Lolium perenne: 2. Effect of defoliation frequency and height

The object of this study was to determine the importance of frequency and height of defoliation on regrowth potential of Lolium perenne. Defoliation interval was based on stage of the regrowth cycle, as indicated by leaves per tiller.
Simulated swards of Lolium perenne cv Yatsyn were grown as individual plants in a glasshouse kept at a day/night temperature of 25°C/15°C.
Treatments imposed were defoliation at 2, 5 or 12 cm residual height, and low and high water soluble carbohydrate (WSC) level obtained by varying defoliation interval, i.e. defoliating at the 1-leaf or 3-leaf stage of the regrowth cycle. Regrowth after frequent short defoliations was only 65% of the less frequently defoliated plants taken over the full regrowth cycle. This was associated with a lower stubble WSC content (2·15 vs 17·5% in stubble) and a twenty-seven-fold difference in the amount of WSC in the stubble per plant. This difference in total WSC was a combined effect of more and heavier tillers and higher WSC content in stubble of plants defoliated less frequently at the end of the regrowth cycle. The regrowth of plants with WSC levels depleted by frequent defoliation when defoliated at 2 cm was significantly below that of those defoliated at 5 and 12 cm.
The results indicate the desirability of defoliating plants at the 3-leaf stage of the regrowth cycle. This not only allows the full regrowth potential to be expressed in that growth cycle, but also in the next cycle, by allowing the replenishment of WSC reserves and optimizing tiller status. The potential to regrow appears then to be based more on the total amount of WSC than the proportion of WSC in stubble.     

THE EFFECT OF AUTUMN MANAGEMENT ON WINTER DAMAGE AND SUBSEQUENT SPRING PRODUCTION OF SIX VARIETIES OF LOLIUM PERENNE GROWN AT CAMBRIDGE

Six perennial ryegrass varieties which were known to differ in winter hardiness were subjected to ten managements comprising two N rates and fire autumn cutting treatments. Autumn yields and soluble carbohydrate contents were measured, also the amount of winter damage in February and subsequent spring yields. The winter was mild, herbage damage was slight and no plant deaths were recorded, but significant differences were found between cutting treatments and varieties. The amount of winter damage increased with the length of the autumn rest period and with the degree of winter dormancy of individual varieties, indicating that natural senescence was a more important causative agent than frost action. Spring yields were affected by the amount of winter damage but there were considerable interactions between varieties and environments. The results are discussed in relation to autumn management practices, choice of variety for different locations and measurements of winter hardiness. The induction of winter dormancy by day length or low temperature reactions is considered in the breeding of varieties for maximum production in different parts of the country.     

The impact of defoliation frequency and nitrogen fertilizer application in spring on summer survival of perennial ryegrass under grazing in subtropical Australia

Abstract This field study investigated the effect of timing of nitrogen (N) fertilizer application in spring on the survival of grazed perennial ryegrass (Lolium perenne cv. Dobson and Yatsyn) over summer in a subtropical environment. There were five N fertilizer treatments: no applied N, 46 kg N ha^?1 on 22 October or 22 November or 22 December, or on 22 October and again on 22 December. Water‐soluble carbohydrate (WSC) concentration of perennial ryegrass plants entering the summer was altered by varying defoliation frequency, with defoliation interval based on the number of leaves per tiller. Frequent defoliation was set at a regrowth level of one leaf per tiller and less frequent defoliation at a regrowth level of three leaves per tiller, over a total of two by three‐leaf per tiller regrowth periods. Application of N fertilizer was found to have no significant effect (P > 0·05) on survival of perennial ryegrass plants over summer. On the other hand, defoliation had a marked effect on perennial ryegrass persistence, with frequent defoliation decreasing ryegrass plant density (51 vs. 88 plants m^?2; P < 0·001) and increasing the density of tropical weed grasses (99 vs. 73 plants m^?2; P < 0·001) by autumn. Frequently defoliated plants had a lower stubble WSC content on a per plant basis than less frequently defoliated plants in spring (103 vs. 201 mg per plant; P < 0·001) and summer (59 vs. 101 mg per plant; P < 0·001). The lower WSC content was associated with a smaller root system in spring (1·50 vs. 2·14 g per plant; P < 0·001) and autumn (1·79 vs. 2·66 g per plant; P < 0·01), and this was reflected in 0·29 more plants being pulled from the soil by livestock between November 1996 and April 1997. Rhizoctonia fungus was associated with roots of pulled plants, but not with roots of seemingly healthy plants, indicating that this fungus may have a role in a weakened root system, which was more prone to sod pulling. Nitrogen applied in October and November resulted in a reduced WSC concentration, although the effect was restricted to 1 month after N application. The present study indicates that survival of perennial ryegrass plants over the summer in a subtropical region is prejudiced by frequent defoliation, which is associated with a lower WSC concentration and a shallower root system. Under grazing, sod pulling is a reflection of this weaker root system and contributes to plant mortality.     

