Potential Dry Matter and Water Import Rates in the Tomato Fruit in Relationship to Fruit Size

The tomato fruit was compared to a sphere with a radius R. Radialgrowth rates in the fruit (F_IW and F_ID) due to water importor to dry matter import, respectively, which are also the waterimport rate or dry matter import rate per unit surface areaof fruit, were calculated from two sets of published results.This data referred to fruits which swelled in such a way thatthe availability of assimilates had little effect on growth.Two varieties differentiated the two series of results and inone series, three trials were differentiated by the salinityof the nutrient solution. In all trials, it was found that F_IW and F_ID decreased whenR increased. Two phases were observed for F_IW: after a firstphase, F_IW decreased more quickly and almost linearly when Rincreased. F_ID was constant or decreased with respect to R.Except at the beginning of growth at the greatest salinity,there were clearly linear regressions between F_IW and F_ID suchas F_ID = aF_IW-b; where b was lower with higher salinity. Thechanges of the concentration of imported dry matter (F_ID/F_IWwere examined in terms of R and F_IW/R. The mechanisms controllingthe changes in F_IW and F_ID were discussed. The results suggestedfruit radius was an important parameter of these mechanisms.Thus, water import rate and dry matter import rate could eachbe considered to be the product of two factors: fruit surfacearea, which is directly dependent on fruit radius, and waterimport rate or dry matter import rate per unit of fruit surfacearea.Copyright 1993, 1999 Academic Press Dry matter, fruit growth, logistic model, sink size, tomato, water transfer     

Hybridization of Senecio squalidus and S. viscosus and Introgression of Genes from Diploid into Tetraploid Senecio Species

Natural triploid hybrids (Senecio x londinensis Lousley) betweenS. squalidus L. (2n = 20) and 5. viscosus L. (2n = 40) are fairlyfrequently found in Britain. Under glasshouse conditions bothnatural and artificial hybrids displayed very low levels ofseed fertility and gave rise to morphologically diverse F₂ plantsat about the triploid or pentaploid chromosome levels. By theF₄ generation, progeny of a F₂ pentaploid plant had somaticchromosome numbers near to the tetraploid level and considerablyincreased pollen and seed fertilities. Such fertile tetraploidsegregants of S. x londinensis permit the introgression of S.squalidus genes into S. viscosus, and may indicate the courseof introgression into other tetraploid species of Senecio. Senecio, hybridization, introgression     

Variations with Age in Net Assimilation Rate and other Growth Attributes of Sugar-beet, Potato, and Barley in a Controlled Environment: With two Figures in the Text

Sugar-beet, potato, and barley plants were grown in a controlledenvironment, for periods of up to 10 weeks from sowing, witha light intensity of 1,8oo f.c. (4·9 cal./cm.²/hr.) anda temperature of 20° C. during the 18-hour photoperiod and15° C. during the dark period, to test whether net assimilationrate varied with age and differed between the three species. Net assimilation rate of all species based on leaf area (E_A)fell approximately linearly with time. During 5 weeks E_A ofsugar-beet decreased by only about 20 per cent. and E_A of potatodecreased by 50 per cent. E_A of barley remained approximatelyconstant for 4 weeks after sowing and was halved during thesubsequent 4 weeks. The average value of E_A for all times wasgreatest for sugarbeet and least for barley. Net assimilation rates based on leaf weight (E_W) and leaf N(E_N) decreased at about 15 per cent. of the initial value perweek for all species; this was similar to the mean rate of decreaseof E_A of potato and barley, but greater than that of E_A of sugar-beet.Mean values of E_W or E_N for potato and barley were similar andless than for sugar-beet. Relative growth rate (R_W), relative leaf growth-rate (R_A), andleaf-area ratio (F) fell with time at similar rates for allspecies. Average values of R_W decreased and of F increased inthe order sugar-beet, potato, barley. R_A was greatest for potatoand least for barley.     

