Exogenous Abscisic Acid Increases Carbohydrate Accumulation and Redistribution to the Grains in Wheat Grown Under Field Conditions of Soil Water Restriction

This work investigates the effects of abscisic acid (ABA) on physiologic parameters related to yield in wheat (Triticum aestivum) grown under field conditions with water restriction ranging between 45.7% and 49.5% of field capacity during anthesis and postanthesis. ABA (300 mg L⁻¹) was sprayed onto the plants at the beginning of shoot lengthening which significantly promoted leaf area and higher concentrations of chlorophylls and carotenoids in flag leaf at anthesis. ABA also increased soluble carbohydrates in shoots at anthesis, which were then re-exported to the grains at maturity. This correlated with a yield increase that was achieved by a higher number and weight of grains per spike, but protein content was not significantly affected.     

氮素形态对小麦花后不同器官内源激素含量的影响

采用盆栽方法,研究了酰胺态氮、铵态氮和硝态氮对小麦(Triticum aestivum)花后根系、旗叶和籽粒内源激素IAA、GA₃、ABA和ZR含量的影响。结果表明,小麦不同器官的内源激素含量对3种氮素形态的响应不同。氮素形态调节籽粒灌浆是通过根系、旗叶和籽粒中内源激素的协同作用而实现的。酰胺态氮与硝态氮处理相比,小麦花后5~15 d,旗叶GA₃含量、籽粒IAA和ABA含量较高,籽粒灌浆速率(Grain-filling rate, GFR)较高;花后15~25 d,根系GA₃含量、旗叶IAA和GA₃含量、籽粒ABA含量较高,籽粒IAA含量较低,GFR较低。铵态氮与硝态氮处理相比,小麦花后5 d,籽粒ZR含量较高;花后15 d前后,籽粒IAA、ABA含量较低,GFR较低;花后20~25 d,根系ZR、GA₃含量较低,旗叶IAA、GA₃含量较低,ABA含量较高,籽粒ABA、GA₃含量较低,IAA含量较高,GFR较高。铵态氮比硝态氮处理的小麦籽粒粒重显著增加。铵态氮和酰胺态氮处理比硝态氮处理增产显著。建议在‘豫麦49’施肥时,使用铵态氮或酰胺态氮并配施硝化抑制剂。     

Abscisic Acid Translocation and Influence of Water Stress on Grain Abscisic Acid Content

The movement of foliar applied [1-¹⁴C]abscisic acid (ABA) inwheat plants (Triticum aestivum L., cv. Kolibri) was investigatedat two stages of grain development (1000 grains, weight 19 and24 g dry matter). [1–¹⁴C]ABA seemed to be readily translocated within 12h into the developing grains as well as in other plant parts.A subsequent rapid metabolism took place leading to a decreasedactivity of the ABA-containing chromatogram fraction in theyounger plants 48 h after application. The metabolism seemodto be less intensive in the older grains, where the activityrunning with the ABA increased over 64 h. Treating the leaves of barley plants (Hordeum vulgare, L., cv.Union) 2 weeks after anthesis with a gentle stream of warm air(36° C) resulted in a significant increase in the ABA contentof all parts of the ear. The results mentioned above indicatethat this may be partially due to translocation from other partsof the plant such as the leaves.     

Effects of crop thinning and reduced grain numbers per ear on grain size in two winter wheat varieties given different amounts of nitrogen

