Proline accumulation and its implication in cold tolerance of regenerable maize callus

Embryogenic callus of maize (Zea mays L.) inbreds B37wx, H99, H99³H95, Mo17, and Pa91 accumulated proline to levels 2.1 to 2.5 times that of control callus when subjected to mannitol-induced water stress, cool temperatures (19°C) and abscisic acid (ABA). A combination of 0.53 molar mannitol plus 0.1 millimolar ABA induced a proline accumulation to about 4.5 times that of control callus, equivalent to approximately 0.18 millimoles proline per gram fresh weight of callus. Proline accumulation was directly related to the level of mannitol in the medium. Levels of ABA greater than 1.0 micromolar were required in the medium to induce proline accumulation comparable to that induced by mannitol. Mannitol and ABA levels that induced maximum accumulation of proline also inhibited callus growth and increased tolerance to cold. Proline (12 millimolar) added to the culture media also increased the tolerance of callus to 4°C. The increased cold tolerance induced by the combination of mannitol and ABA has permitted the storage of the maize inbreds A632, A634Ht, B37wx, C103DTrf, Fr27rhm, H99, Pa91, Va35, and W117Ht at 4°C for 90 days which is more than double the typical survival time of callus. These studies show that proline and conditions which induce proline accumulation increase the cold tolerance of regenerable maize callus.     

Stimulation of in vitro root and shoot growth of potato by increasing sucrose concentration in the presence of fluridone,an inhibitor of abscisic acid synthesis

Fluridone, an inhibitor of abscisic acid (ABA) biosynthesis, strongly stimulated rooting of nodal stem segments of potato (Solanum tuberosum L.) cultivar Arran Banner cultured in darkness on tuberisation medium. Inclusion of 10^-6 M ABA in the culture medium prevented this rooting response, indicating that root proliferation in the presence of fluridone could be due to inhibition of ABA synthesis. The rooting response to fluridone (increased total root number and root fresh weight) was obtained only at high sucrose concentrations (0.175 and 0.234 M) and was demonstrated with two potato cultivars and two culture media; one which favoured tuberisation and one which did not. Shoot numbers were also increased, but to a lesser extent than root numbers, and total fresh weight of plant material per culture was greatly increased by inclusion of both fluridone (10^-6 or 10^-5 M) and 0.234 M sucrose in the culture medium. The role of sucrose was not simply osmotic because when the osmolarity of fluridone medium was increased using mixtures of mannitol and sucrose, no root proliferation occurred unless sucrose predominated in the mixture.     

Mevalonate-activating enzymes in callus culture cells from Nepeta cataria

The 30000 g supernatants from cell-free extracts of Nepeta cataria leaf tissue and leaf callus tissue have mevalonic acid kinase, mevalonic acid phosphate kinase and mevalonic acid pyrophosphate decarboxylase activities. The callus tissue cell-free extract produced mevalonic acid pyrophosphate and isopentenyl pyrophosphate; however, very little mevalonic acid phosphate was observed. The leaf cell-free extracts incubated with [¹⁴C]-mevalonic acid produced higher amounts of mevalonic acid phosphate. When both the leaf cell-free extract and the callus cell-free extract were incubated with [¹⁴C]-mevalonic acid in the presence of iodoacetamide, the ion exchange column elution profile was cleaner, which was confirmed by PC. Apparently the callus tissue 30000 g supernatant contains mevalonic acid phosphorylating enzymes even though there is no production of the methyl cyclopentane monoterpenes.     

Inhibition of abscisic acid biosynthesis inCercospora rosicola by triarimol

The fungicide triarimol was tested for its effect on abscisic acid (ABA) accumulation in growing culturesof Cercospora rosicola. ABA accumulation was reduced by approximately 50% with 10^?8 M triarimol. Growth ofC. rosicola, as measured by dry weight accumulation, was inhibited by triarimol concentrations at or greater than 10^?7 M. These results are compared with those obtained with clomazone, ancymidol, and paclobutrazol, which inhibit ABA accumulation by 50% at concentrations of 5 × 10^?5, 5 × 10^?6, and 5 × 10^?7 M, respectively. Triarimol, therefore, is among the most potent inhibitors of ABA biosynthesis reported to date. Feeding studies with [¹⁴C]mevalonic acid confirmed the inhibition of ABA biosynthesis by 5 × 10^?8 M triarimol. These results support previous suggestions that one or more of the steps in the ABA biosynthetic pathway from mevalonic acid is catalyzed by cytochrome P-450. Feeding studies with 1′-deoxy-[²H]-ABA in resuspended cultures ofC. rosicola show that the conversion of this substrate is not inhibited by triarimol.     