不同小麦品种产量和氮素吸收利用的差异

为明确不同氮肥水平下不同小麦品种产量、氮素吸收和利用的差异,在大田条件下,以山东省主推的21个小麦品种为材料,研究了0、120、180、240和300kg·hm~(-2)共5个氮肥水平下不同小麦品种的产量、氮素利用率、氮素吸收效率和氮素利用效率等指标。结果表明,氮肥、品种及两者的互作效应显著影响冬小麦籽粒产量、氮素利用率、氮素吸收效率和利用效率;氮肥水平不同则品种间的氮效率差异程度不同。大部分供试品种的氮效率类型在不同氮肥水平下的聚类结果不一致,仅临麦4号、鲁原502、泰山28和山农25在各氮肥水平下的产量、氮素利用率均较高,可划为氮高效品种。氮高效品种达到高氮效率的途径不同,临麦4号通过高氮素吸收效率、泰山28通过高氮素利用效率、鲁原502和山农25通过氮素吸收效率和利用效率共同作用实现氮高效。由此可知,应在不同氮肥水平下对冬小麦品种进行产量和氮素利用率的综合评价,以筛选出适应不同地力环境的品种和与品种特性相适应的施肥技术;在氮高效品种的选育和推广中,应针对不同小麦品种的氮素吸收和利用特性进行鉴别和调控,才能最大程度挖掘和利用其高产、高效潜力,实现增产增效。     

De-hardening in contrasting cultivars of timothy and perennial ryegrass during winter and spring

Scenarios of climate changes indicate longer and more frequent spells of mild weather during winter in northern latitudes. De-hardening in perennial grasses could increase the risk of frost kill. In this study, the resistance to de-hardening of different grass species and cultivars was examined, and whether the resistance changes during winter or between years, was tested. In Experiment 1, two cultivars of timothy ( Phleum pratense L.) and perennial ryegrass ( Lolium perenne L.) of contrasting winter hardiness were grown under ambient winter conditions, transferred from the field in January and April 2006 to the laboratory for 9 d with controlled de-hardening conditions of 3°C, 9°C and 15°C. The timothy cultivars were tested at 3°C, 6°C and 9°C in a similar experiment (Experiment 2) in January 2007. De-hardening, measured as decrease in frost tolerance (LT₅₀), was less in timothy than in perennial ryegrass and increased with increasing temperatures. The northern winter-hardy cultivar Engmo of timothy de-hardened more rapidly than the less-hardy cultivar Grindstad, but had higher initial frost tolerance in both experiments, whereas there was less difference between cultivars of perennial ryegrass in Experiment 1. Cultivar Grindstad of timothy lost all hardiness in early spring at all temperatures, whereas cultivar Engmo maintained some hardiness at 3°C. Cultivar Engmo de-hardened at a lower rate in 2007 than in 2006, in spite of similar frost tolerance at the start of de-hardening treatment in both years. This indicates that the rate of de-hardening was controlled by factors additional to the initial frost tolerance and that autumn weather conditions might be important for the resistance to de-hardening.     

