In vitro selection of wheat callus tolerant to high levels of salt and plant regeneration

Summary Embryogenic calli isolated from immature embryos of four wheat cultivars were subjected to three in vitro selection methods for salt tolerance. The effect of NaCl on the selected and unselected cell lines has been investigated. The results indicated that the relative growth rate of callus decreased as the concentration of NaCl increased in both callus lines. The selected callus line gave a higher growth weight in the presence of NaCl in the medium and was highly significant as compared with unselected callus line across medium protocols in all wheat cultivars. The dry weight of both kinds of callus lines of all wheat cultivars increased markedly with increasing NaCl concentration in most cases. The Na⁺ and Cl^- contents of both callus lines were increased with increasing salinity levels while K⁺ content was decreased. The selected callus line of each cultivar at the same salinity level produced significant amounts of Na⁺, K⁺ and Cl^- higher than the unselected callus line in most salinity levels. However, the unselected callus lines of the cultivars Giza-157 and Sakha-90 at the same salinity level produced significant amounts of K⁺ higher than the selected callus line in most salinity levels. The proline content of both kinds of callus lines for all wheat cultivars was increased with increasing salinity level. However, the selected callus line gave a significantly higher proline content than the unselected callus line in all wheat cultivars at the same Salinity level. Results from the in vitro selection for NaCl tolerance showed that the stepwise method of increasing NaCl in the medium was more effective for plant regeneration than other methods.     

Improvement of Salt Tolerance in Maize by Selection and Breeding

Genetic variation for NaCl tolerance at the vegetative stage was assessed in nutrient solution culture in maize (Zea mays L.). Shoot growth, and plant fresh and dry weight of the two cultivars, Akbar and Sadat were severely reduced after three weeks growth in 120, 150, and ISO mol m^?3 NaCl. There was however considerable variability between seedlings. Ten-thousands seeds of cv. Akbar were therefore screened for shoot growth at 180 mol m^?3 NaCl after four weeks growth in sand culture. A selection intensity of 0.42 % was achieved. Eighteen selected plants were polycrossed for estimation of narrow-sense heritability based on female parent-progeny regression. A narrow-sense heritability estimate of 0.54 was obtained. The progeny of the salt-tolerant selection line and selfed progeny plants of the unselccted control lines of cvs Akbar and Sadat were grown for six weeks in 0, 30, 60, 90, 120, 150, and 180 mol m^?3 NaCl in sand culture. The tolerant line produced significantly greater fresh and dry biomass and had greater shoot length than the unselected cv. Akbar, but the selection line derived from cv. Akbar was equal to the salt tolerant cv. Sadat in all the growth parameters measured. These data suggest that in maize, improvement in salt tolerance could be obtained through further cycles of selection and breeding.     

Barley lines showing prominent high green shoot regeneration in cultures derived from immature embryos

Eighty‐four cultivars of barley (Hordeum vulgare) and 95 wild strains (82 of H. spontaneum and 13 of H. agriocrithon) were surveyed for the production of callus, callus growth, and shoot regeneration in cultures derived from immature embryos. All cultivars except for ‘Turkey 381′, induced calli from more than 90% of embryos. On the other hand, the wild lines showed a large variation in the percentage of callus induction from 0 to 100%. Among the cultivars, those with the brittle rachis genotype, bt Bt₂, on chromosome 3H, regenerated shoots with a significantly higher percentage than the cultivars with the Bt bt₂ genotype. Green shoots were produced in a higher ratio (0.84) in the cultivars than in the wild lines (0.52). Among the lines examined,‘Lenins’ regenerated shoots efficiently (90.4%) and produced the highest number of calli with green shoots per embryo (4.77) followed by ‘Golden Promise’ (3.15). Examination of callus growth and shoot regeneration from embryos at different developmental stages revealed that scutellum development affected the quantity and quality of callus and shoot regeneration.     