The Heritability of Abscisic Acid Accumulation in Water-stressed Leaves of Pearl Millet [Pennisetum americanum (L.) Leeke]

Using detached leaves, two cultivars of pearl millet [Pennisetumamericanum (L.) Leeke], B282 and Serere 39, were assessed forvariation in the capacity to accumulate ABA in response to waterstress. Significant differences in ABA accumulation were detectedbetween cultivars and between different inbred lines withina cultivar, but within lines there was much less variation inthis character. In crosses between individual lines of B282(low ABA) and Serere 39 (high ABA), ABA accumulation in theF₁ was mid-way between parental values, indicating additivegenetic control and lack of dominance. Selfed progeny of a B282 x Serere 39 cross were selected forcontrasting ABA accumulation in the F₂ to F₄ generations. Asixfold range in ABA accumulation was found amongst 207 F₂ progeny.This increased to nearly ninefold at F₃ and F₄. Regression analysisindicated high heritability of ABA accumulation and rapid approachto homozygosity. As the cross studied involved a dwarf (B282) and a tall (Serere39) parent, segregation occurred for height as well as for ABA,though not entirely independently. Tall F₃ progeny had significantlyhigher ABA contents than dwarf progeny and high ABA was thereforeassociated with other traits (e.g. large leaves, high leaf percent d. wt) characteristic of tall plants. Nevertheless, therewas a substantial range of ABA content within both groups whichwas uncorrelated with height and other characters. The potential use of the selections in studies on drought responseis briefly discussed. Pennisetum americanum (L.), Leeke, pearl millet, abscisic acid accumulation, water stress, genetic differences, inheritance     

Genetic Analysis of Growth in Tomato: Segregating Generations

Genetical control of certain aspects of plant development hasbeen studied in the segregating generations derived from a diallelcross involving three varieties of L. esculentum and one ofL. pimpinellifolium. In the first experiment, dry-weight relativegrowth-rate was found to be inherited additively with a largedominance component which, however, was probably overestimatedin the F₁ by the inclusion of only one type of reciprocal cross.Gene association in the parents was suggested by the distributionof the points in the diallel regression graph. In a second experiment,the increase in leaf number with time was followed in parental,F₁, F₂, F₃, and backcross generations. Genetical analysis forvarious parameters of rates of production of leaves showed thecontrol to be additive with various degrees of dominance, andsimilar results were found for analysis of results on time-to-floweringand fruit yields. There was, throughout the experiments, agreementbetween the results for each generation. It is believed thatcombined genetical and physiological studies of growth in cropplants will be of value in the selection of parents for breedingimproved varieties.     

Inhibition of growth and photosynthesis of freshwater phytoplankton by ultraviolet A (UVA) radiation and subsequent recovery from stress

The effect of ultraviolet A (UVA) on growth and photosyntheticrate was studied in diatoms (Melosira spp.) of the phytoplanktonof a eutrophic lake and a cultured green alga Chloretla ellipsoidea.The cells were incubated under photosynthetically active radiation(PAR) (–UVA) or PAR + UVA conditions (+UVA). Growth ofC.ellipsoidea was retarded under +UVA, as shown by an increasein the lag period, but the rate of exponential growth was almostthe same in + and –UVA conditions. The photosyntheticrate was depressed markedly by UVA in Chlorella cells grownunder –UVA. In contrast, cells grown in +UVA showed onlyslight inhibition by UVA and after exposure to UVA for 6 daysthere was no inhibition. During the growth experiment, the cellularchlorophyll a content was higher in +UVA than +UVA grown cells.A similar effect was observed in diatoms from the eutrophicLake Suwa. In vivo fluorescence with (F_a) and without 3-(3,4-dichloropheny)-l,l-dimethylurea (DCMU) (F_b) and the photosynthetic rate were measured forC.ellipsoidea and the diatoms for 5 h under + and –UVAconditions. Soon after C.ellipsoidea had been subjected to +UVA,F_b and F_a / F_b decreased quickly and reached minima after 40min and 1 h, respectively. The suppressed in vivo fluorescenceresumed and full recovery was achieved after 4 h. This suggeststhat reactivation of the photosystem is acquired under prolongedexposure to UVA. A similar shift of F_a + F_b, but no change inFb, was found in diatoms by exposure to UVA. Changes in photosyntheticoxygen evolution by C.ellipsoidea under +UVA were similar tochanges in F_a + F_b. Degradation of chlorophyll a extracted inmethanol was enhanced by UVA. The rate of degradation by UVAwas independent of temperature from 15 to 34°C, suggestinga photochemical reaction. The results indicate that C.ellipsoideaand Melosira spp. acclimatize to prolonged UVA exposure by reactivationof the photosystem and enhanced cellular chlorophyll a synthesis.The ecological importance of these results to phytoplanktonproductivity in natural aquatic environments is discussed.     