To study the importance for final grain size in wheat (Triticum aestivum, L.) of assimilate supply and the storage capacity of the grain, two field experiments were done. In 1976 nitrogen was applied in the range from none to 180 kg ha^-1, part of the crop was thinned, and the top halves of some ears of the short variety Hobbit and of the tall variety Maris Huntsman were removed soon after anthesis. In 1977 ears of Maris Huntsman were halved 5 days after anthesis or at 30 days after anthesis when grain volume was maximum. Thinning the crop from 360 to 180 ear-bearing shoots m^-2 30 days before anthesis increased the number of grains per ear, except in the absence of nitrogen fertiliser, but did not increase grain size, grain dry weight per ear or total dry weight per culm. Removing the upper half of ears of Hobbit 5 days after anthesis increased dry weight per grain, but when this treatment was applied to Maris Huntsman either 5 days after anthesis in 1976 and 1977, or when grain volume was maximal in 1977, the grains failed to increase in dry weight. Non-grain dry weight of both varieties was increased by halving the ear. In both varieties the maximum volume of grains in halved ears was larger than in intact ears. Grain dry weight increased relatively less than volume after halving the ear of Hobbit, and the decrease in volume up to maturity was greater in halved than intact ears of both varieties. The larger grain volume in halved ears of Maris Huntsman in 1977 was associated with more endosperm cells.     

Contribution of the wheat (Triticum aestivum L.) flag leaf to grain yield in response to plant growth regulators

In West-Europe, intensive cereal management uses plant growth regulators (PGRs) especially for wheat. A green-house experiment compared the effects of two PGRs on flag leaf characteristics and yield of winter wheat. Chlormequat chloride + choline chloride (CCC) and chlormequat chloride + choline chloride + imazaquin (CCC+I) were applied to winter wheat at growth stage 5 (Feekes Large scale). CCC and CCC+I significantly increased flag leaf surface area at anthesis. Both treatments also enhanced chlorophyll content of the main stem flag leaf. The grain filling period was extended with PGR application by 2 days. CCC and CCC+I significantly increased net CO₂ assimilation rates during the flag leaf life. No effects of PGR spraying were observed on the pattern of ¹⁴C labelled assimilate distribution. Increased grain yield was due to the increase in average grain weight. The results indicate that PGR treatments increased flag leaf contribution to grain filling. The addition of imazaquin (I) to chlormequat (CCC) improved the effects of CCC.     

Distribution of photosynthate produced before and after anthesis in tall and semi-dwarf winter wheat, as affected by nitrogen fertiliser

In 2 years the distribution of radioactivity recovered in entire shoots of field-grown winter wheat was determined at various times after exposing the top two leaves (flag leaf or second leaf) to ¹⁴CO₂ for 30 s. In 1976 when ¹⁴C was supplied to either leaf 14 days before anthesis, 30% was in the ear at anthesis. Less than 5% was in the leaf exposed to ^I4CO₂. The remainder was equally divided between the stem above and below the flag-leaf node when the flag leaf had been exposed, and was mainly in the lower part of the stem when the second leaf had been exposed. By maturity the proportion in the stem had decreased; 20% of the total activity was in the grain and 30% was still in the ear structures. When ¹⁴C was supplied 10 days after anthesis, the proportion in the ear 24 h later ranged from 42 to 69% of that in the whole shoot when the flag leaf was exposed, and from 6 to 28% when the second leaf was exposed. At maturity these proportions increased to 92 and 85% when the ¹⁴C had been supplied to flag leaves and second leaves respectively. When ¹⁴C was supplied 25 days after anthesis to either flag leaves or second leaves, more than 90% of the activity was in the mature ears. Less than 5% of the ¹⁴C remaining at maturity from any treatment was still in the leaf exposed to ¹⁴CO₂. Between 2 and 6% of ¹⁴C supplied after anthesis was in the non-grain parts of the ear. The proportion of the ¹⁴C in the ear was greatest for the semi-dwarf varieties Maris Fundin and Hobbit, less for Maris Huntsman, and least for Cappelle-Desprez. These varietal differences were large 24 h after exposure to ¹⁴CO₂, especially in 1976. They were small and rarely significant at maturity. Nitrogen fertiliser up to 210 kg N ha^-1 had negligible effects on the distribution of ¹⁴C, although it greatly increased growth and yield, especially in 1975.     

