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Published in Crop Sci. 44:812-818 (2004).
© 2004 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

CROP PHYSIOLOGY & METABOLISM

Ethylene Production of Two Wheat Cultivars Exposed to Desiccation, Heat, and Paraquat-Induced Oxidation

Maria Balota*,a, Simona Cristescub, W. A. Paynea, S. te Lintel Hekkertb, L. J. J. Laarhovenb and F. J. M. Harrenb

a Texas A&M Univ. System, Texas Agric. Exp. Stn., 6500 Amarillo Blvd. West, Amarillo, TX 79106
b Dep. of Molecular and Laser Physics, Univ. of Nijmegen, P.O. Box 9010, 6500 GL Nijmegen, the Netherlands

* Corresponding author (m-balota{at}tamu.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS
 DISCUSSION
 REFERENCES
 
Drought and heat are the major limitations to wheat (Triticum aestivum L.) production worldwide. Insufficient water supply, often accompanied by high temperature, reduces plant growth, hastens senescence, and produces considerable yield losses. Ethylene production has been often associated with reduced growth and premature senescence, and may therefore be an indicator of plant susceptibility to stress such as drought and heat. A system of rapid and sensitive measurement of plant ethylene production could have potential to identify cultivar-dependent response to environmental constraint. There have been no studies regarding genetic differences in ethylene production rate (EPR) in wheat. The purpose of this study was to compare EPR of two wheat cultivars previously characterized for field performance under drought with a new system of ethylene measurement. Seedlings of the winter wheat cultivars Dropia (stress resistant) and Delia (stress susceptible) were exposed to optimum and stress conditions and measured for EPR. Stress conditions included desiccation, high temperature, and paraquat (Pq; 1,1'-dimethyl-4,4'-bipyridinium) induced oxidation. Ethylene was measured with a CO2 laser-driven intracavity photoacoustic (PA) detector which could detect ethylene concentrations as low as 10 pL L–1 and a time response of 40 s. Additional physiological parameters related to stress resistance were measured in eight wheat cultivars including Delia and Dropia. Under desiccation, EPR decreased in both cultivars, and then increased by 2 to 5 fold upon rewatering. Ethylene evolution decreased more in Delia than in Dropia under desiccation. However, Delia produced consistently and significantly more ethylene than Dropia under optimum and all other stress conditions. Enhanced EPR by Delia was consistent with all other physiological measurements, indicating that Delia was more stress-sensitive than Dropia.

Abbreviations: ACC, 1-amino-cyclopropane-1-carboxylic acid • DSI, drought susceptibility index • DW0.5, dry weight at –0.5 MPa soil water potential • DWPEG10, dry weight upon 10% polyethylene glycol treatment • EPR, ethylene production rate • HAH, high air humidity • LAH, low air humidity • PA, photoacoustic • PEG, polyethylene glycol • PPFD, photosynthetic photon flux density • Pq, paraquat • RH, relative humidity • RT, residual transpiration • SLDW, specific leaf dry weight • TS, thermal membrane stability


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 NOTES
 RESULTS
 DISCUSSION
 REFERENCES
 
ENHANCED ETHYLENE EVOLUTION has been associated with reduced growth, accelerated senescence, and response to environmental stresses. In wheat seedlings, ethylene reduces coleoptile length and root length and number (Carbone and Beltrano, 1995). During early growth stages of wheat, treatment with ethephon (2-chloroethanephosphonic acid), an exogenous source of ethylene, causes depression of photosynthesis (Rajala and Peltonen-Sainio, 2001). In mature wheat plants, increased ethylene production shortens grain filling period, decreases thousand-grain weight, hastens maturity, and triggers senescence and premature plant death (Beltrano et al., 1999). Studies of wheat on the Mir space station indicated that enhanced ethylene caused male sterility (Levinskikh et al., 2000). Ethylene overproduction in wheat was detected either during or soon after recovery from water stress (Morgan et al., 1990; Narayana et al., 1991; Beltrano et al., 1997; Beltrano et al., 1999), and upon Pq treatment (Weckx et al., 1989). Enhanced ethylene production due to water stress was associated with alteration of pigment content and plasma membrane integrity (Beltrano et al., 1999; El-Shintinawy, 2000). Similarly, Pq-induced ethylene production was related to formation of oxygen free radicals, lipid peroxidation, and reduction of membrane fluidity (Weckx et al., 1989). Although ethylene has been extensively studied in wheat, little information is available on genetic differences in the EPR under environmental stress. Recent data indicate that increased wheat tolerance to salinity may be associated with lower ethylene production (El-Shintinawy, 2000).

