Endogenous Indole-3-acetic Acid Research Articles

Synergistic interaction between coumarin 1,2-benzopyrone and indole-3-butyric acid in stimulating adventitious root formation in Vigna radiata (L.) Wilczek cuttings: I. Endogenous free and conjugated IAA and basic isoperoxidases

Cuttings from 7-day-old Vigna radiata seedlings were treated for 24 h with various concentrations of coumarin and/or indole-3-butyric acid (IBA), applied either alone or in combination, in order to stimulate adventitious root formation (ARF). The effects of treatment on endogenous free and conjugated indole-3-acetic acid (IAA), basic peroxidase (basic PER) activity and its isoperoxidases analysis and their relation to ARF were then investigated at the potential rooting sites during the first 96 h after application. Simultaneously, combined treatments acted synergistically in inducing more adventitious roots in treated cuttings than in those treated with coumarin or IBA individually, as compared with the control. Endogenous free IAA increased transiently in treated cuttings as compared with the control and the maximum increase occurred with the combined treatment. This suggests that coumarin and IBA may act synergistically in increasing the endogenous free IAA level during the induction phase of rooting to initiate more roots. Likewise, higher level of conjugated IAA was also found in treated cuttings than in untreated ones, during the primary events of ARF, with the maximum level occurring in the combined treatment. Comparison of the dynamics of conjugated IAA and activity of basic PERs led to conclusion that the former but not the latter is responsible for downregulation of endogenous IAA levels significantly during the primary events of ARF. A sharp increases in basic PERs occurred during the secondary events of ARF, suggesting their role in root initiation and development rather than root induction.

Brassinolide induces IAA5, IAA19, and DR5, a synthetic auxin response element in Arabidopsis, implying a cross talk point of brassinosteroid and auxin signaling.

Despite numerous physiological studies addressing the interactions between brassinosteroids (BRs) and auxins, little is known about the underlying molecular mechanisms. We studied the expression of IAA5 and IAA19 in response to treatment with indole acetic acid (IAA) or brassinolide (BL), the most active BR. Exogenous IAA induced these genes quickly and transiently, whereas exogenous BL induced them gradually and continuously. We also found that a fusion of DR5, a synthetic auxin response element, with the GUS (beta-glucuronidase) gene was induced with similar kinetics to those of the IAA5 and IAA19 genes in response to both IAA and BL treatment of transgenic plants. These results suggest that the IAA genes are induced by BL, at least in part, via the activation of the auxin response element. Endogenous IAA levels per gram fresh weight did not increase when seedlings of Arabidopsis wild type (WT) or the BR-deficient mutant det2 were treated with BL. Furthermore, the levels of IAA transcripts were lower in the det2 mutant than in the WT, even though endogenous IAA levels per gram fresh weight were higher in the det2 mutant than in the WT. In conclusion, the lack of evidence for auxin-mediated activation of early auxin-inducible genes in response to BL suggests that the BR and auxin signaling pathways independently activate the transcriptional system of the IAA and DR5-GUS genes.

Developmental stages during the rooting of in-vitro-cultured Quercus robur shoots from material of juvenile and mature origin.

In-vitro-cultured shoots of clones initiated from shoots of the basal parts (BS) and the crown (C) of two mature Quercus robur L. trees were subjected to rooting experiments to relate rooting with shoot topophysical origin. The BS shoots exhibited morphologically juvenile characteristics and rooted more easily after indole-3-butyric acid (IBA) treatment than C shoots. When naphthylphthalamic acid (NPA) was applied to BS shoots, rooting capacity decreased and root emergence was delayed at least 2 days compared with shoots treated with IBA only. During the first days of the rooting process, endogenous indole-3-acetic acid (IAA) concentration was higher in C shoots than in BS shoots, regardless of whether the shoots were treated with NPA. Mitotic figures were observed in cells from the basal part of both BS and C shoots 24 h after IBA treatment. After 4 days of IBA treatment, the occurrence of histological events differed between BS shoots and C shoots. Cells of BS shoots became meristematic, giving rise to meristemoids and root primordia, whereas no differentiation of root meristemoids occurred in cells of C shoots. Thus, although adult oak material (C shoots) is capable of responding to the initial stimulus of auxin during the adventitious rooting process, the endogenous IAA concentration is not the factor limiting rooting in adult material.