The winter hardiness of Festuca rubra, Holcus lanatus and Agrostis spp. in comparison with Lolium perenne

Preliminary experiments were conducted to assess the winter hardiness of representative cultivars or populations of Festuca rubra, Holcus lanatus and Agrostis spp. in comparison with a hardy (Premo) and a susceptible (Grasslands Ruanui) cultivar of Lolium perenne. Two controlled-environment tests and a field experiment including two cutting and two fertilizer managements are reported.
Grasslands Ruanui and H lanatus suffered most damage in all tests. In the controlled-environment experiments, Premo was at least as hardy as the F. rubra and Agrostis cultivars but in the field Premo was more susceptible. Spring growth of Premo was affected more by the cutting and fertilizer managements applied the previous summer and autumn than was spring growth of F. rubra or Agrostis spp.
It was concluded that, whereas use off. F. rubra or Agrostis spp. should not be limited by fears of winter damage, poor winter hardiness may limit the value of H. lanatus for hill and upland reseeding.     

部分春性和半冬性小麦品种氮效率差异分析

为了解小麦冬春性与氮素利用的关系,在大田条件下,以江苏省主栽的15个小麦品种为材料,比较了半冬性和春性小麦的氮效率差异。结果表明,相同施氮条件下,半冬性小麦的平均氮肥吸收效率(NUEa)和氮肥生产效率(NGPE)分别比春性小麦高12.19%和9.64%,差异均达显著水平。其中,NUEa和NGPE最高的半冬性小麦品种均为济麦22,春性小麦两个指标最高的品种均为扬麦15。半冬性小麦的平均氮肥表观利用率(NUR)、氮肥农学效率(NAE)、氮素生理效率(NPE)及氮收获指数亦高于春性小麦,但差异均不显著。半冬性或春性类型小麦中各指标都存在氮高效和氮低效的品种,且品种间差异均极显著;不同品种氮效率高或低的机制不同,使得对氮肥的吸收、利用、转运的能力存在差异,生产中应根据不同品种氮效率机制特点采用不同的施肥应对策略。     

Selection for resistance to ryegrass mosaic virus and crown rust (Puccinia coronata Corda) in Italian ryegrass populations

Inoculation in the glasshouse revealed marked differences in resistance to both ryegrass mosaic virus (RMV) and crown rust (Puccinia coronata Corda) among eight unselected populations of Italian ryegrass (Lolium multiflorum Lam.). Two generations of selection for crown rust resistance led to a significant improvement in all eight populations, while one generation of selection for RMV gave a more variable response. Frequency distributions of individual genotype scores suggested that much of the resistance to crown rust was under the control of one or two genes with major effects, while resistance to RMV appeared to be controlled by a large number of genes each having a small effect. In the field, resistance to RMV meeisured after 20 months was strongly correlated with that obtained in the glasshouse. Natural infection by crown rust in the field was too low to reveal any differences between populations, although three populations from the Po Valley region of Northern Italy became heavily infected with Puccinia recondita. Selection for disease resistance significantly reduced the persistency of three populations, but that of four others was unaffected and one showed a significant improvement. Selection reduced water-soluble carbohydrate (WSC) levels in spring slightly but significantly from 27.6 to 25.8% WSC overall. These results emphasize that agronomic performance must be monitored during selection for disease resistance.     

Tolerance to frost and ice encasement in cultivars of timothy and perennial ryegrass during winter

Ice encasement causes major winter damage in grasslands in coastal areas of Northern Scandinavia and may also affect grass performance in a future changing climate. Changes in ice‐encasement tolerance (LD50), frost tolerance (LT50) and water‐soluble carbohydrate (WSC) content were investigated in different cultivars of timothy and perennial ryegrass sampled at three sites with contrasting conditions. Timothy endured ice encasement for 40 d more than ryegrass (maximum LD50 63 vs. 20 d), and a cultivar originating from 69°N tolerated significantly longer periods in ice than a cultivar from 59°N. A similar relationship between cultivar origin and tolerance was observed for ryegrass. The higher LD50 in timothy compared with ryegrass seemed to be associated with a lower rate of change in WSC content during ice encasement, but no apparent relationship was found between WSC content at the start of encasement and plant survival in ice. A significant linear relationship was found between LD50 and LT50 of plants sampled in the field. A differing decline in frost tolerance during ice encasement for the species indicated that timothy is more resistant to dehardening under ice than ryegrass. This study contributes data and functional relationships needed to develop models of grass performance during winter.     

