Bulb Development in Onion (Allium cepa L.) I. Effects of Plant Density and Sowing Date in Field Conditions

Early and later maturing cultivars of both spring- and autumn-sownonions were sown on two dates at 25, 100 and400 plants m^–2.High levels of nutrients and irrigation were applied. Both high plant density and early sowing advanced the date atwhich bulb scales, rather than leaf blades, were initiated atthe shoot apex and so advanced the date of maturity by up to46 d. The later maturing spring-sown cultivar showed greaterresponses to density and sowing date than the earlier maturingspring-sown cultivar which in turn was more responsive thanthe autumn-sown cultivars. For each cultivar, maturity date increased linearly with decreasesin the percentage radiation intercepted by the leaf canopy. Onion, Allium cepa L., bulbing, competition, spacing     

Growth, Dry Matter Partition and Radiation Interception in an Overwintered Bulb Onion (Allium cepa L.) Crop

Growth, bulb development, partition of dry weight between leafblades and bulbs, and the interception of solar radiation weremeasured in overwintered crops of five cultivars of bulb onionwith different maturity dates sown on successive dates in threeseasons. The onset of bulbing was later the later maturity ofthe cultivar. Later sowing did not delay the onset of bulbingbut it did delay maturity. There was little mean differencebetween cultivars in the duration of bulb growth defined asthe interval between onset of bulbing and maturity, but therewere considerable differences between cultivars within a season,and between seasons for a given cultivar. Duration of bulb growthranged from 11 to 46 days with a mean of 35 days. Increases in total shoot dry weight during bulb developmentand, in the absence of much bolting, bulb dry-matter yieldswere linearly related to the total radiation intercepted duringbulb growth. These relationships were similar to those reportedfor other crops in Britain. Radiation interception during thephase of bulb growth was low compared with other crops, witha mean value of 49 per cent and a maximum of 65 per cent. Thepercentage of solar radiation intercepted during bulb developmentwas higher from early sowings than from later ones, particularlyin early maturing cultivars. The harvest index was high, withtypically more than 80 per cent of the shoot dry weight in bulbsat maturity. Allium cepa L., onion, blub, growth, partition of dry matter, radiation interception     

Effects of Nitrogen Supply on Bulb Development in Onion Allium cepa L.

Onions were sub-irrigated with 0.6, 2.4, 4.8 and 12 mol m^–3nitrate while growing in constant photoperiods of 14, 15 or16 h. The lower the nitrogen, the later bulb scale initiationoccurred and in 14 h photoperiods bulb scales initiated onlyat the highest N level. In the early stages of the experiment,the ratio bulb width/neck width was higher in low N than inhigh N treatments, but the reverse was true at the later harvests,by which time bulb scales had formed. In a second experimentonions were sub-irrigated continuously with 0.6 mol m^–3or 12 mol m^–3 nitrate or transferred from the low to high,or from high to low N after 36 d of growth in 14, 15.25 and17 h photoperiods. An initial period of low N delayed bulb scaledevelopment and ripening relative to high N throughout, particularlyunder 14 h photoperiods. Late N deprivation did not preventor delay bulb scale initiation or ripening. In a field experiment,plants not fertilized before sowing but given the total quantityof N late in the growing season formed mature bulbs later thanplants fertilized with N before sowing, or given half the quantityof N before sowing and the second half late in the growing season. Key words: Onion, nitrogen, photoperiod, daylength, bulbing     

Nitrogen fertilization to optimize the greenhouse gas balance of hemp crops grown for biomass

