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CROP PHYSIOLOGY & METABOLISM

Sensitivity of Wheat and Rice to Low Levels of Atmospheric Ethylene

Crop Physiology Laboratory, Dep. of Plants, Soils and Biometeorology, Utah State Univ., Logan, UT 84322-4820

* Corresponding author (bugbee{at}cc.usu.edu)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Ethylene (C₂H₄) gas is produced throughout the life cycle of plants and can accumulate in closed growth chambers to levels 100 times higher than in outside environments. Elevated atmospheric C₂H₄ can cause a variety of abnormal responses, but the sensitivity to elevated C₂H₄ is not well characterized. We evaluated the C₂H₄ sensitivity of wheat (Triticum aestivum L.) and rice (Oryza sativa L.) in five studies. The first three studies compared the effects of continuous C₂H₄ levels ranging from 0 to 1000 nmol mol^-1 (ppb) in a growth chamber throughout the life cycle of the plants. A short-term 1000 nmol mol^-1 treatment was included in which exposure was stopped at anthesis. Yield was reduced by 36% in wheat and 63% in rice at 50 nmol mol^-1 and both species were virtually sterile when continuously exposed to 1000 nmol mol^-1. However, the yield reductions were much less with exposure that stopped at anthesis, suggesting the detrimental effect of C₂H₄ on yield was greatest around the time of seed set. Two additional studies evaluated the differential sensitivity of two wheat cultivars (Super Dwarf and USU-Apogee) to 50 nmol mol^-1 C₂H₄ at three CO₂ levels [350, 1200, 5000 µmol mol^-1 (ppm)] in a greenhouse. Yield of USU-Apogee was not significantly reduced by C₂H₄ but the yield of Super Dwarf was reduced by 60%. Elevated CO₂ did not influence the sensitivity to C₂H₄. A difference in the C₂H₄ sensitivity of USU-Apogee between greenhouse and growth chamber trials suggests that C₂H₄ sensitivity is dependent on the environment. Collectively, the data suggest that relatively low levels of C₂H₄ could induce anomalous plant responses by accumulation in greenhouses and growth chambers with inadequate ventilation. The data also suggest that C₂H₄ sensitivity can be reduced by both genetic and environmental manipulations.

Abbreviations: DW, dry weight • HI, harvest index • PPF, photosynthetic photon flux

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
PLANTS CONTINUOUSLY SYNTHESIZE C₂H₄ throughout their life cycles and it mediates a broad range of physiological responses (Abeles et al., 1992). Several papers have described patterns of C₂H₄ synthesis and efflux in wheat (Beltrano et al., 1994; Labrana et al., 1991; Petruzzeli et al., 1994). Ethylene efflux appears to be correlated with growth rate and may peak during anthesis in grain crops. Wheeler et al. (1996) found that C₂H₄ production from a wheat canopy was several times higher during vegetative growth than during grain fill. Rates of 0.1 to 0.2 nmol C₂H₄ kg^-1 DW s^-1 have been measured in wheat, lettuce (Lactuca sativa L.), and cotton (Gossypium spp.) during vegetative growth (Wheeler et al., 1996; Morgan et al., 1990). We have measured C₂H₄ production rates five times higher than this (1.0 nmol C₂H₄ kg^-1 DW s^-1) in the heads of wheat at anthesis (unpublished data).

Terrestrial atmospheric C₂H₄ levels rarely exceed 10 nmol mol^-1 (Abeles et al., 1992). Levels from 50 to 100 nmol mol^-1 are common in greenhouses with heating or ventilation problems and have resulted in a broad range of crop damage in the horticulture industry (Blankenship and Kemble, 1996; Gibson et al., 2000; Mortensen, 1989). North Carolina State University provides helpful information on how to prevent C₂H₄ problems in greenhouses and a service for checking air samples posted on the web at http://www.ces.ncsu.edu/depts/hort/greenhouse_veg; verified 28 Nov. 2001. Levels as high as 1000 nmol mol^-1 have been measured in controlled environments in both ground and space studies (Abeles et al., 1992; Bingham et al., 2000; Campbell et al., 2001; Salisbury, 1997). Elevated C₂H₄ levels can cause a variety of abnormal responses including shortened height, epinasty, leaf rolling, premature leaf senescence, and sterility (Abeles et al., 1992; Bennet and Hughes, 1972; Morison and Gifford, 1984).

