Effects of NORIN 10 and Tom Thumb Dwarfing Genes on Morphology, Physiology and Abscisic Acid Production in Wheat

The pleiotropic effects of three genetically related dwarfinggenes were investigated in near-isogenic lines of wheat. TheNORIN 10 semi-dwarfing alleles, Rht 1 and Rht 2, and the TomThumb allele, Rht 3, were assessed for effects on some vegetativemorphological and physiological characters. The Rht allelesaffected leaf size with a resultant decrease in leaf area ofthe whole plant. Rht 3, which had the most marked effects, reducedleaf area in young plants by as much as 30 per cent. Althoughflag leaf dimensions and stomatal distributions of the flagleaf were altered, the gene had no effect on its area, stomatalconductance or net CO₂ exchange rate. Comparisons of Rht andtall plants revealed no differences in the abscisic acid (ABA)levels of either turgid or partially dehydrated leaves. Triticum aestivum L., wheat, dwarfing genes, leaf structure, abscisic acid, stomatal conductance, CO₂, exchange, relative growth rate     

The Spatial Distribution of Growth in the Extension Zone of Seedling Wheat Leaves

Two models of the distribution of relative elemental rates ofelongation (RELEL) were tested for the extension zone (EZ) ofthe first foliage leaf of seedling wheat plants, by comparisonto patterns of separation of rings and gyres in the walls ofprotoxylem vessels. One model, containing a defined growth maximumin the basal half of the EZ, is favoured in the literature andwas derived from data published for perennial ryegrass. Theother, containing a flat, broad maximum throughout the regionof the EZ with stomates, was constructed from regressions ofinterstomatal distance against distance along the EZ in thefirst foliage leaf of wheat seedlings. The test strongly favouredthe model with the flat maximum. Although the gibberellic acid(GA) insensitivity alleles Rht1 and Rht2 reduce length of extensionzone (LEZ), leaf extension rate (LER) and final cell and leaflengths, they had no effect on maximum RELEL. Results with aninhibitor of GA synthesis indicated that control of leaf elongationby the control of LEZ may be generalizable as a mechanism bywhich GA controls LER in the grass leaf. Extension zone, elongation, gibberellic acid, Rht, wheat, Triticum aestvum L.     

Biomechanical and Morphometric Differences in Triticum aestivum Seedlings Differing in Rht Gene-Dosage

Biomechanical and morphometric comparisons among coleoptilesfrom wheat seedlings differing in Rht gene-dosage (Rht = 0,2, 4 doses) are presented in an effort to evaluate the influenceof Rht on the mechanics of soil penetration by this organ. Rhtis known to reduce seedling establishment compared to the wildtype. Data from 3–7-day-old seedlings indicate that Rhtreduces tissue elastic modulus E, increases the second momentof area I, and decreases the slenderness ratio (l/r) of coleoptiles.Rht-relatedchanges in E and I are such that the flexural stiffness of coleoptilesfrom Rht plants does not differ significantly from the wildtype-hence the growing coleoptiles of all three genotypes haveequivalent biomechanical capacity to penetrate the soil. Rhtreduction of coleoptile slenderness ratios confers a capacityto safely sustain higher axial compressive loads compared tocoleoptiles with equivalent flexural stiffness but higher ratios.However, wild type seedlings produce longer coleoptiles andlonger subcrown internodes than Rht seedlings. Longer coleoptilesdeliver the crown node closer to the top of the soil beforethe crown node extends beyond the lateral confinement of thecoleoptile. This reduces the potential for buckling of the subcrowninternode and leaves due to the compressive loading of soil.Rht affects a variety of mechanical features whose influenceis dependent upon the stage of seedling growth and the degreeof soil compaction. However, at equivalent depths of burialwhich exceed the maximum length of coleoptiles and moderatesoil compaction, Rht is biomechanically disadvantageous to seedlingestablishment. Wheat, germination, biomechanics, Rht-gene     

