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.     

Diurnal Extension Rates of Wheat Leaves in Relation to Temperatures and Carbohydrate Concentrations of the Extension Zone

The objective of this experiment was to determine how diurnalvariations in rates of leaf extension of wheat plants in anirrigated field crop were related to temperatures and carbohydrateconcentrations of the extension zone. Leaves 3, 4. 5 of themain shoot were studied as each emerged from the encirclingsheath. The carbohydrates in the extension zone of the emergingleaf were analysed by converting them to glucose-6-phosphateand then measuring the reduction of NADP in the presence ofglucose-6-phosphate dehydrogenase. Average hexose concentrations(glucose and fructose) increased each day from 4 up to 5 mgg^–1 fr. wt. and sucrose from 3 up to 7 mg g^–1 fr.wt. with the maximum in mid-afternoon; there were no differencesamong the three leaves. Concentrations of fructans were constantthroughout the day for leaves 3 and 4 but showed a mid-afternoonrise in leaf 5. The average concentrations of fructans in theextension zones increased from 0 to 5 to 11 mg g^–1 fr.wt. for leaves 3, 4, and 5 respectively and was consistent withthe conclusion that there was an increasing over-supply of carbohydratesfor growth of the shoot as the plant increased in size. Ratesof leaf extension were correlated with temperature but not withhexose concentrations. We concluded that the supply of carbohydratesdid not limit the growth of leaves under field conditions buttheir utilization in leaf growth was limited by temperature.The rates of leaf extension increased exponentially with temperatureand the relationship was described by the Arrhenius equation.The Q10 at 15 °C for leaf extension was 2.7 for leaves 4and 5 and 3.2 for leaf 3.     

The Effect of Gibberellic Acid on Fibre-cell Length

Measurements were made of fibre-cells from plants of Corchorusolitorius L., Hibiscus cannabinus L., and Cannabis sativa L.Which had been sprayed with gibberellic-acid solution. Fibre-cellsfrom treated plants showed a highly significant increase inlength, 20–130 per cent, for the whole stem and as muchas 400 per cent. for a single intermode. Gibberellic acid increased the variation in cell-length andthe positive skewness of the distribution of the variate. Differences in cell-length can be related to the developmentalsequence of the shoot and the variation in internode-length.     

The Growth Rate of Wheat Leaves in Relation to the Extension Zone Sugar Concentration Manipulated by Shading

The main objective of this study was to determine the relationshipbetween the relative rate of growth of emerging wheat leavesand the hexose sugar concentration of the extension zone. Shortperiods of intense shading (to 20 or 5% of full sun for up to14 d) were used to decrease hexose concentrations. Shading decreased hexose concentrations to a fraction of thatof controls and also resulted in thin and narrow leaves thatwere less in dry weight than control leaves of the same length.Shading did however increase the length of the zone of extendingtissue at the leaf base by 30%. The effect of hexose concentrations on the relative rate ofleaf growth was evaluated by determining the ratio between growthrates of shaded and control leaves. This ratio declined as hexoseconcentrations declined and the relationship was described bya rectangular hyperbola (r > 0.95, P < 0.01). Combineddata from many leaves on the main shoot and its tillers fromtwo irrigated wheat crops all conformed to the same relationship.The hexose concentrations where the ratio of growth rates washalf the maximum rate were 0.42 mg g^–1 fr. wt. for extensiongrowth and 1.74 mg g^–1 fr. wt. for dry weight growth.These values were significantly (P < 0.01) different. These results were compared with data from emerging leaves offield crops and it was concluded that hexose concentrationshad not limited leaf growth rates, the lowest values recordedbeing 2.5–3.0 mg g^–1 fr. wt. It was further suggestedas unlikely that leaf growth rates of wheat crops in the fieldwould be limited by hexose concentrations.     

