Hormones and Pod Development in Oilseed Rape (Brassica napus)

The endogenous levels of several plant growth substances (indole acetic acid, IAA; abscisic acid, ABA; zeatin, Z; zeatin riboside, [9R]Z; isopentenyladenine, iP; and isopentenyladenosine, [9R]iP were measured during pod development of field grown oilseed Rape (Brassica napus L. var oleifera cv Bienvenu) with high performance liquid chromatography and immunoenzymic (enzyme-linked immunosorbent assay, ELISA) techniques. Results show that pod development is characterized by high levels of Z and [9R]Z in 3 day old fruits and of IAA on the fourth day. During pod maturation, initially a significant increase of IAA and cytokinins was observed, followed by a progressive rise of ABA levels and a concomitant decline of IAA and cytokinin (except iP) levels. The relationship between hormone levels and development, especially pod number, seed number per pod, and seed weight determination, will be discussed.     

Plant growth regulators as putative physiological markers of developmental stage in Prunus persica

Prunus persica plants of different ages and statesof maturation were analysed to compare their phytohormonal status. Endogenouslevels of indole-3-acetic acid (IAA), abscisic acid (ABA), and severalcytokinins (Cks): zeatin (Z), zeatin riboside ([9R]Z), dihydrozeatin ((diH)Z),dihydrozeatin riboside ((diH)Z[9R]), isopentenyl adenine (iP) and isopentenyladenosine ([9R]iP), were measured in order to determine their possible use asphysiological indices of phase change and maturation. A decrease in Ck levels(Z, [9R]Z, (diH)Z, (diH)[9R]Z, and iP, [9RiP) was found from the embryonic tojuvenile stage as well as a decrease in the ratio of iP-type (iP and[9R]iP)/Z-type Cks paralleling the increase in tree age. ABA levels increasedduring maturation in Prunus persica and the ratio ofCks/IAA decreased with tree age. From our results, we propose that the balancesof Cks/IAA and iP-type/Z-type Cks are good indices of different developmentalstates in Prunus persica.     

Thermoperiodic effect on flowering and endogenous hormonal status in Dendrobium (Orchidaceae)

Two year-old cloned plants of Dendrobium Second Love were submitted to 25 °C (light) and 10 °C (dark) under a 12 h photoperiod (60 μmol m⁻¹ s⁻¹) for 30 days. The endogenous levels of IAA, ABA, and the cytokinins Z, [9R]Z, iP, and [9R]iP were measured 15, 22, and 30 days after the start of the thermoperiodic treatment in lateral buds and leaves. The endogenous levels of IAA and cytokinins, especially the zeatin-derived forms, increased significantly in buds after 15 days of treatment. On the other hand, the amount of ABA decreased progressively and significantly throughout the treatment. The treatment conspicuously accelerated flower-bud development. The found correlation suggests that hormones are involved in the signal transduction pathway of thermoperiodic flowering control.     

Transport and metabolism of [3H]iso-pentenyladenine following application to the tips of intact and decapitated tap roots of Pinus pinea

[³H]iso-Pentenyladenine ([³H]iP) was fed for 24 h to the tips of intact and root tip-decapitated Pinus pinea seedlings. Twelve and 24 h after application to the roots of intact plants most of the applied radioactivity (±60%) was transported to the shoot. Root tip removal increased transport of the applied radioactivity to the shoot, but the overall pattern of distribution of radioactivity in the seedling did not change. Large amounts of radioactivity were recovered from the elongation zone of the root. Some radioactivity also accumulated in the older part of the root with well-developed lateral roots. When [³H]iP was applied one day after decapitation, no significant changes in the pattern of radioactivity distribution were found between the intact and decapitated root systems. However, when applied 7 days after decapitation there was a significant increase of radioactivity in the region of the root where lateral roots were emerging. HPLC separation of extracts from the different root sections showed that [³H]iP was extensively metabolized in the root. Six peaks of radioactivity, which co-chromatographed with authentic cytokinin standards, were detected.Abbreviations ABA abscisic acid - ADE adenine - IAA indole-acetic acid - iP iso-pentenyladenine - HPLC high performance liquid chromatography - [OG]DHZ O-glycosyldihydrozeatin - [9R-MP]DHZ ribosyldihydrozeatin monophosphate - [9G]iP iso-pentenyladenine-9-glucoside - [9R]Z ribosylzeatin - [9R]iP iso-pentenyladenosine - TLC thin layer chromatography     

