Sucrose Phosphate Synthase, Sucrose Synthase, and Invertase Activities in Developing Fruit of Lycopersicon esculentum Mill. and the Sucrose Accumulating Lycopersicon hirsutum Humb. and Bonpl

The green-fruited Lycopersicon hirsutum Humb. and Bonpl. accumulated sucrose to concentrations of about 118 micromoles per gram fresh weight during the final stages of development. In comparison, Lycopersicon esculentum Mill. cultivars contained less than 15 micromoles per gram fresh weight of sucrose at the ripe stage. Glucose and fructose levels remained relatively constant throughout development in L. hirsutum at 22 to 50 micromoles per gram fresh weight each. Starch content was low even at early stages of development, and declined further with development. Soluble acid invertase (EC 3.2. 1.26) activity declined concomitant with the rise in sucrose content. Acid invertase activity, which was solubilized in 1 molar NaCl (presumably cell-wall bound), remained constant throughout development (about 3 micromoles of reducing sugars (per gram fresh weight) per hour. Sucrose phosphate synthase (EC 2.4.1.14) activity was present at about 5 micromoles of sucrose (per gram fresh weight) per hour even at early stages of development, and increased sharply to about 40 micromoles of sucrose (per gram fresh weight) per hour at the final stages of development studied, parallel to the rise in sucrose content. In comparison, sucrose phosphate synthase activity in L. esculentum remained low throughout development. The possible roles of the sucrose metabolizing enzymes in determining sucrose accumulation are discussed.     

The occurrence of nitrate reductase in apple leaves

Nitrate reductase utilizing NADH or reduced flavin mononucleotide (FMNH₂) as electron donor was extracted from the leaves, stems and petioles, and roots of apple seedlings. Successful extraction was made possible by the use of insoluble polyvinylpyrrolidone (Polyclar AT) which forms insoluble complexes with polyphenols and tannins. The level of nitrate reductase per gram fresh weight was highest in the leaf tissue although the nitrate content of the roots was much higher than that of the leaves. Nitrite reductase activity was detected only in leaf extracts and was 4 times higher than nitrate reductase activity. Nitrate was found in all parts of young apple trees and trace amounts were also detected in mature leaves from mature trees. Nitrate reductase was induced in young leaves of apple seedlings and in mature leaves from 3 fruit-bearing varieties. An inhibitor of polyphenoloxidase, 2-mercaptobenzothiazole was used in both the inducing medium and the extracting medium in concentrations from 10⁻³ to 10⁻⁵m with no effect upon nitrate reductase activity.     

Seasonal and diurnal variation in nitrate reductase activity of cowpeas

The seasonal and diurnal variations in nitrate reductase activity (NRA) of cowpea cultivars were determined. Mean activity per gram fresh weight per hour averaged 31% higher in the 20 mg N/plant treatment than the control throughout the growing period. The highest activity occurred in the seedling stage and declined towards anthesis. Diurnal variations were marked by an increase in the NRA from 0600 to 1800 hours and then declined to a minimum at 2400 hours.     

Stabilization of nitrate reductase in maize roots by chymostatin

Nitrate reductase (NR) in maize (Zea mays cv W64A × W182E) roots has been stabilized in vitro by the addition of chymostatin to extraction buffer. Contrary to previous observations, levels of NR were higher in the mature root than in root tip sections when chymostatin was included in the extraction buffer. Two forms of NR were identified, an NADH monospecific NR found mainly in the 1cm root tip and an NAD(P)H bispecific NR found predominantly in mature regions of the root. During the first 10 days of seedling growth, NR activity in the root ranged from 50 to 80% of the activities found in the leaf (a maximum of 2.4 micromoles NO⁻₂ produced per hour per gram fresh weight was measured at 4 days).     

Plant morphological and biochemical responses to field water deficits: I. Responses of glutathione reductase activity and paraquat sensitivity

The effects of water deficits on plant morphology and biochemistry were analyzed in two photoperiodic strains of field-grown cotton (Gossypium hirsutum L.). Plants grown under dryland conditions exhibited a 40 to 85% decrease in leaf number, leaf area index, leaf size, plant height, and total weight per plant. Gross photosynthesis decreased from 0.81 to 0.47 milligram CO₂ fixed per meter per second and the average midday water, osmotic, and turgor potentials decreased to −2.1, −2.4, and 0.3 megapascals, respectively.