The Influence of the Rht1 and Rht2 Alleles on the Growth of Wheat Stems and Ears

A field experiment was carried out with a set of near-isogenicspring wheat lines (cv. Triple Dirk) to determine the influenceof the Rht₁ and Rht₂ alleles on the partitioning of dry matterbetween the developing stem and the ear. Each line was sampledtwice weekly and dissected into its component above-ground parts.The rate of change of the dry mass of the individual plant organswas expressed as a proportion of the rate of change of the totalplant dry mass. This ratio was used to assess the relative sinkstrengths of the stem and ear during crop growth. The Rht₁ andRht₂ alleles reduced plant height, but increased grain yield.The greater yield was achieved through a greater grain numberper ear in the Rhtl line, a greater ear number per plant inthe Rht₂ line, and a greater allocation of assimilate to thedeveloping ear than to the developing stem in both Rht lines,particularly at the time of maximum stem growth (17 d beforeanthesis). From the earliest stages of detectable ear growthuntil anthesis, the ear masses per unit area of the Rht₁ andRht₂ lines exceeded that of Triple Dirk (Rht). It was not possibleto determine whether the Rht₁ and Rht₂ alleles were directlyresponsible for increasing grain number per ear and ear numberper plant, respectively, since the increase in these componentsof yield could equally be explained by a greater partitioningof assimilate to developing ears and tillers caused simply bya reduction in plant height. Triticum aestivum L., wheat Rht genes, stem and ear development, dry matter partitioning, allocation ratio     

Effect of the Rht3 dwarfing gene on dynamics of cell extension in wheat leaves, and its modification by gibberellic acid and paclobutrazol

The second leaf of wheat was used as a model system to examinethe effects of the Rht3 dwarfing gene on leaf growth. Comparedto the rht3 wild type, the Rht3allele decreased final leaf length,surface area and dry mass by reducing the maximum growth rates,but without affecting growth duration. Gibberellic acid (GA₃)increased final leaf length and maximum growth rate in the rht3wild type, but was without effect on the Rht3 mutant, whichis generally regarded as being non-responsive to gibberellin(GA). Paclobutrazol, an inhibitor of GA biosynthesis, decreasedfinal leaf length and maximum growth rate in the rht3 wild typeto values similar to those in the untreated Rht3 mutant. NeitherGA₃ nor paclobutrazol affected the duration of leaf growth.The decrease in leaf length was produced by reduction of celllength rather than cell number. The maximum relative elementalgrowth rate (REGR) for cell extension was essentially the samein all treatments, as was the time between the cells leavingthe meristem and achieving maximum extension rate. The differencesbetween the genotypes and treatments were all almost entirelydue to differences in the time taken from the attainment ofmaximum REGR of cell extension to the cessation of extension.This was reflected in the length of the extension zone, whichwas approximately 6–8 per cent of final leaf length. Theeffects of the Rht3 allele, GA₃ and paclobutrazol all appearto be on the processes which promote the cessation of cell elongation. Key words: Cell extension, gibberellin, leaf growth, Rht3 gene, Triticum, wheat     

Gibberellic Acid Sensitivity Determines the Length of the Extension Zone in Wheat Leaves

To test the hypothesis that gibberellic acid (GA) sensitivityaffects the length of the extension zone (LEZ) of leaf No. 1of wheat seedlings, we performed a gene dosage experiment usingRht dwarfing genes that condition GA insensitivity. We utilizednearly isogenic lines, at Rht-dosage levels of 0, 2 and 4 alleles.Anatomical markers (distances between successive stomates) wereused to infer the distribution of growth along the axis of theleaf. Interstomatal distance (ISD) and LEZ were inverse linearfunctions of Rht-dosage. The number of stomates matured perhour was independent of Rht-dosage. The relationship betweenISD and distance along the axis within the extension zone (EZ)was indistinguishable from linear. Rht-dosage did not affectthe slope of the regression of ISD against distance along theEZ. A-REST (AR; ancymidol, a potent GA synthesis inhibitor)reduced LEZ. Wild type was more sensitive to AR than doubledwarf. AR affected growth of leaf No. 1 more than length ofthe coleoptile, regardless of Rht-dosage. AR-dosage affectedcell division, whereas Rht-dosage did not. Extension zone, elongation, gibberellic acid, Rht, wheat, Triticum aesiivum L.     