The Potential for Exploiting Variation in Salinity Tolerance in Pearl Millet (Pennisetum americanum (L.) Leeke)

Shoot and root growth and plant dry weight were determined for twenty four accessions of pearl millet, Pennisetum americanum (L.) Leeke, after two weeks growth in saline solution culture at EC 20 dS m^-1. The EC was achieved using NaCl + CaCl₂ 1:1 by weight in solution culture. Although salinity markedly inhibited growth of all accessions, three, 93613, KAT/PM-2 Kitui, and Kitui local produced significantly greater dry matter, and they and 93612 had longer shoots than the other accessions. Accessions WCA 78 and Bulk 7704 were highly sensitive to salinity in all 3 characters measured. There was considerable variation between the 24 accessions, suggesting that selection for increased tolerance to salinity in pearl millet should be possible. This was examined using normal seed of the cultivar Al/3. A first cycle of selection screened 16,000 seeds after 15 days growth in nutrient solution at EC 26 dS m^-1. 106 individuals were selected to form the S₁ selection line. Selected seedlings were grown to maturity and polycrossed. In a second cycle of selection, 30,000 S₁ polycross progeny were screened as previously, but at EC 30 dS m^-1. A selection intensity of 0.08% was achieved. The selected plants were again grown to maturity and again polycrossed. Efficiency of the selection procedure was assessed from comparison in sand culture of the performance of the selected line with the unselected Al/3 line and Kitui local which from the previous assessment of the 24 accessions was rated as relatively salinity tolerant. The selection line was superior to the other two lines at four salinity levels. The results of this study suggest that further improvement in salinity tolerance in pearl millet may be expected from further selection and breeding, using the methods described here.     

Selection of hydroxyproline-resistant proline-accumulating mutants of cauliflower (brassica oleracea var. botrytis)

Summary A procedure is described by which hydroxyproline-resistant lines could be selected from regenerating curd tissue of cauliflower. Mutagenesis was by N-nitroso-N-ethylurea, supplied as a drop of 0.3 mM solution on each 3 mm diameter curd piece. The mutagen generated numerous morphological and pigment mutations without significantly affecting shoot regeneration from explants. Thirty one resistant shoots were recovered from more than six thousand explants mutagenised on regeneration medium supplemented with 3 mM hydroxyproline, while none was obtained from a similar number of non-mutagenised controls. Out of twenty-three resistant shoots which survived subculture, only one showed consistently elevated levels of endogenous proline. During early shoot culture passages, proline levels were 3.6–4.7 times higher than controls, but this was reduced to 1.6 times after 10–12 culture passages in the absence of hydroxyproline. Possible reasons for this decline are discussed. Leaf strip assays suggest resistant shoots may be chimeras and current efforts are directed towards regenerating solid mutants from resistant sectors. These will then be evaluated for any alteration in frost tolerance.Abbreviations IBA Indole-3-butyric-acid - NEU N-nitroso-N-ethylurea     

Production and performance of potato mini-tubers

Summary Production of mini-tubers as a source for seed potato was investigated by growing in soil micropropagated plants and micro-tubers produced from micropropagated plants. Cultures of several cultivars were initiated from indexed tubers and multiplied on modified MS medium. Cultures were micropropagated by using a modular system which allowed batch handling. Micropropagated plants produced mini-tubers in glasshouse after 70–115 days of growth in soil. A large proportion of the mini-tubers produced were between 9 and 15 mm diameter. Several factors, e.g., explant number, duration of in vitro culture and genotype influenced mini-tubers production. Micropropagated plants after culture of 86 days or longer produced micro-tubers ca. 2 to 10 mm diameter. Plants, which formed micro-tubers in vitro, produced less number of mini-tubers in soil. Micro-tubers produced 1 to 3 mini-tubers when grown in soil in chain-type paper pots, but produced conventional sized tubers when grown in soil under plastic polytunnel. Mini-tuber number varied widely between potato cultivars; cvs. Bintje and British Queen produced more mini-tubers than the other cultivars. Mini-tubers developed green hard skins when kept in light for 3 weeks, and could be stored in dark at 4° C upto 6 months. In a field trial, small mini-tubers ca. 5–10 mm diameter produced more but smaller tubers than mini-tubers ca. 15–20 mm diameter. The micropropagated plants and the plants grown from mini-tubers were genetically stable, and did not show any morphological aberrations except for one variegated plant among the several thousand produced. It is concluded that the production of mini-tubers by soil planting of micropropagated plants is a rapid and efficient method for producing seed potato tubers.The use of any trade mark and other commercial products mentioned in this paper does not constitute any recommendation or advertisement for such products on behalf of the Agriculture and Food Development Authority, Dublin, Ireland     