The effect of abscisic acid on the uptake of potassium and chloride into Avena coleoptile sections

Summary The effect of abscisic acid (ABA) on uptake of potassium (⁸⁶Bb⁺ or ⁴²K⁺) by Avena sativa L. coleoptile sections was investigated. ABA lowered the potassium uptake rate within 30 min after its application and inhibition reached a maximum (ca. 75%) after 2 h. The inhibition of K⁺ uptake increased with ABA concentration over a range of 0.03 to 10 g/ml ABA. At a higher K⁺ concentration (20 mM) the percentage inhibition decreased. The percentage inhibition of K⁺ uptake by ABA remained constant with external K⁺ varied from 0.04 to 1.0 mM. After a loading period in 20 mM K⁺ (⁸⁶Rb⁺), apparent efflux of potassium was only slightly increased by ABA. Experiments in which growth was greatly reduced by mannitol or by omission of indole-3-acetic acid from the medium indicated there was no simple quantitative correspondence between ABA inhibition of coleoptile elongation and ABA inhibition of K⁺ uptake. Chloride uptake was also inhibited by ABA but to a smaller degree than was K⁺ uptake. No specificity for counterions was observed for K⁺ uptake. Uptake of 3,0-methylglucose and proline were inhibited by ABA to a much smaller extent (14 and 11%) than that of K⁺, a result which suggests that ABA acts on specific ion uptake mechanisms.     

Abscisic Acid accelerates adaptation of cultured tobacco cells to salt

Adaptation of tobacco (Nicotiana tabacum L. var Wisconsin 38) cells to NaCl was accelerated by (±) abscisic acid (ABA). In medium with 10 grams per liter NaCl, ABA stimulated the growth of cells not grown in medium with NaCl (unadapted, S-0) with an increasing response from 10⁻⁸ to 10⁻⁴ molar. ABA (10⁻⁵ molar) enhanced the growth of unadapted cells in medium with 6 to 22 grams per liter NaCl but did not increase the growth of cells previously adapted to either 10 (S-10) or 25 (S-25) grams per liter NaCl unless the cells were inoculated into medium with a level of NaCl higher than the level to which the cells were adapted. The growth of unadapted cells in medium with Na₂SO₄ (85.5 millimolar), KCl (85.5 or 171 millimolar), K₂SO₄ (85.5 millimolar) was also stimulated by ABA. ABA (10⁻⁸-10⁻⁴ molar) did not accelerate the growth of unadapted cells exposed to water deficits induced by polyethylene glycol (molecular weight 8000) (5-20 grams per 100 milliliters), sorbitol (342 millimolar), mannitol (342 millimolar) or sucrose (342 millimolar). These results suggest that ABA is involved in adaptation of cells to salts, and is not effective in promoting adaptation to water deficits elicited by nonionic osmotic solutes.     

Chloroplasts and the biosynthesis and catabolism of abscisic acid

Chloroplast preparations from the mesocarp ofPersea gratissima and from light-grown shoots ofPisum sativum were unable to synthesize abscisic acid (ABA) from mevalonolactone, mevalonic acid, or isopentenyl pyrophosphate. Similar plastid preparations transformed [2-¹⁴C]ABA into acidic products that were chromatographically similar to those generated byP. gratissima mesocarp slices and excised shoots ofP. sativum. Attempts to increase ABA catabolism in chloroplast preparations using sedimentation through Percoll to remove associated proteases also reduced the capacity for ABA catabolism, suggesting that such catabolism arose from contaminating, cytoplasmic enzymes. Both lincomycin and chloramphenicol inhibited the catabolism of ABA by excised shoots ofP. sativum but had little effect on either ABA biosynthesis or ABA catabolism in mesocarp fromP. gratissima. These processes were inhibited markedly by cycloheximide.     