Ethylene evolution has been measured with various systems, ranging from airtight containers (Eastwell and Spencer, 1982) to continuous-flow systems that allow precise control of the gaseous environment (Narayana et al., 1991). Fast detection of low levels of gas emission, isolation from other low-molecular-weight hydrocarbon gases, and control of plant environment have always been major concerns for accurate measurement. Photoacoustic trace gas detectors have recently proven to be indispensable in biological and human health research (Harren et al., 1990; Voesenek et al., 1993). This instrumentation allows sensitive, real-time, noninvasive detection of trace gases under rapidly changing environmental conditions from different plant organs and whole plants.

The PA system was used in this study to determine whether ethylene emissions could be used to differentiate the drought and heat susceptibility of two wheat cultivars in the field. Stress-resistant and susceptible cultivars that had been previously characterized (Saulescu et al., 1998) were chosen for study and subjected to desiccation, high temperature, and Pq oxidative stresses.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
NOTES
 RESULTS
 DISCUSSION
 REFERENCES
 
Plant Material
Eight Romanian wheat cultivars were studied, including drought sensitive Delia and tolerant Dropia with drought susceptibility index (DSI) values of 1.21 and 0.88, respectively (Saulescu et al., 1998). The DSI provides an estimate of how well cultivars will yield under drought stress regardless of their yield potential. A high DSI indicates greater drought susceptibility (Fisher and Maurer, 1978). Delia and Dropia were used exclusively for EPR measurements. Stress resistance traits were measured for all eight cultivars.

Seeds of uniform size were briefly surface-sterilized in 1% sodium hypochlorite and rinsed in distilled water. Seeds were germinated in Petri plates at 22°C in an incubator. In the same plates, seedlings were further grown at 22°C and 200 µmol m–2 s–1 photosynthetic photon flux density (PPFD) for 16 h d–1. Approximately 7-d-old seedlings were used to determine EPR under desiccation and high temperature, and for measurements of the physiological parameters related to stress resistance. To determine the effect of Pq-induced oxidation on EPR, 3- to 4-wk-old plants were used. Plants were grown in June 1999 in the glasshouse of the Nijmegen University in 25-cm-diam. by 20-cm-high pots and {approx}2 kg of potting soil. A VitaCo Climate (Hoogendoorn, Vlaardingen, the Netherlands)1 computerized system was used to control and record the temperature and humidity in the glasshouse. Day temperature was 24 ± 1°C with 54 ± 5% relative humidity (RH) and night temperature was 20 ± 0.3°C and 69 ± 3% RH. Natural light filtered through the glass roof and walls of the glasshouse was available to the plants.

Plant Treatments for Ethylene Production Rate Sampling
Desiccation
For the well-watered treatment, rooted seedlings of uniform size were placed on five layers of filter paper moistened with 20 mL of distilled water in Petri plates, one each for Delia and Dropia. Each open Petri plate was then introduced in a 1.2-L glass cuvette and continuously flushed with air for EPR sampling. The cuvettes with seedlings were maintained at 22 ± 0.8°C and 200 µmol m–2 s–1 PPFD artificial light (SON-T AGRO 400, Philips, Eindehovert, the Netherlands). To maintain constant temperature, a 5-L water reservoir with glass walls was placed between the lamp and cuvettes.

Desiccation was induced by replacing the moistened with completely dry filter paper for 24 h. Ethylene production was monitored for 17 h under the well-watered regime, during desiccation, and 17 h during a recovery period when water was reintroduced into the cuvettes containing the seedlings. The experiment was replicated three times with 30 seedlings per cultivar per replication. Each cuvette was a replication. At the start of each experiment, RH was measured in the air stream before entering the cuvettes with a Panametrics Moisture Monitor Series 35 (Panametrics B.V., Hoevelaken, the Netherlands). Relative humidity was 80% in the high air humidity (HAH) treatment.

Two additional sets of 30 seedlings per cultivar were exposed to a 40-h desiccation regime. One set was supplied with HAH and the other with low air humidity (LAH), where air was passed through a CaCl2 scrubber before entering the cuvettes. In the LAH experiment, RH was 2% before entering the cuvettes. Ethylene production was monitored 17 h before desiccation, during the stress regime, and 25 h during a recovery period when water was added into the cuvettes. Primary leaf length was measured at the beginning of each experiment, start and end of desiccation periods, and at the end of the recovery period.

