Crown Tissue Research Articles

Apoplastic Sugars, Fructans, Fructan Exohydrolase, and Invertase in Winter Oat: Responses to Second-Phase Cold Hardening

Abstract Changes in apoplastic carbohydrate concentrations and activities of carbohydrate-degrading enzymes were determined in crown tissues of oat (Avena sativa L., cv Wintok) during cold hardening. During second-phase hardening (−3°C for 3 d) levels of fructan, sucrose, glucose, and fructose in the apoplast increased significantly above that in nonhardened and first-phase-hardened plants. The extent of the increase in apoplastic fructan during second-phase hardening varied with the degree of fructan polymerization (DP) (e.g. DP3 and DP4 increased to a greater extent than DP7 and DP > 7). Activities of invertase and fructan exohydrolase in the crown apoplast increased approximately 4-fold over nonhardened and first-phase-hardened plants. Apoplastic fluid extracted from nonhardened, first-phase-hardened, and second-phase-hardened crown tissues had low levels, of symplastic contamination, as determined by malate dehydrogenase activity. The significance of these results in relation to increases in freezing tolerance from second-phase hardening is discussed.

First Report of Mycoleptodiscus terrestris Infecting Forage Legumes and Soybeans in Wisconsin.

Soilborne plant pathogens are regarded as important causes of failures of newly established and mature stands of forage legumes and reduced yield of soybean (Glycine max (L.) Merr.) in the North Central Region of the U.S. Birdsfoot trefoil (Lotus corniculatus L.) plants exhibited decaying roots and stems in yield trials in 1994 and 1995 at the Arlington Agricultural Research Station, Arlington, WI. After first harvest, over 40% of the plants failed to produce regrowth or regrowth was extremely slow. Dark brown lesions were evident on root and crown tissue and occasionally spread to newly initiated stem tissue. A fungus resembling Mycoleptodiscus terrestris was isolated from birdsfoot trefoil by placing surface-disinfested plant tissue on potato dextrose agar (PDA) amended with 10 ppm chlorotetracycline or tetracycline and 100 μl liter-1 of tergitol. Isolates of M. terrestris produced dark green mycelium and black scerotia on PDA. These same isolates also produced conidia with characteristics similar to those described by Ostazeski (1) for M. terrestris when cultures were grown on gamma ray-sterilized chrysanthemum leaf pieces positioned on the surface of 2% water agar on petri dishes. The setaed conidia were generally two-celled and oval, and measured 24 to 30 × 4.5 to 7.4 μm. Isolates were also obtained from soybean plants that expressed a premature decline symptom at the R6 to R7 growth stage. Soybean plants in two commercial fields initially expressed curled leaves followed by defoliation. Lower stems expressed a gray-tan discoloration of the cortical tissue. Alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), birdsfoot trefoil, and soybean were inoculated with isolates of M. terrestris. All crop species expressed symptoms and M. terrestris was recovered from symptomatic tissue. M. terrestris isolates have also been baited from other soils from southern Wisconsin with red clover, birdsfoot trefoil, and soybean. Although recognized in states south of Wisconsin, M. terrestris has not been implicated previously in poor health of forage legumes and soybeans in Wisconsin. This report provides evidence that M. terrestris inhabits agricultural soils farther north than previously recognized.

Relationships between planting date, winter survival and stress tolerances of soft white winter wheat in eastern Ontario

Reduction of populations of fall planted crops in the course of winter can result in substantial losses in economic yield. Variations in planting date of soft white winter wheat (Triticum aestivum L.) in eastern Ontario are known to influence both survival and grain yield. This study was conducted to determine relationships between fall-accumulated growing degree days (GDD), cold hardiness, ice tolerance and a number of plant characteristics with survival recorded the next spring. Locations were at Ottawa (45°23′N) and Douglas (45°33′N) with four planting dates, 27 August, 10 September, 24 September and 8 October in 4 yr, 1983–1986. Delayed planting was associated with reduced survival at Ottawa in 1987 and in all years at Douglas. Consequently, survival at Ottawa showed little association with cold hardiness and ice tolerance, but there were significant correlations at Douglas. Measurements in 3 yr showed that late planted wheats were single tillered, up to 10 times lower fresh weight than the 3–5 tillered August-planted wheat, and their cold hardiness and ice tolerance were decreased. Moisture content of the crown tissue (CrW) increased with delayed planting despite the growth of the plants in acclimating conditions. Highest CrW developed in late-planted wheat at Douglas and showed a high negative correlation with survival. Cold hardiness and ice tolerance correlated with survival at Douglas and there were significant relationships between the stress tolerances. However, no consistent associations across location-years could be defined to explain winter survival in terms of fall-measured stress tolerances and plant parameters. Key words: Cold acclimation, cold hardiness, crown moisture, winter injury, ice encasement, delayed planting