Expression of the nuclear gene TaF(A)d is under mitochondrial retrograde regulation in anthers of male sterile wheat plants with timopheevii cytoplasm

Alterations of mitochondrial-encoded subunits of the F_oF₁-ATPsynthase are frequently associated with cytoplasmic male sterility(CMS) in plants; however, little is known about the relationshipof the nuclear encoded subunits of this enzyme with CMS. Inthe present study, the full cDNA of the gene TaF_Ad that encodesthe putative F_Ad subunit of the F_oF₁-ATP synthase was isolatedfrom the wheat (Triticum aestivum) fertility restorer ‘2114’for timopheevii cytoplasm-based CMS. The deduced 238 amino acidpolypeptide is highly similar to its counterparts in dicotsand other monocots but has low homology to its mammalian equivalents.TaF_Ad is a single copy gene in wheat and maps to the short armof the group 6 chromosomes. Transient expression of the TaF_Ad–GFPfusion in onion epidermal cells demonstrated TaF_Ad's mitochondriallocation. TaF_Ad was expressed abundantly in stem, leaf, anther,and ovary tissues of 2114. Nevertheless, its expression wasrepressed in anthers of CMS plants with timopheevii cytoplasm.Genic male sterility did not affect its expression in anthers.The expression of the nuclear gene encoding the 20 kDa subunitof F_o was down-regulated in a manner similar to TaF_Ad in theT-CMS anthers while that of genes encoding the 6 kDa subunitof F_o and the subunit of F₁ was unaffected. These observationsimplied that TaF_Ad is under mitochondrial retrograde regulationin the anthers of CMS plants with timopheevii cytoplasm. Key words: CMS, F_Ad subunit, F_oF₁-ATP synthase, retrograde regulation, wheat Received 8 October 2007; Revised 9 January 2008 Accepted 28 January 2008     

Force treadmill for measuring vertical and horizontal ground reaction forces

Weconstructed a force treadmill to measure the vertical, horizontal andlateral components of the ground-reaction forces (F_z,F_y,F_x, respectively) and the ground-reaction force moments(M_z,M_y,M_x), respectively exerted bywalking and running humans. The chassis of a custom-built, lightweight(90 kg), mechanically stiff treadmill was supported along its length bya large commercial force platform. The natural frequencies of vibrationwere >178 Hz for F_z and >87Hz for F_y, i.e., well above thesignal content of these ground-reaction forces. Mechanical tests andcomparisons with data obtained from a force platform runway indicatedthat the force treadmill recordedF_z,F_y,M_x andM_y ground-reaction forces andmoments accurately. Although the lowest natural frequency of vibrationwas 88 Hz for F_x, thesignal-to-noise ratios for F_x andM_z were unacceptable. This devicegreatly decreases the time and laboratory space required for locomotionexperiments and clinical evaluations. The modular design allows forindependent use of both treadmill and force platform.

     

Water relations and hydraulic architecture of woody hemiepiphytes

Several parameters related to the water relations of eight woodyhemiepiphytes with different photosynthetic pathways were studiedin situ and in the laboratory on Barro Colorado Island, Panama.As a group, woody hemiepiphytes tended to have less conductivestems per unit leaf area (lower k_L) and invested less wood cross-sectionper unit leaf area compared to free-standing trees, while theirspecific conductivity (K_s) was significantly higher. Among hemiepiphytes,there were significant differences between C₃ and CAM (CrassulaceanAcid Metabolism) species in respect to leaf characteristics,transpiration rates, diel patterns of water flow through aerialroots, and in hydraulic architecture parameters. Average transpirationrates of the two Clusia species (C₃-CAM) were lower by aboutan order of magnitude compared to the C₃ species. In all C₃species, sap flow through aerial roots (F) closely followedtranspiration (E), whereas E and F were decoupled in time inthe C₃-CAM species Clusia uvitana: considerable long-distancewater flow occurred at night. The hydraulic efficiency of theother C₃-CAM species, C. minor was the lowest of the five speciesinvestigated. Key words: Hemiepiphytes, water relations, hydraulic architecture, aerial roots, CAM     

