Potted Apple Trees Research Articles

CARBON TRANSLOCATION AND ROOT RESPIRATION IN POTTED APPLE TREES DURING CONDITIONS OF MODERATE DROUGHT

CARBON TRANSLOCATION AND ROOT RESPIRATION IN POTTED APPLE TREES DURING CONDITIONS OF MODERATE DROUGHT

Performance of a pyrethroid-resistant strain of the predator mite Typhlodromus pyri (Acari: Phytoseiidae) under different insecticide regimes.

An organophosphate pyrethroid-resistant strain of Typhlodromus pyri Scheuten imported from New Zealand was reared on potted apple trees in an outdoor insectary. From 1988 to 1995, the population was selected one to three times per year with a dilute solution (1.7 ppm) of the pyrethroid cypermethrin. Petri dish bioassays with cypermethrin in 1995 indicated that the insectary-reared T. pyri had an LC50 of 81 ppm versus 0.006 ppm for native T. pyri taken from a research orchard. The bioassays suggested that recommended orchard rates of cypermethrin would cause heavy mortality in native populations of T. pyri but only moderate losses in the imported New Zealand strain. Bioassays in 1996 with the organophosphate insecticide dimethoate indicated both New Zealand and native T. pyri were susceptible and that recommended orchard rates of dimethoate likely would cause high mortality of T. pyri in apple orchards. These findings from bioassays were supported by data from orchard trials. In June and July 1993, insectary-reared New Zealand T. pyri were placed on five apple trees in each of eight 38-tree plots in the research orchard. In late August 1994, New Zealand T. pyri from orchard trees that had been sprayed twice by airblast sprayer with the full recommended rate of 50 g (AI)/ha (83 ppm) cypermethrin were placed on the other 33 trees in each of six plots. In the summers of 1994-1996, plots were treated with one of the following insecticide regimes: (1) conventional integrated pest management (IPM) (registered neurotoxic insecticides considered harmless or slightly toxic to T. pyri); (2) advanced IPM (use of newer, more selective insecticides); (3) pyrethroid (at least one full-rate application of cypermethrin); (4) dimethoate; and (5) dimethoate plus pyrethroid. Densities of European red mite, Panonychus ulmi (Koch), were highest in all plots treated with dimethoate and in pyrethroid plots not yet inoculated with New Zealand T. pyri. Densities of apple rust mite, Aculus schlechtendali (Nalepa), and of the stigmaeid predator Zetzellia mali (Ewing) were highest in plots treated with dimethoate and were nearly absent in the IPM plots. Densities of T. pyri were high enough for effective biocontrol in the IPM plots and in the pyrethroid plots 1-2 yr after release of the New Zealand strain, provided pyrethroid was applied just before the resistant strain was released in the orchard. A recurring theme of this study was the generally negative association between densities of phytophagous mites and those of T. pyri, suggesting the ability of this predator to suppress their prey. In contrast, the positive association between phytophagous mites and Z. mali suggests the inability of this predator to regulate their prey at least under the conditions of this study.

Epidemiology of brown rot (Monilinia fructigena) on apple: infection of fruits by conidia

Effects of fruit maturity, wound age, temperature and the duration of wetness periods on infection of apple fruits by conidia of the brown rot fungus, Monilinia fructigena, were studied. Inoculation of fruits on potted apple trees and harvested mature fruits showed that wounding was essential for infection by M. fructigena. On potted trees, there was a significant difference between the susceptibility of cvs Cox and Gala and this difference depended on wound age. The incidence of brown rot was affected greatly by fruit maturity and wound age. Wounds on younger fruits were more resistant to infection than those on older fruits, whilst the older the wound, the more resistant it was to infection. Furthermore, the degree of wound age‐related resistance was greater on younger fruits than on older fruits. These relationships were well described by regression models. The effect of the duration of wetness periods was very small: increasing the duration of wetness periods reduced the incidence of brown rot on older wounds. For detached fruits, all those wounded were rotted after inoculation, except for those in two treatments under 20°C on fruits with wounds which were 8 days old. The incubation period of the fungus was generally very short. Wound age was the single most important factor influencing the length of the incubation period; the incubation period increased as wound age increased.

Some effects of pre-release host-plant on the biological control of Panonychus ulmi by the predatory mite Amblyseius fallacis.