Effect of shading individual soybean reproductive structures on their abscisic Acid content, metabolism, and partitioning

Pod set in soybean is related to carbon partitioning and may be, at least partially, regulated by abscisic acid (ABA) concentrations. The studies reported here examine the relationship between carbon and ABA partitioning, reproductive abscission and ABA metabolism. The partitioning of radiolabeled ABA and photoassimilates from leaves to flowers and endogenous ABA concentrations were determined in shaded and unshaded reproductive structures. Aluminum foil was gently placed over individual soybean reproductive structures for 48 hours at 0, 4, 12, 17, and 22 days after anthesis (DAA). Shading of flowers at 12, 17, and 22 DAA resulted in significantly reduced concentration of ABA. However, shading had no effect on the catabolism of exogenously supplied [³H] ABA. The shading treatment on the first four of the five dates reduced partitioning of photoassimilates and ABA from the subtending leaf to the flower. Shading of reproductive structures also caused a significant reduction in the amount of assimilate exported from the subtending leaf, at 17 DAA. We conclude that shade-induced premature reproductive abscission in soybean is not stimulated by high levels of ABA within reproductive structures, but that ABA may inhibit abscission of reproductive structures by playing a role in preferential assimilate partitioning.     

Assimilatverteilung in Sommergerstensorten mit unterschiedlicher Resistenz gegenüber dem Echten Mehltau (Erysiphe graminis f. sp. hordei)

Distribution of assimilates in cultivars of spring barley with different resistance against powdery mildew (Erysiphe graminis f. sp. hordei) Transport and distribution of radioactive labelled assimilates in spring barley cultivars with different degrees of resistance to powdery mildew were studied after ¹⁴CO₂-treatment of single leaves. Plants of the cultivars ‘Amsel’ (susceptible), ‘Asse’ (adult plant resistant), and ‘Rupee’ (resistant) were analyzed at the vegetative growth stage (5. leaf unfolded) and the generative growth stage (anthesis). At the vegetative growth stage the assimilate export from the mildew inoculated 5. leaf of ‘Amsel’ and ‘Rupee’ is decreased; in ‘Asse’, there is no considerable change of assimilate distribution due to infection. At the generative growth stage the assimilate export from the infected flag leaf of ‘Amsel’ is reduced when the fungus, is sporulating. In the cultivar ‘Asse’ the assimilates are bound at the infection site until the seventh day after inoculation, then the transport of assimilates to the ear is increased. In ‘Rupee’ mildew inoculation causes an enhanced assimilate transport to the ear. The changes in assimilate distribution due to mildew inoculation are discussed with respect to the different types of host-parasite-interactions and the source-sink-activities in the different cultivars.     

Seed development and vivipary in Zea mays L.

Seed development was investigated in kernels of developing wild-type and viviparous (vp-1) Zea mays L. Embryos and endosperm of wild-type kernels began to dehydrate at approx. 35 d after pollination (DAP); viviparous embryos did not desiccate but accumulated fresh weight via coleoptile growth in the caryopses. Concentrations of endogenous abscisic acid (ABA) in the embryo were relatively high early in development, being approx. 150 ng·g^-1 fresh weight at 20 DAP. The ABA content declined thereafter, falling to approx. 50 ng·g^-1 at 30 DAP. Endosperm ABA content was always low, being less than 20 ng·g^-1. There were no differences between wild-type and vp-1 tissues. Immature kernels did not germinate when removed from the ear until late in development. The ability to germinate was correlated with decreasing moisture content in the endosperm at the time of removal; premature drying of immature kernels resulted in greatly increased germination following imbibition. Excised embryos germinated precociously when removed from the endosperm as early as 25 DAP. Such germination could be prevented by treatment with 10^-5 M ABA or by lowering the solute potential (_s) of the medium with 0.3 M mannitol. Treatment of excised embryos with ABA led to internal ABA concentrations comparable to those in embryos in which germination was inhibited in situ. Mannitol treatment did not have this effect, although water-deficit stress of excised embryos resulted in substantial ABA production. Germinated vp-1 embryos were less sensitive to growth inhibition by ABA or mannitol than germinating wild-type embryos. The vp-1 seedlings were not wilty and their transpiration rates were reduced in response to ABA or water shortage.Abbreviations and symbols ABA abscisic acid - DAP days after pollination - FW fresh weight - vp-1 viviparous genotype - _s solute potential     