Elevated C₂H₄ levels may cause anomalous results in greenhouses and growth chambers, but the threshold for C₂H₄ sensitivity is poorly characterized. Elevated C₂H₄ levels are of particular concern in tightly sealed bioregenerative life support systems, which are being developed for space by the National Aeronautics and Space Administration (NASA). The objectives of a bioregenrative life support system are to provide food, O₂, CO₂ removal, and water purification for long-term space exploration. NASA has recognized that atmospheric C₂H₄ may need to be scrubbed to prevent abnormal plant growth in space. Recent advances in catalytic scrubbing technology have improved significantly our ability to remove C₂H₄ from air (Tibbitts et al., 1998). However, it is difficult to remove C₂H₄ below 50 nmol mol^-1 in closed plant growth chambers and this is still 10 to 50 times higher than levels in the field.

There is considerable genetic variability in C₂H₄ sensitivity. Variation in post harvest flower longevity among carnations has been attributed to genetic variation in both C₂H₄ synthesis and perception (Wu et al., 1991; Brandt and Woodson, 1992). Recent advances in the identification of genes associated with C₂H₄ perception facilitate breeding C₂H₄ tolerant genotypes (Barry et al., 2000; Bleeker and Kende, 2000; Bleeker and Schaller, 1996; Gubrium et al., 2000; Lindstrom et al. 1999). Ethylene insensitive transgenic tomatoes, petunias, and tobacco have been developed by transformation with the Arabidopsis etr1-1 gene (Wilkinson et al., 1997). However, C₂H₄ insensitive Arabidopsis mutants and transgenic plants can have abnormal developmental processes that affect seed germination, flower initiation, flower longevity, and fruit set (Bleeker et al., 1988; Gubrium et al., 2000).

In addition to genetics, environmental factors including light, temperature, O₂, and CO₂ influence C₂H₄ production (Abeles et al., 1992; Finlayson and Reid, 1996; Grodzinski and Woodrow, 1989; Preger and Gepstein, 1984; Sanders et al., 1990; Sisler and Wood, 1988). How these factors influence C₂H₄ perception is not well understood. Gubrium et al. (2000) observed significant differences between the temperature responses of insensitive transgenic petunias (Petunia x hybrida Vilm.) and wild-type plants, suggesting a possible interaction between temperature and C₂H₄ perception. Burg and Burg (1967) classified hypoxia (<5% O₂) as an inhibitor of C₂H₄ responses, but later studies found no effect of hypoxia on C₂H₄ binding activities in plants (Sanders et al., 1990).

CO₂ is of particular interest since it is normally high in space environments and is commercially used in fruit storage to inhibit the ripening action of C₂H₄ (Yang, 1985). Burg and Burg (1967) reported that CO₂ competitively inhibits C₂H₄ action, but only at very high levels (10%). Later studies suggested the inhibitory effects of CO₂ are noncompetitive (Sisler, 1979; Sanders et al., 1990). We know that exposure to elevated CO₂ (>0.2%) at the time of anthesis decreases seed set in wheat (Grotenhuis and Bugbee, 1997). Similarly, we know that exposure to C₂H₄ at anthesis also inhibits seed set in wheat (Klassen et al., 1999). The effect of elevated CO₂ on C₂H₄ sensitivity at levels much lower than required to inhibit C₂H₄ action needs to be examined.

Our previous studies found that 250 nmol mol^-1 C₂H₄ caused a 60% yield reduction in wheat and 500 nmol mol^-1 completely inhibited seed production (Klassen et al., 1999). Microscopic examination of developing heads indicated that failed seed production was caused by an inhibition of anther dehiscence. Exposure to 1000 nmol mol^-1 prior to anthesis did not reduce yields, suggesting wheat is most susceptible to C₂H₄ induced sterility around the time of anthesis. Our results implicated C₂H₄ as the cause of failed seed production in wheat grown on the Russian Space Station MIR in 1996 (Bingham et al., 2000; Campbell et al., 2001; Salisbury, 1997).