Endogenous auxin and cytokinin contents associated with shoot formation in leaves of pineapple cultured in vitro

The in vitro culture of pineapple leaves on a shoot induction medium (SIM) results in the formation of protuberances and further development in shoots, and plantlets. The contents of endogenous indoleacetic acid (IAA) and five cytokinins (Cks), N6(2-isopentenyl)adenine (iP), N6(2-isopentenyl)adenosine (iPR), zeatin (Z), zeatin riboside (ZR) and N6-benzyladenine (BA), present in the basal portion of those leaves, were correlated to the organogenic response that occurs over 15 days of culture. The endogenous auxin/cytokinins ratio was lowest on the 3rd day, mainly due to a strong increase in the iP level. It seems that endogenous iP concentration triggered the induction signal for an organogenic response in pineapple leaf bases. The rise in iP content required the presence of BA and a-naphthaleneacetic acid (NAA) in the medium, suggesting that endogenous iP production is regulated in response to these growth regulator uptakes.

Restart of Lignification in Micropropagated Walnut Shoots Coincides with Rooting Induction

The lignin content of walnut shoots did not change during in vitro shoot multiplication. Lignin content started to increase as soon as shoots were passed to a rooting medium with auxin. Exogenous auxin (applied for rooting) caused a transient elevation of the endogenous free indoleacetic acid (IAA) content with a simultaneous decrease of peroxidase activity. These events typically marked the completion of the rooting inductive phase (before any visible histological event, that is before the cell divisions beginning the rooting initiation phase). This meant that either the given exogenous auxin or the endogenous IAA has served as signal for the stimulation of lignification. Continued increase of lignification in the shoots required completion of root formation; this increase indeed was slown down when root emergence did not occur. It was further shown that lignification varied conversely to the content of the soluble phenol content, itself apparently being related to the activity of phenylalanine ammonia-lyase activity.

Synergistic effect of auxins and polyamines in hairy roots of Cichorium intybus L. during growth, coumarin production and morphogenesis

Hairy roots of Cichorium intybus obtained by infecting with different Agrobacterium rhizogenes strains (LMG-150 and A20/83) were studied for total endogenous indole-3-acetic acid (IAA) levels and indole-3-acetic acid oxidase (IAAO) activity. The roots initiated by LMG- 150 showed higher endogenous IAA levels as well as IAAO activity as compared to the roots from A20/83. Coumarin production in roots obtained by both of these strains strictly correlated with growth, with higher content in the roots obtained by LMG- 150. Moreover roots from LMG-150 showed increased growth index, length of primary roots and number of secondary and tertiary roots. The roots derived from LMG-150 were studied for total endogenous IAA and IAAO activity under the exogenous administration of polyamines and fungal elicitors. The treatment with putrescine (Put) at 1.5 mM level showed maximum endogenous IAA levels and IAAO activity as compared to the control and other polyamine administration, it also supported faster growth in terms of biomass accumulation, and total coumarin production. Of the various treatments, mycelial extract (ME) and culture media filtrate (CMF) of Pythium aphanidermatum and Phytopthora parasitica var. nicotiana, the treatment with 1 % CMF of P. parasitica var. nicotiana, resulted in maximum IAA levels and IAAO activity, which was supported by maximum biomass, coumarin production as compared to the control and other elicitor treatments. Two different regenerants of chicory obtained through A. rhizogenes LMG-150 designated as T-I and T-II, were studied for total endogenous IAA levels and IAAO activity. T-II showed higher titers of IAA with higher activity of IAAO as compared to T-I. Endogenous titer of IAA and IAAO activity was found to be maximum in transformed roots as compared to T-I, T-II, normal roots and normal plants. Our work showed a variation in endogenous auxin levels in these transformed plants. There exists a synergistic effect of endogenous IAA titers and polyamines in regulating root morphogenesis. Fungal elicitors influenced growth and coumarin production and an elicitor preparation of 1 % CMF of P. parasitica var. nicotiana gave spontaneous regeneration of shoots. The implications of these results are discussed.