High Temperature Stress
Under 200 µmol m–2 s–1 PPFD, the temperature was changed from 22°C during the first 14 h to 38°C for the next 10 h, and then returned to 22°C. After 15 h at 22°C, a second treatment at 38°C was applied for 20 h, followed by 14 h at 22°C. Relative humidity at 22°C was {approx}80% before entering the cuvettes. The cuvettes contained rooted seedlings in open Petri plates as described before. The temperature variation was obtained by placing the cuvettes inside a water bath for {approx}20 min. Ethylene was continuously monitored during cyclic temperature change. The experiment was replicated four times with 30 seedlings per cultivar per replication. Each cuvette was a replication.

Paraquat Oxidation
Individual tillers, usually the first tiller, were cut above the soil surface and introduced with the stem base in 20-mL flasks containing different solutions. The entire tiller and flask was placed in a glass cuvette, 5 cm in diameter and 80 cm height, and connected to the PA system. Plants, flasks, and cuvettes were placed in an environmental chamber at 22°C, and 200 µmol m–2 s–1 PPFD for EPR measurements. Air RH was {approx}80% before entering the cuvettes. The control flask contained only nutrient solution. A second flask contained nutrient solution as well as 0.25 mM Pq to induce oxidation. Light intensity was increased to 400 µmol m–2 s–1 after 16 h to enhance membrane peroxidation. Ethylene production was monitored 29 h. An estimate of chlorophyll content (SPAD-502, Minolta Camera Co., Ltd., Osaka, Japan) was recorded before placing the plants in solutions and at the end of each experiment. Units of the SPAD meter correspond to the relative amount of chlorophyll present in the leaf and were calculated on the basis of the amount of light transmitted by the leaf at two wavelengths where chlorophyll absorption is different. Each cuvette and tiller was one replication, and two replications per cultivar and treatment were analyzed.

Measurements of Ethylene Emission with the Laser-Driven Intracavity Photoacoustic Detector System
Ethylene production was measured with a sensitive CO2 laser-driven intracavity PA detector in line with a flow-through sampling system developed at the Department of Molecular and Laser Physics, University of Nijmegen, the Netherlands (Bijnen et al., 1996). A detailed description of the system can be obtained from te Lintel Hekkert et al. (1998) and Harren and Reuss (1997). Briefly, radiation from the laser is absorbed by ethylene molecules and subsequently converted into an acoustic wave with amplitude that is proportional to the gas concentration. A sensitive microphone (Knowles Electret BT-1754, West Sussex, UK) was used to obtain high acoustic resolution and lower detection limits.

Ethylene production was measured from seedlings in glass cuvettes as previously described. The cuvettes were fitted with inlet and outlet ports and flushed with compressed air as carrier gas at a flow rate of 2 L h–1. The compressed air was passed though a humidifier (10-mL flask half filled with water) to raise the RH to 80% before entering the cuvettes. In the LAH experiment the humidifier was replaced with a CaCl2 scrubber to reduce the RH to 2%. All measurements of RH were taken after air flow had stabilized for {approx}1 h before experiments were started. The flow from each cuvette was directed into the PA cell, where a resonator detected the acoustic signal. Three-way electrical valves controlled gas flow through the measuring system to the PA cell (on-position) or into the laboratory (off-position) for purging, so that gas emission from the cuvettes was alternately transported to the PA cell at controlled flow rates, preventing contamination of samples.

The flow rate was adjusted by a flow controller and continuously monitored by a mass flow sensor. A computer program automated the laser-based ethylene detector and the electric three-way valves, permitting continuous measurements for several days. The PA method could detect as little as 10 pL L–1 ethylene (Harren and Reuss, 1997), roughly three orders of magnitude greater than traditional gas chromatography analysis, while the response time was 40 s.

Measurements of Physiological Parameters Related to Stress Resistance
Six physiological parameters associated with drought, heat, and oxidation resistance were measured: (i) residual transpiration (RT), (ii) membrane stability upon osmotic treatment with polyethylene glycol (PEG) 40% (–2.5 MPa) for 24 h, (iii) thermal membrane stability (TS) at 49°C for 1 h, (iv) dry weight accumulation in seedlings under a –0.5 MPa soil water potential (DW0.5) imposed by a system similar to that of Snow and Tingey (1985), (v) dry weight accumulation in seedlings upon a 10% PEG treatment (DWPEG10), and (vi) resistance to Pq oxidation.