小麦开花后iPA和ABA含量的变化及与旗叶光合,根系活力和籽？…

以２个粒重不同的小麦品种为材料,研究了开花后旗叶、根系和籽粒异戊烯基腺苷（ｉＰＡ）和脱落酸（ＡＢＡ）含量的变化及其与旗叶光合速率、根系活力和籽粒样重的关系。结果表明,高粒重的鲁麦２２较低粒重的鲁麦１４,旗叶和根系ｉＰＡ含量高且速降始期推迟,ＡＢＡ含量低且速升始期推迟;与此相对应,旗叶光合速率和根系活力高且高值持续期长。鲁麦２２籽粒ｉＰＡ含量高于鲁麦１４,其ＡＢＡ含量在花后１６ｄ内高于鲁麦１４,１６     

Distribution between parts of the main shoot and the tillers of photosynthate produced before and after anthesis in the top three leaves of main shoots of Hobbit and Maris Huntsman winter wheat

The top three leaves of main shoots in crops of Hobbit and Maris Huntsman winter wheat were exposed to ¹⁴CO₂ at 22 and 16 days before and at 10 days after anthesis in 1978. The distribution of the ¹⁴C recovered in whole plants at anthesis and at maturity was measured. There was negligible loss of ¹⁴C between these two times, but some redistribution. The percentage in the tillers was negligible when the flag leaf (leaf 1) was exposed to ¹⁴CO₂, and otherwise less than 12% except for ¹⁴C absorbed by the third leaf at 16 days before anthesis, when it averaged 26% but was very variable. When ¹⁴C was supplied before anthesis, about 20% reached the grain whichever leaf had been supplied. The ear structures contained about 10% of that absorbed by the third leaf and 35% of that absorbed by the flag or second leaf. When ¹⁴C was supplied after anthesis, the amounts reaching the grain from the different leaves were: flag 82%, second 68%, third 56%. Most of the remainder was in the stem. The exposed leaf never retained more than 6%. The amount of ¹⁴C that moved from the stem to the grain between anthesis and maturity was about 50% greater in the semi-dwarf variety Hobbit than in Maris Huntsman. There was no significant varietal difference in the percentage of post-anthesis ¹⁴C reaching the grain. The ear structures of Hobbit contained about a third more ¹⁴C than those of Maris Huntsman. An additional 90 kg N ha^-1, which increased grain yield by 46%, had negligible effects on the distribution of ¹⁴C.     

Lack of C4 photosynthetic metabolism in ears of C3 cereals

There is continuing controversy over whether a degree of C₄ photosynthetic metabolism exists in ears of C₃ cereals. In this context, CO₂ exchange and the initial products of photosynthesis were examined in flag leaf blades and various ear parts of two durum wheat (Triticum durum Desf.) and two six-rowed barley (Hordeum vulgare L.) cultivars. Three weeks after anthesis, the CO₂ compensation concentration at 210 mmol mol^?1 O₂ in durum wheat and barley ear parts was similar to or greater than that in flag leaves. The O₂ dependence of the CO₂ compensation concentration in durum wheat ear parts, as well as in the flag leaf blade, was linear, as expected for C₃ photosynthesis. In a complementary experiment, intact and attached ears and flag leaf blades of barley and durum wheat were radio-labelled with ¹⁴CO₂ during a 10s pulse, and the initial products of fixation were studied in various parts of the ears (awns, glumes, inner bracts and grains) and in the flag leaf blade. All tissues assimilated CO₂ mainly by the Calvin (C₃) cycle, with little fixation of ¹⁴CO₂ into the C₄ acids malate and aspartate (about 10% or less). These collective data support the conclusion that in the ear parts of these C₃ cereals C₄ photosynthetic metabolism is nil.     