Lateral root formation in rice (Oryza sativa L.): differential effects of indole-3-acetic acid and indole-3-butyric acid

Comparative effects of indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) on lateral root (LR) formation were studied using 2-day-old seedlings of IR8 rice (Oryza sativa L.). Results showed that IBA at all concentrations (0.8–500 nmol/L) increased the number of LRs in the seminal root. However exogenous IAA, failed to increase the number of LRs. On the other hand, both IBA and IAA caused inhibition of seminal root elongation and promotion of LR elongation, but IAA can only reach to the same degree of that of IBA at a more than 20-fold concentration. Exogenous IBA had no effect on endogenous IAA content. We conclude from the results that IBA could act directly as a distinct auxin, promoting LR formation in rice, and that the signal transduction pathway for IBA is at least partially different from that for IAA.

Relationships of endogenous plant hormones to accumulation of grain protein and starch in winter wheat under different post-anthesis soil water statusses

Accumulation of protein and starch in grain is a key process determining grain yield and quality in wheat. Under drought or waterlogging, endogenous plant hormone levels will change and may have an impact on the yield and quality of wheat. In a greenhouse experiment, four winter wheat (Triticum aestivum L.) varieties differing in grain protein content, Heimai 76, Wanmai 38, Yangmai 10 and Yangmai 9, were subjected to drought (SRWC = 45∼50%, DR), waterlogging (WL) and moderate water supply (SRWC = 75∼80%, CK), beginning from 4 days post-anthesis (DPA) to maturity. On the 10 (grain enlargement stage) and 20 (grain filling stage) DPA, endogenous abscisic acid (ABA), gibberellins (GA1+3), indole-3-acetic acid (IAA) and zeatin riboside (ZR) were determined in sink and source organs of wheat plants by enzyme linked immunosorbent assay (ELISA). The patterns of hormonal changes were similar in four varieties. The ABA levels were much higher under DR and WL than under CK. Compared with CK, GA1+3 levels in whole-plant under DR and WL changed a little at 10 DPA, but markedly decreased under DR and WL at 20 DPA. Changes of endogenous IAA level under DR and WL exhibited a complicated pattern, depending on organs and growth stages. Particularly at the 20 DPA, the mean levels of IAA in roots, leaves and grains decreased significantly under DR and WL. In comparison with CK, ZR levels in all organs significantly decreased under DR and WL at both stages. The correlation analyses between yields and contents of starch and protein in grains and levels and ratios of four hormones in source and sink organs indicated that the changes in yield and content of grain starch and protein under DR and WL were associated with the reduced IAA, ZR and GA1+3 levels and elevated ABA level in plants, especially in grains. It was proposed that the changed levels of endogenous hormones under post-anthesis DR and WL might indirectly affect protein and starch accumulation in grains by influencing the regulatory enzymes and processes.