The Net Orientation of Wall Microfibrils in the Outer Periclinal Epidermal Walls of Seedling Leaves of Wheat

Use of the dichroic stain chlor-zinc-iodine revealed that thenet orientation of cellulose wall microfibrils in the outerparadermal wall of the epidermis of seedling wheat leaves isprincipally transverse in the extension zone. The net orientationof microfibrils changes abruptly to principally longitudinalat the end of cell elongation. The net angle of orientationof microfibrils in the extension zone was not a function ofRht-dosage (number of dwarfing alleles), and neither leaf extensionrate nor estimated maximum relative elemental rate of elongationwere functions of microfibril orientation. The results indicatethat elongation rates are not regulated by the net angle oforientation of microfibrils and support the concept that leafextension rate is regulated by the length of the extension zone.Copyright1995, 1999 Academic Press Cellulose wall microfibrils, extension zone, elongation, Rht, wheat, Triticum aestivum L     

Dwarfing genes and cell dimensions in different organs of wheat

A field experiment was conducted under non-limiting water and nutritional conditions with three near-isogenic lines of spring wheat (dwarf, DD; semi-dwarf, SD and standard height, SH) to study the impact of the GA-insensitive alleles Rht1 and Rht2, at the cellular level, on the growth of different vegetative organs and of the pericarp of grains. Cell length and width of blades of different leaves (3, 7 and flag leaf), the flag-leaf sheath and the penultimate internode as well as the pericarp of basal grains from central spikelets of the spike were evaluated. With the exception of the flag leaf, dwarfing genes produced a significant reduction in cell length in all the different vegetative organs analysed. There was no effect on the number of cells nor their width. Therefore, in vegetative organs, the effects of these alleles appeared to be exclusively due to a reduction in cell length. It would appear that dwarfing genes act on cell elongation without affecting cell division.The Rht alleles did not modify cell length nor width in the pericarp. Grain weight was different between the lines and these differences were associated with grain volume at the beginning of linear grain growth. Thus, they reduced the size of individual grains by reducing the total number of cells in the pericarp.It appears that Rht alleles reduced the final sizes of vegetative organs (such as internodes and leaves) and of tissues (pericarp) associated with reproductive structures (grains), but the modes of action in these different organs were different.Keywords: Cell dimensions, plant height, Rht alleles, Triticum aestivum/wheat.      

The effects of temperature and the Rht3 dwarfing gene on growth, cell extension, and gibberellin content and responsiveness in the wheat leaf

The effects of low temperature and the Rht3 dwarfing gene onthe dynamics of cell extension in leaf 2 of wheat were examinedin relation to gibberellin (GA) content and GA-responsivenessof the extension zone. Leaf 2 of wild-type (rht3) wheat closelyresembled that of the Rht3 dwarf mutant when seedlings weregrown at 10C. The maximum relative elemental growth rate (REGR)within the extension zone in both genotypes was lower at 10Cthan at 20C, but the position with respect to the leaf basewas unaffected by temperature. The size of the extension zoneand epidermal cell lengths were similar in both genotypes at10C. Growth at 20C, instead of 10C, increased the lengthof the extension zone beyond the point of maximum REGR in thewild type, but not in the Rht3 mutant. Increasing temperatureresulted in longer epidermal cells in the wild type. Treatingwild-type plants at 10C with gibberellic acid (GA₃) also increasedthe length of the extension zone, but the Rht3 mutant was GA-non-responsive.However, the concentrations of endogenous GA₁ and GA₃ remainedsimilar across the extension zone of wild-type plants grownat both temperatures, despite large differences in leaf growthrates. The period of accelerating REGR as cells enter the extensionzone, and the maximum REGR attained, are apparently not affectedby GA. It is proposed that GA functions as a stimulus for continuedcell extension by preventing cell maturation in the region beyondmaximum REGR and that low temperature increases the sensitivitythreshold for GA action. Key words: Cell extension, gibberellin, Rht3 dwarfing gene, temperature, wheat leaf     

Effect of Temperature on Gibberellin (GA) Responsiveness and on Endogenous GA(1) Content of Tall and Dwarf Wheat Genotypes