First Report of Sclerotinia sclerotiorum on Coneflower.

Purple coneflower (Echinacea purpurea (L.) Moench; Asteraceae), a perennial herb originating from North America, is used as a garden ornamental and is grown commercially for use in medicinal preparations as an immunostimulant. In October 1996, a previously undescribed stem rot disease was observed in a research plot of 6-month-old echinacea plants at Brooks. Seedlings had been raised in small rockwool cubes (2 × 2 × 5 cm3) in a greenhouse, then transplanted into the field in early June. By late August, dead and dying plants were observed throughout the stand. They had dark brown to black stem lesions above and at the soil level and dead leaves with bleached petiole lesions that extended ca. 15 cm above the axil. Diseased stems and petioles often disintegrated, leaving only fibrous tissues intact. Roots were rotted and black. Superficial white mycelium developed over the basal part of affected stems. Black, oblong to irregular-shaped sclerotia, 5.1 to 17.6 mm in size, formed externally on the crown areas after plant death. Sclerotinia sclerotiorum (Lib.) de Bary (1) was isolated from the diseased plants. Five isolates were selected to fulfill Koch's postulates with 3-month-old echinacea seedlings grown in 12-cm pots of soilless mix. Sclerotia from wilted, field-grown echinacea plants were transferred onto potato dextrose agar medium for 2 days at 20°C. Agar disks were cut with a 1-cm cork borer and two plugs containing sclerotial and mycelial tissues were inserted into the soilless mix 0.5 cm deep and 0.5 cm from the opposite sides of stems of test plants. Inoculated plants were enclosed in transparent plastic bags for 5 days and incubated in a growth chamber at 15/18°C (night/day) with a 12-h photoperiod. One to four lower leaves per plant wilted within 1 week after inoculation and aerial mycelia appeared on the petioles. Infected leaves quickly withered, dried, and dropped off the plant after the bags were removed. Plants often died 3 weeks after inoculation and S. sclerotiorum was reisolated from infected crown tissues. This disease was also found on 3-year-old plants of E. pallida (Nutt.) Nutt. var. angustifolia (DC.) Cronq. in Vernon, British Columbia, Canada, in May 1997. This is the first report of sclerotinia stem rot on Echinacea spp., a disease that could have a significant impact on the longevity and productivity of this crop in the field and greenhouse. Reference: (1) L. H. Purdy. Phytopathology 69:875, 1979.

Tissue Specificity of `Chandler' Strawberry Peroxidase Isozymes

Peroxidase activity in extracts from freeze-dried tissue of Fragaria × ananassa Duch. cv. Chandler was highest in tissue-cultured (TC) plants, followed by field-grown (FG) and lowest in greenhouse (GH) plants. Among tissue types, activity was highest in petioles, with leaves second highest. Fruit, root, and crown tissue all exhibited low or no activity. When subjected to isoelectric focusing (IEF), petiole tissue extracts exhibited more isozymes than extracts from other organs regardless of staining substrate. Using 4-chloro-1-naphthol and H2O2 as substrates, anionic and cationic isozymes were observed in TC petiole extract with nine isozyme bands ranging in pI from 3.9 to 9.5. In TC leaf extract an isozyme at pI 7.4 was observed that was not present in other organ extracts when H2O2 and benzidine, p-phenylenediamine or 3-amino-9-ethylcarbazole were used as substrates. Specific isozymes and number of isozymes varied according to plant organ and developmental stage. Mature leaves and over-ripe fruit appeared to exhibit more activity and a larger number of isozymes than developing tissues of those plant organs.