Pollination and CPPU Treatment Increase Endogenous IAA and Decrease Endogenous ABA in Muskmelons during Early Development

An investigation was conducted to determine how pollination and CPPU treatment influence endogenous IAA and ABA content in netted muskmelon [Cucumis melo L. (Reticulatus Group) `Crest Earl's'], and to clarify their roles in fruit set and development in relation to these endogenous plant hormones. CPPU treatment at anthesis significantly increased the fresh weight of ovaries, whether the flowers were pollinated or not, but from 6 days after anthesis (DAA) the growth rate in the nonpollinated + CPPU treatment tended to be lower than the growth rates in the pollination treatment plots. Ovaries of nonpollinated flowers not treated with CPPU failed to grow and turned brown within 4 DAA. IAA content in the placenta of fruit from pollinated flowers increased rapidly from the day of anthesis to 2 DAA and remained at relatively high levels. IAA content in the placenta of parthenocarpic fruit induced to develop by CPPU treatment was lower than that of fruit from pollinated flowers but the pattern was almost the same as that in fruit of pollinated flowers. Conversely, IAA content in the placenta of fruit from nonpollinated flowers not treated with CPPU decreased sharply after anthesis. IAA content in the mesocarp of CPPU-treated fruit, whether or not the flowers were pollinated, increased significantly from the day of anthesis to 2 DAA, then decreased to almost the same level as that of the pollination-only treatment by 10 DAA, while the IAA content of nonpollinated CPPU-treated fruit declined even further. IAA content in the mesocarp of fruit from nonpollinated flowers not treated with CPPU decreased sharply. ABA contents in both the placenta and mesocarp of muskmelon that would set decreased after anthesis while the ABA content of muskmelon that would not set increased rapidly. Results suggest that pollination and CPPU treatment increased endogenous IAA content and decreased endogenous ABA content to promote the set and growth of fruit during early development. Chemical names used: [1-(2-chloro-4-pyridyl)-3-phenylurea] (CPPU); indole-3-acetic acid (IAA); abscisic acid (ABA).

The Role of auxin, pH, and stress in the activation of embryogenic cell division in leaf protoplast-derived cells of alfalfa.

Culturing leaf protoplast-derived cells of the embryogenic alfalfa (Medicago sativa subsp. varia A2) genotype in the presence of low (1 microM) or high (10 microM) 2, 4-dichlorophenoxyacetic acid (2,4-D) concentrations results in different cell types. Cells exposed to high 2,4-D concentration remain small with dense cytoplasm and can develop into proembryogenic cell clusters, whereas protoplasts cultured at low auxin concentration elongate and subsequently die or form undifferentiated cell colonies. Fe stress applied at nonlethal concentrations (1 mM) in the presence of 1 microM 2,4-D also resulted in the development of the embryogenic cell type. Although cytoplasmic alkalinization was detected during cell activation of both types, embryogenic cells could be characterized by earlier cell division, a more alkalic vacuolar pH, and nonfunctional chloroplasts as compared with the elongated, nonembryogenic cells. Buffering of the 10 microM 2,4-D-containing culture medium by 10 mM 2-(N-morpholino)ethanesulfonic acid delayed cell division and resulted in nonembryogenic cell-type formation. The level of endogenous indoleacetic acid (IAA) increased transiently in all protoplast cultures during the first 4 to 5 d, but an earlier peak of IAA accumulation correlated with the earlier activation of the division cycle in embryogenic-type cells. However, this IAA peak could also be delayed by buffering of the medium pH by 2-(N-morpholino)ethanesulfonic acid. Based on the above data, we propose the involvement of stress responses, endogenous auxin synthesis, and the establishment of cellular pH gradients in the formation of the embryogenic cell type.

