The association of sexuality with plant traits and tuber yield in white yam

Summary One 3-to 5-year clonal and two recent seedling-derived populations of white yam cultivars were grown and studied for the association of sex, flowering intensity, number of fruits and seed yield with vegetative characters and tuber yield. Yield was lowest in non-flowering plants and increased in the following order: male, monoecious and female plants; with females yielding almost twice as much as the non-flowering plants. The plants produced from small tuber setts, with higher leaf virus infection, low plant leafiness either had low flowering intensity, fewer fruits and seeds or even failed to flower. The sequence and timing of vegetative and reproductive processes in relation to the difference in yield between male and female plants is discussed. There is a strong indication that the intensity of flowering and fruiting is lower in plants from previous vegetative propagation compared to recent seedling-serived clones.     

Ploidy variation in the cultivated yams (Dioscorea cayenensis-Dioscorea rotundata complex) from Cameroon as determined by flow cytometry

The ploidy levels of the twenty-two yam (Dioscorea cayenensis-D. rotundata complex) cultivars within germplasm Cameroon Guinea yam were determined by flow cytometry. Three different ploidy levels (4x,6x, 8x) were detected within the samples analysed. Fifteen cultivars were tetraploids, five were hexaploids, and two were octoploids. The cultivar group EKOTO showed a high level of ploidy variation with tetraploid, hexaploid and octoploid cultivars. The hexaploid nature of cultivars Dobnawo and Bilougnou supported the hypothesis that they are hybrids between cultivars of the EKOTO group and either the KPE or BAKOKAE groups. This revised version was published online in July 2006 with corrections to the Cover Date.     

Seed set and seedling variation in greater yam (Dioscorea alata L.)

Summary A high degree of fruit and seed set was obtained in Dioscorea alata L. by hand pollination. The sexual progeny was studied in comparison with the clonal plants. In general, the seedlings were poor in growth vigour, flowering and tuber production but the variations observed in the different characters were unprecedented, indicating scope for efficacious genetic improvement in this vegetatively propagated crop.     

Ploidy analysis in water yam, Dioscorea alata L. germplasm

Poor reproductive development in yams (Dioscorea spp.) has often been attributed to the polyploid nature of the crop. In this study, flow cytometry was used to determine the ploidy level of 53 accessions of Dioscorea alata, mostly from West African countries, Chad and Puerto Rico. Nuclei were isolated from young leaf material and stained with DAPI(4,6-diamidino-2-phenylindole). The nuclear genome size (2C) was measured as an indicator of the ploidy level. Dioscorea rotundata genotypes with known ploidy levels were used as standards. The results showed that the majority of plants were hexaploid (84.9%) with a smaller percentage of tetraploids (15.1%). A higher number of male plants were hexaploid than tetraploids. This is at variance with earlier findings, which reported that hexaploid male plants are rare. Higher ploidy levels were not directly related to sparse or erratic flowering as previously reported as profuse flowering occurred in some male hexaploid accessions. These findings have important implications for yam breeding in relation to yam genetic resources. This revised version was published online in August 2006 with corrections to the Cover Date.     

The reproductive biology of three perennial Sesbania species (Leguminosae)

Summary The reproductive biology of Sesbania sesban, S. goetzei and S. keniensis (Leguminosae; Papilionoideae) was studied. Fifty percent flowering was observed at 102 to 153 days after germination for S. sesban accessions; 96 to 146 days for S. goetzei and 131 to 176 days for S. keniensis accessions. Flowers opened in the afternoon and remained fresh for 2–3 days. Bee species including Xylocopa sp., Apis mellifera, Megachile bituberculata and Chalicodoma sp. visited the flowers. Hand pollination experiments showed that all three species were self and cross compatible. The percentage of fully developed seeds was greater in pods formed after cross pollination compared to self pollination. No evidence was found for stigmatic or stylar self incompatibility. Outcrossing is probably the common method of reproduction under natural conditions, although in isolated trees substantial seed set by selfing might occur. Pod production under natural conditions was 34% for S. sesban; 49% for S. goetzei and 39% for S. keniensis. Considerable variation was found in pod production under open pollination between accessions of the same species. Selective abortion was observed within pods, with more mature seeds formed at the distal end of the pod.     

The reproductive biology of okra. 2. Self-fertilization kinetics in the cultivated okra (Abelmoschus esculentus), and consequences for breeding

Summary The use of Cruden's index (1977), for studying okra sexual reproductive allocations, indicates a facultative autogamy mode. The purpose of the research described is to obtain more accurate information on the self-fertilization process. For this, self-pollen grain germination was stopped at different times of the day. Twelve varieties of Abelmoschus esculentus were used. Self-fertilization kinetics—expressed by the setting rate—displayed an increase between 7.00 to 16.00 hr. Study of obtained progenies, by stimulated allogamy, confirmed the process and showed that allo-pollen grains deposited on a stigma after midday had only a very little chance of contributing to fertilization. As a result, the polymorphism of the flower structure and particularly the distance between anther and stigmas, insect types and movement habits play major roles in governing okra allogamy. We also show that self-fertilization kinetics can be used to improve controlled hybrid production.     