Residual transpiration is a measure of water diffusion across the waxy cuticle of the epidermal cells and the partially closed stomata, and is expressed as water loss after 5 h of dehydration of detached primary leaves (Jaradat and Konzak, 1983). Membrane stability and TS were calculated as ratios of initial and final conductivity readings with an approach similar to that of Blum and Ebercon (1982) and Balota and Saulescu (1994). After either PEG or temperature treatments, electrolyte leakage from 5-cm2 leaf segments placed in 25 mL of distilled water was recorded as initial conductivity. Final conductivity readings were taken after autoclaving the leaf segments for 20 min for complete release of the cellular electrolytes. The DW0.5 and DWPEG10 were expressed as differences between control and stressed plants after 30 d of treatment. Paraquat-induced oxidation was quantified as leaf water loss after application of 2 x 10–4 M Pq for 24 h in the nutrient solution in which seedlings were grown hydroponically. Under optimum growth conditions, specific leaf dry weight (SLDW) and relative chlorophyll content (SPAD units) were measured as well. Each measurement was replicated four times with five seedlings per cultivar and replication.

Statistical Analysis
Effects of cultivars, time of observation, treatments, and their interaction on the EPR were analyzed with the ANOVA according to the PROC MIXED procedure in SAS (SAS Institute, 1999) with replications treated as random effects. Time was used as a continuous concomitant variable. The two RH treatments from the 40-h desiccation experiment were combined and analyzed as blocks; that is, homogeneous variation within and heterogeneous between. In experiments where data showed less linearity, quadratic interaction of hour and cultivar was analyzed. Effect of cultivar was tested by ANOVA according to the GLM (general linear model) procedure for all physiological parameters related to stress resistance, and cultivar means were separated by the LSD test at the 0.05 probability level.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS
DISCUSSION
 REFERENCES
 
Ethylene Production in Response to Desiccation
Delia had significantly higher EPR than Dropia when well watered and after recovery from desiccation, and significantly lower EPR following 24 h of desiccation (Table 1). In a well-watered regime, significant cultivar (P = 0.003) and hour (P = 0.0009) effects on the EPR were observed, indicating that cultivars produced ethylene at different rates with an apparent change across time. However, the rate of change was the same for both cultivars because there was a nonsignificant cultivar x hour interaction (Fig. 1) . Under desiccation, the mean EPR decreased by 0.191 nmol g–1 DW h–1 in Dropia and by 0.343 nmol g–1 DW h–1 in Delia. The two cultivars produced ethylene at different rates across time; that is, similar rates at the beginning of desiccation but significantly different rates after 10 h of stress, as indicated by a significant (P < 0.001) cultivar x hour interaction (Fig. 1). Upon recovery from desiccation, the mean EPR of Dropia rose to 0.352 nmol g–1 DW h–1, compared with 0.397 nmol g–1 DW h–1 for Delia. At this time, the cultivar effect was less significant (P = 0.086) and the hour and hour x cultivar interactions were not significant (Fig. 1).


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  		Table 1. Average ethylene production rates and standard errors (in parentheses) from the PROC MIXED ANOVA (SAS Institute, 1999) table of means adjusted for the effect of time. Factors were cultivar (Delia and Dropia) and growth condition.

 


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  		Fig. 1. Effects of cultivar, water regime, and time of observation on the ethylene production rate of wheat seedlings. Means ± SD of three replications are presented. Rectangles at the top of the figure show the well-watered (the first 17 h) and the recovery (the last 17 h), and the line in between shows the desiccation (24 h) regime. Probabilities for cultivar effect and cultivar x hour interaction (in parentheses) are presented at the top of the figure for each water regime. DW = dry weight.

 
Similarly, upon exposure to prolonged (40 h) desiccation, Delia had higher EPR than Dropia when well watered and during recovery from desiccation, and significantly lower EPR during desiccation (Table 1). Under HAH, Dropia mean EPR decreased from 0.310 nmol g–1 DW h–1 under the well-watered regime to 0.184 under desiccation, while the EPR of Delia decreased from 0.421 to 0.115 nmol g–1 DW h–1 (Fig. 2) . Under LAH, Dropia mean EPR decreased from 0.237 to 0.133 nmol g–1 DW h–1, and the EPR of Delia declined from 0.303 to 0.088 nmol g–1 DW h–1. Upon recovery, both cultivars had an EPR of {approx}0.700 nmol g–1 DW h–1 under HAH and {approx}0.500 nmol g–1 DW h–1 under LAH, or {approx}40% greater than the EPR values before exposure to prolonged desiccation. The quadratic interaction of cultivar x hour was significant at P < 0.0001 under both desiccation and recovery, showing that the time response was curvilinear and that the curves were cultivar dependent. Cultivar x hour interaction was not significant when plants were well watered. Delia developed significantly shorter primary leaves than Dropia during desiccation and recovery (Fig. 3) .