P-CPA increases the endogenous IAA content of parthenocarpic muskmelon fruit

We examined the effects of seed formation andpara-chlorophenoxyacetic acid (p-CPA)treatment on the growth and endogenous indole acetic acid (IAA) content ofmuskmelon fruit. The growth of parthenocarpic muskmelon fruit induced by 1-(2-chloro-4-pyridyl)-3-phenylurea (CPPU) declined 15 days after anthesis (DAA),resulting in smaller fruit than those pollinated at harvest.p-CPA improved the growth of parthenocarpic fruit thatweretreated between 10 and 25 DAA. Endogenous IAA levels in the seedsof artificially pollinated fruit were at their highest at 10 DAA,then decreased, and increased again after 30 to 45 DAA, whereas,the levels in the empty seeds of parthenocarpic fruit were significantly lowerthroughout development. Although endogenous IAA levels in the placenta ofpollinated fruit were lower than those in the seeds, the changing patterns werevery similar to those in the seeds. Endogenous IAA levels in the mesocarp ofpollinated fruit remained lower than those in the placenta throughout fruitgrowth, and the pattern of change was similar to that of the placenta. Levelsinthe seed, placenta and mesocarp of p-CPA-nontreatedparthenocarpic fruit stayed lower than those in pollinated fruit.p-CPA increased the levels of IAA in the seeds, placenta,and mesocarp of parthenocarpic fruit after the first treatment (10DAA) to 15 DAA, while those in the mesocarp increasedsignificantly after the second treatment (25 DAA), but did notincrease in empty seed and placenta.

Distribution and transport of IAA in tomato plants

We determined as to indole-3-acetic acid (IAA), endogenous levels byliquid chromatography-mass spectrometry using 13C6-IAA, diffusible levels byfluorometric detection using indole-propionic acid, and polar transportactivityby radioactive IAA in 3-month-old tomato plants (stems, leaves or roots). TheIAA concentration in the apoplast (AP) solution was higher than those in thesymplast (SP) solution in both the upper and lower parts of stems, showing thatIAA analysis of AP solution is important. Younger leaves exported much morediffusible IAA than older leaves. The IAA concentration in the main roots wasalmost at the same level as in the stems. The results suggested that thetransport capacity of IAA is probably the limiting factor for the amount of IAAtransported in stems and the amount of polar IAA transport might be only 19% ofthe endogenous IAA amount in stems.

Effect of light on peroxidase and lignin synthesis in mungbean hypocotyls

Two days of light irradiance reduced the growth of mungbean hypocotyls as well as the levels of endogenous indole-3-acetic acid (IAA). In hypocotyls, both peroxidase and laccase activities were enhanced by light. The lignin content in mungbean hypocotyls was enhanced twofold by light. The inhibition of mungbean hypocotyl growth caused by light might be due to the decline of endogenous IAA, which could be degraded by a cationic peroxidase. The higher levels of lignin were correlated with the increased anionic peroxidase activity in light-treated tissues.

Physiological Factors Affecting Response of Mature `Valencia' Orange Fruit to CMN-Pyrazole. II. Endogenous Concentrations of Indole-3-Acetic Acid, Abscisic Acid, and Ethylene

Endogenous concentrations of IAA and ABA in the peel, pulp, seed, and abscission zone of mature `Valencia' oranges [Citrus sinesis (L.) Osbeck] were determined by high-performance liquid chromatography and enzyme-linked immunosorbent assay from early November 1998 to mid-June 1999. Ethylene production of mature `Valencia' oranges during the same period was determined by gas chromatography. IAA concentrations in the pulp and seed were three to five times lower than those in the peel over the 7-month observation period. IAA concentration in the abscission zone and peel was high from late April to mid-May, the period of less responsiveness to abscission chemicals. ABA concentration in the pulp was low over the entire observation period. ABA concentration in the abscission zone and peel was low during the less responsive period. Ethylene production was always low except for a slight increase during late December and early February. The IAA to ABA ratio was high in the fruit abscission zone during the less responsive period. Fruit detachment force of CMN-pyrazole-treated fruit was positively correlated with the ratio of endogenous IAA to ABA or endogenous IAA, but negatively to endogenous ABA in the fruit abscission zone. These data suggest the balance between IAA and ABA in the fruit abscission zone may be an important factor in determining sensitivity and thereby the response of mature `Valencia' orange fruit to abscission chemicals. Chemical names used: abscisic acid (ABA); indole-3-acetic acid (IAA); 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMN-pyrazole).

Effects of salinity on endogenous ABA, IAA, JA, AND SA in Iris hexagona.

