High Density Plots Research Articles

Relation between characteristics and rooting activity of rice seedlings (Oryza sativa L.) with special reference to amylase activity.

The authors examined the difference in the characteristics of rice seedlings (Oryza sativa L.) grown under different growth conditions, especially the relation between rooting activity and amylase activity. The characteristics ot rice seedlings on 14 days after seeding were judged by the rooting activity test. In the low density and high temperature plot the qualities of seedlings were good: in the upland, 1/2N, 3/2N and low temperature plots qualities were the same as in the control plot, but in the high density, lowland and shaded plots they were poorer than those in the control plot. While on 28 days after seeding, in the low density, upland and low temperature plots the qualities were better than those in the control plot: in the high density, lowland and high temperature plots the qualities were not so good, and in the shaded plot they were even poorer. On 14 days after seeding, rooting activity showed a significant correlation with leaf age, plant length, leaf sheath diameter, leaf dry weight root dry weight and amylase activity. While on 28 days after seeding, rooting activity showed a highly significant correlation with nitrogen content, starch content and amylase activity. Seedlings with higher starch accumulation, that is higher amylase activity, showed better rooting activity, and good quality.

Relationship of Pacific Madrone Sprout Growth to Productivity of Douglas-Fir Seedlings and Understory Vegetation

Abstract We studied the development of Pacific madrone (Arbutus menziesii) sprout clumps of various initial densities and their effect on Douglas-fir (Pseudotsuga menziesii) seedling growth and understory vegetation. Five years after density treatments, average leaf area index (LAI) of 9-year-oldmadrone sprouts ranged from 3.6-1.0 m²/m² and total aboveground biomass from 25,630-8,390 kg/ha on the high- and low-density plots, respectively. Diameter of 9-year-old Douglas-fir was inversely related to madrone LAI and ranged from about 27 mm on the high-density plots to 54 mmin the absence of madrone. Analyses of diameter growth trends also indicated that, in the absence of madrone, Douglas-fir grew significantly (P = 0.001 to 0.023) faster than in other treatments. An index of shrub, forb, and grass density was inversely related to madrone LAI, suggesting thatunderstory species are quickly excluded from young madrone stands during secondary succession. We provide equations relating the 5-year growth of 9-year-old Douglas-fir to measures of madrone density and seedling size made when the plantation was 5 years old. West. J. Appl. For. 5(1):20-24.

Preliminary investigation of Cyanella hyacinthoides (Tecophilaeaceae) as an edible crop plant

The literature search, herbarium studies and field work associated with revision of the southern African genus Cyanella Royen ex L. suggest that C. hyacinthoides Royen ex L. may have potential as an edible crop. Preliminary investigations of seed germination in various soil types and under different temperatures regimes, coupled with plant counts in disturbed areas indicate that it may be possible to grow this species in high-density plots as a supplementary food crop. However, the economic viability of such a venture would depend on the time taken for corms to reach marketable size and information as to this aspect is at present lacking.

Regulatory Mechanisms of Reproductive Effort in Plants III. Plasticity in Reproductive Energy Allocation and Propagule Output of Two Grass Species, Oryza sativa cv. Akihikari and Coix ma‐yuen Cultivated at Varying Densities and Nitrogen Levels, and the Evolutionary‐Ecological Implications