The Growth Rate of Successive Leaves of Wheat Plants in Relation to Sugar and Protein Concentrations in the Extension Zone

The growing part of a wheat leaf (the extension zone) is loocatedat the leaf base and following from this it was proposed thatthe absolute leaf extension rate (L_er) can be partitioned intotwo components: the length of the extension zone (L_ez) and therelative extension rate of that extension zone (R_ez). R_ez isan appropriate measure of the efficiency of leaf growth forcomparing different leaves. This model of a wheat leaf was thenused to investigate whether differences in growth rates betweensuccessive leaves on wheat plants were due to differences inhexose sugar or protein concentrations within the extensionzone. Measurements were done in an irrigated field crop suppliedwith 0, 3, 10, or 30 kg N ha^–1 per week. The mean values of L_er at 15 °C increased with leaf numberand with nitrogen supply as did values for L_ez. In contrastR_ez at 15 °C declined from 0.9 d^–1 for the first leavesto 0.3 d^–1 for the flag leaf. Nitrogen supply had littleeffect on R_ez. A separate measure of the efficiency of leafgrowth, the responsiveness of L_er to temperature (measured asthe slope of the temperature response curve), also decreasedwith leaf number by the same order as R_ez and was similarlyunaffected by nitrogen supply. The protein concentrations in the extension zones of the firstleaves were around 40 mg g^–1 fr. wt. and this declinedto approximately 20 mg g^–1 fr. wt for leaves emergingafter tillers emerged and remained low thereafter. Protein concentrationswere not correlated with the external supply of nitrogen. Hexosesugar concentrations followed a reverse pattern of increasingin the later order leaves and these also increased as nitrogensupply decreased. Both R_ez and the responsiveness of L_er to temperature were positivelycorrelated with the protein concentration, the relationshipin each case being described by a rectangular hyperbola equation(P < 0.01), and negatively correlated with hexose concentrations.It was concluded that internal competition between growing pointsfor reduced nitrogen caused the observed effects. However, differencesin protein concentrations may not simply reflect differencesin enzyme concentrations; rather these differences may indicatechanges in some other character such as cell numbers.     

Regulation of Apoplastic pH in Source Leaves of Vicia faba by Gibberellic Acid

Leaf discs of broad bean (Vicia faba L.), peeled on the spongy mesophyll side, rapidly altered the pH of the surrounding medium (apoplast). Using pH indicator paper appressed against the leaf, immediately after peeling, initial apoplastic pH was estimated to be 4.5. Changes in the apoplastic pH were measured with a microelectrode placed into a 100-microliter drop of an unbuffered solution (2 millimolar KCl, 0.5 millimolar CaCl₂, and 200 millimolar mannitol) on the peeled surface. Discs acidified the medium until the pH stabilized at about 5.0 (about 10 minutes). Acidification was inhibited by 50 micromolar sodium vanadate, an inhibitor of the plasmalemma H⁺-ATPase and attenuated by omitting the osmoticum or potassium ions from the medium. Fusicoccin (10 micromolar) greatly enhanced the rate of acidification. The presence of 0.1 to 1 micromolar gibberellic acid resulted in a slower rate of medium acidification. Gibberellic acid appeared to modulate the activity of the H⁺-translocating ATPase located at the plasma membrane of the mesophyll cells.     

Effects of Age and Gibberellic Acid on the Flavonol Content of Pisum sativum Leaves

Changes in the content of quercetin and kaempferol during leafdevelopment in Pisum sativum cv. Meteor are described. As theleaves matured the quercetin content decreased and that of kaempferolincreased during a period from 37 to 48 days from planting.Maximum flavonol content per leaf was found in leaves higheron the shoot as development proceeded, but total leaf flavonolcontent per plant remained relatively constant. There was noevidence to suggest that the quantitative changes in the individualflavonols were the result of conversion of quercetin into kaempferol. Treatment with gibberellic acid led to an accelerated fall inquercetin content in the mature leaves. This change was detectable24 h after treatment and before any apparent growth response.     

Effect of Gibberellic Acid on Rate of Extension and Maturation of Pea Internodes

Gibberellic acid (GA) does not delay maturation of pea internodes;on the con-trary, maturation (i.e. cessation of extension) takesplace slightly earlier. Thus the increased length of internodesresulting from GA treatment is due entirely to increased rateof extension. In this experiment, GA treatment of plants accelerated the visibleproduction of the first flower bud by about 4 days: the nodebearing the first flower was not altered. The total number offlower buds produced by the end of the experiment was increasedas a result of GA treatment, but many of those first formedon plants receiving high doses (I-IOµg.) withered beforeopening.     