Morphological and Physiological Characteristics Associated with Tolerance to Summer Irrigation Termination in Alfalfa

Deliberately withholding irrigation to alfalfa (Medicago sativa L.) during summer, a management strategy referred to as summer irrigation termination (SIT), has been suggested to conserve water in long‐season desert environments. Summer irrigation termination may reduce productivity of alfalfa stands, although such effects may be lessened if cultivars with improved tolerance to SIT could be developed. This study was conducted to identify characteristics associated with low crown mortality as the primary component of tolerance to SIT. Using such traits in selection may be superior to direct selection for post‐SIT yield, a trait potentially characterized by low heritability. Single spaced nondormant alfalfa plants were grown in a field trial in Tucson, AZ, and subjected to two SIT treatments during two years (Treatment 1, 84 d in 1994 and Treatment 2, 42 d in 1994 and 75 d in 1995). Traits evaluated prior to SIT (forage yield, stem number, plant height, leaf area, and root diameter) were not significantly associated with crown mortality. Significant but weak correlations were detected between crown mortality and days to wilting (r = −0.25*) and plant height during SIT (r = −0.29*). Accumulation of osmotically active substances, such as proline and sucrose, was not related to crown mortality. However, crown mortality was significantly correlated with concentration of total nonstructural carbohydrates (TNC) in crown tissue on Days 28 (r = − 0.57*) and 35 (r = − 0.79*) of SIT. The TNC reserves may be essential for continued respiration during periods of drought. Depletion of TNC reserves could have caused plant death in intolerant plants. Concentrations of TNC in crown tissue may represent a criterion in indirect selection for decreased crown mortality during SIT in selected environments.

Seasonal changes in the physical state of crown water associated with freezing tolerance in winter wheat

The relationship between freezing tolerance (expressed as LT50, the lethal freezing temperature for 50% of plants) and the amount and physical state (as determined by spin‐lattice [T1] and spin‐spin [T2] relaxation times of protons) of water in crown tissue was examined in contrasting winter wheat (Triticum aestivum L.) varieties grown under field conditions from 1992 to 1994. During acclimation, the LT50 values decreased from around ‐7 to ‐17, ‐20 and ‐27°C in PI 173438, Chihokukomugi and Valuevskaya, respectively. Tissue water content decreased continuously through autumn to reach a plateau around 3 g H2O (g dry weight)‐1 in early winter when LT50 was still decteasing, and then gradually increased under snow cover. A significant negative correlation was found between mean minimum air temperatures and freezing tolerance prior to the establishment of continuous snow cover. In contrast, a positive association between mean minimum temperatures and crown tissue water content was significant only when air temperatures were above 0°C, as water content did not decrease further at sub‐zero temperatures. Seasonal changes in T1 were closely related to changes in freezing tolerance. T1 decreased until January even though water content stopped decreasing. Further tests on 15 field‐grown varieties confirmed a strong negative association between freezing tolerance and T1. The results suggest that cold hardening is comprised of two stages, with the transition occurring at ca 0°C. Development of hardiness was related to (1) a reduction in water content in the first stage (at minimum temperatures > 0°C), and (2) a change in physical state of water without much reduction in water content in the second stage. Varietal differences in hardiness thus arise due to changes in both water content and physical state of water.

Susceptibility of ‘Skyline’ Honeylocust to Cankers Caused by Nectria cinnabarina Influenced by Nursery Field Management System

Abstract The influence of five nursery field management treatments including alternative, sustainable practices (ie. companion crops, allelopathic cover crops/mulches) on the incidence of stem cankers caused by Nectria cinnabarina (Tode: Fr.) Fr. on ‘Skyline’ thornless honeylocust (Gleditsia triacanthos L. var. inermis Willd. ‘Skyline’) was monitored in Minnesota. Although considered a stress related disease, field management treatments that reduced tree vigor (ie. growth), decreased susceptibility to attack by N. cinnabarina while treatments which promoted vigorous growth increased susceptibility. The observed vulnerability of honeylocust trees to N. cinnabarina may be related to plant cold hardiness and subsequent winter injury to root and crown tissue as affected by nursery field management treatment. Moisture stress late in the growing season, resulting from root injury during the previous winter, mechanical root injury from cultivation, and high summer soil temperatures may have increased susceptibility to N. cinnabarina for honeylocust trees grown in bare soil field production systems. Moisture stress in late summer may not be reflected in plant growth, but may increase the susceptibility of honeylocust trees to attack by N. cinnabarina.