Abstract Plasticity in growth, reproductive energy allocation (RA), reproductive output (PN), propagule weight (Pw), fecundity, and relative cost of producing single propagules (RA) and their interlinked properties were critically investigated in two grass species, Oryza sativa L. ev. Akihikari and Coix ma‐yuen Roman., grown under varying densities and soil nitrogen levels.In these two species, exceedingly marked plastic but parallel responses were detected in all significant yield and reproductive traits under changing growing conditions. The results obtained also corresponded very well with those for Coix ma‐yuen, Helianthus annuus and Glycine max reported in our previous works in this series (Kawano and Hayashi, 1977; Kawano and Nagai, 1986; Nagai and Kawano, 1986).In Oryza sativa, plants cultivated at higher densities exhibited proportionately lower individual biomasses, lower seed outputs, and smaller grain size in response to increasing density, although RA remained more or less constant throughout all five different densities, and fecundity showed an abrupt decrease in the overcrowded cohort beyond 400 plants/m2. In this experiment, Coix ma‐yuen was grown only under conditions of extraordinarily overcrowded density (over ca. 4,500 plants/m2) with extremely broad nitrogen fertilizer levels from 20 N (20 kg ammonium sulfate/10 ARE) to 100 N (100 kg/10 ARE). This species likewise demonstrated a remarkably sharp responsive plasticity in significant reproductive parameters with increasing N‐levels, just as was found with changing density (Kawano and Hayashi, 1977).One common significant conclusion obtained from the present series of studies was that sexual propagule production under limited resource availability—for example, a sharp decline in solar radiation and soil nutrients, due to strong interference in higher density plots—is exceedingly costly, implying that there occurs a significant change and variance in this important fitness component under changing environmental conditions. This was most conspicuously demonstrated by a very sharp increase in relative energy partitioning to a single propagule in response to increased density and decreasing nitrogen levels, the relative energy cost to a single seed (RA) increasing from 1.00 to 31.02, and from 1.00 to 10.31 in both paddy field and pot populations of Oryza sativa cv. Akihikari, respectively, and from 1.00 to 4.23 in field populations of Coix ma‐yuen. The adaptive significance of induced changes in several reproductive traits under changing environmental conditions was also discussed in the light of all the evidence available today.

Effects of Resource Density on Mycophagous Fly Dispersal and Community Structure

I censused adult flies visiting 1-m2 plots containing high or low densities of a natural host-mushroom analog (Agaricus bisporus). I also conducted a mark-recapture experiment with visiting drosophilids. More individuals and species were captured in high density plots than low density plots, and species used high density plots more frequently. Also, mycophagous drosophilids localized activity in rich resource areas and dispersed more readily from poor resource areas. These patterns are consistent with the 'trapping effect' predicted for dense resource patches in the resource concentration hypothesis. I also collected mushrooms from plots and censused flies that emerged. In contrast to the resource concentration hypothesis, there was no evidence of the establishment of a dominance hierarchy in response to increased resource density. Single mushrooms in the high density plots produced more diverse, species-rich communities, with less dominance and greater resource overlap, than single mushrooms in low density plots. Persistent patches of mushrooms may accumulate species, but the ephemerality of individual resource units may preclude the establishment of large populations and maintain communities in a high diversity state.

The population biology of the arctic dwarf birch, Betula glandulosa: seed rain and the germinable seed bank

The seed rain and seed bank of arctic dwarf birch, Betula glandulosa, were studied at Kuujjuaq, Que., and Tarr Inlet, near Iqaluit, N.W.T. At each site dispersing seeds were captured in sticky traps in three adjacent plots having locally high and low birch densities; soil cores were collected in parallel. Trap data indicate that birch samaras disperse principally during late fall and early spring. The number of samaras dispersed was significantly greater at Kuujjuaq than near Iqaluit and was greater in high- than in low-density areas. Samaras collected at Kuujjuaq were significantly heavier (x = 0.19 mg) than those from Tarr Inlet (x = 0.10 mg). Germination showed an asymptotic exponential weight dependence. From the weight distribution of samaras, we expected germination of 69.7% of samaras from Kuujjuaq. Few viable propagules were found in collections from the Iqaluit area; from their weight distribution 0.38% were expected to germinate. We predicted emergence from soil cores, using seed-trap and germination data, and tested predictions under greenhouse conditions. Observations and predictions agreed for low-density areas. At Kuujjuaq 248 emergences/m2 were predicted and 229 observed; at Iqaluit no emergences were predicted and none observed. In high-density plots at Kuujjuaq 9292 emergences/m2 were predicted and 2164 observed. Corresponding rates for Iqaluit were 2 and 17 emergences/m2, respectively. Differences in seed banks and recruitment indicate differences in the structure, dynamics, and reproductive biology of populations at the two sites.