Near-isogenic wheat (Triticum aestivum L.) lines differing in height-reducing (Rht) alleles were used to investigate the effects of temperature on endogenous gibberellin (GA) levels and seedling growth response to applied GA₃. Sheath and lamina lengths of the first leaf were measured in GA treated and control seedlings, grown at 11, 18, and 25°C, of six Rht genotypes in each of two varietal backgrounds, cv Maris Huntsman and cv April Bearded. Endogenous GA₁ levels in the leaf extension zone of untreated seedlings were determined by gas chromatography-mass spectrometry with a deuterated internal standard in the six Maris Huntsman Rht lines grown at 10 and 25°C. Higher temperature increased leaf length considerably in the tall genotype, less so in the Rht1 and Rht2 genotypes, and had no consistent effect on the Rht1+2, Rht3 and Rht2+3 genotypes. In all genotypes, endogenous GA₁ was higher at 25°C than at 10°C. At 10°C the endogenous GA₁ was at a similar level in all the genotypes (except Rht2+3). At 25°C it increased 1.6-fold in the tall genotype, 3-fold in Rht1 and Rht2, 6-fold in Rht3, and 9-fold in Rht1+2. Likewise, the genotypic differences in leaf length were very conspicuous at 25°C, but were only slight and often unsignificant at 11°C. The response of leaf length to applied GA₃ in the Rht1, Rht2, and Rht1+2 genotypes increased significantly with lowering of temperature. These results suggest the possibility that the temperature effect on leaf elongation is mediated through its effect on the level of endogenous GA₁ and that leaf elongation response to endogenous or applied GAs is restricted by the upper limits set by the different Rht alleles.     

The Influence of the Rht1 and Rht2 Alleles on the Deposition and Use of Stem Reserves in Wheat

A field experiment was undertaken with a set of near-isogenicspring wheat lines (cv. Triple Dirk) to determine the influenceof the Rht₁ and Rht₂ alleles on the deposition of carbon inthe stem, and the subsequent use of these reserves during graingrowth. The amount of dry matter stored and mobilized was estimatedby the measurement of changes in masses of stem from frequentharvests. Deposition or absolute reserve was defined as thesum of the increments in mass in each segment of the large culmbetween the time that the segment ceased extending and the timethat it reached maximum mass. The incorporation of the Rht₁and Rht₂ alleles into a Triple Dirk background reduced the absoluteamount of stored carbon in the stem by 35 and 39%, respectively.This was a consequence of the 21% reduction of stem height inRht₁ and Rht₂ lines. Use or mobilization of reserve was definedas the sum of the decrements in mass in each segment of thelarge culm between maximum and maturity. The alleles did notconfer an ability to mobilize more of the stored stem reservesin absolute terms, although the efficiency of use of stem reserves(i.e. use as a proportion of deposition) was higher in Rht₁than in rht or Rht₂ . The possible contribution of stored carbonin the stem to final grain yield was estimated to be 22, 18and 14% in the rht, Rht₁ and Rht₂ lines. In these estimates,the loss of mass was adjusted by 33% to allow for respiration.It was concluded that the larger stem reserves in rht wheatsare of no real advantage under favourable environmental conditions,and may in fact be a disadvantage if the accumulation of thatextra dry matter results in a reduction of sink size.Copyright1993, 1999 Academic Press Triticum aestivum L., Rht genes, stem reserves, deposition, mobilization, grain growth     

Gibberellins and leaf expansion in near-isogenic wheat lines containing Rht1 and Rht3 dwarfing alleles