Early stomatal closure in waterlogged pea plants is mediated by abscisic acid in the absence of foliar water deficits

Abstract Soil waterlogging decreased leaf conductance (interpreted as stomatal closure) of vegetative pea plants (Pisuin sativum L. cv. ‘Sprite’) approximately 24 h after the start of flooding, i.e. from the beginning of the second 16 h-long photo-period. Both adaxial and abaxial surfaces of leaves of various ages and the stipules were affected. Stomatal closure was sustained for at least 3 d with no decrease in foliar hydration measured as water content per unit area, leaf water potential or leaf water saturation deficit. Instead, leaves became increasingly hydrated in association with slower transpiration. These changes in the waterlogged plants over 3 d were accompanied by up to 10-fold increases in the concentration of endogenous abscisic acid (ABA). Waterlogging also increased foliar hydration and ABA concentrations in the dark. Leaves detached from non-waterlogged plants and maintained in vials of water for up to 3 d behaved in a similar way to leaves on flooded plants, i.e. stomata closed in the absence of a water deficit but in association with increased ABA content. Applying ABA through the transpiration stream to freshly detached leaflets partially closed stomata within 15 min. The extractable concentrations of ABA associated with this closure were similar to those found in flooded plants. When an ABA-deficient ‘wilty’ mutant of pea was waterlogged, the extent of stomatal closure was less pronounced than that in ordinary non-mutant plants, and the associated increase in foliar ABA was correspondingly smaller. Similarly, waterlogging closed stomata of tomato plants within 24 h, but no such closure was seen in ‘flacca’, a corresponding ABA-deficient mutant. The results provide an example of stomatal closure brought about by stress in the root environment in the absence of water deficiency. The correlative factor operating between the roots and shoots appeared to be an inhibition of ABA transport out of the shoots of flooded plants, causing the hormone to accumulate in the leaves.     

Accounting for sap flow from different parts of the root system improves the prediction of xylem ABA concentration in plants grown with heterogeneous soil moisture

When soil moisture is heterogeneous, sap flow from, and ABA status of, different parts of the root system impact on leaf xylem ABA concentration ([X-ABA]leaf). The robustness of a model for predicting [X-ABA]leaf was assessed. 'Two root-one shoot' grafted sunflower (Helianthus annuus L.) plants received either deficit irrigation (DI, each root system received the same irrigation volumes) or partial rootzone drying (PRD, only one root system was watered and the other dried the soil). Irrespective of whether relative sap flow was assessed using sap flow sensors in vivo or by pressurization of de-topped roots, each root system contributed similarly to total sap flow during DI, while sap flow from roots in drying soil declined linearly with soil water potential (Psisoil) during PRD. Although Psisoil of the irrigated pot determined the threshold Psisoil at which sap flow from roots in drying soil decreased, the slope of this decrease was independent of the wet pot Psisoil. Irrespective of whether sap was collected from the wet or dry root system of PRD plants, or a DI plant, root xylem ABA concentration increased as Psisoil declined. The model, which weighted ABA contributions of each root system according to the sap flow from each, almost perfectly explained [X-ABA] immediately above the graft union. That the model overestimated measured [X-ABA]leaf may result from changes in [X-ABA] along the transport pathway or an artefact of collecting xylem sap from detached leaves. The implications of declining sap flow through partially dry roots during PRD for the control of stomatal behaviour and irrigation scheduling are discussed.     