Preliminary Test of the Ecological Trap Hypothesis

We tested factors associated with predation near forest-field edge and with the ecological trap hypothesis using artificial bird nests containing Japanese quail (Coturnix japonica) eggs. Predation rates were determined for 759 nests placed in a systematic pattern within plots perpendicular to a forest-field ecotone created by commercial timber harvest in northern Idaho. One plot contained an abrupt edge and 1 contained a wide feathered edge of partial timber removal. Nests of 2 diameters (80 and 100 mm) were located on the ground and above ground in shrubs, and were placed in high(20 nests/ha) and low-density (9 nests/ha) patterns. No difference was detected for predation rate with respect to nest location or size. Nests placed in a low-density pattern received higher percent predation than nests in a high-density pattern. The highdensity plot had greater shrub cover, which may.have restricted predator effectiveness. Predation rates were >4 x higher in forest plots than field plots; these results may reflect relatively infrequent use of field subplots by avian predators requiring perch sites. Our data do not support the ecological trap hypothesis; we found no relationship between distance from edge and predation rate. However, our abrupt-edge subplots had higher predation than the feathered-edge subplots. These data support earlier hypotheses that birds are poorly adapted to abrupt, artificial-edge habitats, and that these habitats may have a barrier effect and create a travel lane for predators. J. WILDL. MANAGE. 52(3):484-491 The edge concept is a firmly established maxim in ecology (Leopold 1933:131, Emlen 1973: 394, Smith 1977:160). Species diversity and density increase at community junctions (Odum 1971:157). Numerous ornithological studies have supported the edge effect concept (Lay 1938, Kendeigh 1946, Johnston 1947, Johnston and Odum 1956, Hagar 1960, Marten 1960, Hogstad 1967, Johnston 1970, Gates and Gysel 1978). Edge effect results from vegetative heterogeneity created by the junction or juxtaposition of 2 uniform habitats, and is correlated with increased avian diversity and density (MacArthur and MacArthur 1961, Willson 1974, Roth 1976). However, several recent reports suggest that increased avian density and species richness are not universal characteristics of edges (Laudenslayer and Balda 1976, Ferris 1979, O'Meara et al. 1981, Morgan and Gates 1982). Gates and Gysel (1978) and Chasko and Gates (1982) reported that nests within edge zones had greater predation and lower fledging success than nests in zones more distant from the edge. Gates and Gysel (1978) proposed the ecological trap hypothesis; i.e., that passerines are attracted to the vegetative diversity of edge habitats but experience greater predator activity at the edge. They further postulated that abrupt-edge habitats are associated with relatively recent environmental perturbations by man, and that birds are poorly adapted to predation near artificial edge zones. Increased predator activity at edges is assumed to be the combined result of increased prey density (Johnston and Odum 1956) and the natural travel lane created by the abrupt change in vertical vegetational structure (Bider 1968). Yahner and Wright (1985) and Angelstam (1 86) measured predation rates on artificial nests in plots near edge zones. Neither study revealed an edge effect consistent with the ecological trap ypothesis. O r objectives were to examine several questions related to predation near the edge zone and the ecological trap hypothesis: (1) do predation rates differ between forest and field habitats, (2) does nest density influence predation te, (3) does nest predation vary with distance from the edge zone, and (4) does nest predation vary with edge structure (edge zone width)? We acknowledge J. W. Thomas for suggesting this research topic. We thank B. M. Janosik and J. J. Parker for assistance with data collection. J. Besser and E. Scott provided quail eggs. H. L. Osborne helped delineate the study area. J. A. Blakesley, J. Linman, and J. Nelson assisted with data analysis. E. O. Garton, J. E. Gates, B. D. Griffith, J. M. Scott, J. Verner, and R. H. Yahner reviewed the manuscript and provided helpful comments. This project was funded