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  		Fig. 2. Effects of cultivar, water regime, and time of observation on the ethylene production rate of wheat seedlings under high air humidity (above) and low air humidity (below). Rectangles at the top of the figure show the well-watered (the first 17 h) and the recovery (the last 25 h), and the line in between shows the desiccation (40 h) regime. Probabilities for cultivar effect and cultivar x hour interaction (in parentheses) are presented at the top of the figure for each water regime. The two levels of humidity were used as blocks (replications). DW = dry weight.

 


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  		Fig. 3. Primary leaf length of wheat cultivars Delia and Dropia at the beginning of well-watered regime (Start Exp), beginning of desiccation (Start Str), end of desiccation (End Str), and end of recovery (End Exp). Data are based on seedlings from all desiccation experiments. Means ± SD of five replications are presented.

 
Ethylene Production in Response to High Temperature
For both cultivars, temperature variation from 22 to 38°C produced changes in the EPR. At both temperatures, Delia produced significantly more ethylene than Dropia (Table 1). The cultivar x temperature interaction was not significant. The mean EPR for Delia was 0.332 nmol g–1 DW h–1 at 22°C and 0.310 at 38°C, while the mean EPR for Dropia was 0.204 nmol g–1 DW h–1 at 22°C and 0.195 at 38°C (Fig. 4) . During the first temperature increase at 14 h, EPR increased by 0.030 nmol g–1 DW h–1 in Dropia and by 0.106 nmol g–1 DW h–1 in Delia. During the second temperature increase at 40 h, EPR increased by 0.080 nmol g–1 DW h–1 in Dropia and by 0.134 nmol g–1 DW h–1 in Delia. Maximum EPR in both cultivars developed briefly after the end of the second exposure at 38°C, at 64 to 68 h, when seedlings were returned to 22°C (Fig. 4). At this time, the quadratic interaction of hour x cultivar was significant (P = 0.01), showing that the response across time was curvilinear and that the curves were different; that is, the peaks were different and cultivar-dependent (Fig. 4).



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  		Fig. 4. Effects of cultivar, temperature, and temperature duration on the ethylene production rate in wheat seedlings. Means ± SD of four replications are presented. To make graph easy, only 1/4 of the means have SD shown. Rectangles at the top of the figure show the 38°C and the lines the 22°C temperature regimes. Probabilities for cultivar effect and cultivar x temperature interaction (in parentheses) are presented at the left top of the figure. DW = dry weight.

 
Ethylene Production in Response to Paraquat Oxidation
Delia had significantly greater EPR than Dropia in both control and Pq treatment (Table 1). Significant effects of cultivar (P = 0.0015), treatment (P = 0.0001), and time of observation (P = 0.0001) were detected. The cultivar x treatment interaction was not significant. The hour x cultivar interaction was significant for control (P = 0.09) and Pq (P ≤ 0.05), showing that the treatments affected ethylene production differently across time; that is, the EPR decreased in controls and increased in oxidized plants with time (Fig. 5) . The effect was apparent within one hour after plants were transferred to Pq solution, and maximum EPR was obtained after plants were exposed to increased light intensity (Fig. 5). The mean EPR difference between Delia and Dropia was 0.065 nmol g–1 DW h–1 in control and 0.196 nmol g–1 DW h–1 in Pq-treated plants. Oxidized plants rapidly desiccated before any degradation of chlorophyll was detectable (data not shown).



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  		Fig. 5. Effects of cultivar (Ctr), paraquat-induced oxidation (Pq), and time of observation on the ethylene production rate in wheat seedlings. Means ± SD of two replications are presented. Arrow indicates application of additional light to 400 µmol m–2 s–1 photosynthetic photon flux density. Probabilities for cultivar effect and cultivar x treatment interaction (in parentheses) are presented at the left top of the figure. DW = dry weight.

 
Cultivar Effect on Physiological Parameters Related to Stress Resistance
For all physiological parameters, the cultivars differed significantly (Table 2). Consistent with field measurements (Saulescu et al., 1998), Dropia and Delia were the most contrasting cultivars. Delia exhibited greater membrane disruption under both PEG-osmotic and high temperature stresses, higher water loss through RT, and produced less biomass under reduced water availability. When exposed to Pq, Delia lost 49% more water than Dropia, demonstrating a higher sensitivity to oxidative stress.