Mitochondrial Complementation and Grain Yield in Hybrid Wheat

ADP/O ratios, cytochrome c oxidase and adenosine triphosphatasehave been measured in mitochondria and mixtures of mitochondriaisolated from two day-old shoots of wheat of known F₁ hybridgrain yield performance. Mixtures of mitochondria from two varieties,Peko and Cappelle-Desprez, which have considerable F₁ hybridyield heterosis, showed a significantly increased ADP/O ratioover the mean value for mitochondria from the varieties assayedindividually, i.e. these varieties showed ‘mitochondrialcomplementation’. No mitochondrial complementation wasdetected for cytochrome c oxidase or adenosine triphosphatase.In other mitochondrial mixtures no complementation in ADP/Oratios were found even when the varieties showed F₁ hybrid yieldheterosis. Mitochondrial ADP/O ratios were studied in six varietiesindividually and in mixtures. In only one mixture was any significantcomplementation detected. However, when all the results wereconsidered together, mitochondrial complementation was significantlycorrelated with F₁ hybrid grain yield heterosis when the plantswere grown at a low seed density but not at a high seed density.New hypotheses are offered to account for mitochondrial complementationand its statistical relationship with yield heterosis.     

黄腹山雀的鸣唱特征分析

2006年4—5月及2007年6月,在北京小龙门林区录制了黄腹山雀(Parus venustulus)的鸣唱,利用Avisoft-SASLab Pro鸟声声谱分析软件(德国)测量鸣唱特征参数后进行统计分析,发现该地区的黄腹山雀种群鸣唱句法简单,鸣唱句子均为相同音节的不断重复。所采集的音节曲目中包含了56种音节型,音节类型数与采样个体数(r=0.973,P=0.000<0.05)呈显著正相关,不同个体的领域性鸣唱存在显著差异。与同域分布的大山雀、褐头山雀、煤山雀、沼泽山雀相比较,黄腹山雀的鸣唱句子最短(Dv=0.83±0.48),频率较高(Fmax=7.64±1.01,Fmin=3.27±1.13),句子中音节的重复次数最少(Ns.v=2.0±0.2)。     

Mast cell activation is not required for induction of airway hyperresponsiveness by ozone in mice

Exposure to ambient ozone(O₃) is associated withincreased exacerbations of asthma. We sought to determine whether mastcell degranulation is induced by in vivo exposure toO₃ in mice and whether mast cellsplay an essential role in the development of pulmonarypathophysiological alterations induced byO₃. For this we exposed mastcell-deficientWBB6F₁-kit^W/kit^W-v(kit^W/kit^W-v)mice and the congenic normalWBB6F₁ (+/+) mice to air or to 1 or 3 parts/million O₃ for 4 h andstudied them at different intervals from 4 to 72 h later. We foundevidence of O₃-induced cutaneous,as well as bronchial, mast cell degranulation. Polymorphonuclear cellinflux into the pulmonary parenchyma was observed after exposure to 1 part/milllion O₃ only in mice thatpossessed mast cells. Airway hyperresponsiveness to intravenousmethacholine measured in vivo under pentobarbital anesthesia wasobserved in bothkit^W/kit^W-vand +/+ mice after exposure to O₃.Thus, although mast cells are activated in vivo byO₃ and participate inO₃-induced polymorphonuclear cellinfiltration into the pulmonary parenchyma, they do not participate detectably in the development ofO₃-induced airwayhyperresponsiveness in mice.

     

Dependence of Yields of Wheat Varieties on their Leaf Area Durations

In a field experiment three wheat varieties: autumn-sown CappelleDesprez, Prestige, both autumn-sown and spring-sown, and spring-sownJufy I, each supplied with 0.5 or 1.0 cwt nitrogen/acre (6₃or 126 kg/ha), had grain yields nearly proportional to theirLeaf Area Durations (D) during grain development. Squarehead'sMaster had a smaller grain yield relative to its total D, butwith 0.5 cwt N/acre the ratio of its grain yield to D of partsabove the flag leaf node (D_F) was similar to the other varieties.The difference in this ratio between varieties was less whenD_Fwas calculated from anthesis than from ear emergence to ripening.Squarehead’s Master with 1.0 cwt N/acre had a smallerratio of grain yield to D_F than other treatments, implying lessgrain per ear relative to D_Y per shoot, perhaps because of lodging,or because factors limiting size of ears restricted their abilityto accept all the assimilate the shoots could produce with thisamount of N.     