Phytohormones play critical roles in regulating plant responses to stress. We investigated the effects of salinity on abscisic acid (ABA), indole-3-acetic acid (IAA), salicylic acid (SA), and jasmonic acid (JA) in leaves, stalks, fruits, and seeds of Iris hexagona, a native wetland species. Using gas chromatography-mass spectroscopy with selected ion monitoring, our experiments demonstrated significant and different short- and long-term changes in iris phytohormones. ABA and JA generally increased and IAA and SA declined in response to salinity. We conclude that these phytohormones may have separate and interactive effects on how plants respond and adapt to stress in natural environments.

Effects of Day and Night Temperatures on Hypocotyl Elongation and Endogenous Levels of Indole-3-acetic and Abscisic Acids in Radish (Raphaus sativus L.).

The effect of the difference between day and night temperatures (DIF), under a 12-hr photoperiod on hypocotyl elongation (HE) of radish (Raphanus sativus L.) and the roles of endogenous indole-3-acetic acid (IAA) and abscisic acid (ABA) on this growth process were investigated. At 10 DIF, HE was rapid enhanced during the day (DP) but slow at night (NP). Hence, the elongation pattern showed a stepped curve. At 0 DIF, HE during DP was only slightly shorter than that during NP so that the slope of the growth curve changed gradually. At -10 DIF, HE was enhanced at NP, whereas it nearly stopped during DP, so that the stepped curve became exaggerated. In all DIF treatments, changes in IAA content paralleled those of HE ; the highest level of IAA was found at 10 DIF at night. Contrarily, a rapid increase and the highest level in ABA occurred after 36 hr after treatment started at -10 DIF during which HE was retarded. These data indicate that DIF regulates HE in radish by controlling endogenous IAA and ABA synthesis.

The relative importance of tryptophan-dependent and tryptophan-independent biosynthesis of indole-3-acetic acid in tobacco during vegetative growth.

A quantitative study of indole-3-acetic acid (IAA) turnover, and the contribution of tryptophan-dependent and tryptophan-independent IAA-biosynthesis pathways, was carried out using protoplast preparations and shoot apices obtained from wild-type and transgenic, IAA-overproducing tobacco (Nicotiana tabacum L.) plants, during a phase of growth when the level of endogenous IAA was stable. Based on the rate of disappearance of [13C6]IAA, the half-life of the IAA pool was calculated to be 1.1 h in wild-type protoplasts and 0.8 h in protoplasts from the IAA-overproducing line, corresponding to metabolic rates of 59 and 160 pg IAA (microg Chl)(-1) h(-1), respectively. The rate of conversion of tryptophan to IAA was 15 pg IAA (microg Chl)(-1) h(-1) in wild-type protoplasts and 101 pg IAA (microg Chl)(-1) h(-1) in protoplasts from IAA-overproducing plants. In both instances, IAA was metabolised more rapidly than it was synthesised from tryptophan. As the endogenous IAA pools were in a steady state, these findings indicate that IAA biosynthesis via the tryptophan-independent pathway was 44 pg IAA (microg Chl)(-1) h(-1) and 59 pg IAA (microg Chl)(-1) h(-1), respectively, in the wild-type and transformed protoplast preparations. In a parallel study with apical shoot tissue, the presumed site of IAA biosynthesis, the rate of tryptophan-dependent IAA biosynthesis exceeded the rate of metabolism of [13C6]IAA despite the steady state of the endogenous IAA pool. The most likely explanation for this anomaly is that, unlike the protoplast system, injection of substrates into the apical tissues did not result in uniform distribution of label, and that at least some of the [2H5]tryptophan was metabolised in compartments not normally active in IAA biosynthesis. This demonstrates the importance of using experimental systems where labelling of the precursor pool can be strictly controlled.

Root hair elongation is inhibited by hypaphorine, the indole alkaloid from the ectomycorrhizal fungus Pisolithus tinctorius, and restored by indole-3-acetic acid.