In near-isogenic lines of winter wheat (Triticum aestivum L. cv. Maris Huntsman) grown at 20° C under long days the reduced-height genes, Rht1 (semi-dwarf) and Rht3 (dwarf) reduced the rate of extension of leaf 2 by 12% and 52%, respectively, compared with corresponding rht (tall) lines. Lowering the growing temperature from 20° to 10° C reduced the rate of linear extension of leaf 2 by 2.5-fold (60% reduction) in the rht3 line but by only 1.6-fold (36% reduction) in the Rht3 line. For both genotypes, the duration of leaf expansion was greater at the lower temperature so that final leaf length was reduced by only 35% in the rht3 line and was similar in the Rht3 line at both temperatures. Seedlings of the rht3 (tall) line growing at 20° C responded positively to root-applied gibberellin A₁ (GA₁) in the range 1–10 μM GA₁; there was a linear increase in sheath length of leaf 1 whereas the Rht3 (dwarf) line remained unresponsive. Gibberellins A_{1, 3, 4, 8, 19, 20, 29, 34, 44} and ₅₃ were identified by full-scan gas chromatography-mass spectrometry in aseptically grown 4-d-old shoots of the Rht3 line. In 12-d-old seedlings grown at 20° C, there were fourfold and 24-fold increases in the concentration of GA₁ in the leaf expansion zone of Rht1 and Rht3 lines, respectively, compared with corresponding rht lines. Although GA₃ was present at a similar level to GA₁ in the rht3 (tall) line it accumulated only fivefold in the Rht3 (dwarf) line. The steady-state pool sizes of endogenous GAs were GA₁₉ ? GA₂₀ = GA₁ in the GA-responsive rht3 line whereas in the GA non-responsive Rht3 line the content of GA₁₉≈ GA₂₀ ? GA₁. It is proposed that one of the consequences of GA₁ action is suppression of GA₁₉-oxidase activity such that the conversion of GA₁₉ to GA₂₀ becomes a rate-limiting step on the pathway to GA₁ in GA-responsive lines. In the GA-non-responsive Rht lines it is suggested that GA₁₉ oxidase is not downregulated to the same extent and GA₁ accumulates before the next rate-limiting step on the pathway, its 2β-hydroxylation to GA₈. The steady-state pool sizes of GA_{19, 20, 1, 3} and ₈ were similar in developmentally equivalent tissues of the rht3 (tall) line growing at 10° C and 20° C despite a 2.5-fold difference in the rate of leaf expansion. In contrast, in the Rht3 (dwarf) line, the extent of accumulation of GA₁ reflected the severity of the phenotype at the two temperatures with slower growing tissues accumulating less, not more, GA₁. These results are interpreted as supporting the proposed model of regulation of the GA-biosynthetic pathway rather than previous suggestions that GA₁ accumulates in GA-insensitive dwarfs as a consequence of reduced growth rates.     

GA-Responsive Dwarfing Gene Rht12 Affects the Developmental and Agronomic Traits in Common Bread Wheat

Opportunities exist for replacing reduced height (Rht) genes Rht-B1b and Rht-D1b with alternative dwarfing genes, such as the gibberellin-responsive gene Rht12, for bread wheat improvement. However, a comprehensive understanding of the effects and mode of action of Rht12 is lacking. In the present study, the effects of Rht12 were characterized by analyzing its effects on seeding vigour, seedling roots, leaf and stem morphology, spike development and carbohydrate assimilation and distribution. This was carried out in the four genotypes of F_2:3 lines derived from a cross between Ningchun45 and Karcagi (12) in two experiments of autumn sowing and spring sowing. Rht12 significantly decreased stem length (43%∼48% for peduncle) and leaf length (25%∼30% for flag leaf) while the thickness of the internode walls and width of the leaves were increased. Though the final plant stature was shortened (40%) by Rht12, the seedling vigour, especially coleoptile length and root traits at the seedling stage, were not affected adversely. Rht12 elongated the duration of the spike development phase, improved the proportion of spike dry weight at anthesis and significantly increased floret fertility (14%) in the autumn sowing experiment. However, Rht12 delayed anthesis date by around 5 days and even the dominant Vrn-B1 allele could not compensate this negative effect. Additionally, grain size was reduced with the ability to support spike development after anthesis decreased in Rht12 lines. Finally, grain yield was similar between the dwarf and tall lines in the autumn sowing experiment. Thus, Rht12 could substantially reduce plant height without altering seeding vigour and significantly increase spikelet fertility in the favourable autumn sowing environment. The successful utilization of Rht12 in breeding programs will require careful selection since it might delay ear emergence. Nonetheless, the potential exists for wheat improvement by using Rht12.     

Gibberellic Acid Regulates Cell Wall Extensibility in Wheat (Triticum aestivum L.)

Mutations (Rht genes) blocking sensitivity to gibberellic acid (GA) were used to examine phytohormone mediated cell wall expansion in wheat (Triticum aestivum L.). Irreversible extensibility of immature leaf segments, as determined by stress/strain (instron) measurements, declined with Rht gene dose. Exogenous GA₃ significantly increased wall extensibility in the nonmutant controls but had no effect on the near-isogenic GA-insensitive genotypes. Furthermore, ancymidol, an inhibitor of gibberellin biosynthesis, diminished wall extensibility in the nonmutant control. Extensibility of immature segments was highly correlated with mature leaf sheath length (R = +0.95). The results indicate that wall yielding properties of expanding wheat leaves are associated with leaf cell expansion potential and that GA is involved in the determination of those properties.     