Macroalgal mats and species abundance: a field experiment

A field experiment was carried out whereby the density of macroalgae ( Enteromorpha spp.) was manipulated and the resultant changes in sediment infaunal density were monitored. Four densities of Enteromorpha spp. were used: 0,0·3, 1, and 3 kg FW m −2, corresponding to control, low-, medium-, and high-density plots. The experiment ran from May to October 1985 and was sampled on three occasions. By July, the density of Corophium volutator was reduced at all weed levels when compared to control plots, whereas densities of Hydrobia ulvae, Macoma balthica, Nereis diversicolor, and Capitella capitata, all increased. Samples taken in October when the weed mats were buried in the sediment showed fewer differences than in July. Macoma, Nereis, and Capitella were still significantly more abundant at medium and high weed densities. Corophium showed no significant treatment effect. There was, however, a highly significant difference in population size structure for Corophium. Measurements of sediment redox potential and silt content under medium- and high-density plots revealed rapid anoxia with a significant increase in siltation.

Interactions Between a Generalist Herbivore, the Moose Alces alces, and its Food Resources: An Experimental Study of Winter Foraging Behaviour in Relation to Browse Availability

SUMMARY (1) The browsing behaviour of free-ranging moose Alces alces in winter was evaluated by experimentally altering the availability of their most important food resource, the birch Betula pubescens. Plots of birch saplings (2-5-3-5 m high and about 15 years old) were artificially made at ten different densities in early winter. The browsing behaviour was studied by following the moose tracks in the snow during mid-winter. Intake, measured as dry weight per sapling, was estimated from the number of bites per sapling and bite diameter. (2) The moose responded to these small-scale variations in the available food supply. At high densities of birch saplings, the moose ate less on each sapling, selected twigs of better quality and browsed on more saplings per plot. However, the high-density plots were not visited more frequently than the low-density plots. Instead the moose moved longer distances in the high-density plots and probably spent more time there. (3) The proportion of the available browse exploited decreased curvilinearly as food availability increased. Thus, the exploitation of browse was strongly determined by its spatial distribution. (4) It is suggested that factors that influence the spatial organization of plants may also affect both their potential as food supply for herbivores and the type of anti-herbivores strategy which the plant is likely to evolve.

ウンシュウミカンの栽植密度と栽培条件が収量に及ぼす影響

In 1967, two-year-old trees of‘Miyagawa Wase’, an early ripening satsuma mandarin, were planted at various planting densities and grown under different cultural treatments until 19 years old.Planting densities were 1, 250, 2, 500, 5, 000 and 10, 000 trees/ha. Cultural treatments adopted were shallow plowing+less fertilization, or deep plowing+standard fertilization and non pruning+non fruit thinning, or pruning+fruit thinning.The effect of planting density on fruit yield per unit area was determined from data obtained during the period from 1969 to 1984 (tree age from 4 to 19 years old). The effect of the cultural treatments on yield was also determined.1. The yields of high density plots, 5, 000 and 10, 000 trees/ha, increased rapidly with tree age, and then started to decrease gradually. The yields of the lower density plots, 1, 250 and 2, 500 trees/ha, gradually increased in the early years, but soon exceeded those of the high density plots.This trend was almost the same under all of the cultural treatments.2. Planting densities which gave the maximum yield were 10, 000 trees/ha after 4-5 years; 5, 000 trees/ha after 6-7 years; 2, 500 trees/ha after 8-13 years; 1, 250 trees/ha after 14-19 years. The average maximum yield for the planting densities at each tree age (4-19 years old) was 68t/ha.3. With non pruning+non fruit thinning, the trees allowed to bear the first crop showed the alternate bearing habit as early as the following year.With the pruning+fruit thinning, the lower the planting density, the later the alternate bearing habit occurred.4. The yield fluctuations from year to year were larger with the deep plowing +standard fertilization. There was no significant difference in the average yield between the two treatments.The yield fluctuations were apparently larger with the non pruning+non fruit thinning than with the pruning+fruit thinning. The average yield was slightly higher in the former than in the latter.