Amblyseius fallacis Garman has been selected for pyrethroid resistance and mass reared for experimental release as a biological control agent for tetranychid mites on a number of crops in Canada. Several releases of this predator onto apple and peach trees have failed to result in the establishment of A. fallacis, or in the biological control of Panonychus ulmi Koch. Here, we test the hypothesis that the change of host-plant at the time of release is a critical factor in the establishment of A. fallacis for biological control of P. ulmi. Functional and numerical response studies were undertaken on two populations of A. fallacis: a wild strain collected from the canopy foliage of an apple orchard near Vineland, Ontario; and a second strain reared on bean plants in a commercial insectary with Tetranychus urticae as prey. Each population consumed significantly more P. ulmi and produced significantly more eggs when on leaf disks from the plant species they were reared on, than on leaf disks from the novel host plant. A further experiment was conducted to determine if establishment and biological control of mass-reared A. fallacis could be affected by rearing a population for a short term on apple leaves prior to release on apple trees. Three release treatments were made into potted apple trees in a glasshouse, using predators commercially mass-reared on bean and T. urticae: A. fallacis released directly; A. fallacis reared in the laboratory for four weeks on bean and T. urticae; A. fallacis reared on apple leaves and T. urticae for four weeks. They were compared with a control treatment lacking predator release. Contrary to results of the functional and numerical response studies, no difference was observed between release treatments. All release treatments adding A. fallacis resulted in a similar, if limited, degree of biological control of P. ulmi. These results indicated that there may be short-term effects of host plant on the establishment of A. fallacis and biological control of P. ulmi, which in our study were observed as an initial reduction of the predatory response. However, in a test, the predators appeared to overcome these short-term effects and successfully established on the new host-plant to control P. ulmi.

Gas Exchange Characteristics of Apple at Various Light Intensities under Two Levels of European Red Mite Damage

Single leaf gas exchange measurements were taken at a range of light intensities from 20 to 1500 μmol·m-2·s-1 PAR under greenhouse conditions on `Washington Spur'/EMLA seven potted apple trees subjected to either 1500 cumulative mite days (CMD) European Red mite (ERM) damage or no mite damage. 1500 CMD ERM damage significantly reduced assimilation (A) over all light intensities for leaves present during mite damage at 6 days after the mite population had reached the 1500 CMD level and the mites were killed. Mite damage did not significantly affect A of either leaves present during mite damage or leaves produced after the mites were killed on any other sampling date. However, a trend of reduced A of leaves present during mite infestation on the mite-damaged trees was apparent on all sampling dates after the mites were killed. Evapotranspiration (E) was not affected by mite damage. The mite damage by light intensity interaction did not have a significant effect on A or E on any sampling date.

Insectary Rearing and Initial Testing in Canada of an Organophosphate/Pyrethroid-Resistant Strain of the Predator Mite Typhlodromus pyri (Acari: Phytoseiidae) from New Zealand

A strain of Typhlodromus pyri Scheuten resistant to pyrethroids and to several organophosphate insecticides was imported into Nova Scotia, Canada, from New Zealand in 1988. T. pyri reared on potted apple trees treated with dilute cypermethrin in outdoor insectaries remained at densities >1 per leaf throughout the summers of 1988–1994 and were able to suppress European red mite, Panonychus ulmi (Koch), and twospotted spider mite, Tetranychus urticae Koch. Leaf disk bioassays in 1988 indicated that native T. pyri from an orchard regularly sprayed with several organophosphates had low mortality when exposed to azinphosmethyl, malathion, and phosmet. However, the New Zealand strain had significantly lower mortality and less repellancy than did native T. pyri when exposed to the pyrethroid insecticide cypermethrin or the organophosphate azinphosmethyl. In an orchard trial, the New Zealand strain colonized trees and exerted effective control of P. ulmi and apple rust mite, Aculus schlechtendali (Nalepa), despite minimum winter temperatures of −20 to −22°C from 1988 to 1992 and −30°C in 1993. In late August 1988, New Zealand T. pyri were placed on trees that were treated with cyperemethrin in 1988, 1989, and 1991. Maximum densities of motile P. ulmi decreased from 32 per leaf in 1988 to 10 in 1989 and to <1 per leaf in 1990 and 1991. By August 1990 the T. pyri had dispersed at least 3 rows north from the release row. Slide dip bioassays indicated that T. pyri on the release trees had higher survival of cypermethrin than did T. pyri from control trees 10 rows to the north in the same orchard, suggesting the former were the New Zealand Strain (pyrethroid resistant) and the latter were native (pyrethroid susceptible). Evidence of retention of pyrethroid resistance was seen in a 2nd orchard trial. Cypermethrin was applied in mid-May 1992 to eliminate native T. pyri . New Zealand T. pyri released in late May and early July colonized the release trees, dispersed to other trees in the same rows, and suppressed P. ulmi and A. schlechtendali despite pyrethroid residues on all trees. Suppression of phytophagous mites continued in 1993 despite application of dilute cypermethrin in May of that year.