Hypaphorine, the major indolic compound isolated from the ectomycorrhizal fungus Pisolithus tinctorius, controls the elongation rate of root hairs. At inhibitory concentrations (100 microM), hypaphorine induced a transitory swelling of root hair tips of Eucalyptus globulus Labill. ssp. bicostata. When the polar tip growth resumed, a characteristic deformation was still visible on elongating hairs. At higher hypaphorine concentrations (500 microM and greater), root hair elongation stopped, only 15 min after application. However, root hair initiation from trichoblasts was not affected by hypaphorine. Hypaphorine activity could not be mimicked by related molecules such as indole-3-acetic acid (IAA) or tryptophan. While IAA had no activity on root hair elongation, IAA was able to restore the tip growth of root hairs following inhibition by hypaphorine. These results suggest that hypaphorine and endogenous IAA counteract in controlling root hair elongation. During ectomycorrhiza development, the absence of root hairs might be due in part to fungal release of molecules, such as hypaphorine, that inhibit the elongation of root hairs.

The Role of Free Indole-3-acetic Acid (IAA) Levels, IAA Transport, and Sucrose Transport in the High Temperature Inhibition of Primary Root Development in Pea (Pisum sativum L. cv. Alaska)

Growth of Alaska pea (Pisum sativum L.) at high temperature results in general inhibition of the root system. As part of a study on the effects of temperature on developmental timing, we tested the possibility that reductions in free indole-3-acetic acid (IAA) levels, IAA transport, or sucrose transport might be responsible for inhibition of primary roots. Seedlings were grown at 25 and 32°C. For transport studies, pea cotyledons were inoculated with 3 H-sucrose and 14 C-IAA. Free endogenous IAA in tips of pea primary roots was determined at three stages of development by gas chromatography-mass spectrometry (GC-MS) using a 13 C-IAA internal standard. Differences in mean free IAA concentration between temperature treatments were not significant during the early stages of development (28 to 58 ng/g FW). IAA concentration increased significantly in root tips during the late stage at both temperatures (108-165 ng/g FW) when elongation was declining. Differences in sucrose and IAA transport rates between temperatures were minor. Differences at each temperature over time were consistent with stage-specific growth patterns. Neither transport failure nor differences in free IAA level appear responsible for developmental inhibition of pea primary roots. Results suggest that pea primary root inhibition at high temperature is not due to heat-induced stress on transport systems but rather to shifting of developmental timing.

Indole acetic acid and abscisic acid levels in new shoots and fibrous roots of citrus scion-rootstock combinations

Citrus rootstocks are important for the growth of scion varieties, but it is not clear how they regulate scion vigor. We studied three citrus rootstock cultivars; ‘Flying Dragon’ ( Poncirus trifoliata var. monstrosa) a dwarfing rootstock, ‘Swingle’ citrumelo ( P. trifoliata x Citrus paradisi) a vigorous rootstock, and trifoliate orange ( P. trifoliata) as the control. Tender buds from new shoots of ‘Eureka’ lemon were cleft grafted on etiolated rootstock seedlings. Eighteen months after grafting, the dry matter of each part of the young grafts was measured. Top weight was the greatest on ‘Swingle’ citrumelo and smallest on ‘Flying Dragon’, but there was no distinct difference in root growth among the rootstocks. Endogenous indole acetic acid (IAA) and abscisic acid (ABA) were measured in the new shoots and fibrous roots. The IAA level in the new shoots was highest in ‘Swingle’ citrumelo and lowest in ‘Flying Dragon’. The ABA level in the new shoots was highest in ‘Flying Dragon’ and lowest in ‘Swingle’ citrumelo. Both the IAA and ABA levels in the fibrous roots were highest in the strains of Poncirus trifoliata and lowest in ‘Swingle’ citrumelo. The vigorous rootstock ‘Swingle’ citrumelo had the highest T–R and IAA-ABA ratios in the new shoots.