Inhibition of abscisic acid biosynthesis inCercospora rosicola by triarimol

The fungicide triarimol was tested for its effect on abscisic acid (ABA) accumulation in growing culturesof Cercospora rosicola. ABA accumulation was reduced by approximately 50% with 10^–8 M triarimol. Growth ofC. rosicola, as measured by dry weight accumulation, was inhibited by triarimol concentrations at or greater than 10^–7 M. These results are compared with those obtained with clomazone, ancymidol, and paclobutrazol, which inhibit ABA accumulation by 50% at concentrations of 5 × 10^–5, 5 × 10^–6, and 5 × 10^–7 M, respectively. Triarimol, therefore, is among the most potent inhibitors of ABA biosynthesis reported to date. Feeding studies with [¹⁴C]mevalonic acid confirmed the inhibition of ABA biosynthesis by 5 × 10^–8 M triarimol. These results support previous suggestions that one or more of the steps in the ABA biosynthetic pathway from mevalonic acid is catalyzed by cytochrome P-450. Feeding studies with 1-deoxy-[²H]-ABA in resuspended cultures ofC. rosicola show that the conversion of this substrate is not inhibited by triarimol.     

Influence of Abscisic Acid on Phosphorus Metabolism and Respiration in Potato Tuber Discs. Comparison with the Cycloheximide Effect

Ageing of discs of potato tuber by incubation in an aerated medium, produces an increase in the rates of respiration and of phosphate uptake. The presence of cycloheximide (CHM) or abscisic acid (ABA) in the uptake medium, does not change uptake by fresh tissue over 3–4 h. On the other hand, CHM causes an inhibition of the rate of uptake by aged tissue although ABA does not. The addition of CHM or ABA to the ageing medium, prevents totally (CHM) or only partially (ABA) the increase in phosphate uptake. The analysis of ³²P-incorporation into the various phosphorylated fractions after 24 h of ageing with CHM or ABA show that CHM induces a large inhibition of the rate of uptake with an almost complete inhibition of ³²P-incorporation into the various phosphorylated fractions. By contrast, ABA produces equal inhibition of ³²P labelling of all fractions including all acid-soluble components. CHM prevents the increase in the rate of respiration, whereas ABA causes a slight stimulation. In both cases, no important effect on ATP content was observed. These results are discussed in terms of a comparison of ABA and CHM actions. They lead to the hypothesis of a specific effect of ABA on the development of the uptake mechanism.     

Callus culture from hypocotyls of Kosteletzkya virginica (L.) seedlings

It was shown that callus established from Kosteletzkya virginica (L.) Presl. (Malvaceae) can grow in salinities higher than 200 mM NaCl if previously accomodated stepwise. Callus lines developed from seedlings of different harvests or of the same harvest at different times, all showed the same pattern of growth and sensitiviy to salinity. The absorption of Na⁺ into the callus increased with increasing external NaCl concentration. In the callus, Na⁺ was apparently distributed outside and inside a cellular membrane (possibly the plasmalemma). This membrane was, apparently, capable of regulating the Na⁺ concentration in the protoplast. Outside this membrane Na⁺ accumulated to concentrations higher than in the external growth medium. Exogenously supplied proline or glycine-betaine did not affect the growth of the callus. Externally applied ABA stimulated growth under saline conditions and increased the accumulation of proline. Growth and proline content were positively correlated in callus exposed to salinity, but in the presence of ABA they were negatively correlated. ABA was involved in both growth and proline accumulation, but there was no clear relationship between these two effects. Both ABA and proline, if added to the growth medium, improved the appearence of the callus.Abbreviations ABA abscisic acid - B₅ Gamborg's medium - BA benzylalanine - 2,4-d 2,4-dichlorophenoxy acetic acid - FW fresh weight - G B₅ medium without growth regulators - GH B₅ medium supplemented with growth regulators - NAA naphthalene acetic acid - PGR plant growth regulators - Q _T total amount of a certain ion in the tissue - Q _s amount of the ion that has leaked out - QAC Quaternarty Ammonium Compounds - RGR mean relative growth rate - W₁ and W₂ fresh weight at times t₁ and t₂     