Maize genotypes with deep root systems tolerate salt stress better than those with shallow root systems during early growth

Maize (Zea mays L.) is susceptible to salinity but shows genotypic variation for salt tolerance. How maize genotypes with contrasting root morphological traits respond to salt stress remains unclear. This study assessed genotypic variation in salinity tolerance of 20 maize genotypes with contrasting root systems exposed to NaCl for 10 days (0, 50 mM or 100 mM NaCl, added in four increments every other day from 14 days after transplanting, DAT) in a semi-hydroponic phenotyping system in a temperature-controlled greenhouse. Considerable variation was observed for each of the 12 measured shoot and root traits among the 20 genotypes under NaCl treatments. Salt stress significantly decreased biomass production by up to 54% in shoots and 37% in roots compared with the non-saline control. The 20 genotypes were classified as salt-tolerant (8 genotypes), moderately tolerant (5) and salt-sensitive (7) genotypes based on the mean shoot dry weight ratio (the ratio of shoot dry weight at 100 mM NaCl and non-saline control) ± one standard error. The more salt-tolerant genotypes (such as Jindan52) had less reductions in growth, and lower shoot Na⁺ contents and higher shoot K⁺/Na⁺ ratios under salt stress. The declared salt tolerance was positively correlated with shoot height, shoot dry weight and primary root depth, and negatively correlated with shoot Na⁺ content at 100 mM NaCl. Primary root depth is critical for identifying salt responsiveness in maize plants and could be suggested as a selection criterion for screening salt tolerance of maize during early growth. The selected salt-tolerant genotypes have potentials for cultivation in saline soils and for developing high-yielding salt-tolerant maize hybrids in future breeding programmes.     

CIMMYT's approach to breed for drought tolerance

Summary About 32% of the 99 million ha wheat grown in developing countries experiences varying levels of drought stress. Three major drought types have been identified: Late drought (LD) is common in the Mediterranean region, early drought (ED) is found in Latin America and wheat is produced on residual soil moisture (RM) in the Indian subcontinent and part of Australia. Until 1983, CIMMYT selected all germplasm under near optimum conditions for its yield potential and tested only advanced lines under drought. In spite of many critics, this approach proved to be successful, since in the mid 80's CIMMYT germplasm was grown on 45% of the wheat area in LC with annual rainfall from 300–500 mm and on 21% in areas with less than 300 mm. Since 1983, CIMMYT's drought breeding methodology is to alternate segregating populations between drought stressed and fully irrigated conditions (FI) and to test advanced lines under a line source irrigation system. To compare the efficiency of these approach, yield of four, mostly leading varieties, from each of the regions with LD, ED, RM, and FI and twelve recent CIMMYT cultivars selected for high yield under FI and RM conditions (ALT) were compared under four different moisture regimes (FI, LD, ED, and RM) in 89–90 and 90–91 in Yaqui Valley, Mexico. Genotypic correlation between yield and days to flowering, days to maturity, height, grains m^-2, TKW, test weight and grain fill period were calculated.Mean grain yield of the four best lines in the ALT group was highest under all moisture stress regimes, followed by the FI-group. However, the highest yielding cultivar within each moisture regime was from the FI-group under FI, from the LD-group under LD, and from the ALT-group under ED and RM conditions. Estimates for genetic advance suggest that FI is the best environment for increasing grain yield even in all three drought environments. This indicates that yield potential per se is beneficial also in drought environments. The highest yield in drought environments was realized by the CIM cultivars selected under FI and RM. Simultaneous evaluation of the germplasm under near optimum conditions, to utilize high heritabilities and identify lines with high yield potential, and under stress conditions to preserve alleles for drought tolerance seem at present the best strategy.     