Uniconazole-induced tolerance of soybean to water deficit stress in relation to changes in photosynthesis, hormones and antioxidant system

This study investigated whether uniconazole confers drought tolerance to soybean and if such tolerance is correlated with changes in photosynthesis, hormones and antioxidant system of leaves. Soybean plants were foliar treated with uniconazole at 50 mg L-1 at the beginning of bloom and then exposed to water deficit stress at pod initiation for 7 d. Uniconazole promoted biomass accumulation and seed yield under both water conditions. Plants treated with uniconazole showed higher leaf water potential only in water-stressed condition. Water stress decreased the chlorophyll content and photosynthetic rate, but those of uniconazole-treated plants were higher than the stressed control. Uniconazole increased the maximum quantum yield of photosystemand ribulose-1,5-bisphosphate carboxylase/oxygenase activity of water-stressed plants. Water stress decreased partitioning of assimilated 14C from labeled leaf to the other parts of the plant. In contrast, uniconazole enhanced translocation of assimilated 14C from labeled leaves to the other parts, except stems, regardless of water treatment. Uniconazole-treated plants contained less GA3, GA4 and ABA under well-watered condition than untreated plants, while the IAA and zeatin levels were increased substantially under both water conditions, and ABA concentration was also increased under water stressed condition. Under water-stressed conditions, uniconazole increased the content of proline and soluble sugars, and the activities of superoxide dismutase and peroxidase in soybean leaves but not the malondialdehyde content or electrical conductivity. These results suggest that uniconazole-induced tolerance to water deficit stress in soybean was related to the changes of photosynthesis, hormones and antioxidant system of leaves.     

Shoot‐derived abscisic acid promotes root growth

The phytohormone abscisic acid (ABA) plays a major role in regulating root growth. Most work to date has investigated the influence of root‐sourced ABA on root growth during water stress. Here, we tested whether foliage‐derived ABA could be transported to the roots, and whether this foliage‐derived ABA had an influence on root growth under well‐watered conditions. Using both application studies of deuterium‐labelled ABA and reciprocal grafting between wild‐type and ABA‐biosynthetic mutant plants, we show that both ABA levels in the roots and root growth in representative angiosperms are controlled by ABA synthesized in the leaves rather than sourced from the roots. Foliage‐derived ABA was found to promote root growth relative to shoot growth but to inhibit the development of lateral roots. Increased root auxin (IAA) levels in plants with ABA‐deficient scions suggest that foliage‐derived ABA inhibits root growth through the root growth‐inhibitor IAA. These results highlight the physiological and morphological importance, beyond the control of stomata, of foliage‐derived ABA. The use of foliar ABA as a signal for root growth has important implications for regulating root to shoot growth under normal conditions and suggests that leaf rather than root hydration is the main signal for regulating plant responses to moisture.     

A hormonal misunderstanding in Acca sellowiana embryogenesis: levels of zygotic embryogenesis do not match those of somatic embryogenesis

Endogenous levels of IAA, ABA and four types of CKs were analyzed in zygotic and indirect (ISE) and direct somatic embryogenesis of Acca sellowiana. Zygotic and somatic embryos at different developmental stages were sampled for morphological and hormonal analysis. Both embryo types showed substantial asymmetry in hormone levels. Zygotic embryos displayed a conspicuous peak of IAA in early developmental stages. The results outlined the hormonal variations occurring during zygotic and somatic embryogenesis regarding the timing, nature and hormonal status involved in both processes. The short transient pulse of IAA observed on the 3rd day in culture was suggested to be involved with the signaling for the induction of somatic embryogenesis. Fertilized ovule development was associated with increased IAA levels 21?C24?days after pollination, followed by a sharp decrease in the cotyledonary stage, both in zygotic and somatic embryos. There was a prominent increase in ABA levels in cultures which generated ISE 24?C30?days after pollination, a period that corresponds to the heart and torpedo stages. The levels of total CKs (Z, [9R]Z, iP and [9R]iP) were also always higher in zygotic than in somatic embryogenesis. While zygotic embryogenesis was dominated by the presence of zeatin, the somatic process, contrarily, was characterized by a large variation of the other cytokinin forms and amounts studied. The above results, when taken together, could be related to the previously observed high frequency formation of anomalous somatic embryos formed in A. sellowiana, as well as to their low germination ability.     