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  		Table 2. Physiological parameters of stress resistance measured in seedlings of eight wheat cultivars, including Dropia and Delia, identified by the most contrasting field drought and heat resistance according to Saulescu et al. (1998). Shoot dry weight under osmotic stress induced with 10% polyethylene glycol (DWPEG10) and under reduced soil water potential (DW0.5) is presented as difference from nonstressed plants. Leaf membrane stability upon 40% polyethylene glycol treatment (MS) and thermal stability (TS) at 49°C for 1 h are presented as ratios between two conductivity readings of electrolyte leakage, after stress and after complete destruction of membranes. Leaf residual transpiration is presented as water loss after 5 h per gram DW. Tolerance to paraquat-induced oxidation (Pq) is presented as loss of water per gram DW of leaf. Means of five plants per replication and four replications per cultivar are presented.

 
Primary leaf characteristics under optimal growth conditions are presented in Table 3. Delia had significantly (P ≤ 0.05) reduced SLDW (14%), with 23% less chlorophyll content than Dropia.


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  		Table 3. Specific leaf dry weight (SLDW) and relative chlorophyll content of eight wheat cultivars. Means of five primary leaves per replication and four replications per cultivar are presented.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS
 DISCUSSION
REFERENCES
 
The reduction of EPR under water stress, its recovery to prestress levels after rehydration, and a transitory higher increase during stress recovery shown in Fig. 1 and 2 are consistent with other studies of EPR in wheat (Morgan et al., 1990; Narayana et al., 1991; Beltrano et al., 1997). Beltrano et al. (1997) explained that the cessation of ethylene production under low leaf water potential (–1.9 MPa) might be due to temporary enzyme folding and low leaf conductance (52 ± 17 mmol m–2 s–1). The same authors showed that an abrupt increase in ethylene production after rehydration was due to a significant increase in free radicals during water depletion and a corresponding increase in catalase and superoxid dismutase activities. Their explanation is supported by findings that ethylene was generated by peroxidation and formation of reactive oxygen species through enhanced conversion from the 1-amino-cyclopropane-1-carboxylic acid (ACC) (McRae et al., 1982; Mayak et al., 1983; Gardner and Newton, 1987; Wang and Yang, 1987; Krizek et al., 1993).

Ethylene emission significantly increased in both cultivars, although the increase in Delia was greater, under both high temperature- and Pq-induced oxidation (Fig. 4, 5). Other studies showed that abiotic stresses, including heat and drought, generated reactive oxygen species and membrane lipid peroxidation which enhanced ethylene production in leaves and ears of wheat (Paulin et al., 1986; Weckx et al., 1989; Navari-Izzo et al., 1993; Beltrano et al., 1997). In this experiment, in addition to desiccation and high temperature stresses, we used the herbicide Pq to increase oxidative stress and provide a relative test of how each cultivar responded. Since reactive oxygen species may convert ACC to ethylene (McRae et al., 1982; Mayak et al., 1983), the higher ethylene production of Delia under Pq oxidation may be caused by increased membrane peroxidation compared with Dropia.

In absence of stress, Delia had significantly higher EPR than Dropia (Fig. 1, 2, 4, 5) and smaller SLDW and lower chlorophyll content (Table 3). Even though we have studied only two cultivars, our results are generally consistent with those of Quarrie (1987), that excessive ethylene induces such morphological symptoms as reduced leaf growth. The greater EPR by Delia was consistent with data in Table 2 that showed Delia had more electrolyte leakage than Dropia during desiccation and under both water and high temperature stresses. Field observations also confirmed that Delia is more drought and heat sensitive (Saulescu et al., 1998).


   ACKNOWLEDGMENTS
 
The authors thank Mr. Tom Popham, statistician with the USDA-ARS, Southern Plains Area, Stillwater, OK, for assistance with the statistical analysis, and Dr. Nick N. Saulescu, wheat breeder with the Research Institute for Cereals and Industrial Crops (RICIC), Fundulea, Romania, for supplying seeds of the wheat cultivars.


   NOTES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
RESULTS
 DISCUSSION
 REFERENCES
 
1 Mention of a trade name or product does not constitute endorsement to the exclusion of other products that may also be suitable. Back

Received for publication October 8, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS
 DISCUSSION
 REFERENCES
 


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Crop Science 2004 44: 707-710. [Full Text]  




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