Effects of Nitrogen Fertilizer on the Distribution of Photosynthate during Grain Growth of Spring Wheat

The distribution of photosynthate labelled with ¹⁴C was studiedin spring wheat grown with different amounts of nitrogen fertilizerin the three years 1972–4, after exposing the flag leafor the leaf below the flag leaf to ¹⁴CO₂ at 6–10 or 19–26days after anthesis. The movement of ¹⁴C to ears was unaffectedby nitrogen fertilizer except after early exposure in 1973,when nitrogen increased the retention of ¹⁴C in stems at maturity The concentration of sugar in the top part of the shoot at theend of the day was unaffected by nitrogen in 1973, but at 22days after anthesis in 1974 the concentration of sucrose inthe glumes and rachis, and in the flag leaf lamina was increasedby nitrogen. Loss of sugar by translocation and respirationduring the night may explain why this increase in concentrationwas not reflected in the ¹⁴C distribution 24 h after supplying¹⁴C. The proportion of the total ¹⁴C content of the shoot that wasin the ear at maturity ranged from 68 to 95 per cent dependingon when and to which leaf the ¹⁴CO₂ was supplied. Less than5 per cent remained in the leaf exposed to ¹⁴CO₂. The proportionof the final ear weight contributed by the leaf below the flagleaf was about half that contributed by the flag leaf. In 1974 about 24 per cent of the ¹⁴C absorbed by the flag leaf,and 56 per cent of that absorbed by the second leaf, was lostby maturity, presumably by respiration. Most loss occurred inthe first 24 h.     

Effects of variation in temperature and light intensity at different times on growth and yield of spring wheat

Spring wheat grown in pots outdoors was transferred to growth rooms for various periods to study the effect of increasing the temperature from 14-4 to 20-3 ^oC (Expt 1) or from day/night values of 15-0/15-2 to 20-0/15-2 ^oC (Expt 2) and of increasing the amount of visible radiation in a 16 h day from 424 to 792 J cm^-2 (Expt 1) or 374 to 740 J cm^-2 (Expt 2). There were no interactions between temperature and radiation. In Expt 1 neither the increase in temperature nor extra radiation, applied for 14 days immediately after the appearance of double ridges on the stem apex, or 14 days later, increased grain yield at maturity. Warmth early, but not late, increased dry weight, leaf area and the number of floret primordia immediately after treatment, but these effects had disappeared by anthesis, 30 days later. Dry weight but not leaf area was increased by extra radiation but the effects had disappeared 2 wk after treatment. An increase in temperature imposed for 16 days starting 5 or 21 days after anthesis (Expt 2) increased dry weight of the ear and decreased that of the rest of the plant immediately after treatment, and decreased leaf area at all times. When plants from the two temperatures were put together in the same conditions, ear growth of plants that had been in the warm was slower than that of plants from the cold treatment, so that the difference in ear weight observed after 16 days of treatment reversed and grain yield was decreased by warmth applied in either period; the component of yield decreased by warmth was grain size. Additional radiation in either post-anthesis period increased dry weight of all parts of the plant and had negligible effects on leaf area. Final grain yield was increased by c. 15% because the individual grains were larger. Early treatment also increased grain number slightly. The effects of treatment during the two post-anthesis periods were similar in size, and additive.     

Mechanism of monocarpic senescence in rice

During grain formation stage (90 to 110 days), the youngest flag leaf of rice (Oryza sativa L. cv. Jaya) remained metabolically most active (as indicated by cellular constituents and enzyme activities) and the third leaf the least active. At the grain development stage (110 to 120 days) the above pattern of age-related senescence of the flag leaf completely changed and it senesced at a faster rate than the second leaf which remained metabolically active even up to grain maturation time (120 to 130 days), when both the flag and the third leaf partially senesced. Removal of any leaf temporarily arrested senescence of the remaining attached leaves, that of flag leaf did not hasten senescence of the second leaf, while that of either the second or the third accelerated senescence of the flag. Removal of the inflorescence after emergence or foliar treatment of intact plant with kinetin equally delayed senescence and produced an age-related, sequential mode of senescence or leaves. Both translocation and retention of ³²P by the flag leaf were maximum at the time of grain formation and that by the second leaf was maintained even up to grain maturation time. The induction of senescence of the flag leaf was preceded by a plentiful transport of ³²P to the grains. Kinetin treatment decreased the transport of ³²P, prolonged its duration, and almost equally involved all of the leaves in this process. The pattern of senescence of isolated leaf tips was similar to that of attached leaves. The level of endogenous abscisic acid-like substance(s) maintained a close linearity with the senescence behavior of the leaves of intact and defruited plants during aging, and the rise in abscisic acid in the flag leaf was also preceded by higher ³²P transport to the grains.     