Water-soluble carbohydrates in perennial ryegrass breeding

Perennial ryegrasses showing genetic differences in water-soluble carbohydrate content (WSC) were assessed for dry matter digestibility (DMD), crude protein (CP) and dry matter production (DM) in both spaced plant and small plot trials. Among 100 accessions grown as spaced plants there appeared to be more genetic variation for WSC than for DMD or CP. In both spaced plants and plots, WSC was consistently positively correlated with DMD, negatively correlated with CP and showed little correlation with DM. Correlations between CP and DMD were rather inconsistent and some negative correlations were obtained between CP and DM, suggesting that environmental effects, particularly with regard to nitrogen availability, affected CP more than WSC content. It was concluded that there is considerable potential for using ryegrasses with high WSC contents to develop varieties with improved feeding value without sacrificing production.     

Freeze tolerance and physiological changes during cold acclimation of giant reed [Arundo donax (L.)]

Arundo donax L. (Poaceae), giant reed, is a rhizomatous shrubby grass that is cultivated in subtropical and warm temperate regions for a multitude of uses. Recently, it has been identified as a leading sustainable, non‐food crop for lignocellulosic biofuels due to its low ecological and agronomic demands. Lack of cold hardiness may limit its diffusion into colder areas of the transition zone. The objectives of this study were to (i) quantify freeze tolerance (LT₅₀) of cold‐acclimated and non‐acclimated Arundo donax L. plants using reproducible, controlled environment conditions, (ii) determine the effect of prolonged exposure to freeze stress on tolerance by keeping plants at a constant, sublethal temperature and (iii) study the relationship of non‐structural carbohydrates (total soluble sugars, glucose, fructose and sucrose) and proline accumulation with cold hardiness. In vitro‐propagated plants of the Honduran and Hungarian ecotypes of Arundo donax L. were chosen for this study. Cold acclimation treatment was imposed for 1 week using a controlled environment chamber set at 10°C and with a 12‐h photoperiod of 200 μmol m^?2 s^?1 photosynthetically active radiation. Freeze tolerance ranged from ?12·8°C (Honduran) to ?16·4°C (Hungarian ecotype). In all the organs analysed, total soluble sugars significantly increased during cold acclimation, with concentrations between 1·8‐ and 4·7‐fold higher than in non‐acclimated plants. The higher concentrations of sugars and proline in cold‐acclimated plants were positively associated with enhanced giant reed freeze tolerance (2·9°C lower). Our results confirm that during cold acclimation, metabolic changes related to increased freezing tolerance occur in giant reed.     

低氮胁迫对不同氮效率玉米品种苗期叶片碳氮代谢的影响

研究低氮胁迫对不同氮效率玉米品种幼苗生长及叶片碳氮代谢的影响,分析比较不同氮效率玉米品种对低氮胁迫响应的差异。采用水培方法,以氮高效品种郑单958、正红311和氮低效品种川单428、先玉508为试验材料,在低氮胁迫(0.04 mmol/L)、正常氮(4 mmol/L)和低氮恢复处理3个氮水平下,分别测定苗期干物质积累量、叶片碳氮代谢产物含量及碳氮代谢相关酶活性,比较不同氮效率品种间的差异。结果表明,低氮胁迫显著降低玉米叶片和整株干物质重量、叶片氮代谢指标(硝酸还原酶和谷氨酰胺合成酶活性、可溶性蛋白和游离氨基酸含量),提高根系干重、根冠比、叶片碳代谢指标(蔗糖合成酶和蔗糖磷酸合成酶活性、可溶性总糖和淀粉含量)。不同品种间各指标的变化幅度随着时间的延长表现出不同规律,氮高效品种在氮素胁迫下对碳氮代谢的平衡能力更强,受低氮环境影响较小,植株的恢复能力较强,对胁迫环境具有更强的耐性和适应性。