Biosynthesis of the prenyl side chains of plastiquinone and related compounds in maize and barley shoots

The incorporation of ¹⁴C by etiolated maize and barley shoots exposed to light of ¹⁴CO₂ and [2-¹⁴C]mevalonic acid into phylloquinone, plastoquinone, ubiquinone, α-tocopherolquinone and α-tocopherol was examined. In maize (the principal tissue studied) it was demonstrated that ¹⁴C from [2-¹⁴C]mevalonic acid is incorporated into phylloquinone, plastoquinone and ubiquinone. α-Tocopherol and α-tocopherolquinone, although undoubtedly labelled from this substrate, were not purified completely. As expected, ¹⁴C from ¹⁴CO₂ was incorporated into all components examined. Ozonolytic degradation studies showed that ¹⁴C from [2-¹⁴C]mevalonic acid was incorporated specifically into the prenyl side chains of plastoquinone and ubiquinone, and from this it was inferred that mevalonic acid can be regarded as the specific distal precursor to the prenyl portions of all terpenoid quinones occurring in plant tissues. From a comparison of the relative incorporation of ¹⁴C from ¹⁴CO₂ and [2-¹⁴C]mevalonic acid into the intra- and extra-chloroplastidic terpenoids evidence was obtained consistent with the tenet that the prenyl portions of the chloroplastidic quinones phylloquinone and plastoquinone, along with β-carotene, are biosynthesized within the confines of the chloroplast, the side chain of the extraplastidic ubiquinone and phytosterols being synthesized elsewhere within the cell. The results obtained for the incorporation of ¹⁴C from ¹⁴CO₂ and [2-¹⁴C]mevalonic acid into α-tocopherol and α-tocopherolquinone were not readily interpretable with regard to the site of synthesis of these compounds.     

Cell biological mechanism for triggering of ABA accumulation under water stress inVicia faba leaves

Water stress-induced ABA accumulation is a cellular signaling process from water stress perception to activation of genes encoding key enzymes of ABA biosynthesis, of which the water stress-signal perception by cells or triggering mechanism of the ABA accumulation is the center in the whole process of ABA related-stress signaling in plants. The cell biological mechanism for triggering of ABA accumulation under water stress was studied in leaves ofVicia faba. Mannitol at 890 mmol ·kg^-1 osmotic concentration induced an increase of more than 5 times in ABA concentration in detached leaf tissues, but the same concentration of mannitol only induced an increase of less than 40 % in ABA concentration in protoplasts. Like in detached leaf tissues, ABA concentration in isolated cells increased more than 10 times under the treatment of mannitol at 890 mmol · kg^-1 concentration, suggesting that the interaction between plasmalemma and cell wall was essential to triggering of the water stress-induced ABA accumulation. Neither Ca²⁺-chelating agent EGTA nor Ca²⁺channel activator A23187 nor the two cytoskeleton inhibitors, colchicine and cytochalasin B, had any effect on water stress-induced ABA accumulation. Interestingly water stress-induced ABA accumulation was effectively inhibited by a non-plasmalemma-permeable sulfhydryl-modifier PCMBS (p-chloromercuriphenyl-sulfonic acid), suggesting that plasmalemma protein(s) may be involved in the triggering of water stress-induced ABA accumulation, and the protein may contain sulfhydryl group at its function domain.     