The effect of cycloheximide on IAA-stimulated transport of 14C-ABA and 14C-sucrose in long pea epicotyl segments

The effect of cycloheximide (CH) on the indol-3yl-acetic acid (IAA)-stimulated transport of ¹⁴C-labelled abscisic acid (ABA) and ¹⁴C-labelled sucrose was studied in 110 mm long pea epicotyl segments. IAA application resulted in elongation growth of the segments. This effect was decreased by CH treatment which also reduced [¹⁴C] ABA and [¹⁴C] sucrose accumulation in the growing apical part of the segments. A reduction in [¹⁴C] IAA uptake and in protein synthesis in this part of the segments was also observed. The simultaneous inhibition of protein synthesis and reduction of [¹⁴C] ABA and [¹⁴C] sucrose transport suggests that IAA can stimulate the transport of ABA and sucrose through a protein synthesis-based elongation growth.     

Effects of water stress on cellular ultrastructure and on concentrations of endogenous abscisic acid and indole-3-acetic acid in Fatsia japonica leaves

Ultrastructural alterations in mesophyll cells as well as variations in bulk leaf endogenous ABA and IAA concentrations were studied in water-stressed field-grown plants of Fatsia japonica. Under water deficit cellular membranes were modified and an increase in vesicles was observed. The main damage to the chloroplasts included thylakoid swelling and disruption of the chloroplast envelope. Concomitant variations in abscisic acid and indole-3-acetic acid were observed. Despite the expected increased in endogenous ABA concentration in relation to water stress, after the highest concentration of ABA, observed at predawn in severely stressed plants (29-1), there was a sharp decline from 2768 pmol g fw^–1 to 145 pmol g fw^–1; thus in severely stressed plants ABA levels were not related to changes in bulk leaf ABA contents. Water stress did not influence the concentrations of indole-3-acetic acid, although the increase in the endogenous abscisic acid concentration could be related with the ultrastructural changes.Abbreviations ABA abscisic acid - IAA indole-3-acetic acid - leaf water potential     

Physiological Factors Limiting Grain Size in Wheat

The effects on grain size of changing the supply of assimilates,by thinning before anthesis or by shading the plants or by halvingthe ears either early or late in grain growth, were studiedin two glasshouse experiments with Kleiber spring wheat (Triticumaestivum L.), in 1976 and 1977. Late treatments had no effect,presumably because little grain growth occurred thereafter.Thinning the plants before anthesis increased, and shading theplants soon after anthesis decreased grain size. Halving theears soon after anthesis increased the size of the remaininggrains, but grain weight per ear decreased. The effect on grainsize of halving the ear tended to be smaller under conditionsmore favourable for photosynthesis, except when the plants werethinned before anthesis. Shading decreased the total amountof nitrogen per culm and the proportion of total nitrogen recoveredin the ear. Halving increased the retention of nitrogen in thestem of unshaded shoots and had no effect on nitrogen distributionwithin shaded shoots. In 1977 halving the ear increased the rate of dry matter accumulationin the grain throughout the grain filling period, but in 1976the increase in dry weight was faster in the grains of halvedears only during early grain growth. Later the grains in halvedand intact ears increased in dry weight at the same rate, eventhough the supply of photosynthate and the capacity of the grains(as measured by volume) were greater in the halved ears. Theseresults are discussed in relation to the influence on finalgrain weight of assimilate supply and the storage capacity ofthe grain.     