Thidiazuron influences the endogenous levels of cytokinins and IAA during the flowering of isolated shoots of Dendrobium

This study reports the effects of thidiazuron (TDZ) on the endogenous levels of indoleacetic acid (IAA), zeatin, zeatin riboside ([9R]Z), isopentenyladenine and isopentenyladenosine ([9R]iP) as well as structural changes in the apical meristem of Dendrobium Second Love shoots during flower induction and initial development in vitro. The results revealed that the presence of 1.8microM TDZ had a profound effect on the endogenous cytokinins (CKs) and IAA levels of the explants, when compared to those grown on a TDZ-free medium. A significant increase in CKs (especially [9R]iP and [9R]Z) and IAA in the first samples (taken at day 5) grown on TDZ-enriched medium was associated with flower induction, while a second increase in the level of these hormones after 25d of culture was related to flower development. The histological changes detected in the shoot apical meristem of explants grown in the presence of 1.8microM TDZ during 30d of culture are also described. Based on these findings, it is suggested that both auxin and CKs seem to be involved with the floral transition of Dendrobium Second Love in vitro. However, a possible direct effect of TDZ on flower formation is not discarded.     

NaCI Reduces Indole-3-Acetic Acid Levels in the Roots of Tomato Plants Independent of Stress-Induced Abscisic Acid

Indole-3-acetic acid (IAA) was measured in leaves and roots of tomato (Lycopersicon esculentum) genotypes subjected to salt stress. An abscisic acid (ABA)-deficient mutant of tomato (sitiens), the genetic parent (Rheinlands Ruhm, RR), and a commercial variety (Large Cherry Red, LCR) of tomato were treated with 50 to 300 mM NaCl in nutrient culture. Both LCR and RR had significantly higher levels of IAA in the roots compared with that in sitiens prior to treatment. The initial levels of IAA in the roots of LCR and RR declined by nearly 75% after exposure to NaCl, whereas those in roots from the sitiens mutant remained unchanged. IAA levels in the leaves of all genotypes remained unchanged or increased slightly in response to NaCl. ABA was highest in leaves from the normal genotypes after exposure to NaCl. ABA levels in the roots of sitiens were similar to the levels in the normal genotypes, whereas levels in the leaves were only 10% of the levels found in normal genotypes regardless of the salt treatment. Treatment of LCR and sitiens with exogenous ABA increased the ABA levels in leaves and roots, but there were no measurable changes in endogenous IAA. Therefore, the reduction in IAA appears to result from an ABA-independent effect of NaCl on IAA metabolism in the roots of stressed plants.     

Activation of abscisic acid biosynthesis in the leaves of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> in response to water deficit

It is well known that endogenous abscisic acid (ABA) levels increase rapidly in response to drought stress and that this induces stomatal closure. In Arabidopsis thaliana, ABA levels increased rapidly in the leaves and roots when intact wild-type whole plants were exposed to drought stress. However, if the leaves and roots were separated and exposed to drought independently, the ABA level increased only in the leaves. These results suggest that, under our experimental conditions, ABA is synthesized mainly in the leaves in response to drought stress and that some of the ABA accumulated in the leaves is transported to the roots. Tracer experiments using isotope-labeled ABA indicate that the movement of ABA from leaves to roots is activated by water deficit in the roots. We also demonstrate that the endogenous ABA level in the leaves increased only when the leaves themselves were exposed to drought stress, suggesting that leaves play a major role in the production of ABA in response to acute water shortage.     

激动素和丁二酸拌种对玉米衰老过程中抗氧化系统和植物激素的影响

以‘郑单958’（晚衰型品种）和‘豫单2002’（早衰型品种）为实验材料,采用盆栽方式,0．03μg·kg-1的外源激动素（KT）和300mg·kg-1丁二酸复合剂进行拌种处理,研究拌种后玉米根叶衰老指标的变化及其化学调控效应。结果表明：激动素和丁二酸混合拌种后根系与叶片中超氧阴离子（O2-）产生速率、丙二醛（MDA）及脱落酸（ABA）含量低于其对照,而超氧化物歧化酶（SOD）活性和生长素（IAA）含量却较高,据此认为膜脂过氧化得到缓解,根叶的生理功能期延长;且在整个生育期内各个叶位叶的MDA含量、SOD活性、ABA和IAA含量高于根系,但其O2-和IAA／ABA较低,表明根系的衰老早于叶片。综上可以推测,激动素和丁二酸拌种能有效防止根叶早衰,为提高玉米产量打下基础。     