Evaluation of the U.S. cucumber germplasm collection for root size using a subjective rating technique

Summary Root size in cucumber (Cucumis sativus L.) has received little research attention until recently. However, a large root system could improve nutrient and water uptake, and ultimately yield. Knowledge of root size could help cucumber breeders develop cultivars with improved performance. A study was conducted in the greenhouse to evaluate 857 cultigens (827 plant introduction accessions, 22 breeding lines, and 8 cultivars) of cucumber for root length. Complete data were obtained for 794 cultigens. Plants were grown from seeds in 2 replications in the greenhouse. Plants were rated for root length using a 1 to 9 scale (1–3 = small, 4–6 = medium 7–9 = large). Also, root length was measured from soil surface to the tip of the longest main root. In addition, all cultigens were evaluated for root length and dry weight to provide meaning to the subjective rating. Root rating was highly correlated with both root length (r=0.92) and dry weight (r=0.64) of roots. Root length averaged 74 mm and most cultigens (579, or 68%) had roots 40 to 110 mm long. The cultigen with the longest roots was PI 183056 (193 mm), and the shortest was PI 176954 (2 mm). The subjective rating technique provided a simple, accurate and rapid method to estimate root growth in the greenhouse.     

Evaluation of seedling selection for salinity tolerance in potato (Solanum tuberosum L.)

Summary Salt-tolerance in potato (Solanum tuberosum L.) was selected on the basis of germination and survival of seed sown in trays of perlite suspended in either 75 or 150 mM NaCl. Salinity reduced the germination of seed. Genetic differences in salt-tolerance were apparent with salt reducing germination more in seed collected from cv. Cara than in that collected from cv. Maris Piper. Progeny from the seedling selection were then grown to maturity to produce tubers.The relative tolerance of the parental cultivars and of unselected and selected progeny to long-term exposure to salinity was examined in a pot experiment in which plants were irrigated with either fresh water or 50 mM NaCl solution from one week after plant emergence. In this experiment, salinity significantly reduced leaf conductance, total dry matter production and partitioning of assimilate to tubers. Salinity reduced dry matter production and assimilate partitioning to tubers to a greater extent in Cara than in M. Piper. Progeny selected for short-term salt-tolerance did not exhibit greater long-term salt-tolerance than unselected progeny, and both were more sensitive than M. Piper.These results demonstrate genetic variation in salt-tolerance in potato. However, although there was a correlation between the performance of the parent to long-term salinity and survival of progeny in the seedling selection, there was no correlation between short- and long-term salt-tolerance. This suggests that characters underlying short-term tolerance may contribute to long-term tolerance but do not of themselves confer long-term tolerance. Future progress in selecting for improved salt-tolerance depends on understanding the effects of salinity on the physiological processes underlying growth and carbon partitioning.     

Clonal propagation of rice through proliferation of axillary shoots

Summary Shoot base segments have been explanted from seedlings of rice (Oryza sativa L. subsp. Japonica, cv. Arborio) and grown on agar-solidified MS medium supplemented with different concentrations of four cytokinins: kinetin, BAP, 2iP and zeatin. After one month, segments were explanted from proliferated shoots and subcultured on their respective media. BAP was by far the most effective in inducing shoot proliferation. Highest rates were achieved at the higher concentration used: 5 mg 1^–1. Shoot base segments were subcultured fifteen times consecutively on seven different concentrations of BAP. Shoots grown in the presence of 5 mg 1^–1 of BAP proliferated an average of 12 normal shoots for each base segment throughout the fifteen subcultures. The shoots rooted easily on hormone-free medium. The technique does not require any particular skill, it is very effective and, therefore, can be suggested as suitable for clonal propagation of rice.     