First Report of Stevia as a Host of Sclerotinia sclerotiorum.

Stevia (Stevia rebaudiana Bertoni; Asteraceae), an annual plant originating from Paraguay, contains glucosides of a diterpenoid (2), which is used as a low-caloric sweetener in some South American and southeast Asian countries. The main active ingredient, stevioside, is 100 to 300 times as sweet as sucrose. Stevia has been experimentally grown under field conditions in central and western Canada and has the potential to become a commercially viable alternative crop. In August 1996, a previously undescribed stem rot disease was observed on stevia plants at the Crop Diversification Centre South, Brooks, Alberta. The disease was found in research plots where 4-month-old plants were growing in loam soil. Diseased stems showed dark brown lesions above and at soil level when plant height reached approximately 30 cm. Under dry conditions, mild stem lesions caused plant stunting with lower leaves turning black and curling downward. Wilted leaf symptoms gradually spread upward in affected plants. Partial wilting symptoms appeared when girdling was restricted to branches. The entire plant collapsed when girdling of the crown and roots occurred. Superficial white mycelium developed over the basal part of affected stems under moist conditions, especially after rainy periods. Black, round to oblong sclerotia, 3.5 to 10.1 mm in size, formed externally on the crown areas after plant death. Sclerotinia sclerotiorum (Lib.) de Bary (1) was consistently isolated from the diseased plants. To confirm pathogenicity, 4-week-old stevia seedlings were obtained from shoot cuttings and grown in 12-cm pots of soilless mix. Sclerotia produced on potato dextrose agar were inserted into the mix 0.5 cm deep and 0.5 cm from the stems of test plants. Plants were placed in a growth chamber at 22°C with a 12-h photoperiod and 95% relative humidity. Two weeks after soil infestation, plants wilted and S. sclerotiorum was reisolated from the diseased crown tissues. This is the first report on stevia of sclerotinia stem rot, a disease that could significantly reduce foliar growth and stevioside production in field plantings.

Growth Responses and Allocation of Assimilates of Rice Seedlings by Paclobutrazol and Gibberellin Treatment

Paclobutrazol [(2RS,3RS)-1-(4-chlorophenyl)methyl-4,4-dimethyl-2-(1h-1,2,4-trizol-1-yl)penten-3-ol] effectively decreased vegetative growth of rice (Oryza sativa L.) seedlings and increased the chlorophyll content. The number of veins in a leaf, the calculated number of stomata per leaf, and the length of guard cells were not altered by the paclobutrazol treatment, suggesting an effect on cell elongation. The allocation pattern of carbohydrates was changed by either gibberellin (GA) or paclobutrazol treatment. GA3 induced more shoot growth and less accumulation of starch than the control and paclobutrazol-treated seedlings. Photosynthetic ability was not affected by either paclobutrazol or GA3 treatment. Paclobutrazol-treated plants allocated a smaller amount of photosynthates for vegetative shoot growth and stored more as starch in the crowns than the control and GA3-treated plants. The same starch degrading activity in the crown tissue of paclobutrazol-treated seedlings as in control plants suggests that the accumulated starch is utilized in a normal activity for growth including leaf emergence, tiller formation, and root production, resulting in improved seedling quality.