Acclimation of potato plants to polyethylene glycol-induced water deficit I. Photosynthesis and metabolism

The acclimation of photosynthesis and metabolism in response to water deficit is characterized using hydroponically grown potato plants (Solanum tuberosum cv. Désirée). Plants were subjected to a reduced water potential of the nutrient solution by adding 10% (w/v) PEG 6000. PEG-treated plants were retarded in growth. Leaves which had been fully developed before the PEG treatment and leaves grown during the PEG treatment showed different phenotypes and biochemical and physiological properties. Photosynthesis of all leaves decreased during the whole treatment. However, the decrease of photosynthesis in the two types of leaves had different causes indicated by differences in their metabolism. Leaves which were fully developed at the beginning of the PEG treatment began to wilt starting from the leaf rim. The apoplastic ABA content increased, coinciding with a decreased stomatal conductance. Increased energy charge of the cells indicated impaired chloroplastic metabolism, accompanied by a decrease of amounts of chloroplastic enzymes. The apoplastic and the symplastic ABA content were increased during water deficit and because ABA was concentrated in the cytosolic compartment it is suggested that ABA is involved in decreasing photosynthetic enzyme contents in old leaves. Young leaves, grown after the imposition of water deficit, were smaller than control leaves and had a curly surface. In young leaves apoplastic and cytosolic ABA contents were identical with control values. Carboxylation efficiency of photosynthesis was decreased, but the water use efficiency remained unchanged. Metabolic data of the photosynthetic pathways indicate a down-regulation of chloroplastic metabolism. It is concluded that in young leaves photosynthesis was non-stomatally limited. This limitation was not caused by ABA.     

Abscisic acid, indole-3-acetic acid and cytokinin changes in buds of Pseudotsuga menziesii during bud quiescence release

Bud quiescence release, considered as the ultimate dormancy breaking phase, was achieved in Pseudotsuga menziesii (Mirb.) Franco by a 9-week cold (5°C) treatment, under short daylength (9 h) followed by a transfer to mild temperature (22°C) under long daylength (16 h). Indole-3-acetic acid (IAA), abscisic acid (ABA), zeatin-type (Z) and isopentenyladenine-type (iPA) cytokinin (CK) levels were measured by means of an ELISA technique performed on HPLC-fractionated extracts of terminal and axillary buds. During the cold period, all hormones except IP-type CK levels decreased, whereas the opposite observation was made after transfer to mild temperature and long daylength, when buds started to grow. Some other immunoreactive compounds were also detected and quantified. The ABA-glucosyl ester (ABA-GE) level pattern was similar to that of ABA, but no accumulation occurred at mild temperatures. A putative IAA conjugate, more polar than IAA, was also detected. Its level increased transiently like IAA in terminal buds and, to a lesser extent, in axillary buds during the 10th week of the experiment. In terminal buds, isopentenyladenosine ([9R]-iP) was released by alkaline hydrolysis of a polar immunoreactive compound detected with anti-[9R]iP antibodies. This compound accumulated during the cold period and quickly dropped at 22°C. Relationships between environmental conditions and endogenous hormones are discussed.     