Nitrogen trials were carried out on hemp crops grown in Ireland over a 3 year period to identify nitrogen fertilization strategies which optimize the greenhouse gas (GHG) and energy balances of hemp crops grown for biomass. Nitrogen rates up to 150 kg N ha^?1 were used in the study. Yield increased with nitrogen rate up to 120 kg N ha^?1 for early (Ferimon), mid (Felina 32) and late maturing (Futura 75) varieties. Variety had a significant effect on yield with yields increasing with maturation date. In 2 years of the study, certain application rates of nitrogen were applied either at sowing, at emergence, after emergence or split between these dates to determine if nitrogen rates could be reduced by delaying or splitting the applications. The application of nitrogen at times later than sowing or in splits during the early part of the growing season had no significant effect on biomass yield compared with the practice of applying nitrogen at the time of sowing. Late applications of nitrogen reduced leaf chlorophyll content and height early in the growing season. Later in the growing season, there was no difference in height between treatments although the highest concentrations of chlorophyll were found in the leaves of the late application treatment. Nitrogen rate and the timing of nitrogen application had no effect on plant density. Biomass yield, net energy and net GHG mitigation increased up to an application rate of 120 kg N ha^?1, this result was independent of soil type or soil nitrogen level. Net GHG and energy balance of hemp crops grown for biomass are optimized if late maturing varieties are used for biomass production and a nitrogen rate of 120 kg ha^?1 is applied at sowing.     

Bulbing in Onions: Photoperiod and Temperature Requirements and Prediction of Bulb Size and Maturity

Bulb size and maturity are key characteristics of an onion cropand the onset of bulbing is an important determinant of these.In this paper we describe an experiment in which bulb and neckdiameter and leaf number were measured in onion crops (cultivarsPukekohe Longkeeper and Early Longkeeper) with different sowingdates planted at two different locations in New Zealand. A sensitiveindicator of earliest time of bulbing was developed using theratio of bulb and neck diameters and the statistical techniqueof cusums. Bulb diameter at bulbing was related to thermal timeaccumulated prior to bulbing. Bulbing only occurred when dualthresholds of a minimum thermal time of 600 degree days anda photoperiod of 13.75 h were reached. Mathematical relationshipswere developed between leaf number, sowing date, bulbing dateand bulb growth and maturity. Final bulb size could be predictedfrom bulb size at bulbing and number of leaves produced afterbulbing. Bulb maturity date could be predicted by number ofleaves after bulbing. Allium cepa L.; onion; temperature; photoperiod; bulb-neck ratio; leaf number; bulbing     

Replicating Life Cycle of Early‐Maturing Species in the Timing of Restoration Seeding Improves Establishment and Community Diversity

Tallgrass prairie is among the most endangered ecosystems in North America. High‐diversity restorations protect remnant habitat and expand native communities. Excluding land acquisition, the most expensive step in restoration is procuring seed. Given this cost, managers want to maximize seedling establishment. Native species that flower and ripen early in the growing season are included in a diverse seeding mix but as a group they have not successfully established. For early‐maturing species, the practice of storing seeds in a cold room from harvest until sowing in the dormant season effectively eliminates exposure to the summer conditions seeds would naturally have in the wild. In this study, we compared the effect of summer sowing timing and winter sowing timing on establishment in field conditions. In August 2004 and December 2004, we broadcast a seed mix of seven early‐maturing species: Antennaria plantaginifolia (L.) Richardson. (Pussy toes), Arabis lyrata L. (Sand cress), Carex swanii (Fernald) Mack. (Downy green sedge), Hymenopappus scabiosaeus L'Hér. (Old plainsman), Lupinus perennis L. ssp. perennis var. occidentalis S. Watson. (Wild lupine), Phlox bifida Beck. (Sand phlox) and Hesperostipa spartea (Trin.) Barkworth. (Porcupine grass). We collected data on establishment and reproductive success at 12 time points from June 2005 until October 2008. Species established one growing season sooner when planted at the summer sowing time, and diversity in the summer sowing plots was higher after 4 years. Quicker establishment may have benefits such as providing early competition from weeds that may outweigh additional effort required to ensure timely planting.     