Crown moisture and prediction of plant mortality in drought-stressed alfalfa

Withholding alfalfa (Medicago sativa L.) irrigations during the summer, a practice referred to as summer irrigation termination (SIT), can conserve substantial amounts of water in long-season desert environments; however, plant mortality associated with SIT may be substantial. Proper timing of re-irrigation is critical for minimizing mortality and yield reductions following SIT. Procedures that would permit probable mortality prediction during drought stress would improve management efficiency with SIT. This study was conducted to determine (1) whether plant mortality occurs once the moisture content of the plant woody stem portions (crown) falls below some critical threshold and (2) if such a threshold could be used to predict the likelihood of plant mortality during SIT. Crown samples were taken from single, spaced, field-grown plants in Tucson, Arizona, at the end of a 84-day SIT period in 1994. A crown moisture content of about 42% was identified as a likely threshold critical for crown tissue SIT survival. This value was then used to predict whole-plant mortality of alfalfa grown in solid-seeded plots comparable to commercial fields. Crown samples were taken at five locations within the field along a solid gradient that was related to plant mortality. At each sampling location, the proportion of samples with less than 42% crown moisture was used to predict plant mortality. Predicted mortality slightly overestimated actual mortality but differences between predicted and observed mortality were significant for only one of five sampling locations. Alfalfa growers may be able to use this simple method of crown moisture determination to prevent permanent yield reductions by initiating irrigation before substantial portions of crowns fall below the threshold moisture content of 42%.

Crown and root rots of table grapes caused by Phytophthora spp. in Chile

Phytophthora crown and root rot of grapevine ( Vitis vinifera L.) occurs frequently in Chile, where grapes are cultivated on their own roots. P. cinnamomi, P. cryptogea , and P. drechsleri were isolated from diseased root or crown tissue of the table grape cvs Flame Seedless, Italia, Red Globe, and Thompson Seedless with P. cryptogea being the most frequently isolated species. These pathogens were identified on the basis of standard morphological and cultural features and were compared by SDS-PAGE profiles of mycelial proteins. All isolates were pathogenic on 1-year-old cane segments and 1-year-old rooted grapevines of cvs Thompson Seedless and Red Globe. The latter cultivar was more susceptible than Thompson Seedless. P. cinnamomi and P. cryptogea have been associated with root rot of grapevines elsewhere, but this is the first report of P. drechsleri as a pathogen of grapevines and the first report of Phytophthora spp. affecting table grapes in Chile.

Influence of Moving Height, Nitrogen Source, and Iprodione on Brown Patch Severity in Perennial Ryegrass

Brown patch (Rhizoctonia solani Kühn) is a destructive disease of turfgrasses, but more is known regarding its management with fungicides than by cultural practices. This field study was conducted to determine the influence of mowing height and nitrogen (N) source on brown patch severity. ‘Caravelle’ perennial ryegrass (Lolium perenne L.) was subjected to two mowing heights (1.7 versus 4.5 cm) and fertilized with one of eight N sources at rates of 196 kg N ha−1 yr−1 from 1991 to 1993. Plots were split with one‐half receiving iprodione [3‐(3,5‐dichlorophenyl)‐N‐(1‐methylethyl)‐2,4‐dioxo‐1‐imidazolidine‐carboxamide)] (3.1 kg ai ha−1 applied on a 21‐d interval) and the other half receiving none. The complete combination of treatments were assessed at Silver Spring, MD, and in a second trial the N sources alone were assessed on a perennial ryegrass fairway in Pikesville, MD. In non‐fungicide‐treated plots, turf maintained at 1.7 cm was blighted more by brown patch in 1991, but in 1992 and 1993 blighting was more severe in plots mowed to a height of 4.5 cm. Blighting was reduced by iprodione at both mowing heights, and summer turfgrass quality was improved in all plots receiving N plus iprodione. Ammonium sulfate in 1991 and Sustane and Ringer Lawn Restore‐treated turf in 1992 were associated with less blighting among non‐fungicide‐treated plots at Silver Spring. Non‐fungicide‐treated Sustane and Ringer Lawn Restore plots had blight levels equivalent to most fungicide‐treated plots in 1992, but the quality of non‐fungicide‐treated plots was inferior. No N source had any significant effect on brown patch at Pikesville in any year. No single N source combined with iprodione was consistently associated with improved disease suppression or turf quality. There was no apparent relationship between brown patch severity and soil pH or crown tissue carbohydrates. Plant parasitic nematode population densities were not influenced by any N source.