Ascospore Release and Infection of Apple Leaves by Conidia and Ascospores of Venturia inaequalis at Low Temperatures

ABSTRACT Mills' infection period table describes the number of hours of continuous leaf wetness required at temperatures from 6 to 25 degrees C for infection of apple leaves by ascospores of Venturia inaequalis and reports that conidia require approximately two-thirds the duration of leaf wetness required by ascospores at any given temperature. Mills' table also provides a general guideline that more than 2 days of wetting is required for leaf infection by ascospores below 6 degrees C. Although the table is widely used, infection times shorter than those in the table have been reported in lab and field studies. In 1989 a published revision of the table eliminated a potential source of error, the delay of ascospore release until dawn when rain begins at night, and shortened the times reported by Mills for ascospore infection by 3 h at all temperatures. Data to support the infection times below 6 degrees C were lacking, however. Our objective was to quantify the effects of low temperatures on ascospore discharge, ascospore infection, and infection by conidia. In two of three experiments at 1 degrees C, the initial release of ascospores occurred after 131 and 153 min. In the third experiment at 1 degrees C, no ascospores were detected during the first 6 h. The mean time required to exceed a cumulative catch of 1% was 143 min at 2 degrees C, 67 min at 4 degrees C, 56 min at 6 degrees C, and 40 min at 8 degrees C. At 4, 6, and 8 degrees C, the mean times required to exceed a cumulative catch of 5% were 103, 84, and 53 min, respectively. Infection of potted apple trees by ascospores at 2, 4, 6, and 8 degrees C required 35, 28, 18, and 13 h, respectively; substantially shorter times than previously were reported. In parallel inoculations of potted apple trees, conidia required approximately the same periods of leaf wetness as ascospores at temperatures from 2 to 8 degrees C, rather than the shorter times reported by Mills or the longer times reported in the revision of the Mills table. We propose the following revisions to infection period tables: (i) shorter minimum infection times for ascospores and conidia at or below 8 degrees C, and (ii) because both ascospores and conidia are often present simultaneously during the season of ascospore production and the required minimum infection times appear to be similar for both spore types, the adoption of a uniform set of criteria for ascosporic and conidial infection based on times required for infection by ascospores to be applied during the period prior to the exhaustion of the ascospore supply. Further revisions of infection times for ascospores may be warranted in view of the delay of ascospore discharge and the reduction of airborne ascospore doses at temperatures at or below 2 degrees C.

Use of 13CO2 as a Tool to Investigate Carbon Partitioning in Field and Greenhouse-grown Apple Trees

Instrumentation to measure soil respiration is currently readily available. However, the relationship between soil respiration and root activity or root mass is not known. Herein we report on preliminary result using a 13CO2 pulse to the foliage to determine if 13C respiration can be related to either root activity or root mass. An experiment was performed in the field on a 5-year-old apple tree (cv. Jonagold on M7). The tree canopy was enclosed in a Mylar® balloon and 2.1 g 13CO2 were pulsed in the balloon for 1 hr. After the pulse, air emitted by the soil and selected roots was collected every 6 hr for 8 days, by bubbling it in 2 M NaOH. 13C/12C ratios were measured with the mass spectrometer. The emission of 13CO2 from the roots gradually increased after the pulse reaching a peak after 100 hr. The emission trend was not linear, but it seemed related to soil temperature. Leaves and fruit were also collected daily. 13C content in leaves was 1.15% right after the pulse, but it progressively decreased to 1.09% at the end of the experiment. The experiment was then repeated on 12 potted apple trees (cv. Redcort on M7) in greenhouse conditions. Six of them were maintained well-watered, whereas six plants were subjected to a mild water stress, by watering them with half of the volume of water used for well-watered plants. After the two soil moisture levels were achieved, the tree canopies of all the 12 trees were pulsed. Leaves, stems, and roots were ground and run in the mass spectrometer. The results of root emission rate were found to be similar to the field experiment. Results also indicated that, in our experiment, stress did not affect root respiration rate. Specific details of the physiology data will be presented.