There was a progressive increase in glutathione reductase activity and in the cellular antioxidant system in the leaves of stressed plants compared to the irrigated controls. The stress-induced increases in enzyme activity occurred at all canopy positions analyzed.

Irrigation of the dryland plots following severe water stress resulted in a 50% increase in leaf area per gram fresh weight in newly expanded leaves of both strains over the leaves which had expanded under the dryland conditions. Paraquat resistance (a relative measure of the cellular antioxidant system) decreased in the strain T25 following irrigation. Glutathione reductase activities remained elevated in the T25 and T185 leaves which were expanded fully prior to irrigation and in the leaves which expanded following the irrigation treatment.

     

Nitrate Reduction in Solanum tuberosum L.: Development of Nitrate Reductase Activity in Field-grown Plants

Nitrate reductase activity (in vivo method, substrate non-limiting)in unshaded leaves from the top of the canopy has been determinedfor field-grown potato plants over the course of the growingseason. The pattern of change was almost identical for plantsreceiving no added fertilizer and those receiving 24 g N m^–2.Activity increased to a peak at about 90 days after plantingand declined thereafter. On a fresh weight basis activity wasalways higher in fertilized plants. Nitrate reductase activitywas positively and significantly correlated with leaf proteincontent in high N plants (r² = 0.71; P = 0.05), but poorly correlatedwith both the nitrate content of the leaf lamina and the nitrateconcentration in petiole sap. Up until 90 days after planting(mid-July) there appeared to be a positive relationship betweenincreased activity of nitrate reductase and solar radiation.However, results obtained over two seasons showed that the declinein activity after this time was not consistently linked witha fall in the level of solar radiation. Remobilization of reduced-Nand stored nitrate from leaves and stems accompanied this declinein nitrate reductase activity and in the latter part of theseason appeared to account for all of the N gained by growingtubers. In unfertilized plants nitrate-N accounted for 5 per cent orless of total plant N. Fertilized plants contained up to 25per cent nitrate-N. While nitrate availability limited growthin unfertilized plants, sub-optimal rates of nitrate assimilationin fertilized plants, particularly during the early stages ofpost-emergence growth, may contribute to inefficient use ofacquired nitrate. The carbohydrate status of leaf lamina and petiole sap weremodified by N supply. The soluble sugar and starch contentsof low N leaves were higher than in their high N counterparts.By contrast, the concentration of soluble sugars in petiolesap increased to a higher value in high N samples. Althoughsap sugar levels declined in both treatments towards the endof the season, N application delayed this decline for severalweeks. Solanum tuberosum, nitrate reductase, nitrate assimilation, senescence     

Leaf Carbohydrate Status and Enzymes of Translocate Synthesis in Fruiting and Vegetative Plants of Cucumis sativus L

Carbon partitioning in the leaves of Cucumis sativus L., a stachyose translocating plant, was influenced by the presence or absence of a single growing fruit on the plant. Fruit growth was very rapid with rates of fresh weight gain as high as 3.3 grams per hour. Fruit growth was highly competitive with vegetative growth as indicated by lower fresh weights of leaf blades, petioles, stem internodes and root systems on plants bearing a single growing fruit compared to plants not bearing a fruit. Carbon exchange rates, starch accumulation rates and carbon export rates were higher in leaves of plants bearing a fruit. Dry weight loss from leaves was higher at night from fruiting plants, and morning starch levels were consistently lower in leaves of fruiting than in leaves of vegetative plants indicating rapid starch mobilization at night from the leaves of fruiting plants. Galactinol, the galactosyl donor for stachyose biosynthesis, was present in the leaves of fruit-bearing plants at consistently lower concentration than in leaves of vegetative plants. Galactinol synthase, and sucrose phosphate synthase activities were not different on a per gram fresh weight basis in leaves from the two plant types; however, stachyose synthase activity was twice as high in leaves from fruiting plants. Thus, the lower galactinol pools may be associated with an activation of the terminal step in stachyose biosynthesis in leaves in response to the high sink demand of a growing cucumber fruit.     

Measurement of Indole-3-Acetic Acid in Peach Fruits (Prunus persica L. Batsch cv Redhaven) during Development

The amount of indole-3-acetic acid (IAA) was measured in peach fruits by gas chromatography-mass spectrometry-selective ion monitoring using an isotope dilution assay with [¹³C₆]IAA as an internal standard throughout the growing season. Ethylene evolution of the fruit was also measured. IAA levels were 25 nanograms per gram fresh weight, 18 days after anthesis. Both IAA levels and rates of ethylene evolution declined to their lowest levels (7 nanograms IAA per gram fresh weight and 0.01 nanoliter ethylene per gram per hour) in the second stage of fruit growth. Endogenous levels of free-IAA and ethylene evolution increased in the last stage of peach fruit development to 32 nanograms per gram fresh weight and 0.27 nanoliter per gram per hour, respectively. IAA amounts peaked in the ovules 67 days after anthesis.     