Regulatory Mechanisms of Reproductive Effort in Plants II. Plasticity in Reproductive Energy Allocation and Propagule Output of Glycine max Merr. (Leguminosae) Cultivated at Varying Densities and Nitrogen Levels

Abstract Plasticity in growth, reproductive energy allocation (RA), and reproductive output were studied in Glycine max Merr. Cv. Enrei (Leguminosae) grown under varying densities and soil nitrogen levels.Marked plastic responses were detected in individual biomass, the patterns of resource allocation to total reproductive structures (RA) and also to propagules, reproductive outputs, and propagule weight under changing densities and soil nitrogen levels. Plants cultivated at higher densities exhibited proportionately lower individual biomass, lower RA, lower seed output, and smaller seed size in response to increasing density and decreasing soil nitrogen levels, although some deviations were observed, especially in the highest density plot with no fertilization. Differences due to different N‐levels were not as great as those to changing density, which may in part be due to the fact that soybean has nitrogen‐fixing bacteria in root tubercles, just as in any other Leguminosae. Fecundity was also maintained at the similar high rates of 80–97% in all plots examined, although slight but steady decreases were noted with increasing density. This resemblance in fecundity may be due to its strong inbreeding system.Another important finding was that seed production under limited resource availability, notably lack of ample solar radiation due to strong interference at higher density plots, is exceedingly costly. This was most clearly exhibited by a sharp increase in relative energy partitioning to a single propagule in response to the increased density, the relative energy cost to a single propagule (RA) increasing from one to seven‐fold. The results obtained in this study coincide well with the findings made in other plants, e.g., Helianthus annuus, Oryza sativa, and Coix ma‐yuen, with the same experimental designs.

Regulatory Mechanisms of Reproductive Effort in Plants 1. Plasticity in Reproductive Energy Allocation and Propagule Output of Helianthus annuus L. (Compositae) Cultivated at Varying Densities and Nitrogen Levels

Abstract Plasticity in growth, reproductive energy allocation (RA), and reproductive output were investigated in Helianthus annuus L. cultivar. Russia (Compositae) grown under varying densities and soil nitrogen levels. The role and behaviour of pollinators in seed production was also examined.Exceedingly marked plastic responses were detected in individual biomass, the patterns of resource allocation to total reproductive structures (RA) and also to propagules, fecundity, reproductive outputs, and propagule size and weight under changing densities and soil nitrogen levels. Plants cultivated at higher densities exhibited proportionately lower individual biomass, lower RA, lower fecundity, lower seed output, and smaller seed size in response to increasing density and decreasing soil nitrogen levels. However, differences due to different N‐levels were not as great as those to changing density.One of the most significant findings was that seed production under limited resource availability, i.e., lack of ample solar radiation and soil nutrients, due to strong interference at higher density plots, is exceedingly costlty, This was most clearly demonstrated by a very sharp increase in relative energy partitioning to a single propagule in response to the increased density and decreasing nitrogen levels, the relative energy cost to a single achcne (RA) increasing from one to twenty‐fourfold. Reproduction was also affected by pollinator‐limitations, and seed size showed a marked position effect.