Tests of fungicides for post-germination activity against Nectria galligena, causal agent of canker and fruit rot of apple

Twelve fungicides were screened in vitro for post-germination activity against Nectria galligena, the causal agent of European apple canker and Nectria fruit rot. Colony size was recorded 5 and 12 days after the application of fungicides. Fungicides differed in their ability to inhibit colony growth. Type of fungicide and concentration interacted. In general, inhibition was greater the higher the concentration of fungicide. Also, the shorter the delay (0, 6 and 30 h) between seeding the plates with conidia and applying the fungicides, the greater the inhibition, although this effect was far less than that due to concentration. Four fungicides were selected for further in vitro testing at lower concentrations and longer delays (36 and 96 h) after seeding plates with conidia. On the basis of these in vitro screens, carbendazim, fenpropimorph and prochloraz, each with and without a chemical penetrant, were applied to pruned shoots as curative treatments on potted apple trees in a polythene tunnel. These fungicides failed to prevent infection when applied 48 h after the inoculation of fresh pruning cuts. In a second experiment, these fungicides, especially prochloraz, slightly reduced the incidence of cankered shoots and lengthened the incubation period when applied 36 h after inoculation of 1-day-old pruning cuts. The results are discussed in relation to the rate of the wound-healing process, which is likely to be a factor influencing the degree of contact between fungicides and the invading fungus.

‘Golden Delicious’ apple fruit shape and russeting are affected by light conditions

The effect of light on apple fruit shape was studied by exposing 5-year-old potted apple trees cultivar ‘Golden Delicious’ Clone B in open air to six lighting regimes obtained by altering the natural daylight (control) by means of Ultra-violet (UV)-Blue sunlamps and Red-Infra-Red (R-IR) lamps and/or by shading with corrugated fiberglass-polyester sheets to cut off UV-B and violet wavelengths. Shading increased the length/diameter ( L D ) ratio of the fruit and decreased the incidence of russeting. Natural light produced the lowest L D ratio and greatest amount of russeting. The Blue/UV-B ratio had no consistent effect on shape or russeting. The Red/Far-Red ratio did not vary greatly among treatments and was not related to differences among fruit.

Water Relations, Stomatal Conductance, and Abscisic Acid Content of Young Apple Trees in Response to Antitranspirant Treatment

Potted apple trees (Malus domestica L. `Gala') were drenched with either water or an antitranspirant (N-2001). After treatment, no additional water was applied to the plants. Abscisic acid (ABA) content of immature and mature leaves was determined by radioimmunoassay after 0, 1, 3, and 5 h and 1, 2, 4, 7, 8, and 9 days after treatment. ABA content of mature and immature leaves of antitranspirant-treated plants peaked 1 and 4 days after treatment, respectively, and remained constant thereafter. In contrast, with increasing water stress, the ABA content of mature and immature leaves of control plants without antitranspirant peaked at 7 and 8 days, respectively. The overall level of ABA in mature leaves of both treatment groups was significantly greater than in immature leaves. The water saturation deficit increased, water and turgor potentials of leaves decreased, and stomatal conductance decreased in response to antitranspirant application. The changes in water relations parameters and stomatal conductance were highly correlated with changes in leaf ABA content.

Water Relations, Stomatal Conductance, and Abscisic Acid Content of Young Apple Trees in Response to Antitranspirant Treatment

Potted apple trees (Malus domestica L. `Gala') were drenched with either water or an antitranspirant (N-2001). After treatment, no additional water was applied to the plants. Abscisic acid (ABA) content of immature and mature leaves was determined by radioimmunoassay after 0, 1, 3, and 5 h and 1, 2, 4, 7, 8, and 9 days after treatment. ABA content of mature and immature leaves of antitranspirant-treated plants peaked 1 and 4 days after treatment, respectively, and remained constant thereafter. In contrast, with increasing water stress, the ABA content of mature and immature leaves of control plants without antitranspirant peaked at 7 and 8 days, respectively. The overall level of ABA in mature leaves of both treatment groups was significantly greater than in immature leaves. The water saturation deficit increased, water and turgor potentials of leaves decreased, and stomatal conductance decreased in response to antitranspirant application. The changes in water relations parameters and stomatal conductance were highly correlated with changes in leaf ABA content.