Genotypic variation in the responsiveness to GA3 within tall (rht1) and semi-dwarf (Rht1) spring wheat

The effect of GA₃ on coleoptile-and first leaf elongation of tall (rht1) and semi-dwarf (Rht1) nearly-isogenic genotypes, within each of 25 random F₉ wheat families, was determined on seedlings grown in a growth room at 18 °C. Conspicuous and very significant inter-family variation in the response of the first leaf to GA₃ application was found in both the rht1 and Rht1 genotypes. The magnitudes of the response of the different families within genotypes to GA₃ were not related to the leaf length of their untreated seedlings. It is suggested that, under given environmental conditions, background genotypic effects, inducing inter-family variation in responsiveness to GA₃, regulate the elongation growth up to the limits set by the Rht alleles.     

Identification and characterization of <Emphasis Type="Italic">Rht25</Emphasis>, a locus on chromosome arm 6AS affecting wheat plant height,heading time,and spike development

Key message

This study identified Rht25, a new plant height locus on wheat chromosome arm 6AS, and characterized its pleiotropic effects on important agronomic traits.

Abstract

Understanding genes regulating wheat plant height is important to optimize harvest index and maximize grain yield. In modern wheat varieties grown under high-input conditions, the gibberellin-insensitive semi-dwarfing alleles Rht-B1b and Rht-D1b have been used extensively to confer lodging tolerance and improve harvest index. However, negative pleiotropic effects of these alleles (e.g., poor seedling emergence and reduced biomass) can cause yield losses in hot and dry environments. As part of current efforts to diversify the dwarfing alleles used in wheat breeding, we identified a quantitative trait locus (QHt.ucw-6AS) affecting plant height in the proximal region of chromosome arm 6AS (<?0.4 cM from the centromere). Using a large segregating population (~?2800 gametes) and extensive progeny tests (70–93 plants per recombinant family), we mapped QHt.ucw-6AS as a Mendelian locus to a 0.2 cM interval (144.0–148.3 Mb, IWGSC Ref Seq v1.0) and show that it is different from Rht18. QHt.ucw-6AS is officially designated as Rht25, with Rht25a representing the height-increasing allele and Rht25b the dwarfing allele. The average dwarfing effect of Rht25b was found to be approximately half of the effect observed for Rht-B1b and Rht-D1b, and the effect is greater in the presence of the height-increasing Rht-B1a and Rht-D1a alleles than in the presence of the dwarfing alleles. Rht25b is gibberellin-sensitive and shows significant pleiotropic effects on coleoptile length, heading date, spike length, spikelet number, spikelet density, and grain weight. Rht25 represents a new alternative dwarfing locus that should be evaluated for its potential to improve wheat yield in different environments.

     

Effects of Rht-dosage on the breaking strength of wheat seedling leaves

We report the effect of the reduced-height alleles Rht1 and Rht2 on the breaking strength and breaking stress of the first foliage leaves of wheat seedlings. Transverse size variables such as blade transverse area and blade width were positively related to the breaking strength and breaking stress of leaves, as was the total wall area of the principal fiber strands. A negative correlation was found for the Rht-dosage with respect to the breaking strength. Because Rht plants had greater transverse dimensions and equal amounts of principal fiber strands compared to wild type, we posit that Rht may affect the mechanical quality of leaf tissues per se. Rht may serve as an example of a simple genetic system that profoundly affects plant morphology, development, and biomechanics.     