Sowing date and phosphorus utilization by wheat

The uptake and utilization of phosphorus (P) by cereal crops is influenced by the growing period of the crop. In this article the effect of sowing date on the utilization of P by wheat crops grown in southern NSW is reviewed. Crops sown early in the accepted sowing period require smaller inputs of P fertilizer to reach the maximum yield but produce grain with a higher concentration of P than crops sown late in the sowing season. For later sowings a higher rate of applied P is required to achieve the yield potential but this is not associated with a high grain P concentration or a high rate of removal of P from the soil. If grain with a high P concentration is required as seed for subsequent crops, then sowing early, even with little or no applied P fertilizer, is preferable, although crops sown early in the season are likely to remove more P from the soil than the amount applied in fertilizer.     

Studies in the Physiology of the Onion Plant: VI. A SENSITIVE MORPHOLOGICAL TEST FOR BULBING AND ITS USE IN DETECTING BULB DEVELOPMENT IN STERILE CULTURE

Bulb development in the onion plant, which occurs in responseto the stimulus of long days, has two main morphological features:swelling of the base of the pseudostem and the formation ofscales from leaf initials produced at the stem apex. The swellingmay be expressed as the ‘bulbing ratio’, i.e. theratio of the maximum diameter at the base to the minimum atthe neck, but this is a somewhat insensitive measure wider adverselight conditions and no ratio can be specified at which bulbingis first apparent. In the absence of bulbing, leaf initialshave at all stages of development a blade longer than the leafsheath, giving a ‘leaf ratio’ of blade/sheath atall times greater than unity. In scale formation, when bulbdevelopment begins, the sheath of a small initial starts togrow sooner than in foliage leaf formation and blade growthis suppressed; the leaf ratio therefore falls rapidly belowunity and this is diagnostic of bulbing. Characteristic graphsof leaf ratio plotted against leaf number from the apex demonstratethe stage of bulbing reached and a fall in leaf ratio from nearthe apex in such a graph can indicate the beginning of bulbingbefore any actual scales (with ratios less than unity) havebeen formed. In normally grown plants, differentiation intofoliage leaves or scales appears to take place when the initialsreach about 1 mm in length. When the incidence of bulbing within a treatment is very variable,difficulties arise in averaging the leaf ratio curves for thedifferent plants. Mean number of scales per plant, or mean minimumleaf ratio, may then be used as measures of bulbing. Heavy shading in winter was found to delay but not prevent thebulbing of those plants which survived. ‘Buds’, consisting of a portion of stem with theapex surrounded by leaf initials, but no scales, were dissectedout from onion sets and cultured aseptically in tubes. Limitedgrowth was obtained and rooting occurred in some cases. In thepresence of sucrose, swelling of leaf sheaths and reductionof the leaf ratios took place; these responses increased withsucrose concentration. Glucose similarly stimulated bulbingbut high osmotic pressure caused by mannitol or mineral nutrientshad no effect. Rooting appeared to reduce the degree of bulbing,possibly by competing for carbohydrate supply. The bulb developmentin these sterile cultures was most unusual in occurring in shortphotoperiods and the possibility of a residue of ‘bulbinghormone’ being stored in sets is discussed.     

Interaction among planting dates,transgenic maize,seed treatment,corn rootworm damage and grain yield

A study was conducted from 2006 to 2008 at South Charleston and Wooster, Ohio, USA, to evaluate the potential use of planting dates in combination with Bacillus thuringiensis transgenic maize and insecticidal seed treatments to manage the root feeding of western corn rootworm (Diabrotica virgifera virgifera, Coleoptera: Chrysomelidae). Three planting dates (early, middle and late), targeting late April/early, mid‐May and early June, respectively, were used. We planted six hybrid treatments consisting of two seed‐treated hybrids with seed treatment, two transgenic hybrids and two untreated hybrids, each set represented by one short and one full season maturity hybrid. When root injury was high, significant lodging and stunted growth were observed on untreated maize and declined when planting was delayed. Root injury by rootworm larval feeding was significantly reduced by later planting maize, that is, early June. The use of transgenic maize and seed treatment also significantly reduced root injury by rootworm larvae. The influence of planting date on grain yield was inconsistent from year to year. Grain yields from short season hybrids were comparable to full season hybrids especially on later plantings. These results showed that the use of a seed treatment and transgenic maize might be beneficial only when rootworm population is high and planting is early.     