Competition between annual weeds and vining peas grown at a range of population densities: effects on the weeds

SummaryLinear regression of dry weight of weeds against crop density, together with the use of diversity indices and principal component analysis were used to derive information about changes in the behaviour of annual weeds over the growing season and in response to a wide range of crop densities in vining peas Pisum sativum L. Using linear regression it was possible to quantify reductions in weed dry weights per unit increase in crop plant density The ‘weed losse’ factor was acceptably consistent between experiments. Indices examining richness and evenness showed that numbers of weed species declined with increasing crop density and as the season progressed, but although species evenness became less at successive sampling dates the presence of a pea crop, whatever its density, did not radically alter the composition of the weed flora. Principal component analysis demonstrated that although there was competition within the weed flora, the crop did not replace the dominant weed species on high density plots, but reduced growth of all weed species alike.

Influence of intraspecific density and cover on home range of a plethodontid salamander.

Hypotheses about the influence of intraspecific density and available cover on home range size (=mean activity radius) of a plethodontid salamander (Desmognathus monticola) were tested by field experimentation from June 1980 to October 1981. A second order mountain stream in southwestern North Carolina was marked into five sections (two control and three experimental) and three replicated treatments were performed: Treatment I, low and high density additions of D. monticola; Treatment II, addition of cover objects (rocks); Treatment III, addition of cover and D. monticola.In Treatment I, home ranges of D. monticola on high density addition plots increased significantly relative to those on control and low density addition plots. Mean stomach content biomass and number of prey items of salamanders from high density plots were less than controls in all comparisons throughout the experiment but only one comparison revealed a statistically significant difference. Mean stomach contents of salamanders from low density plots were less than controls in 60% of all comparisons and only one statistically significant difference was found. Home range size was not affected significantly by Treatments II and III.Results of these experiments suggest that home range size of D. monticola may vary as a function of the number of conspecifics and cover objects; i.e., when the number of D. monticola increases where cover is limited, home range size increases.

Flowering plant density and pollinator visitation in Senecio.

It has commonly been assumed that pollinator energy intake increases with flowering plant density, and visitation to flowers should therefore be higher in denser stands. I therefore investigated the relationship between flight distance and flight time for bumblebees and butterflies foraging on Senecio integerrimus and S. crassulus in the Colorado Rocky Mountains. I also compared patterns of pollinator visitation and seed set in two adjacent 15x 15 m plots in a population of S. integerrimus; one plot was experimentally thinned of flowering stalks, while the other was left at natural density. Mean flight distance had no effect on mean flight time or the number of florets or heads visited per unit time. There were no significant differences between the two plots in the rate at which plants received visits, although visitation rates varied through the season, with greatest activity at peak flowering. Pollinators were more selective in their visits to plants in the high-density plot, however. Bumblebee-visited plants in the dense plot had a lower variance in stalk height than the plant population average, while butterfly-visited plants in the dense plot had more heads than the population average. Plant density had no effect on number of heads visited per plant, but number of heads visited by bumblebees was correlated with number of heads per plant. Efficiency of visitation (percentage of flowering heads visited), declined with inflorescence size. Flowering plant density had no effect on seed production, and inflorescence size did not affect the percentage of florets setting seed. In Senecio, flowering phenology patterns and differences among pollinators in foraging behavior may have more important consequences for seed set and gene flow patterns than plant density or plant size.

Competition between annual weeds and vining peas grown at a range of population densities: effects on the crop

SummaryCompetition between annual weeds and vining peas (Pisum sativum L.) at five target population densities between 11 and 194 plants/m2 was examined by means of periodic destructive sampling of weedy and weed‐free plots. A further area of each plot was cut and vined to assess yields. Weeds impaired vegetative development, particularly by reducing tillering in low density crops. This resulted in weedy plots having fewer pods per plant at harvest but a lower proportion of flat pods than weed‐free plots. Weeds had no effect on numbers or weights of peas in full pods nor on tenderometer values of samples of vined peas.Adverse effects of weeds on the growth of individual crop plants decreased with increasing crop density. However, at lower crop densities many of the additional pods on weed‐free plots contributed little to vined yield, while at higher densities, direct or indirect effects of weeds increased the problem of maintaining sufficient photosynthetic area during pod swelling to prevent pod abscission and poor ovule development. Regression analysis of yield on crop density and of yield on numbers of pods per plant showed that vined yield per hectare was reduced by weeds by a constant amount across the range of densities and numbers of pods examined. Vining throughput was also reduced in weedy as compared with weed‐free crops, even on high density plots where little or no weed vegetation remained at harvest.In general, weed presence had effects similar to those caused by increasing crop density, but without the additional contribution to yield made by extra plants. Higher density crops suppressed weeds very effectively but were no less vulnerable to yield loss than those of lower density; they therefore merit just as much attention to effective weed control as crops suffering more visibly from competition by weeds.