不同生育期花生叶片蛋白质含量及氮代谢相关酶活性分析

以5个珍珠豆型花生(Arachis hypogaea Linn.)品种(系)‘汕E’(‘Shan E’)、‘汕G’(‘Shan G’)、‘TH’、‘TJ’和‘泉花7号’(‘Quanhua No.7’)为研究对象,分析了花针期、结荚期和饱果期花生叶片中可溶性蛋白质含量及硝酸还原酶(NR)、谷氨酰胺合成酶(GS)和谷氨酸脱氢酶(GDH)活性的变化趋势,并比较了5个品种(系)荚果和秆产量的差异。结果表明:在3个生育期内,5个花生品种(系)叶片可溶性蛋白质含量和GDH活性的变化趋势基本一致,而NR和GS活性的变化趋势则有差异。其中,可溶性蛋白质含量均呈"低—高—低"的变化趋势,在结荚期最高;GDH活性均逐渐升高,至饱果期达最高;‘泉花7号’叶片NR活性呈"高—低—高"的变化趋势,而其他4个品种(系)叶片NR活性均逐渐降低;‘汕E’、‘TJ’和‘泉花7号’叶片GS活性呈逐渐降低趋势,而‘汕G’和‘TH’叶片GS活性呈"低—高—低"的变化趋势。总体上看,5个品种(系)中,‘汕G’和‘泉花7号’叶片的可溶性蛋白质含量及NR和GDH活性、‘汕E’叶片的NR和GS活性以及‘TH’叶片的GDH活性均较高。5个品种(系)的2个产量指标(单株荚果鲜质量和单株秆鲜质量)均有明显差异,总体上看,‘汕G’、‘泉花7号’和‘TH’的2个产量指标均较高,而‘汕E’和‘TJ’的2个产量指标均较低。综合分析结果显示:‘汕G’和‘泉花7号’叶片可溶性蛋白质含量及NR和GDH活性均相对较高,其荚果和秆产量也均较高,表明花生荚果和秆产量与不同生育期叶片氮代谢水平有一定关系。     

The Effect of Soil Water Regimes on Leaf Water Potential, Growth and Development of Soybeans

The growth and development of soybeans (Glycine max L. cv. Amsoy) was studied at soil matric potentials of ?0.1 to ?1.0 bars. Chlorophyll, photosynthesis, and leaf nitrogen per plant was greatest at ?4 bars leaf water potential. Leaf area, number of internodes, plant height and dry weight of vegetative parts declined as leaf water potential decreased from ?2 to ?19 bars. Nitrogen content and nitrate reductase activity per g fresh weight determined the percentage protein of individual seeds but nitrogen content and nitrate reductase activity per plant determined the amount of total seed protein. The protein synthesized in the seed changed little in amino acid composition with changes in leaf water potential. Leaf water potentials above or below ?4 bars decreased yield, total protein and total lipid but plants produced the largest percentage of individual seed protein at ?19 bars leaf water potential.     

Nitrate reductase activity in vegetation below an arctic bird cliff,Svalbard, Norway

Abstract. Vegetated sites below bird-nesting cliffs are uniquely nutrient-rich habitats in the otherwise nutrient-poor arctic environment. Plants from six distinct vegetation zones below such a cliff at 79° N, Svalbard, Norway, were collected for analysis under greenhouse conditions. Leaf nitrate reductase activity (NRA) was analysed in 42 species representing 25 % of the Svalbard vascular flora. The species mean NRA values ranged from 0.37 to 8.34 μmols of nitrite ions formed per gram of plant fresh weight per hour. Species in the vegetated zone growing closest to recent guano deposits had the highest NRA values, (mean = 4.47) whereas plants growing farther below the cliff had significantly lower values (mean = 0.55). A similar pattern was detected in a duplicate set of plants induced with 15 mM KNO₃; vegetation zone means for NRA ranged from 5.08 to 0.98 μmols of nitrite ions formed per gram of plant fresh weight per hour. Maximally induced species NRA values were highest in the first zones below the cliff and decreased downslope. This gradient paralleled the steep soil nitrate gradient, which decreased from 13.84 mg/l at the cliffbase to 1.03 mg/l downslope. Correspondingly, soil ammonium ions in the vegetation zones ranged between 1.96 mg/l at the cliff-base to 0.03 mg/l downslope. Correlations between NRA and soil nitrate provide a systematic basis for assigning scalar ‘nitrogen figures’ as indicators of habitat preference, here for the first time applied to arctic species.     