Gas exchange is related to the hormone balance in mycorrhizal or nitrogen-fixing alfalfa subjected to drought

The beneficial effect of mycorrhization on photosynthetic gas exchange of host plants under drought conditions could be related to factors other than changes in phosphorus nutrition and water uptake. Our objective was to study the influence of drought on phytohormones and gas exchange parameters in Medicago sativa L. cv. Aragón associated with or in the absence of arbuscular mycorrhizal (AM) fungi and/or nitrogen-fixing bacteria. Four treatments were used: (1) plants inoculated with Glomus fasciculatum (Taxter sensu Gerd.) Gerdemann and Trappe and Rhizobium meliloti 102 F51 strain (MR); (2) plants inoculated with only Rhizobium (R); (3) plants inoculated with only mycorrhizae (M); and (4) non-inoculated plants (N). When endophytes were well established, treatments received different levels of phosphorus and nitrogen in the nutrient solution in order to obtain plants similar in size. Sixty days after planting, plants were subjected to two cycles of drought and recovery. Midday leaf water potential (Ψ), CO₂ exchange rate (CER), leaf conductance (g_w) and transpiration (T), as well as leaf and root abscisic acid (ABA) and cytokinin concentrations were measured after the second drought period. Gas exchange parameters were determined by infrared gas analysis. Cytokinins and ABA levels in tissues were analysed by ELISA and HPLC, respectively. Nodulated R and MR plants had the lowest ABA concentrations in roots under well-watered conditions. Water stress increased ABA concentrations in leaves of N, R and MR plants, while ABA concentration in M plants did not change. The highest production of ABA under water deficit was in the roots of non-mycorrhizal plants. The ratio of ABA to cytokinin concentration strongly increased in leaves and roots of non-mycorrhizal plants under drought. By contrast, this ratio was lowered in roots of M plants and remained unchanged in leaves and roots of MR plants when stress was imposed. The highest leaf conductances and transpirational fluxes under well-watered conditions were those of nitrogen-fixing R and MR plants, but these results were not impaired with increased CO₂ exchange rates. Photosynthesis, leaf conductance and transpiration rates decreased in all treatments when stress was imposed, with the strongest decrease occurring in non-mycorrhizal plants. The relationships found between these gas exchange parameters and the hormone concentrations in stressed alfalfa tissues suggest that microsymbionts have an important role in the control of gas exchange of the host plant through hormone production in roots and the ABA/cytokinin balance in leaves. The most relevant effect of mycorrhizal fungi was observed under drought conditions.     

Growth of Zea mays L. plants with their seminal roots only. Effects on plant development, xylem transport, mineral nutrition and the flow and distribution of abscisic acid (ABA) as a possible shoot to root signal

Maize (Zea mays L.) was grown in quartz sand culture eitherwith a normal root system (controls) or with seminal roots only(‘single-rooted’). Development of adventitious rootswas prevented by using plants with an etiolated mesocotyl andthe stem base was positioned 5–8 cm above the sand. Eventhough the roots of the single-rooted plants were sufficientlysupplied with water and nutrients, the leaves experienced waterdeficits and showed decreased transpiration as trans plrationalwater flow was restricted by the constant number of xylem vesselspresent in the mesocotyl. As a consequence of this restriction,transpirational water flow velocities in the metaxylem vesselsreached mean values of 270 m h^–1 and phloem transportvelocities of 5.2 m h^–1. Despite limited xylem transportmineral nutrient concentrations in leaf tissues were not decreasedin single-rooted plants, but shoot and particularly stem developmentwas somewhat inhibited. Due to the lack of adventitious rootsthe shoot:root ratio was strongly increased in the single-rootedplants, but the seminal roots showed compensatory growth comparedto those in control plants. Consistent with decreased leaf conductance,ABA concentrations in leaves of single-rooted plants were elevatedup to 10-fold, but xylem sap ABA concentrations in these plantswere lower than in controls, in good agreement with the well-wateredconditions experienced by the seminal roots. Surprisingly, however,ABA concentrations in tissues of the seminal roots of the single-rooted plants were clearly increased compared to the controls,presumably due to increased ABA import via phloem from the water-stressedleaves. The results are discussed in relation to the role ofABA as a shoot to root signal. Key words: Zea mays, seminal roots, plant development, xylem transport, mineral nutrition, ABA, shoot-to-root signal     

The role of cytokinins in responses to water deficit in tobacco plants over-expressing trans-zeatin O-glucosyltransferase gene under 35S or SAG12 promoters