Distribution ofLeptosphaeria maculans in two fields in southern Ontario as determined by the polymerase chain reaction

The distribution of the highly virulent and weakly virulent types ofLeptosphaeria maculans, causal agent of blackleg of oilseed rape, was studied in two fields in southern Ontario. Using a polymerase chain reaction-based assay with primers specific for these virulence types, plant tissues were directly examined for the occurrence of the pathogen. The highly virulent type was detected in leaf, stem and crown tissue at most of the sampling sites. The weakly virulent type was detected only in leaf lesions at 50% of the sites in field 1 and 30% of the sites in field 2. Of 96 leaf lesions examined, 48 contained the highly virulent type, 12 contained the weakly virulent type and 16 contained both the highly virulent and weakly virulent types.Sclerotinia sclerotiorum was isolated from all leaf lesion that did not react withL. maculans virulence type-specific primers. Coinfection of single blackleg leaf lesions by bothL. maculans virulence types thus occurred in oilseed rape plants in the field. Only the highly virulent type was detected in pseudothecia on stubble. Approximately 1% of the seed collected from these two fields containedL. maculans, and both the highly virulent and weakly virulent types were detected. The highly virulent type was more prevalent and non-specific in the kind of plant tissue it infected, whereas the weakly virulent type appeared to be limited to infecting leaves and seed. This study illustrates an application of polymerase chain reaction with virulence type-specific oligonucleotide primers to study the epidemiology of blackleg of oilseed rape.

Expression of cold‐inducible genes and frost hardiness in the crown meristem of young barley (Hordeum vulgare L. cv. Igri) plants grown in different environments

ABSTRACTThe purpose of this work was to examine environmental control of expression, at the mRNA level, of cold‐inducible genes and to test the relationship of the expression of the genes to cold acclimation. Barley plants (Hordeum vulgare L. cv. Igri) at the three‐ to four‐leaf stage were (a) grown in different temperature environments between 20/15°C and +4/‐4°C or (b) transferred between 20/15°C and 6/2°C or (c) grown under drought or nutrient stress conditions. Frost hardiness (using a regrowth method) and mRNA levels for three cold‐induced genes, blt4‐9, blt14 and blt101, from meristematic crown tissue (vegetative shoot meristem plus subtending stem and associated root initials) were measured. Hardiness and levels of blt4‐9, blt14 and blt101 mRNAs increased with lower growth temperatures, below a maximum inductive temperature. Prior temperature environment and plant age affected the rate of change in mRNA levels of these genes in response to a change of temperature environment. Hardiness was strongly correlated with mRNA levels of these genes in plants grown in different temperature environments. This correlation did not extend to plants exposed to drought or nutrient stresses. Implications are drawn for plant responses to a warmer climate.

Nutrient dynamics in crown tissues of cork-oak (

Nutrient dynamics in crown tissues of cork-oak (

Seasonal Cold-acclimation Patterns of Sedum spectabile × telephium L. `Autumn Joy' and Sedum spectabile Boreau. `Brilliant'

Electrolyte leakage and regrowth were measured from September through January to determine cold hardiness of Sedum spectabile × telephium L. `Autumn Joy' and Sedum spectabile Boreau. `Brilliant' plants grown outdoors in central Iowa. Crowns were subjected to 0, –3, –6, –9, –12, –15, –18, –21, –24, or –27C. Regrowth tests were performed on whole crowns and electrolyte leakage was determined on excised tuberous root and crown tissue. Both cultivars were killed at –3C in September, but they acclimated gradually through January. Maximum hardiness was achieved in January, with killing temperatures of –27C for `Autumn Joy' and –21C for `Brilliant'. Regrowth quality ratings were significantly correlated with crown and tuberous root electrolyte leakage measurements, although the relationship was stronger for `Autumn Joy'.

Immunolocalization of freezing-tolerance-associated proteins in the cytoplasm and nucleoplasm of wheat crown tissues.