Control of Apple Maggot Flies (Diptera: Tephritidae) with Pesticide-Treated Red Spheres

The effectiveness of baited pesticide-treated red spheres in controlling Rhagoletis Pomonella (Walsh) flies was studied under both field-cage and field (orchard) conditions. Spheres (8-cm diameter) were treated with a mixture containing dimethoate, feeding stimulant, and residue-extending agent. Under field-cage conditions, where potted apple trees containing ‘Gravenstein’ apples were placed Singly inside field cages, a baited fresh pesticide-treated sphere was as effective as a baited fresh sticky sphere in killing released flies as well as reducing oviposition. A pesticide-treated sphere hung in a fruiting tree was as attractive to wild R. pomonella flies as a sticky sphere. Field studies conducted in four commercial orchards showed variation in the efficacy of baited pesticide-treated spheres in controlling R. pomonella . In one orchard with ‘Gravenstein’ apple trees, a commercial level of control <0.5% fruit damage) was not achieved using either baited dimethoate-treated spheres (2.5% fruit damage) or baited sticky spheres (4.0% fruit damage). In three other orchards with ‘Liberty’ (and some ‘McIntosh’) trees, nearly acceptable commercial-level control was achieved using either baited pesticide-treated spheres (1.0% fruit damage) or two foliar applications of azinphosmethyl (0.8% fruit damage).

An improved stem heat balance method using analog heat control

This paper describes a modified sap flow meter appropriate for sap flow measurements even on plants having an irregular stem and/or heat-sensitive bark, such as apple trees. The modifications build on the plant-friendly but not very user-friendly design of Weibel and De Vos (Plant and Soil, 1994). Three important improvements are achieved (1) the number of heat-controlling gauges that can be connected to one ‘Campbell 21X’ datalogger is increased from three to 32 by using an analog algorithm together with additional circuity consisting of sample-and-hold and power amplifiers; (2) the sap flow gauge is easier to install; (3) the gauge has a better adaptability to stem growth in the radial direction and to diurnal stem shrinking. Accuracy tests with the modified system were carried out under glasshouse conditions with potted apple trees having stem diameters between 15 and 45 mm. Using gravimetric weight loss as a reference method we found good reliability of all technical components and excellent measuring accuracy, with errors of less than 4% over a 24 h measuring interval and less than 15% over a period of 15 min. Also, in a 4 month field experiment with sap flow measurements on six 3-year-old apple trees rooted in the ground, the modified system proved highly reliable. After removing the gauges the stems were not ‘strangled’, nor was the tissue of the bark affected. Error analyses on the need to include a term for stored energy in the heat balance equation showed that the importance of stored energy increases substantially when the tree stem diameter is more than 30 mm.

Drought stress of apple trees alters leaf emissions of volatile compounds

Actively growing potted apple trees (Malus domestica [L.] Borkh. ev. Delicious) unacclimated to drought stress were subjected to drought to determine changes in emissions of leaf volatile compounds. Drought stress was imposed over a 2‐week period by weighing pots every 2 or 3 days and adding water hack to an arbitrary and decreasing traction of the original pot weight. Stem water potential was ‐2.7. ‐2.0 and ‐0.8 MPa for the severely stressed, moderately stressed and control trees, respectively. 13 days alter watering treatments were begun. Water use the last 4 days of the experiment was about one‐half for the moderately and severely stressed trees compared to that of the controls Twenty‐nine volatile compounds were identified by using gas chromatography and mass spectroscopy. Emission rates of hexanal, (E)‐2‐hexenal, (E)‐2‐hexen‐1‐ol, 1‐hexenol, hexyl acetate and (E)‐2‐hexenyl acetate were 5 to 310 times higher for severely stressed trees compared to those of the controls with the moderately stressed trees intermediate. The large increases in hexanal. (E)‐2‐hexenal and l‐hexanol may be related. In enhanced lipoxygenase activity. Volatile compounds are products of metabolism and measurement of their changes after biotic or abiotic stresses will increase understanding of the relationship of changes in plant metabolism by those stresses.