Endogenous Auxin and Ethylene in the Lichen Ramalina duriaei

Indole-3-acetic acid (IAA) levels and ethylene evolution rates were measured in a fruticose lichen Ramalina duriaei collected from carob trees growing in northeast Israel. IAA levels were estimated by gas liquid chromatography with electron capture detection of the pentafluorobenzyl ester and also by enzyme-linked immunosorbent assay following methylation. The identity of the isolated IAA was confirmed by gas chromatography-mass spectrometry of both the methyl and the pentafluorobenzyl ester. IAA levels in lichens 1 year after transplanting to an air-polluted urban site were found to be lower than in the control thalli left at a nonpolluted, rural site. The material from the latter contained about 2.5 micrograms per gram fresh weight free IAA and 0.5 microgram per gram fresh weight conjugated IAA, while the urban material contained 0.3 microgram per gram each of free and conjugated IAA. Ethylene production rate was 1.0 nanoliter per gram fresh weight per hour in the material from the rural site and 1.5 nanoliters per gram fresh weight per hour in material from the urban site.     

Physiological changes accompanying senescence in the ephemeral daylily flower

The daylily flower, Hemerocallis hybrid cv Cradle Song, develops from the opening bud to full senescence in 36 hours. Unlike other ephemeral flowers studied to date, it does not respond to ethylene, but other senescence phenomena are similar. There was a small respiration climacteric coinciding with early flower senescence, and it was also observed in isolated petals and petal slices. Cycloheximide abolished the climacteric and delayed senescence in all three systems. Petal apparent free space increased from 30% at bud opening to 38% at the onset of senescence, and sugar efflux increased from 0.2 to 2.8 milligrams per gram of fresh weight per hour during the same period. A sharp increase in ion efflux from 0.8 to 4.0 micromoles of NaCl equivalents per gram of fresh weight per hour, coinciding with the climacteric, was abolished by cycloheximide. Uptake of radiolabeled inorganic phosphate by petal slices from 100 micromolar solution increased during onset of senescence from 6 to 10 nmoles per gram of fresh weight per hour. Half was esterified; of this, 14% went into ATP, and the cellular energy charge remained high at 0.86 during senescence. The proportion incorporated into phospholipid (2.2%) did not change during senescence, but the proportion in phosphatidyl choline increased and in phosphatidyl glycerol decreased during senescence. The general phosphate ester pattern in presenescent slices closely resembled that in other plant tissues except that phospholipid precursors were more prominent (approximately 20% of total organic ³²P versus 5%). In senescent slices, the proportion of hexose phosphates decreased from 40 to 15% of total organic ³²P and that of phospholipid precursors increased to approximately 50%, suggesting that phospholipid synthesis was blocked early in senescence.     

Studies on Genetic Male-Sterile Soybeans : II. Effect of Nodulation on Photosynthesis and Carbon Partitioning in Leaves

Soybean (Glycine max L. Merr.) germplasm, essentially isogenic except for loci controlling male sterility (ms₁) and nodulation (rj₁), were developed to study the effects of reproductive development and nitrogen source on certain aspects of photosynthesis. Plants were sampled from flowering (77 days after transplanting) until maturity (150 days after transplanting). With all four genotypes, net carbon exchange rates were highest at flowering and declined thereafter. Photosynthetic rates of the sterile genotypes (nodulated and non-nodulated) declined more rapidly than the fertile genotypes, and after 105 days, both sterile genotypes maintained low but relatively constant carbon exchange rates (<3 milligrams CO₂/gram fresh weight per hour). Photosynthetic rates and starch accumulation (difference between afternoon and morning levels) declined with time. The sterile genotypes attained the highest morning starch levels, which reflected reduced starch mobilization. After 92 days, the proportion of photosynthetically fixed carbon that was partitioning into starch (relative leaf starch accumulation) in the sterile genotypes increased dramatically. In contrast, relative leaf starch accumulation in the fertile genotypes remained relatively constant with time. Throughout the test period, all four genotypes maintained leaf sucrose levels between 5 and 15 micromoles glucose equivalents per gram fresh weight.