Comparison of Growth Rates and Sugar and Protein Concentrations of the Extension Zone of Main Shoot and Tiller Leaves of Wheat

The objective of this study was to determine, for wheat plantsgrowing in an irrigated, well fertilized crop, whether therewere any differences in leaf growth rates and sugar and proteinconcentrations of the extension zone between leaves on the mainshoot and those on the first two primary tillers. High nitrogensupply was used to accentuate any basic differences that mayexist and to ensure minimum carbohydrate concentrations. Final leaf length, leaf extension rates, the responsivenessof leaf extension to temperature, and the sugar and proteinconcentrations of the extension zone did differ between successiveleaves on a shoot but the values for these parameters were similarfor leaves growing on separate shoots at the same time. Theseresults were consistent with the assumptions that the growthof tiller leaves was integrated with the activity of the mainshoot and that there were no basic physiological differencesbetween the leaves on the separate shoots. The differences ingrowth rates between leaves with time were not related to hexoseconcentrations. The variation in protein concentrations wasless than recorded previously and insufficient to establisha significant relationship with leaf growth rates.     

Interrelationship of Abscisic Acid and Gibberellic Acid in the Promotion of Callus Formation in the Abscission Zone of Citrus Bud Cultures

The mechanism of ABA-induced callus formation was studied in sterile bud cultures of Citrus [Citrus sinensis (L.) Osbeck] on defined media. ABA was found to promote callus formation in the abscission zone between the petiole and the branch while inhibiting bud growth. The promoting effect of ABA was dependent on the physiological state of the shoot from which buds were excised, and on the size of the explant. Callus formation was highest in autumn and summer (i.e. younger) buds, and lowest in older buds excised from previous summer flush. GA was only slightly active in promoting callus formation when applied separately, but showed a highly synergistic effect when applied with ABA: maximal callus formation was attained at a combination of 10^?5M ABA and 10^?6 MGA in the medium. Subcultures of ABA-induced callus revealed that ABA inhibited the growth of isolated subcultured callus, while IAA and kinetin, and especially GA, promoted its rapid proliferation. A general decrease in protein synthesis was found in the abscission zone during the first 5 days of induction, while total protein content changed only slightly. The results suggest that ABA-induced callus formation in Citrus bud explants is a multiphasic phenomenon involving, at least, two stages: (1) activation of certain cells in the abscission zone by ABA, resulting in the formation of callus layers, and (2) subsequent proliferation of the callus tissue, which is dependent on the hormonal balance in the explant. This growth-promoting effect of ABA seems to be a general phenomenon in explants exposing a cut-surface.     

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.     

Gibberellic Acid-induced Increase in Activity of a Particular Isozyme of Acid Phosphatase in Wheat Half-seeds

Embryoless half-seeds of Triticum aestivum L. contain at leastnine acid phosphatase isozymes of isoelectric pH ranging from4.0 to 7.2. Treatment with GA₃ resulted in activation of a particularisozyme of pI 4.0. Three major isozymes (pi 4.0, 4.9 and 6.2)differed in their relative specificities. A similar increaseof the pI 4 isozyme was also observed in the endosperm of germinatingwheat seeds. (Received April 7, 1981; Accepted July 1, 1981)     

Gibberellic Acid-stimulated de Novo Synthesis of a Particular Isozyme of Acid Phosphatase in Wheat Half-seeds

GA₃ stimulated incorporation of radioactive methionine intoan acid phosphatase isozyme possessing an isoelectric pH at4.0 (pI 4 isozyme). This effect of GA₃ was completely inhibitedby cycloheximide, but not by cordycepin. The results suggestthe existence of preformed mRNA coding for pI 4 isozyme in thewheat aleurone layer and the stimulation of its translationby GA₃. (Received April 7, 1981; Accepted July 1, 1981)     

Suspensor Length Determines Developmental Progression of the Embryo in Arabidopsis