Competitive ability and tolerance of organically grown wheat cultivars to natural weed infestations

Competitive ability of a traditional winter wheat cultivar (Maris Widgeon) was compared with two modem cultivars (Hereward and Genesis) when grown organically in each of four seasons in Gloucestershire, UK. In two seasons, cultivars were compared at two sowing dates (September and October or November). Cv. Maris Widgeon was the tallest cultivar and intercepted most photosynthetically active radiation (PAR), particularly if sown early. Cv. Hereward was taller than cv. Genesis when sown early. Cv. Maris Widgeon accumulated most nitrogen and dry matter early in the season (until the end of March) when sown early, but not when sown late. Relative nitrogen and dry matter accumulation by cultivars later in the season depended on season and sowing date. In one season cv. Maris Widgeon had significantly more early season ground cover. Cultivars did not differ significantly in early tiller production. Although most of these indirect measures of competitive ability were greater for the older variety cv. Maris Widgeon, infestation of Veronica spp. was greater in plots of this cultivar than in plots of either one or both of the shorter cultivars in two successive seasons. In the 1993/94 season, the soil seedbank from plots previously cropped with cv. Maris Widgeon produced more seedlings of Sinapis awensis. In the 1994 / 95 season, cv. Maris Widgeon plots contained less weed in terms of ground cover, numbers, dry matter and nitrogen than the other cultivars, and soil sampled from plots previously cropped with cv. Hereward sown late gave rise to larger numbers of weeds than cv. Maris Widgeon. Sowing date affected weed burdens and emergence to a much greater extent than wheat cultivar. In years and sowing date treatments with relatively low weed presence the shorter cultivars tended to yield more than cv. Maris Widgeon. When weed burdens were severe cv. Maris Widgeon yields were equal to or greater than those of either cvs Hereward or Genesis. The results suggest that the traditional tall cv. Maris Widgeon could be used beneficially to tolerate weeds in organic systems when high weed infestations were anticipated, but could not be relied upon to suppress weed development, and in some circumstances could actually encourage certain species.     

Applications of a Model to Predict the Time to Maturity of Calabrese Brassica oleracea

A model that predicts the time from sowing to maturity of calabreseBrassica oleracea cv. Corvet is used to design a sowing scheduleto give a series of crops maturing at weekly intervals. Thepredicted performance of the sowing schedule is evaluated usingmeteorological data for 14 years recorded at the Scottish CropResearch Institute. In E. Scotland sowings could be made inthe second week of April and continued through until the endof June to provide a regular supply of mature calabrese fromlate July through to early October. There was an increasingrisk of crops failing to mature with progressively later sowingsin July. Continual updating of forecast of harvest date andintervals between harvests, using the current year's weathercombined with long-term average weather, is described. A dynamicsowing plan predicting the best dates to sow, based on theseforecasts, reduced the spread in intervals between harvestsfrom 2·5 to 1·8 days. Model, weather, calabrese, broccoli, thermal time, solar radiation     

Effects of sowing date and volunteers on the infectivity of soil infested with Gaeumannomyces graminis var. tritici and on take-all disease in successive crops of winter wheat