The activities of sucrose phosphate synthase (SPS) in leaf extracts of the four genotypes declined from 77 to 147 days. Nodulated genotypes tended to maintain higher activities (leaf fresh weight basis) than did the non-nodulated genotypes. In general, relative leaf starch accumulation was correlated negatively with the activity of SPS (normalized with leaf net carbon exchange rate) in leaf extracts for all four genotypes during early reproductive development, and for the fertile genotypes at all sampling dates. In contrast, leaf sucrose content was correlated positively with SPS activity during early reproductive development. These results suggested that a direct relation existed between the activity of SPS and starch/sucrose levels in soybean leaves. However, the interaction between these processes also may be influenced by other factors, particularly when leaf photosynthetic rates and plant demand for assimilates is low, as in the sterile genotypes.

     

Levels of Indole-3-Acetic Acid in Lemna gibba G-3 and in a Large Lemna Mutant Regenerated from Tissue Culture

Large changes in indole-3-acetic acid (IAA) levels occur during growth of Lemna gibba G-3 in sterile culture. The levels of IAA were measured in plants during a 45 day growth cycle using HPLC and isotope dilution analysis followed by selected ion current monitoring GC-MS analysis with ¹³C₆-IAA as the internal standard. Even though the rate of plant growth remained constant over the entire growth period, IAA levels ranged from a high of 222 to a low of 6 nanograms per gram fresh weight. A Lemna mutant (jsR₁) which has a giant phenotype was obtained by regeneration from primary callus cultures. Microspectrofluorometry of diamidino-2-phenylindole stained cells showed that jsR₁ has the same amount of DNA per nucleus as the parent line (PL). All jsR₁ cell types measured are about 1.5 times larger than in PL. The endogenous levels of IAA per gram fresh weight were higher in jsR₁ at several stages of the plant culture cycle as compared to PL. This difference ranged from 1.2 to over 100 times as much. While PL showed only one high peak at day 9, jsR₁ had IAA levels of 480 and 680 nanograms per gram fresh weight at days 9 and 45, respectively. Throughout the midculture stage of the growth cycle (20-28 days) both jsR₁ and PL had IAA levels in the range of 9 to 14 nanograms per gram fresh weight. In contrast to PL, at day 45, jsR₁ had no detectable ester or amide conjugates of IAA. These changes in IAA levels were determined in sterile plant cultures and thus cannot be attributed to bacterial or fungal activity.     

Nitrate Reductase in Barley Roots under Sterile, Low Oxygen Conditions

Levels of nitrate reductase activity (EC 1.9.6.1.) as high as 11 μmoles nitrite produced/hour gram fresh weight were found in barley (Hordeum vulgare cv. Compana) roots grown under low oxygen conditions. Roots of plants given identical treatment under sterile conditions did not develop the high levels of nitrate reductase activity. The results suggest that the buildup of particulate, reduced viologen-utilizing nitrate reductase reported in barley roots may be caused by bacterial contamination. The nitrate reductase activity in roots grown under low oxygen conditions was not specific for reduced nicotinamide adenine dinucleotide like the assimilatory nitrate reductase (EC 1.6.6.1.) normally found in aerated plant roots.     

Biochemical Mechanism for Regulation of Sucrose Accumulation in Leaves during Photosynthesis

It is not known why some species accumulate high concentrations of sucrose in leaves during photosynthesis while others do not. To determine the possible basis, we have studied 10 species, known to differ in the accumulation of sucrose, in terms of activities of sucrose hydrolyzing enzymes. In general, acid invertase activity decreased as leaves expanded; however, activities remaining in mature, fully expanded leaves ranged from low (<10 micromoles per gram fresh weight per hour) to very high (>100 micromoles per gram fresh weight per hour). In contrast, sucrose synthase activities were low and relatively similar among the species (4-10 micromoles per gram fresh weight per hour). Importantly, leaf sucrose concentration, measured at midafternoon, was negatively correlated with acid invertase activity. We propose that sucrose accumulation in vacuoles of species such as soybean and tobacco is prevented by acid invertase-mediated hydrolysis. Initial attempts were made to characterize the relatively high activity of acid invertase from mature soybean leaves. Two apparent forms of the enzyme were resolved by Mono Q chromatography. The two forms had similar affinity for substrate (apparent K_m [sucrose] = 3 millimolar) and did not interconvert upon rechromatography. It appeared that the loss of whole leaf invertase activity during expansion was largely the result of changes in one of the enzyme forms. Overall, the results provide a mechanism to explain why some species do not accumulate sucrose in their leaves. Some futile cycling between sucrose and hexose sugars is postulated to occur in these species, and thus, the energy cost of sucrose production may be higher than is generally thought.     