Flag Leaf Photosynthesis of Triticum aestivum and Related Diploid and Tetraploid Species

Rates of net photosynthesis of the flag leaves of 15 genotypesof wheat and related species were measured throughout theirlife, using intact leaves on plants grown in the field. At thestage when rates were maximal, they were in general highestfor the diploid species, intermediate for the tetraploidspeciesand lowest for Triticum aestivum (means of 38, 32 and 28 mgCO₂ dm^–2 h^–1 respectively). Rates were stronglynegatively correlated with leaf area, leaf width and the meanplan area per mesophyll cell and positvely correlated with stomatalfrequency and number of veins per mm of leaf width. The differencesamong species in these attributes were mainly related to ploidylevel. It was not possible to determine the relative importanceof each anatomical feature, though the changes in stomatal frequencyhad only slight effects on stomatal conductance and the observeddifferences in rates of photosynthesis were much greater thanwould be expected from those in stomatal conductance alone. There was genetic variation in rates of light dependent oxygenevolution of isolated protoplasts and intact chloroplasts butno difference attributable to ploidy. The mean rate, 91 µmolO₂ mg^–1 chlorophyll h^–1, equivalent to 3.9 mg CO₂mg^-1chlorophyll h^-1 was considerably less than the rate of photosynthesisin comparable intact leaves, which was 7.2 mg CO₂ mg^–1chlorophyll h^–1. The total above-ground dry matter yields were least for thewild diploids T. urartu and T. thauodar and the wild tetraploidT. dicoccoides, but the other wild diploids produced as muchdry matter as the hexaploids. The prospects of exploiting differences in photosynthetic ratein the breeding of higher yielding varieties are discussed. Triticum aestivum L., wheat, Aegilops spp, photosynthesis, stomatal conductance, stomatal frequency, polyploidy     

Temperature Response of Vernalization in Wheat: A Developmental Analysis

The vernalization response of wheat ( Triticum aestivum L.)was reinterpreted from a developmental perspective, using currentconcepts of the developmental regulation of wheat morphologyand phenology. At temperatures above 0 °C, the effects ofthe process of vernalization per se in wheat are confoundedby the effects of concurrent vegetative development. These effectsare manifested by differences in the number of leaves initiatedby the shoot apex prior to floral initiation, which in turnaffects the subsequent rate of development to ear emergenceand anthesis. Leaf primordia development during vernalizationand total leaf number at flowering were used to develop criteriato define both the progress and the point of saturation of thevernalization response. These criteria were then used to reinterpretthe results of Chujo ( Proceedings of the Crop Science Societyof Japan 35 : 177–186, 1966), and derive the temperatureresponse of vernalization per se for plants grown under saturatinglong day conditions. The rate of vernalization increased linearlywith temperature between 1 and 11 °C, such that the timetaken to saturate the vernalization response decreased from70 d at 1 °C to 40 d at 11 °C. The rate declined againat temperatures above 11 °C, and 18 °C was apparentlyineffective for vernalization. Total leaf number at saturation,however, increased consistently with temperature, as a resultof the balance between the concurrent processes of leaf primordiuminitiation and vernalization. Total leaf number at saturationincreased from 6 at 1 °C to 13.3 at 15 °C, which inturn influenced the time taken to reach ear emergence. The advantagesof using this developmental interpretation of vernalizationas the basis for a mechanistic model of the vernalization responsein wheat are discussed. Triticum aestivum L.; wheat; vernalization; rate; temperature; primordia; leaf number; flowering     

The Effects of Copper Supply and Shading on Retranslocation of Copper from Mature Wheat Leaves

Plants of Gamenya wheat (Triticum aestivum L.) were grown inpots of a Cu-deficient sand at two levels of Cu (deficient andsufficient), and harvested on days 13, 22, 28 and 38. In 50per cent of the pots in each Cu treatment, the oldest leaf andleaf 2 of the main stem were shaded when they reached full expansion. The Cu content of the oldest leaf of Cu-sufficient, unshadedplants was high at day 13 and declined rapidly to day 38. Thatof Cu-deficient, unshaded plants was initially relatively lowand declined much more slowly, so that at day 38 it resembledthat of Cu-sufficient plants. Shading the oldest leaf acceleratedthe loss of its Cu in both Cu-deficient and Cu-sufficient plants.The effects of shading and of Cu supply on the loss of Cu fromthe oldest leaf paralleled their effects on the loss of N andchlorophyll. The results suggest that most of the Cu in theoldest leaf does not move out until the leaf senesces. In Cu-deficient plants retention of Cu by old green leaves accentuatedCu deficiency. The release of Cu, resulting from shading theold leaves of Cu-deficient plants, stimulated the growth ofnew leaves. In Cu-sufficient plants, shading depressed growth. copper, shading, retranslocation, wheat, Triticum aestivum L.