Comparative efficiency of honeycomb and conventional pedigree methods of selection for yield and fiber quality in cotton (Gossypium spp.)

The effectiveness of honeycomb pedigree selection (HPS) as compared to conventional pedigree selection (CPS) was studied in one intraspecific (G. hirsutum L. × G. hirsutum L.) cross population (population I) and one interspecific (G. hirsutum L. × G. barbadense L.) cross population (population II). Combined selection for yield and lint quality traits was applied for four cycles at two locations for population I and atone location for population II. Finally, the best F₆ lines derived by each method, together with the unselected population derived by single seed descent (SSD) and three check cultivars, were tested in comparative experiments, separately for each cross, at the same locations. In both populations the analysis of variance indicated significant differences among the groups of the material tested for seed cotton yield, mean boll weight, micronaire reading, staple length, and uniformity ratio. No significant differences were found with respect to plant height, lint percentage, and fiber strength in population I and with respect to lint percentage and fiber strength in population II. In population I, on the basis of mean seed cottonyield and number of superior lines derived by each method as compared to the check varieties and the unselected SSD population, HPS-lines were superior to CPS-lines for seedcotton yield, fiber length and boll weight. HPS-lines, however, did not differ significantly in seedcotton yield from the best check cultivar Eva. Finally, the material derived by CPS was earlier in maturity than the material derived by HPS and SSD. Also in population II, on the basis of mean seedcotton yield and number of superior lines derived by each method as compared to the unselected SSD population, HPS-lines were superior to CPS-lines. No significant differences, however, were identified between the material of the two methods for lint quality traits. It was concluded that in both populations HPS was more effective than CPS in identifying lines with high yielding ability and good lint quality. This superiority of HPS is attributed, at least partially, to its effectiveness in early generation selection. This revised version was published online in August 2006 with corrections to the Cover Date.     

In vitro techniques for selecting wheat (Triticum aestivum L.) for Fusarium-resistance. II. Culture filtrate technique and inheritance of Fusarium-resistance in the somaclones

Summary Calli of resistant, intermediary and susceptible wheat (Triticum aestivum L.) varieties were selected using culture filtrates of Fusarium graminearum and F. culmorum and the regenerants were evaluated for resistance up to R₃. Czapek-Dox broth medium was inoculated with mycelia of Fusarium isolates and incubated for 2–6 weeks. Filtrates were added to MS callus growing medium, then 5 weeks-old calli were transferred onto this medium (MST) for 4–5 weeks. MST containing 30% filtrate was found to be suitable for selection. Resistant calli were transferred again to fresh MST for further two selection cycles. The surviving calli produced less fertile regenerated lines (R₀) than the non-selected ones. Among 18 R₁ lines tested for Fusarium-resistance in the seedling stage by artificial inoculation in the greenhouse, two (11.1%) were significantly more resistant, one (5.6%) was more susceptible than the original cultivar and the rest (83.3%) behaved similarly to the donor plants. Among unselected R₃ lines of three varieties, practically the same number of resistant plants were found as among the related selected ones. When the R₃ selfed generations obtained through double-layer and culture filtrate selection techniques were tested for Fusarium-resistance, 35.7% of the lines were found to be more resistant than the original cultivars, none was more susceptible and 64.3% had a reaction similar to that of the source materials. Thus, inheritance of the disease reaction was not stable in all cases. Success of in vitro selection for Fusarium-resistance depended also on the genotype, and toxin analysis showed that although being effective, the selective media contained deoxynivalenol only exceptionally. In selecting wheat for Fusarium-resistance in vitro, the culture filtrate technique proved better than the double-layer procedure.     