The effect of varying seed rate on the yield and yield components of oil-seed rape (Brassica napus)

SUMMARYTwo varieties of oil-seed rape were drilled in 1979–81 at a range of seed rates. Percentage establishment, plant weight, height, and the ratio of seed to total aboveground dry matter were lower in high-density plots. Seed yields were not increased when the seed rate was raised from 9·0 to 13·5 kg/ha (Expts 1 and 2) or from 4·5 to 18·0 kg/ha (Expt 3) owing to a compensating reduction in the number of pods per plant. Seed quality and 1000-seed weight were not influenced by the seed-rate treatments.

SEED AND VEGETATIVE REPRODUCTION IN RELATION TO DENSITY IN FRAGARIA VIRGINIANA (ROSACEAE)

Transplants of Fragaria virginiana Duchesne, the wild strawberry, from field and woods sites, were studied under controlled laboratory conditions to determine their seed and vegetative reproduction responses under different density conditions. The following results were obtained: (1) Increased competition among high density grown plants resulted in lower total biomass when compared to low density grown plants; (2) Increased shading in high density plots resulted in a greater percentage of biomass in leaves; (3) The percentage of total biomass in reproductive organs (seed and vegetative) was higher in low density grown plants; (4) Vegetative “reproductive effort” was higher in low density plots than in high density plots while seed “reproductive effort” remained constant between the two density treatments; (5) No significant difference between plants from habitats of different successional maturity was observed under the experimental treatments. These results are discussed in relation to reproductive density response models.

Fruit Set as Related to Girdling, Early Cluster Thinning and Pruning of ‘Anjou’ and ‘Comice’ Pear1

Abstract Tests with pear on P. communis L. and ‘E. M. Quince C’ (Cydonia oblonga Mill.) rootstocks showed that early fruit thinning to 1 fruit per cluster increased ultimate % fruit set of ‘Comice’; thinning to 2 fruits per cluster did not increase ultimate set Limb girdling 3 weeks after bloom did not effectively increase set, but when used in combination with cluster thinning, increased ‘Anjou’ set beyond either treatment alone. Heading-back pruning of ‘Comice’ on ‘Quince C’ in a high density plot increased both fruit set and ultimate yield relative to thinning-out pruning.

Basic Studies on Utiliization of Hybrid Vigor in Rice : IV. Heterosis under Different Cultural Conditions

F1 hybrids of eleven cross combinations were tested under 4 different cultural conditions, i.e., standard fertilizer-level with standard planting density (plot A), standard fertilizer-level with high planting density (plot B), high fertilizer-1evel with standard planting density (plot C), and high fertilizer-1evel with high planting density (plot D). The F1 hybrids of Norin 22×Araki, Suzu-nari×Zenith CI 7787 and Aikoku×Zenith CI 7787 showed marked heterosis in grain yield. Within the scope of the present experiment, the best cross combination was Suzunari × Zenith CI 7787 which significantly outyielded the highest yielding variety, Benisengoku. It was concluded that practical utilization of heterosis would be possible in rice, if hybrid seeds could be economically produced. In grain yield, spikelet number per panicle and culm length, the degrees of heterosis obtained under the four conditions were signifi-cantly correlated with one another.