Nitrate Assimilation and Crassulacean Acid Metabolism in Leaves of Kalanchoë fedtschenkoi Variety Marginata

The enzymes necessary to assimilate ammonia either via glutamine synthetase and glutamate synthase or via the glutamate dehydrogenase pathways are present in both green and white leaf tissues of Kalanchoë fedtschenkoi. Nitrate reductase activity develops to a maximum in a Crassulacean acid metabolism (CAM) plant canopy before either ribulose 1,5-bisphosphate carboxylase, or phosphoenolpyruvate carboxylase, or CAM. Nitrate reductase also is activated each morning and is inactivated late in the day as in other plants. However, there does not appear to be any direct relationship between nitrate reductase activity and the level of acid, its daily pattern or the amplitude of CAM. Though nitrate reductase is activated maximally each day by light, in Kalanchoë leaves for six days the activity followed a precise daily pattern independent of continuous light or dark.     

Metabolism of Monoterpenes in Cell Cultures of Common Sage (Salvia officinalis) : Biochemical Rationale for the Lack of Monoterpene Accumulation

Leaves of common sage (Salvia officinalis) accumulate monoterpenes in glandular trichomes at levels exceeding 15 milligrams per gram fresh weight at maturity, whereas sage cells in suspension culture did not accumulate detectable levels of monoterpenes (<0.3 nanograms per gram fresh weight) at any stage of the growth cycle, even in the presence of a polystyrene resin trap. Monoterpene biosynthesis from [U-¹⁴C]sucrose was also virtually undetectable in this cell culture system. In vitro assay of each of the enzymes required for the sequential conversion of the ubiquitous isoprenoid precursor geranyl pyrophosphate to (+)-camphor (a major monoterpene product of sage) in soluble extracts of the cells revealed the presence of activity sufficient to produce (+)-camphor at a readily detectable level (>0.3 micrograms per gram fresh weight) at the late log phase of growth. Other monoterpene synthetic enzymes were present as well. In vivo measurement of the ability to catabolize (+)-camphor in these cells indicated that degradative capability exceeded biosynthetic capacity by at least 1000-fold. Therefore, the lack of monoterpene accumulation in undifferentiated sage cultures could be attributed to a low level of biosynthetic activity (relative to the intact plant) coupled to a pronounced capacity for monoterpene catabolism.     

Involvement of ethylene in the action of the cotton defoliant thidiazuron

The effect of the defoliant thidiazuron (N-phenyl-N′-1,2,3-thiadiazol-5-ylurea) on endogenous ethylene evolution and the role of endogenous ethylene in thidiazuron-mediated leaf abscission were examined in cotton (Gossypium hirsutum L. cv Stoneville 519) seedlings. Treatment of 20- to 30-day-old seedlings with thidiazuron at concentrations equal to or greater than 10 micromolar resulted in leaf abscission. At a treatment concentration of 100 micromolar, nearly total abscission of the youngest leaves was observed. Following treatment, abscission of the younger leaves commenced within 48 hours and was complete by 120 hours. A large increase in ethylene evolution from leaf blades and abscission zone explants was readily detectable within 24 hours of treatment and persisted until leaf fall. Ethylene evolution from treated leaf blades was greatest 1 day posttreatment and reached levels in excess of 600 nanoliters per gram fresh weight per hour (26.7 nanomoles per gram fresh weight per hour). The increase in ethylene evolution occurred in the absence of increased ethane evolution, altered leaf water potential, or decreased chlorophyll levels. Treatment of seedlings with inhibitors of ethylene action (silver thiosulfate, hypobaric pressure) or ethylene synthesis (aminoethoxyvinylglycine) resulted in an inhibition of thidiazuron-induced defoliation. Application of exogenous ethylene or 1-aminocyclopropane-1-carboxylic acid largely restored the thidiazuron response. The results indicate that thidiazuron-induced leaf abscission is mediated, at least in part, by an increase in endogenous ethylene evolution. However, alterations of other phytohormone systems thought to be involved in regulating leaf abscission are not excluded by these studies.