In a field experiment on winter wheat, take‐all on plants and the infectivity of the soil were studied in crop sequences with different combinations of sowing dates. Take‐all was negligible in the first wheat crop, but thereafter the mean disease intensity (measured using a take‐all rating, TAR, with a maximum of 300) was 108, 190, 118 and 251 in the second to fifth successive crops. In each growing season, the disease differed amongst sequences and built up more rapidly and was more intense on plants sown in mid‐September than on plants sown in mid‐October. In late‐sown plots, where volunteers had been present during the mid‐September to mid‐October period, take‐all reached an intensity intermediate between that in early‐sown plots and that in late‐sown plots that had been kept free of volunteers. Volunteers, therefore, partially offset the expected beneficial effect of decreased disease with later sowing. Differences in take‐all amongst sequences were most pronounced in the second wheat crop and early sowing of the previous wheat increased intensity of disease. In the following (third) crop, differences in disease intensity amongst sequences were smaller. Soil infectivity (measured by seedling bioassay after harvest) built up progressively from a low level after the first crop to peak after the third crop. In this build‐up phase, soil infectivity estimates were always numerically greater after harvest of early‐sown treatments than after later‐sown treatments, although never significant at P= 0.05. The greatest difference (P= 0.06) was recorded in October before sowing of the third crop, where the comparison was between soil after two previous early sowings and soil after two previous later sowings and control of volunteers. In the same autumn, presence of green cover (i.e. volunteers) was associated with a smaller loss of soil infectivity between harvest and later sowing than occurred in an absence of green cover. In 2^nd–4^th crops, where comparisons were available and mean TARs indicated moderate levels of take‐all, sowing later had no yield benefit, despite more take‐all and greater soil infectivity associated with early sowing. Important considerations for the management of crops at risk of take‐all are 1) choosing appropriate sowing dates to minimize take‐all or to encourage take‐all decline and 2) controlling volunteers and weed hosts where crops are sown late to minimise take‐all.     

基于统计和过程模型的河南省夏玉米最适播种期时空分布特征

播种期是影响夏玉米产量的重要因素,研究夏玉米最适播种期的时空分布特征对指导夏玉米生产有重要意义.本文应用统计模型和APSIM Maize过程模型分析了河南省夏玉米最适播种期的时空分布特征.结果表明: 河南省夏玉米的最适播种期为5月30日至6月13日,南早北晚,北部地区以6月4日至13日播种为宜,西部山区应在5月30日左右播种,南部地区应尽量保证在6月8日前播种.晚熟品种‘农大108’应比中熟品种‘丹玉13’至少提前播种2 d,气候变暖背景下若收获期可推迟1周,则最适播种期将至少推迟3 d.在生长季降水偏少年型下,夏玉米应较正常年型晚播7 d左右;而在生长季降水偏多年型下,夏玉米应早播7 d左右.1971—2010年,河南省夏玉米最适播种期变化趋势不显著,但是由于温度变化和品种改良对冬小麦成熟期的影响,导致河南省驻马店以南地区、中部的伊川、内乡、南阳,以及北部的林州和西部的三门峡地区夏玉米可播种期提前,可播种范围扩大.统计方法和APSIM模型计算的夏玉米最适播种期在76.7%的研究站点无显著差异.结合两种方法,北部地区应保证需水关键期降水充足和灌浆期温度适宜,做到“见雨即播”.南部地区在满足上述两个指标的条件下,应在播种期降水达到一定有效值时进行播种,对于南部和偏南部地区,该有效值分别为3.9和8.3 mm.     

Effect of mineral nitrogen on nitrogen nutrition and biomass partitioning between the shoot and roots of pea (Pisum sativum L.).

The effect of mineral N availability on nitrogen nutrition and biomass partitioning between shoot and roots of pea (Pisum sativum L., cv Baccara) was investigated under adequately watered conditions in the field, using five levels of fertiliser N application at sowing (0, 50, 100, 200 and 400 kg N ha^–1). Although the presence of mineral N in the soil stimulated vegetative growth, resulting in a higher biomass accumulation in shoots in the fertilised treatments, neither seed yield nor seed nitrogen concentration was affected by soil mineral N availability. Symbiotic nitrogen fixation was inhibited by mineral N in the soil but it was replaced by root mineral N absorption, which resulted in optimum nitrogen nutrition for all treatments. However, the excessive nitrogen and biomass accumulation in the shoot of the 400 kg N ha^–1 treatment caused crop lodging and slightly depressed seed yield and seed nitrogen content. Thus, the presumed higher carbon costs of symbiotic nitrogen fixation, as compared to root mineral N absorption, affected neither seed yield nor the nitrogen nutrition level. However, biomass partitioning within the nodulated roots was changed. The more symbiotic nitrogen fixation was inhibited, the more root growth was enhanced. Root biomass was greater when soil mineral N availability was increased: root growth was greater and began earlier for plants that received mineral N at sowing. Rooting density was also promoted by increased mineral N availability, leading to more numerous but finer roots for the fertilised treatments. However, the maximum rooting depth and the distribution of roots with depth were unchanged. This suggested an additional direct promoting effect of mineral N on root proliferation.     