Cell biological mechanism for triggering of ABA accumulation under water stress inVicia faba leaves

Water stress-induced ABA accumulation is a cellular signaling process from water stress perception to activation of genes encoding key enzymes of ABA biosynthesis, of which the water stress-signal perception by cells or triggering mechanism of the ABA accumulation is the center in the whole process of ABA related-stress signaling in plants. The cell biological mechanism for triggering of ABA accumulation under water stress was studied in leaves ofVicia faba. Mannitol at 890 mmol ·kg^-1 osmotic concentration induced an increase of more than 5 times in ABA concentration in detached leaf tissues, but the same concentration of mannitol only induced an increase of less than 40 % in ABA concentration in protoplasts. Like in detached leaf tissues, ABA concentration in isolated cells increased more than 10 times under the treatment of mannitol at 890 mmol · kg^-1 concentration, suggesting that the interaction between plasmalemma and cell wall was essential to triggering of the water stress-induced ABA accumulation. Neither Ca²⁺-chelating agent EGTA nor Ca²⁺channel activator A23187 nor the two cytoskeleton inhibitors, colchicine and cytochalasin B, had any effect on water stress-induced ABA accumulation. Interestingly water stress-induced ABA accumulation was effectively inhibited by a non-plasmalemma-permeable sulfhydryl-modifier PCMBS (p-chloromercuriphenyl-sulfonic acid), suggesting that plasmalemma protein(s) may be involved in the triggering of water stress-induced ABA accumulation, and the protein may contain sulfhydryl group at its function domain.     

Chloroplasts and the biosynthesis and catabolism of abscisic acid

Chloroplast preparations from the mesocarp ofPersea gratissima and from light-grown shoots ofPisum sativum were unable to synthesize abscisic acid (ABA) from mevalonolactone, mevalonic acid, or isopentenyl pyrophosphate. Similar plastid preparations transformed [2-¹⁴C]ABA into acidic products that were chromatographically similar to those generated byP. gratissima mesocarp slices and excised shoots ofP. sativum. Attempts to increase ABA catabolism in chloroplast preparations using sedimentation through Percoll to remove associated proteases also reduced the capacity for ABA catabolism, suggesting that such catabolism arose from contaminating, cytoplasmic enzymes. Both lincomycin and chloramphenicol inhibited the catabolism of ABA by excised shoots ofP. sativum but had little effect on either ABA biosynthesis or ABA catabolism in mesocarp fromP. gratissima. These processes were inhibited markedly by cycloheximide.     

Abscisic Acid accumulation in spinach leaf slices in the presence of penetrating and nonpenetrating solutes

Abscisic acid (ABA) accumulated in detached, wilted leaves of spinach (Spinacia oleracea L. cv Savoy Hybrid 612) and reached a maximum level within 3 to 4 hours. The increase in ABA over that found in detached turgid leaves was approximately 10-fold. The effects of water stress could be mimicked by the use of thin slices of spinach leaves incubated in the presence of 0.6 molar mannitol, a compound which causes plasmolysis (loss of turgor). About equal amounts of ABA were found both in the leaf slices and in detached leaves, whereas 2 to 4 times more ABA accumulated in the medium than in the slices. When spinach leaf slices were incubated with ethylene glycol, a compound which rapidly penetrates the cell membrane causing a decrease in the osmotic potential of the tissue and only transient loss of turgor, no ABA accumulated. Ethylene glycol was not inhibitory with respect to ABA accumulation. Spinach leaf slices incubated in both ethylene glycol and mannitol had ABA levels similar to those found when slices were incubated with mannitol alone. Increases similar to those found with mannitol also occurred when Aquacide III, a highly purified form of polyethylene glycol, was used. Aquacide III causes cytorrhysis, a situation similar to that found in wilted leaves. Thus, it appears that loss of turgor is essential for ABA accumulation.

When spinach leaf slices were incubated with solutes which are supposed to disturb membrane integrity (KHSO₃, 2-propanol, or KCl) no increase in ABA was observed. These data indicate that, with respect to the accumulation of ABA, mannitol caused a physical stress (loss of turgor) rather than a chemical stress (membrane damage).

     

Abscisic acid and auxin accumulation in Catasetum fimbriatum roots growing in vitro with high sucrose and mannitol content

Endogenous contents of indolyl-3-acetic acid (IAA) and abscisic acid (ABA) were quantified in excised roots of Catasetum fimbriatum (Orchidaceae) cultured in vitro on solidified Vacin and Went medium with 1, 2, 4, 6, 8 and 10 % sucrose, as well as 2 % sucrose plus mannitol. Maximum root growth was observed in media with 4 % sucrose and 2 % sucrose plus 2.2 % mannitol, suggesting that a moderate water or osmotic stress promotes orchid root growth. Contents of both ABA and IAA increased in parallel to increasing sucrose concentration and a correlation between root elongation and the ABA/IAA ratio was observed. Incubating isolated C. fimbriatum roots with radiolabeled tryptophan, we showed an accumulation of IAA and its conjugates.     