Status of yam (Dioscorea spp.) in the Democratic Republic of Congo

Yam is an important tuber crop with enormous potential to enhance rural sustenance and livelihood in DRC. However, studies to enhance its genetic improvement are very far from sufficient with only a handful of information available on the crop. Yam has been treated as an orphan crop compared to contemporary crops such as cassava and sweet potato which have adapted to different cropping systems and become widespread in production. The lack of research attention to address the major production challenges has further decreased the value and potential of the crop compared to its contemporaries. These production constraints include lack of adequate quality planting materials, low yield potential, poor resistance/tolerance to yam mosaic and anthracnose diseases and ultimately poor tuber quality attributes focusing on tuber taste, flesh oxidation and dry matter contents of the majority of the farmers preferred varieties. In this review, we evaluated the status of yam in DRC and presented the needful activities to be incorporated for its improvement. Diversity has however been maintained mainly through ennoblement efforts in house gardens and small farmlands using traditional farming methods. Studies from other nations where yam has been successful with prominence in characterization and genetic improvement brought to light the need for DRC to consider yam as a staple carbohydrate food source, even to the extent of modifications in food public policy. Reversal of the yam's current stigma is a challenge to the scientific community and the population in general.     

Ploidy level studies on the <Emphasis Type="Italic">Dioscorea cayenensis</Emphasis>/<Emphasis Type="Italic">Dioscorea rotundata</Emphasis> complex core set

The Guinea yams, Dioscorea cayenensis Lam. and D. rotundata Poir. (D. cayenensis–D. rotundata complex), represent a highly important crop, widely distributed in the humid and semi-humid tropics. The ploidy levels of 170 accessions of the core set of Guinea yams from West African countries was determined using flow cytometry with propidium iodide staining. One hundred and eight of the genotypes were found to be tetraploid, 47 were hexaploid and five were octoploid. One mixoploid individual containing tetraploid and hexaploid nuclei was also detected. A deeper analysis considering each separate taxon revealed that while for D. rotundata the majority of individuals were tetraploid, for D. cayenensis this ploidy level was not detected in any of the accessions. Also, no association between ploidy level and place of cultivation was found for the evaluated germplasm. The obtained data is highly valuable for breeding programs of Guinea yam, especially for the optimization of future hybridization experiments directed to the genetic improvement of this economically important crop.     

The reproductive biology of okra. 1. Study of the breeding system in four Abelmoschus species

Summary The breeding system of Abelmoschus species is not well known. Although being self-compatibles allogamy reaches 63%. The aim of the study was to place four Abelmoschus species on the log P/O scale devised by Cruden (1977). On this scale P = pollen grains per anther × anthers per flower and O = ovules production. Pollens and ovules productions were therefore assessed for 68 accessions of two cultivated species, A. esculentus and A. caillei, and two wild, A. manihot and A. moschatus. Extreme log P/O values ranged from 1.69 for the variety Clemson Spineliss (A. esculentus) to 2.94 for ORS 278 (A. manihot). Averages of wild species A. moschatus (2.17) and A. manihot (2.19) indicate facultative autogamy. Averages for cultivated species A. esculentus (2.00) and A. caillei (2.05) suggest more autogamy.     

10种长山药花粉形态及分类研究

利用扫描电子显微镜对10个品种的长山药花粉形态进行了观察和对比研究,结果表明:长山药花粉形状为扁球形,两端平截,2沟,极轴长11.51 μm(10.02～ 13.90 μm),赤道轴长18.53 μm(16.21 ～21.05 μm),属于小型的花粉(10 ～ 25 μm).10种长山药品种花粉在极轴、赤道轴和P/E比...     

Floral biology and artificial pollination in Dioscorea alata L.

Summary Floral biology of Dioscorea alata L. was studied. Anthesis occurred in male flowers by 12.30 h and in female flowers between 13.00 h and 14.30 h. Male flowers remained open for 4–5 hours and female flowers for 9–11 days.Natural pollination was practically absent but parthenocarpic fruit development was observed in rare cases. For hand pollination a simple and efficient pencil method was standardised. Pollen remained viable for 4–5 hours whereas stigma receptivity lasted for 9–10 days. The optimum time of pollination was found to be between 12.00 h and 15.00 h. Several factors like high relative humidity, atmospheric temperatures below 30°C and non-bagging the female flowers were found to promote fruit set.     