Thaumatotibia leucotreta and the Navel orange: ovipositional preferences and host susceptibility

False codling moth, Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae), presents a significant threat to the South African citrus industry. To limit income loss due to direct larval damage or from fruit rejection due to the phytosanitary status of this pest, additional pre‐harvest control techniques are required for Navel oranges, which are known to be susceptible to T. leucotreta damage. A number of Navel orange cultivars have been developed, and differences in female T. leucotreta ovipositional preference and susceptibility of cultivars to larval penetration are known to exist. Navel orange cultivars were grouped according to time of maturity (early, mid‐ and late season). Female T. leucotreta were subjected to choice and no‐choice tests with these cultivars, measured by oviposition. Host susceptibility was tested by allowing neonate T. leucotreta larvae to penetrate the different Navel cultivars. In the early maturing group, Fischer Navels were least preferred for oviposition and the least susceptible to larval penetration. The mid‐ and late season maturing groupings showed limited differences in oviposition preference, although host susceptibility did appear to be an important factor in assessing the vulnerability of fruit to T. leucotreta. Despite being widely planted in South Africa, the mid‐season Palmer Navels were highly susceptible to larval penetration, while for the late season cultivars, Cambria and Glen Ora Late were the least susceptible to T. leucotreta. As a result of these laboratory trials, it is recommended that farmers increase cultivation of Fischer Navels as the principal early season cultivar, avoid Palmer Navels in favour of other mid‐season maturing cultivars and give preference to the late maturing Cambria and Glen Ora Late cultivars, to limit T. leucotreta damage.     

In vitro bulb development in shallot (Allium cepa L. Aggregatum Group): effects of anti-gibberellins,sucrose and light

Bulbing was studied in shallot plants cultured in vitro. Bulbing occurred under a 16 h photoperiod with fluorescent + incandescent light and 30-50 g 1(-1) sucrose in the culture medium. Exogenous gibberellin (10 microM GA3) inhibited leaf and root growth and bulbing. When added to the medium at a concentration of 10 microM, three inhibitors of gibberellin biosynthesis (ancymidol, flurprimidol and paclobutrazol) promoted bulb formation and the percentage of bulbing. When ancymidol was used in combination with GA3, it did not reverse the effect of GA3 applied alone. Under treatments with 30-70 g l(-1) sucrose, bulbing ratios greater than those found in control plants were achieved by addition of ancymidol, and bulb fresh weight was increased in the same way. Ancymidol caused a 66% decrease in sucrose content in leaf bases but greatly increased the glucose, fructose and fructan contents. The increase in fructan content by ancymidol could result from the three-fold rise in total [14C]sucrose uptake per plant from the culture medium associated with a marked increase in leaf base labelling at the expense of root labelling. The possible role of ancymidol is discussed and evidence supports a major regulatory role for gibberellins in bulbing.     

Seasonal trends in body size of adult male mosquitofish,Gambusia affinis,with evidence for their social control

Synopsis Collections from a natural population of mosquitofish,Gambusia affinis, in southern Indiana showed that males born early in the breeding season mature rapidly at a small body size, while males born later in the breeding season delay maturation and achieve larger body sizes. A field experiment, involving removal of males from pond populations, was conducted to test the hypothesis that delayed maturation by late summer males is, at least in part, under social control. Mean total length (TL) of adult and maturing males in late summer in ponds from which large numbers of males had previously been removed was significantly lower than mean TL of males in control ponds. These data support the hypothesis of social control and support Sohn's competition-predation hypothesis for control of adult male body size inGambusia.     