Effect of Potato-2 Medium on Anther Culture of Interspecific Rice Hybrids

The effect of two culture media, potato-2 and N₆ supplementedwith kinetin and either 2,4-dichlorophenoxyacetic acid (2,4-D)or -naphthalene acetic acid (NAA) on anther culture responseof two interspecific rice hybrids was studied. While calluscould be successfully induced and plants regenerated from theF₁ of O. saliva x O. rufipogon, the other hybrid, O. salivax O. longistaminala did not respond to the anther culture. Nevertheless,some success in callus induction was achieved when anthers froma few selected F₂ plants were cultured from the latter cross.No interaction effects between the media (potato-2, N₆ and growthhormones (2,4-D and NAA) for anther response to callusing wereobserved. Potato-2 medium proved to be superior to N₆ in termsof increased anther response, early callus induction, multiplecalli formation and also overall green plant regeneration Oryza saliva L., O. rufipogon Griff., O. longistaminata A. Chev. et Roehr, interspecific hybrid, anther culture, potato-2 medium, N₆ medium     

Subunit Proteins of Photosystem I

Photosystem I (PS I) is a supramolecular complex in thylakoidmembranes and mediates the light-driven electron flow from plastocyanin(or cytochrome c₅₅₃) to ferredoxin. It has been establishedthat the PS I complex consists of more than 10 different subunitproteins that ligate 100 to 200 molecules of chlorophylls includingP700, two molecules of phylloquinone and three iron-sulfur centers(F_X, F_A, F_B). The identity and properties of these PS I subunitproteins have been extensively studied, and their genes haverecently been cloned and mutagenized. The current status ofthese investigations is summarized. (Received May 8, 1992; )     

Cl- currents activated via purinergic receptors in Xenopus follicles

Ionic currents elicited via purinergic receptors located in themembrane of Xenopus follicles werestudied using electrophysiological techniques. Follicles responded toATP-activating inward currents with a fast time course(F_in). InRinger solution, reversal potential (E_rev) ofF_in was 22mV, which did not change with external substitutions ofNa⁺ orK⁺, whereas solutions containing50 or 5% of normal Clconcentration shiftedE_rev to about +4and +60 mV, respectively, and decreasedF_in amplitude,indicating thatF_in was carriedby Cl.F_in had an onsetdelay of ~400 ms, measured by application of a brief jet of ATP froma micropipette positioned near the follicle (50 µm).F_in was inhibitedby 50% in follicles pretreated with pertussis toxin. This suggests a Gprotein-mediated receptor channel pathway.F_in was mimickedby 2-MeSATP and UTP, the potency order (half-maximal effectiveconcentration) was 2-MeSATP (194 nM) > UTP (454 nM) > ATP(1,086 nM). All agonists generatedCl currents and displayedcross-inhibition on the others.F_in activation byacetylcholine also cross-inhibitedF_in-ATPresponses, suggesting that all act on a common channel-activationpathway.

     

Effects of Seasonal Variation in Radiation and Temperature on Net Assimilation and Growth Rates in an Arid Climate

The net assimilation rate (E_A), relative growth-rate (R_w), andleaf-area ratio (F_A) were measured for rape (Brassica napus),sunflower (Hetianthus annuus), and maize (Zea mays) at varioustimes of year in an arid climate, using young plants grown widelyspaced on nutrient culture. Multiple regression analysis accountedfor 90–95 per cent of the variation in E_A and R_W in termsof two climatic variables: mean temperature and radiation receipt. E_A rose linearly with radiation in all three species; increasein E_A with temperature was greatest in maize and least (notsignificant) in rape. R_Wrose with radiation and temperature,the latter being the more important variable especially in coolweather; a temperature optimum was shown at 24° C in rape.F_A rose with increase in temperature or decrease in radiation;its variation was due to change in leaf area/leaf weight ratherthan in leaf weight/plant weight. Multiple regression analyses can lead to faulty interpretationif the independent variables are correlated (as are climaticvariables in nature), but conclusions can be checked by controlled-environmentstudies in which climatic factors are not correlated. The presentconclusions are supported by such studies. The regression equations, coupled with average weather records,indicate seasonal cycles of growth parameters. E_A is maximalnear midsummer and minimal near midwinter, following the radiationcycle. Maxima and minima in R_W are about a month later, becauseR_W is affected by the temperature cycle and this lags behindthe radiation cycle. F_A is maximal in autumn and minimal inspring. E_A is highest where radiation receipts near 750 cal cm^–2day^–1 coincide with high temperatures. This combinationoccurs only in clear midsummer weather at low latitudes, andis maintained over long periods only in arid regions. The fact that E_A rose linearly with radiation suggests thatleaf water deficits arising under high radiation had littleeffect on E_A and that saturating levels of light were very high.     