Selection for root hair length in white clover (Trifolium repens L.)

Summary Results are given which show that the root hair length of a white clover population can be increased and decreased by selection. An increase of 50 m in root hair length over the mean of an unselected population of the cultivar Tamar was achieved resulting in an 11% calculated increase in the volume of soil explored by root hairs. The realised heritability for selection in the long root hairs was 0.33, and for the short root hairs 0.44.     

Floral biology in the Zanzibar type clove (Syzygium aromaticum) in Indonesia

Summary Investigations of the floral biology of Zanzibar type cloves were carried out on five mature trees. The majority of flowers opened in the afternoon. Anther dehiscence occurred shortly after anthesis and stamens were shed almost 2 days later. Nectar appeared around the base of the stigma as anthers dehisced and remained in evidence for approximately 60 hours. Three days after anthesis a constriction around the tip of the stigma became apparent. The stigma tip subsequently withered, turned brown and was shed after a further four days. Maximum pollen viability and stigma receptivity were attained simultaneously, 48 hours after anthesis. Indications concerning the breeding system in clove are discussed and a procedure for artificial pollination is suggested     

The inheritance of the cold induction response with respect to floral initiation in white clover (Trifolium repens L.)

Summary The inheritance of the cold induction response in the UK cultivar of white clover Menna, was assessed in controlled environments.Only 50% of the population flowered without prior cold induction. The cold period increased the number of stolons which became reproductive.All families from the crosses between induction and non-induction requiring genotypes produced progeny lacking a cold requirement for flowering.Heritability estimates for the number of flowers per plant, reproductive buds per stolon and stolon number were high.The necessity of eliminating the cold induction requirement in potentially valuable white clover cultivars in relation to seed production at lower latitudes is discussed.     

Genetic variation in a Norwegian germplasm collection of white clover (Trifolium repens L.)

Genetic variation and genotype × environment interactions for important agronomic characters were estimated for eleven Norwegian populations of white clover (Trifolium repens L.), originating from between 58^°52′N and 69^°30′N latitude, and from altitudes of 10 to 450 m a.s.l. The populations were evaluated in monoculture clonal field plots for two years at two locations in Norway. The medium-leaved Danish cultivar Milkanova was also included in the experiment as a standard market cultivar. Considerable variation between populations was observed for winter survival, spring growth, morphological characteristics, dry matter yield, general performance, earliness and seed yield. Winter survival, measured as plant cover after two years, was significantly different at the two locations. All the populations had sufficient winter survival at the southern, continental highland site (61^°07′N).At the northern coastal location (69^°30′N),where the winter climate is unstable with physical stresses such as frost, ice-encasement and water-logging frequently occurring, only the northernmost, coastal populations showed sufficient winter survival. Highly significant interactions were found between populations and locations, and between populations and years for all characters measured at both locations and in both years, respectively. Interaction effects between populations, locations and years were significant only for leaflet length. The largest leaf lamina's were observed in the most southern populations. However, genotypes possessing relatively long leaflets were also found in the northernmost populations, although at low frequencies. In general, the populations originating from the northernmost locations had lower biomass production, and were smaller-leaved, more prostrate and exhibited shorter internodes and stolons compared to the populations from South and Middle Norway. These populations also initiated inflorescences earliest, and were among the lowest seed yielding populations. Otherwise, no clear clinal variation in growth characteristics was observed which could be related to the latitude or altitude of origin of the populations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cultivar variation for seed development in white clover (Trifolium repens L.)

Summary A controlled environment study was undertaken to clarify the factors responsible for poor seed set and to study seed development, ovule degeneration and seed abortion, both morphologically and cytologically, in three Japanese cultivars of white clover. Although the mean number of ovules per floret was 4.2–5.1, the average number of seeds per floret was found to be only 2.3–2.7. Microscopic examination of carpels from 0 to 28 days following floret maturity and pollination showed that 26–33% and 8–17% of the total seeds lost occurred within the first three days and the third through fifth day following pollination, respectively. Beyond this period occasional seed abortion was observed at all stages of seed development, but this represented a very small proportion (2–7%) of the total seeds lost.A stain clearing technique was used to examine the cytoplasmic state of the embryo sac in intact, unfertilized, mature ovules and embryos of the ovules at 3 and 5-day periods following pollination. It was found that 20–22% of unfertilized and matured ovules were sterile, suggesting that ovule degeneration before fertilization was the major cause for the high percentage of seeds lost within a 0 to 3-day period following pollination. Cytological observations revealed that abortion of developing seed was due to a sudden arrest in embryo growth and that the early development of the embryo of such aborting seed was normal. Either nutrient shortage or meiotic irregularities may be the cause for high ovule sterility or post-fertilization abortion of developing seeds.