基于ORYZA2000模型的北京地区旱稻适宜播种期分析

确定适宜播种期是制定合理的作物栽培管理方案的关键内容之一。在作物模型ORYZA2000有效性验证的基础上,以北京地区为例,利用该模型结合长期历史气候资料,对确定旱稻适宜播种期做了初步研究。结果表明：在不考虑水分因子条件下,北京地区旱稻297安全播期的范围较广,多年平均为3月26日-6月4日;受温度升高的影响,最早播期有提前趋势,而最晚播种期有延后趋势。在同一年份内,播期不同旱稻的产量也有一定的变化,呈现为先升高而后降低的趋势。播期过早或过晚导致生育期平均温度偏低是影响穗干物质累积且造成减产的主要原因,在适宜的播期范围内才能获得高产。以90%-100%当年最高产量潜力作为适宜播期的产量指标,确定北京地区旱稻297的适宜播期变化在5月11日-5月19日之间,相应的产量变化在6689-7257 kg/hm2范围内。研究方法可为其他地区旱稻的播期研究提供借鉴。     

The parasitism of Striga hermonthica Benth. on Sorghum spp. under irrigation: I. Preliminary results and the effect of heavy and light irrigation on Striga attack

Investigations were carried out at the Gezira Research Farm in the Anglo-Egyptian Sudan. The greater majority of the seeds of Striga hermonthica can only be germinated by excretions from roots of certain plants not all of which can act as hosts for this parasite. Unless it becomes attached to a host plant, the Striga seedling dies. The growing of those plants whose roots can stimulate the germination of Striga seed but cannot be parasitized by it, may be a means of ridding infested land of this parasite. Green ovaries picked from flowering plants produce viable seeds if left to dry. Sorghum is sown in the field during the rainy season, viz. June to October: the earlier the sowing date within this period the greater is the Striga attack. In the field Striga seeds are distributed in the soil to a depth of at least 15 in. When Striga seeds are evenly distributed through the soil, the number of Striga seedlings attached to a sorghum root is proportional to the root's development. Size of sorghum seed has no effect on the root size of a sorghum plant and consequently no effect on the degree of parasitism. The effect of severe Striga attack on the sorghum plant is to produce a reduction of about 60% in leaf and root weight. No reduction of Striga attack is obtained when various micro-elements are coated on sorghum seeds before sowing. Field and laboratory experiments show that light irrigation of the sorghum crop during the normal sowing period increases the Striga attack and heavy irrigation decreases it. This result was not obtained in laboratory experiments when sorghum was sown out of season. Striga attack is lessened when conditions favouring growth of the sorghum crop are improved.     

Effects of temperature on the development and growth of winter wheat roots

Winter wheat was sown on 2 dates with 3 levels of nitrogen fiertiliser (0, 50 and 200 kg N ha⁻¹) in one year and on 2 sites in a followign season. Shoot and root development and growth were measured between emergence and anthesis in the first season and emergence and 7 mainstem leaves in the second. Differences in temperature and light regime led to significant differences in shoot and root development and growth between sowing dates. A thermal time-scale, based on soil surface or air temperatures, with a base of 0°C, adequately described the production of mainstem leaves and nodal root axes over all treatments. Autumn applied nitrogen had little effect on development. Shoot growth and green area index increased exponentially with thermal time prior to spring nitrogen application and the completion of canopy development. Early-sown crops had larger root systems than late-sown crops prior to winter and this divergence was retained until anthesis. The relationship between root growth and thermal time was little better than with days after sowing and was not improved by either varying the site of temperature measurement or the base temperature used for calculation. Differences in soil texture and drainage, between sites, led to significant changes in root length distribution. Although spring applied nitrogen generally increased root length, its effects were inconsistent. There was a curvilinear relation between root length and the amount of photosynthetically active radiation (PAR) intercepted; this relation was unaffected by sowing date or nitrogen treatment. The amount of root produced per unit PAR decreased as the season progressed, reflecting the decrease in the proportion of total dry matter partitioned to the root system.