The impact of water deficit progression on cytokinin (CK), auxin and abscisic acid (ABA) levels was followed in upper, middle and lower leaves and roots of Nicotiana tabacum L. cv. Wisconsin 38 plants [wild type (WT)]. ABA content was strongly increased during drought stress, especially in upper leaves. In plants with a uniformly elevated total CK content, expressing constitutively the trans -zeatin O-glucosyltransferase gene ( 35S::ZOG1 ), a delay in the increase of ABA was observed; later on, ABA levels were comparable with those of WT.
As drought progressed, the bioactive CK content in leaves gradually decreased, being maintained longer in the upper leaves of all tested genotypes. Under severe stress (11 d dehydration), a large stimulation of cytokinin oxidase/dehydrogenase (CKX) activity was monitored in lower leaves, which correlated well with the decrease in bioactive CK levels. This suggests that a gradient of bioactive CKs in favour of upper leaves is established during drought stress, which might be beneficial for the preferential protection of these leaves.
During drought, significant accumulation of CKs occurred in roots, partially because of decreased CKX activity. Simultaneously, auxin increased in roots and lower leaves. This indicates that both CKs and auxin play a role in root response to severe drought, which involves the stimulation of primary root growth and branching inhibition.     

Effects of Age and Water Stress on Endogenous Levels of Plant Growth Regulators in Euphorbia lathyrus L.

The effect of droughting and waterlogging on changes in endogenouslevels of hormones have been studied in Euphorbia lathyrus L.with particular reference to plant age and leaf age. Younger tissues (i.e. immature leaves and young plants) synthesizedsignificantly greater amounts of ABA per unit fresh weight thandid corresponding older tissue. The increased ABA levels correlatedwell with corresponding increases in water saturation deficit(WSD) and stomatal resistance in a given treatment but the relationshipbetween the absolute values of these three parameters variedgreatly between seedlings of different ages. Endogenous ethylenelevels in leaves and stems were unaffected by droughting irrespectiveof age. Imposition of waterlogging led to marked increases in ABA levelsin young plants though this was not strictly related to changesin WSD or stomatal resistance. Endogenous levels of ethyleneincreased in response to waterlogging, this being marked inthe older seedlings which also had a higher incidence of senescenceand abscission. The more complex effects of waterlogging, the significance ofage of tissue on hormonal responses to stress, and the adaptivevalue of such responses to younger tissues are discussed.     

Manganese accumulation in leaves of Hakea prostrata (Proteaceae) and the significance of cluster roots for micronutrient uptake as dependent on phosphorus supply

When grown at a low P supply, Hakea prostrata R.Br. (Proteaceae) develops dense clusters of determinate branch roots, termed 'proteoid' or 'cluster' roots and accumulates Mn in its leaves. The aim of this study was to vary the production of cluster roots and assess the relationship between Mn uptake and cluster-root mass. We collected native soil from a location inhabited by H . prostrata and amended this with 'high' and 'low' amounts of insoluble or soluble P. After 14 months, we measured the impact of the treatments on cluster-root development and the [P], [Mn], [Fe], [Zn] and [Cu] in young (expanding) and mature leaves. Dry mass and leaf area increased with increasing P availability in the soil, but growth decreased at the highest soluble [P], which caused symptoms of P toxicity. The [P] in young leaves (1.3–2.7 mg g⁻¹ DM) exceeded that in older leaves (0.28–0.85 mg g⁻¹ DM), except when plants were grown with soluble P (3.2–21 mg g⁻¹ DM). Cluster-root formation was inhibited when leaf [P] increased; [P] in young leaves, rather than that in old leaves, appeared to be the factor that determined the proportion of the root mass invested in cluster roots. Old leaves of all treatments had [Mn] from 90 to 120 µg g⁻¹ DM, except for plants grown at high levels of soluble P, when [Mn] decreased below 30 µg g⁻¹ DM. The [Mn] and [Zn] in old leaves and the [Cu] in young leaves were positively correlated with the fraction of roots invested in cluster roots. These findings support our hypothesis that cluster roots play a significant role in micronutrient acquisition, and also provide an explanation for Mn accumulation in leaves of H . prostrata , and presumably Proteaceae in general.