PEP‐Carboxylase Activity: A Comparison of its Role in a C4 and a C3 Species Under Salt Stress

In our experiments, we found an increase of PEP‐carboxylase activity in young shoots of maize under salt stress. Within this study, several analyses were carried out to identify the function of this enhanced enzyme activity during salt stress. In our first experiment, we analysed plants of the salt‐resistant maize hybrid SR 03 (Zea mays L.), whereas in the second experiment, we compared maize and wheat (Triticum aestivum L. cv. Thasos), which were grown under two different light intensities for 20 days. In the saline treatments, NaCl was applied up to 100 mm, while control plants grew under non‐saline conditions (1 mm NaCl). Analyses of shoot fresh weight revealed a comparable reduction for both genotypes and suggested salt resistance at a similar level. Analyses of sugar concentrations showed an increase in the saline treatment for both genotypes independent of the light intensity. Results for sucrose concentrations led to the conclusion that an increase in PEP‐carboxylase activity was not required for sugar metabolism. Independent of light intensity, alkalinity and malate concentrations were decreased only in wheat. It can be concluded that an enhancement of PEP‐carboxylase activity in young shoots of maize supports organic acid metabolism under salt stress.     

Genetic variation in dry weight of carnation cuttings: its causes and consequences

Summary Carnation cultivars vary considerably in the average weight of their cuttings. Cutting weight shows a high negative correlation with mean relative growth rate. This was initially attributed to differences in age of the cuttings when taken from the stock plants, age being defined as time elapsed since start of visible shoot growth. Shoots on a stock plant are removed as cuttings when they have reached an adequate size and number of leaf pairs. Cuttings from cultivars with a lower rate of growth are removed later, usually resulting in a higher dry weight. This could explain why a high cutting weight is associated with a low relative growth rate.The validity of this tentative explanation is tested in 13 carnation cultivars grown under controlled conditions in two trials, one with plants grown from pinched cuttings and one with plants grown from unpinched cuttings for early flowering (forcing). An analysis of the data for plant weight and leaf pair number at successive stages of development suggests that the primary cause of the negative correlation between cutting weight and mean relative growth rate is not genetic variation in age of the cuttings, but variation in the size of the fully developed axillary bud on the stock plant. Larger buds produce thicker shoots that grow more slowly. Their later harvesting as cuttings only enhances their initial weight advantage, and also the negative correlation with mean relative growth rate.When plants are pinched 20 days after planting, as is normal in a summer-grown crop, the newly developing shoots still show variation in age. This is attributed to variation in the developmental stage of the dormant axillary buds at the time of pinching, causing variation in the time interval between pinching and the start of visible shoot growth.     

Improved seed yield in perennial ryegrass (Lolium perenne L.) from two generations of phenotypic selection

Two generations of recurrent phenotypic selection for seed yield per plant under controlled pollination were carried out to combine improved seed yield with improved forage grass performance in perennial ryegrass (Lolium perenne L.). Selected and unselected varieties (AberDartand AberElan respectively) and control varieties were grown for seed in pots in a glasshouse experiment and in two field plot experiments over 5 harvest years. Seed yield components and seed yield were measured. Significant improvements in seed yield of the selected AberDart over the unselected AberElan were observed both in individual plants grown in the glasshouse and in field plots. Detailed observations of the individual components of seed yield showed that the increased seed yield of AberDart can be attributed to a higher proportion of ovules forming seeds (% seedset), greater seed number per tiller and more reproductive tillers per plant. This improvement in seed yield of AberDart was also observed when it was grown in field plots with AberElan, experimental selection lines and commercial varieties. AberDart produced more seeds per tiller than the other commercial varieties and selection lines studied. The implication of this approach for the breeding of perennial ryegrass varieties that combine good agronomic performance and acceptable seed yields is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Response to selection for root dimension in a Mediterranean natural population of Lolium rigidum Gaud