The first structure that differentiates during plant embryogenesis is the extra-embryonic suspensor that positions the embryo in the lumen of the seed. A central role in nutrient transport has been ascribed to the suspensor in species with prominent suspensor structures. Little is known, however, about what impact the size of the rather simple Arabidopsis (Arabidopsis thaliana) suspensor has on embryogenesis. Here, we describe mutations in the predicted exo-polygalacturonase gene NIMNA (NMA) that lead to cell elongation defects in the early embryo and markedly reduced suspensor length. Mutant nma embryos develop slower than wild-type embryos, and we could observe a similar developmental delay in another mutant with shorter suspensors. Interestingly, for both genes, the paternal allele has a stronger influence on the embryonic phenotype. We conclude that the length of the suspensor is crucial for fast developmental progression of the embryo in Arabidopsis.Annual, self-pollinating weeds such as Arabidopsis (Arabidopsis thaliana) benefit from a short life cycle in their natural ephemeral habitats (Snell and Aarssen, 2005). A rapid progression through embryogenesis is a prerequisite for early seed maturation, and it is therefore not surprising that Arabidopsis sets up its body plan already after a very limited number of cell divisions (Aarssen, 2000; Lau et al., 2012).Arabidopsis embryogenesis starts with the fertilized egg cell, the zygote, which elongates about 3-fold before it divides asymmetrically. The smaller apical cell is the founder of the embryo proper that contributes to most of the later seedling, while the larger basal cell develops into a support structure called the suspensor (Jeong et al., 2011a). The suspensor is formed by a series of transverse cell divisions followed by longitudinal cell expansion forming a stalk-like structure. Only the upper-most suspensor cell, the hypophysis, will contribute to parts of the root meristem, while the rest of the suspensor remains extraembryonic and will cease its growth at the heart stage of the embryo (Yeung and Meinke, 1993). The suspensor is thought to be important for pushing the embryo into the lumen of the seed, where the embryo is surrounded by the nourishing endosperm. In addition, a key function in nutrient and hormone transport to the embryo is assigned to the suspensor (Kawashima and Goldberg, 2010).The Arabidopsis suspensor achieves its maximum length with a minimum number of cells by having a rod-shaped structure built by a single cell file. Although several mutants with distorted or shorter suspensors have been described in Arabidopsis, little is known about what impact suspensor length has on embryo development (Schwartz et al., 1994; Vernon and Meinke, 1994; Lukowitz et al., 2004; Breuninger et al., 2008; Bayer et al., 2009; Jeong et al., 2011b).As in all plant cells, the size and shape of suspensor cells is primarily determined by the elasticity of the cell wall. While rigid cellulose microfibrils determine the direction of cell expansion, it is the pectin matrix that plays a major role in determining the stiffness of the cell wall (Peaucelle et al., 2012). Several pectin-modifying enzymes have been described that modulate the elasticity of the cell wall. Among these, pectin methyl esterases that determine the degree of homogalacturonan methylation seem to be major players (Palin and Geitmann, 2012). Pectin-degrading enzymes such as polygalacturonases (PGs) have been implicated with cell expansion, but their role in this process is much less clear (Hadfield and Bennett, 1998).The pectin-rich middle lamella serves as cement to aid cell adhesion, and it is therefore not surprising that several PG genes are involved in dehiscence and abscission events (Rhee et al., 2003; González-Carranza et al., 2007, 2012; Ogawa et al., 2009). The expression profile of many PG genes correlates temporally and spatially with cell elongation and therefore implies a role in cell expansion (Sitrit et al., 1999). The exact function of PGs during cell elongation is still unclear, and the lack of elongation defects in loss-of-function mutants further complicates matters.Here, we describe mutations in a PG gene, which we called NIMNA (NMA), that severely affect cell elongation in the embryo and suspensor.We demonstrate that embryos with shorter suspensors show a significant delay in development during embryogenesis, possibly caused by reduced access to nutrients from the endosperm. Interestingly, reciprocal crosses reveal an unequal parental contribution to NMA function in cell elongation of the suspensor.     

The Effect of Single and Repeated Gibberellic Acid Treatment on Internode Number and Length in Dwarf Peas

In long-term experiments comparing the results of single and repeated treatment of dwarf peas with gibberellic acid, both once-treated and weekly-treated plants formed more internodes than control plants. The initial high rate of internode formation in the once-treated peas dropped rapidly reaching that of control plants three weeks after treatment. Weekly-treated plants maintained a higher rate of internode formation. The ratio of internode lengths of once-treated plants to control plants was high (over 3) for the internodes which were elongating during or shortly after the single treatment but dropped rapidly and approached unity in later-developed internodes. The ratio of internode lengths of weekly-treated plants to control plants continued to be high throughout the experiment.