An Analysis of Growth of Oil Palm Seedlings in Full Daylight and in Shade

Growth of seedling oil palms (Elaeis guineensis), in full daylightand under three levels of shade, was studied using growth analysistechniques. In full day-light, net assimilation rates (E_A) betweeno.15 and 0.32 g./dm.²/week were obtained associated with lowleaf-area ratios (F) giving relative growth-rates (R_W) rangingfromI I.8 to 3.2 per cent, per day. There were no indicationsof seasonal differences within the small range of values found. The plants take about 90 days to adapt to shade conditions becausethe mean plastochron is 24 days, and shading effects are beststudied on plants grown since germination under the shade treatments.Very different pictures of response to shade were obtained usingplants grown initially in full light followed by 90 days' shadebefore sampling compared with plants grown under shade sincegermination. In the latter, except at the lowest light levelused, I I.I per cent, of full day-light, there was very littleeffect of light on E_A or R_w, although the F values decreasedas light increased. Extrapolation of the F values to the extinctionpoint gave values similar to those obtained in another experimenton the effect of a number of shade levels on F. The physiological and ecological implications of these findings,particularly the low growth-rates and shade tolerance, are discussed.     

Differences in Rehydration of Three Desiccation-tolerant Angiosperm Species

The rehydration characteristics of the desiccation-tolerantplantsCraterostigma wilmsii andMyrothamnus flabellifolia (homoiochlorophyllous)andXerophyta viscosa (poikilochlorophyllous) were studied todetermine differences among them. A desiccation-sensitive plant(Pisum sativum) was used as a control. Recovery of water content,quantum efficiency (F_V/F_M), photosynthetic pigments and chloroplastultrastructure as well as damage to the plasmamembrane werestudied. P. sativum did not recover after desiccation and considerabledamage occurred during rehydration. The desiccation-tolerantplants appeared to differ in their responses to dehydrationand rehydration. The small herbaceousC. wilmsii generally showedlittle damage in the dry state and recovered faster than theother tolerant species.M. flabellifolia took longer to recoverthanC. wilmsii probably due to the presence of a woody stemin which dehydration-induced xylem embolisms slowed the rateof recovery. The poikilochlorophyllous speciesX. viscosa tookthe longest to recover because it took longer to reconstitutethe chloroplasts and the photosynthetic pigments. Quantum efficiencyrecovered in all species before water content and chlorophyllcontent recovered to control levels. The significance of thesedifferent responses to desiccation and recovery from desiccationis discussed. Desiccation-tolerant; F_V/F_M; homoiochlorophyllous; poikilochlorophyllous; chlorophyll; chloroplast; ultrastructure; Craterostigma wilmsii ; Myrothamnus flabellifolia ; Xerophyta viscosa ;Pisum sativum     

Genetic Variation in Sorghum for the Inhibition of Maize Pollen Tube Growth

Aniline blue fluorescence was used to study the growth of maizepollen tubes in the stigmas of 13 diverse sorghum accessions.In 12, only short maize pollen tubes were formed, but in thesingle exception (Sorghum nervosum Nr481) maize pollen tubesgrew at least as far as the base of the style. The S. bicolorgenotypes S9B and CMS (a cytoplasmic male sterile line) werehybridized with Nr481, and analysis of maize pollen tube growthin F₁ plants, and BC₁ plants using Nr481 as the recurrent parent,suggested that differences in inhibition of pollen tube growthwere due to variation at a single locus, which we propose todesignate lap (Inhibition of alien pollen tubes). AccessionNr481 appears to be homozygous for a recessive allele permittingmaize pollen tube growth. Attempts were made to produce sorghumx maize hybrids using Nr481 and CMS derivatives which were knownto allow maize pollen tube growth to the base of the style.A putative hybrid endosperm was obtained in one Nr481 x Seneca60 maize cross, but this was not repeatable and no hybrid plantswere produced. A fundamental problem may be the large size ofthe maize pollen tube, which could have difficulty growing throughthe sorghum ovary and in entering the micropyle. Sorghum bicolor spp. bicolor (L.) Moench, Zea mays L, sorghum, maize, pollen tube growth, hybridization barriers