In the Mediterranean environment, root systems are very important for water utilization. For this reason, the effects of selection for increased root dimensions upon root and shoot characteristics of a natural population of Lolium rigidum Gaud, were analysed. Two cycles of phenotypic recurrent selection were carried out on a population collected in Nurra (north-western Sardinia, Italy). Plants derived after two cycles of selection, together with unselected plants, were grown hydroponically in a greenhouse. Data on root volume, longest root length, number of stems and leaves, leaf length, shoot and root dry weight were collected from the selected material and also from unselected material for use as a control. An analysis of variance was performed and correlation coefficients were calculated. The increased size of the root system did not have a negative effect on the above-ground biomass and there was genetic variation for root traits in the ‘Nurra’ population. The improved population will be utilized for further breeding work to obtain L. rigidum varieties capable of withstanding dry periods during the vegetative phase.     

The inheritance of nitrate content in lettuce (Lactuca sativa L.)

Summary The inheritance of nitrate content in lettuce was analysed using 16 F₂ populations and three F₃ populations. Frequency distributions of nitrate content in F₂ and F₃ populations were unimodal and symmetrical, indicating a quantitative inheritance. Both significant positive and negative deviations of the F₂ mean from the mid-parent value were found, indicating dominance or epistasis. Deviations towards low nitrate content were more frequent than deviations in the other direction. Estimates of heritabilities for nitrate content in the F₂ populations ranged from 18% to 69% and were in most cases above 50%. Crosses between low nitrate cultivars did not have lower estimates of heritability in the F₂ than crosses between cultivars with larger differences in nitrate content. In one case a genotype x experiment interaction for nitrate content of parental cultivars was found.Three F₃ populations of crosses between cultivars with low nitrate content were analysed. Estimates of heritabilities for F₃ line means ranged from 78% to 91% and estimates of the genetic standard deviation of nitrate content in unselected advanced generations ranged from 0.24–0.33 g kg^–1. The estimates of heritabilities and of genetic variation in advanced generations offer good prospects of selection of low nitrate genotypes in lettuce. A comparison of efficiency of selection in the F₂ generation and F₃ line selection is made.     

Heritability and gene effects for root characteristics in peas measured at flowering

Summary Heritability and gene effects for root weight, root volume and root to shoot weight ratio were determined in peas (Pisum sativum L.) at flowering. The populations used were developed from four crosses between lines and cultivars differing in size of the root systems. Broad-sense heritability was between 0.41 and 0.81 for root weight, and between 0.44 and 0.77 for root volume. Additive and dominance effects were important in the genetic control of root weight and volume in all populations, while epistatic effects were important only in two populations. The importance of the genetic parameters in the control of root to shoot weight ratio was unclear. Assuming high correlations between root growth in soil-filled pots in the greenhouse and growth in the field, with the presence of large additive effects and high heritability estimates for root weight and volume, selection for superior pure lines with large root systems should be effective.     

Crown depth and susceptibility to crown rot in wheat

Sources of partial resistance to crown rot caused by Fusarium pseudograminearum are detected in mature plants grown in artificially inoculated soil in the field. The resistance in most but not all of these sources can also be detected in seedlings. In order to determine whether partial resistance is related to depth of crown formation, this character was measured in 13 cultivars/lines with a range of reaction to crown rot. It was also measured in doubled haploid plants from the cross, Batavia/2–49. Crown depths varied from 17.1 mm to -2.3 mm (above ground) in pots in a waterbath at 25 ^°C and from 43.5 mm to 20.7 mm when plants were grown in the field. The correlation coefficient between relative susceptibility to crown rot (Field test) and crown depth of 13 cultivars/lines was 0.57 (p ≤ 0.05). With the exception of the cultivars, Sunco and Pelsart, partial resistance to crown rot was inversely related to depth of crown formation. This indicates that depth of crown formation may be partly responsible for the reaction of a cultivar/line to crown rot. Depth of crown formation was also measured in susceptible and partially resistant cultivars/lines grown from seed planted at different depths. As depth of seeding increased, depth of crown formation in partially resistant and susceptible cultivars/lines increased in similar proportions. This revised version was published online in August 2006 with corrections to the Cover Date.