半夏缓慢生长法保存及体细胞变异的ISSR检测

以添加了不同浓度的甘露醇、PP333和ABA的培养基对半夏试管苗进行缓慢生长法保存,并对保存材料再生后代的体细胞变异进行检测。结果显示,甘露醇、PP333和ABA均能有效抑制试管苗生长,且存活率高;最佳浓度分别为甘露醇2%～4%,PP3332.0mg·L-1,ABA 2.0～4.0mg·L-1。保存在添加了2%～4%甘露醇或2.0mg·L-1 PP333培养基上的植株未检测到变异,而保存在添加了2.0～4.0mg·L-1 ABA培养基上的植株检测到1条新增标记和1条缺失标记,位点变异率为1.7%,个体变异率为30%。研究表明,ABA不宜用于半夏试管苗的缓慢生长法保存,但有助于新突变体的产生,在种质创新上具有特殊意义。     

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     

Levels of short-chain fatty acids and of abscisic acid in water-stressed and non-stressed leaves and their effects on stomata in epidermal strips and excised leaves

Straight-chain saturated fatty acids (C6-C11) and abscisic acid (ABA) accumulate in the leaves of Phaseolus vulgaris L. and Hordeum vulgare L. under water stress. ABA and certain of the fatty acids, particularly decanoic and undecanoic acid, can inhibit stomatal opening and cause stomatal closure in epidermal strips of Commelina communis L. depending on the incubating medium used. 10^-4 M (±)-ABA inhibits opening in media containing either high or relatively low concentrations of KCl but causes closure only in the latter medium. The fatty acids (at 10^-4 M) prevent opening in both media while significant closure of open stomata was caused only by undecanoic acid in both media and, additionally, by decanoic acid in the low-KCl medium. 10^-4 M formic acid also caused stomatal closure and prevented opening to significant extents in the low-KCl medium (it was not tested in the high-KCl medium). The efficacy of undecanoic acid in causing 50% inhibition of opening is about three orders of magnitude lower than that of ABA. At a concentration of 10^-3 M, nonanoic, decanoic and particularly undecanoic acid and all-trans-farnesol cause increased cell leakage in Beta vulgaris L. root tissue. Undecanoic acid (10^-4 M) also causes some loss of guard cell integrity in C. communis within 1.5 h of treatment. ABA (10^-4 M) reduces transpiration rates in barley and C. communis leaves when applied via the transpiration stream but decanoic and undecanoic acids did not have this effect. Transpiration was not affected when ABA or the fatty acids were applied to the leaf surfaces.Abbreviations ABA abscisic acid - RWC relative water content - SCFA short-chain fatty acids Deceased May 1977     

Light-induced inhibitors from intact and cultured caps of Zea roots

Growth inhibitors were assayed from extracts of intact (attached) and of excised (cultured) root caps of Zea mays L., cv. Merit, the roots of which show a positive geotropic response only after exposure to light. If caps are intact at the time of illumination, at least two inhibitory substances are produced, an acid inhibitor and a neutral inhibitor, whereas if caps are detached from roots, placed in culture and then illuminated only the neutral inhibitor is formed. Cycloheximide retards inhibitor production in both intact and cultured caps. When [¹⁴C]mevalonic acid is included in the culture medium and the caps are illuminated, 15–25% of the recoverable ¹⁴C cochromatographs with the neutral inhibitor, whereas in caps cultured in the dark, this radiolabelling pattern is not observed. Cyloheximide in the light reduces the incorporation of ¹⁴C into compounds cochromatographing at the R_f of the neutral inhibitor. It is suggested that the neutral inhibitor may be important in the light-induced bending of roots.Abbreviations ABA abscisic acid - CH cycloheximide