A protein family associated with the development of freezing tolerance in wheat has been identified. This protein family is Gramineae-specific and coordinately regulated by low temperature. Antibodies directed against the 50 kDa (WCS120) protein recognize at least 5 members of this family. Using these antibodies, the cellular content and location of this protein family was determined in cold-acclimated wheat seedlings. Western analyses of subcellular fractions indicated the presence of all members of the family in the cytosolic and purified nuclear fractions. These proteins accumulated to 0.9% of soluble proteins after 21 days of cold acclimation in winter wheat. This represents a cellular concentration of 1.34 microM. Immunohistochemical localization showed that these proteins are highly expressed in the vascular transition zone. No detectable expression was found in mature xylem, in the shoot apical meristem or lateral root primordia. This differential tissue expression suggests that the sensitive cells near the regions where water tends to freeze first require a higher amount of these proteins. This observation is consistent with the fact that regrowth after freezing stress is highly dependent on the viability of this region of the crown. Electron microscopy analysis using immunogold labelling showed that these proteins are present in the cytoplasm and in the nucleoplasm. They are not found in cell walls or other organelles. In vitro cryoprotective assays indicated that the WCS120 protein (PD50 of 10 micrograms ml-1 or 0.2 microM) are as effective as BSA and sucrose (at 250 mM) against freezing denaturation of lactate dehydrogenase. These results suggest that this protein family may be involved in a general mechanism of protection in the soluble fraction of the cell. Their presence in the nucleoplasm may also suggest a possible protective function of the transcriptional machinery. The high hydrophilicity, the abundance and stability of these proteins to boiling suggest that they may provide a particular micro-environment needed for cell survival in the sensitive vascular transition zone during freezing stress.

Nitrogen Assimilation of Micropropagated Strawberry Plantlets during the Transition from Heterotrophy to Photoautotrophy

A study was conducted to determine the implication of nitrate reductase (NR) and glutamine synthetase (GS) during the transition of micropropagated plantlets from heterotrophy to photoautotrophy to document how nitrogen metabolism interfaces with photosynthetic and anaplerotic CO2 fixation. The activity of the two enzymes was determined in different tissues at different organogenic stages during the development of plantlets transferred onto rooting media containing varying quantities of sucrose. Under 3% sucrose, NR activity was much higher in leaves than in crown tissues. When roots are initiating, there is a shift in the proportion of nitrate reduction from leaves to crown. As roots mature, the proportion of nitrate reduction increases in roots. Similar trends were observed under 5% sucrose. In contrast, under 1% sucrose, a higher proportion of the nitrate is reduced in the leaf tissues throughout the culture period. This suggests that nitrate is reduced mainly in leaves in photoautotrophic plantlets, while it is reduced in crowns and root tissues for mixotrophic plantlets. In general, the GS activity follows the pattern of NR, but is always in excess, to enable rapid assimilation of ammonium derived from metabolism and medium absorption.

Biological control of Rhizoctonia solani by binucleate Rhizoctonia spp. and hypovirulent R. solani agents

Despite research on biological control dating back to the early 1930s, reliable, economical biocontrols of diseases caused by Rhizoctonia solani are not generally available commercially. New approaches for detection and use of novel agents and development of broadly applicable biological control management systems are needed, especially for field crops. During the past 10 years, new sources of agents from within the diverse groups of binucleate Rhizoctonia spp. and hypovirulent R. solani isolates have been demonstrated to be effective in biocontrol of a range of host — R. solani disease combinations. Although these agents include isolates from several different binucleate Rhizoctonia anastomosis groups (AG) and hypovirulent R. solani AG, neither mycoparasitism nor antibiosis is involved in biocontrol of R. solani by any of these isolates. Postulated mechanisms of biocontrol include induction of systemic host resistance, and/or competition for recognition and invasion sites or nutrients. Tested collections of binucleate Rhizoctonia spp. and hypovirulent R. solani differ markedly in effectiveness as biocontrol agents for diseases caused by R. solani. Reportedly, plant surface-colonizing isolates (i.e. on or superficially within outer tissues of roots, crowns, hypocotyls, stems or petioles) are effective biocontrol agents, whereas, non-colonizers are ineffective.