Influence of young potted apple tree water status on European Red Mite (Acari: Tetranychidae) abundance

The effect of water-stress on population growth of the European Red Mite (ERM) Panonychus ulmi (Koch) was tested in a plastic shelter using young potted apple trees under three water treatment. Mite abundance was significantly greater with higher water supply. The relationship between plant water status and mite number was non-linear, both treatments with leaf water potential from-30 to about -5 bars bearing the lower mite populations. Over the threshold value of-5 bars, mite number sharply increased four times. A non-linear regression analysis showed that the stomatal conductance explained 49.7% of the variance in mite number. Low leaf surface humidity resulting from reduced leaf transpiration of water-stressed trees may increase mite egg and larval mortalities. Daily leaf surface temperatures on the water-stressed trees averaged 1.5°C higher than on the other trees, but did not result in an increase in mite populations, because of the negative influence of low leaf humidity.

Transpiration measurements on apple trees with an improved stem heat balance method

Since the late eighties a handy and user-friendly sap flow meter (Dynagage®) is on the market which can quantify 0205 the sap flow through intact plant stems, based on the stem heat balance method. The documentation about its accuracy and reliability, however, is still too limited to use it as a standard method in field experiments with apple trees. We therefore tested this commercial system on potted apple trees (Malus domestica L.; cv. ‘Red Elstar’ and ‘Jonagold’; on rootstock ‘M9’ vf) with stem diameters of 1.8 to 4 cm. The measured sap flow was compared with mass loss measured by an automated balance, supposing the total mass loss of the trees was equal to the water loss by transpiration. The results revealed three major problems: 1. When there was no optimum contact of the elements of the gauge with the stem, which is typically very irregular on young apple trees, the calculated sap flow rates (accumulated through 24 h) showed errors >20%. 2. On ≥4 year-old trees the calculated sap flow rate showed considerable time lags in periods with abruptly changing transpiration rates, mainly because this sap flow method does not account for energy which is stored in the heated stem section. 3. The constant power input to the stem given with this sap flow meter caused heat damages to the bark tissue after >6 days of continuous measurements.

Respiratory Oxygen Response and Respiratory Quotient of Apple Stem Sections during Chilling

During natural leaf abscission, 2-year-old potted apple trees (Malus domestica Borkh. cv. MM.111 EMLA) were placed in a room at 6C for chilling [0,600, 900, or 1400 chilling units (CU)]. After each chilling treatment, respiration of shoot segments was measured as CO2 evolved and O2 consumed at 22C in several O2 concentrations. Respiration increased with oxygen concentration after all CU treatments. Carbon dioxide evolved at the several O2 levels did not show a pattern related to CU, but O2 consumed decreased at a decreasing rate with additional CU. Respiratory quotient was &lt;1 at 0 and 600 CU and equal to 1 at 900 and 1400 CU, indicating a possible shift in respiratory substrate with chilling.

Influence of root pruning and water stress on growth and physiological factors of potted apple, grape, peach and pear trees

Apple, grape, peach and pear trees were grown with roots divided between two pots for 2 months and then assigned to the following treatments: (1) control, receiving 100% of total plant transpiration (TPT) distributed equally between both pots; (2) pruned, receiving 100% of TPT in one pot with removal of roots in the second pot; (3) stressed, receiving 100% of TPT in one pot with water withheld from the other. Shoot growth of all species except peach was reduced only by root pruning (20%, 30% and 40% less than control in grape, pear and apple, respectively). New root growth of pruned and stressed plants was generally less than control (25% on average). During the first 15 days after treatment, both root pruning and water stress depressed transpiration and net photosynthesis. Thereafter, stressed plants did not differ from control; transpiration and net photosynthesis of pruned plants approached those of control only after 50 days. At the end of the experiment unchanged shoot:root ratios were found in stressed trees of each crop compared with control, while root pruning caused a variable shoot:root ratio depending on the degree of canopy reduction. The findings indicate the ability of the examined crops to adapt to drastic manipulations of the root system.

DROUGHT STRESS ALTERS BIOGENESIS OF APPLE LEAF VOLATILES

Potted apple trees were severely (S) or moderately (M) droughted and compared to a well-watered control (C) to determine changes in biogenesis of leaf volatile compounds. Total available water (TAW) of the soil was allowed to decline to near 0% TAW, 20% TAW, and 100% TAW, for S, M and C, respectively, by the end of a two-week drying period. Twenty-nine volatile compounds were identified by GC-MS using headspace sampling of detached leaves. Concentrations of (E)-2-hexenal, (E)-2-hexenyl acetate, l-hexanol, (E)-2-hexen-1-ol and hexyl acetate were 5 to 310 times higher for S than C. It is suggested that the large drought induced increase in C-6 compounds was related to enhanced lipoxygenase activity.