Commercially available plant growth regulators and promoters modify bulk tissue abscisic acid concentrations in spring barley, but not root growth and yield response to drought

Field and lysimeter experiments were conducted in 2002 to investigate the effects of an antigibberellin growth regulator (Moddus, active ingredient trinexapac‐ethyl, Syngenta Crop Protection UK Ltd, Whittlesford, Cambridge, UK) and an auxin‐stimulating (Route, active ingredient zinc ammonium acetate, De Sangosse Ltd, Swaffham Bulbeck, Cambridge, UK) growth promoter on root growth, soil water extraction and the drought response of spring barley. The effects on root growth and distribution were investigated in the field. The effects on the drought response were studied in 1.2‐m‐deep lysimeters packed with a loamy sand subsoil and sandy loam topsoil. Lysimeters were located under a fixed rain shelter, and drought was imposed by withholding irrigation. In both field and lysimeter experiments, growth regulator/promoters were applied to cv. Optic at early tillering according to the manufacturers’ recommendations. After withholding irrigation from lysimeters at Zadoks growth stage (GS) 21 (37 days after sowing), 50% of the profile available water had been depleted by flag leaf emergence (GS 37/39; 62 days after sowing). Drought significantly reduced stem biomass at ear emergence (GS 59; 78 days after sowing) but not leaf or ear dry weight; this was before there was any significant reduction in leaf water potential or stomatal conductance to water vapour. The reduction in stem biomass may reflect a change in partitioning between shoot and root in response to soil drying. When averaged over growth regulator/promoter treatments, drought reduced grain yield by approximately 1 t ha^?1. This was associated with a reduction in both ears per m² and grains per ear. The mean grain weight was not reduced by drought, in spite of significant reductions in stomatal conductance and canopy lifespan post‐anthesis. Route, and to a lesser extent Moddus, significantly increased abscisic acid accumulation in the stem base of droughted plants, and there was some indication of a possible delay in stomatal closure in Route‐treated plants as the soil moisture deficit developed. However, there was no significant effect on the amount of soil water extracted or grain yield under drought. Similarly, in field experiments, neither Route nor Moddus significantly altered total root length, biomass or distribution. There is little evidence from these experiments or in the literature to support the use of antigibberellin or auxin‐simulating growth regulator/promoters to modify root growth and drought avoidance of spring barley.     

Photosynthesis and transpiration of the flag leaf in four spring-wheat cultivars

Co₂ exchange and transpiration rates of the flag leaves of four spring wheat (Triticum aestivum L.) cultivars, namely Glenlea, Neepawa, Opal and Kolibri, were compared using infra-red gas-analysis technique. The plants were grown in a controlled environment under an 18-h photoperiod, with day and night temperatures of 20 and 15° C, respectively. The time course of the CO₂-exchange rate (CER) of the flag leaf differed among cultivars. CER began to decrease rapidly some 2 weeks after ear emergence in Glenlea, Neepawa and Kolibri, but only after 4 weeks in Opal. The decline in CER of Glenlea, Neepawa and Opal was continuous throughout the period of grain development whereas in Kolibri CER was maintained at a constant level between the 4th and 6th weeks after ear emergence. The transpiration rates of the flag leaves of the 4 cultivars did not change markedly until 6–7 weeks after ear emergence, indicating that the reduction in CER was not primarily a response to increased stomatal resistance to the diffusion of CO₂. Removing the ear of the main shoot of intact plants failed to depress CER of the subtending flag leaf until 5 weeks after ear removal. Removing the ears of all the tillers of plants in which all but 3 tillers had been removed at ear emergence did not depress CER until 4 weeks after ear emergence, but removal of the ear of the main shoot of plants where all the tillers had been removed at ear emergence reduced the CER of the flag leaf 2 weeks after ear removal. Removal of tillers at ear emergence had a marked effect on the time course of CER and transpiration rates of the flag leaf. Both CER and transpiration rates of a 4-tiller plant were maintained at a higher level throughout ear development as compared to those of a one-tiller plant. The transpiration rate of the flag leaf of Glenlea increased during the later part of the life of the leaf even for one-tiller plants with no ear, indicating that such a stomatal response may be part of the normal course of leaf aging and not a response to a feedback stimulus from the ear.