The objective of these studies was to evaluate further the effects of low C₂H₄ levels on wheat and rice. Specifically, we sought to determine (i) the threshold level at which yield is not reduced, (ii) the differential sensitivity of similar species and cultivars, and (iii) if sensitivity to low C₂H₄ levels (50 nmol mol^-1 C₂H₄) is influenced by elevated CO₂ levels typical of space environments (1200–5000 µmol mol^-1 CO₂).

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Growth Chamber Studies
Three trials were conducted in six cylindrical acrylic plastic (Lucite, ICI Acrylics, Wilmington, DE) chambers within a growth chamber (Fig. 1) . Light was supplied with cool white fluorescent lamps providing a photosynthetic photon flux (PPF) of 21.6 and 48.5 mol m^-2 d^-1 in the two wheat trials and 21.6 mol m^-2 d^-1 in the rice trial (Table 1). CO₂ was enriched to 1200 µmol mol^-1 in all three trials. CO₂ levels remained constant in the wheat trials (24-h photoperiod) and nighttime levels in the rice were less than 50 µmol mol^-1 higher than daytime values. The six chambers shared a common recirculating nutrient solution. Hydroponics methodology was previously described in detail (Bugbee and Salisbury, 1988). Airflow through each cylinder was maintained at 3.3 x 10^-4 m³ s^-1 (20 L min^-1) to provide a rapid rate of air turnover, and thus maintain a constant gas composition in each chamber. Pure C₂H₄ from a compressed gas cylinder was diluted with air and introduced to the chambers at flows necessary to maintain steady-state C₂H₄ levels in each treatment. Chamber CO₂ levels were monitored every 60 min with an automated infrared gas analyzer. Chamber C₂H₄ concentrations were monitored every 30 min by automated gas chromatography (Shimadzu GC17-A, Columbia, MD). The C₂H₄ concentration of ambient air used for the clean air control remained below the detection limit of 5 nmol mol^-1 for the duration of these studies and the levels of all other treatments remained within 10% of set point.

View larger version (35K): [in this window] [in a new window]   Fig. 1. Diagram of one of six cylindrical chambers in the growth chamber system.

Greenhouse Studies
Two replicate studies were conducted in a greenhouse in 12 acrylic (Lucite) chambers. Supplemental high pressure sodium (HPS) lighting was used overnight to provide a 24-h photoperiod and to ensure that both studies had a similar average PPF of 51 mol m^-2 d^-1 (Table 1). In one study, the lights were on for 16 h per day providing 45 mol m^-2 d^-1 and natural light provided 6 mol m^-2 d^-1. In the replicate study, lights were on for 12 h per day providing 34 mol m^-2 d^-1 and natural light provided 17 mol m^-2 d^-1. Each cultivar was planted (500 plants per m^-2) in adjacent plots (0.45 by 3.60 m) lengthwise along each of two replicate benches filled with soilless media (1:1; peat:Perlite) 23 cm deep (Fig. 2, 3) . Following emergence, the 12 chambers were placed so that equal areas (23.5 by 36.0 cm) of each cultivar were included in each chamber (Fig. 2, 3). A 30-cm-tall reflective metal barrier (Coil-Zac, Environmental Growth Chambers, Chagrin Falls, OH) was placed between the two cultivars to prevent shading of the shorter cultivar. Each chamber had a water-cooled condenser for temperature control and air sampling lines for automated CO₂ and C₂H₄ analysis. Airflow into each chamber was maintained at 6.6 x 10^-4 m³ s^-1 (40 L min^-1) to provide a rapid rate of air turnover. Air, CO₂, and C₂H₄ were supplied as in the growth chamber trials. Drip irrigation lines buried in the media supplied nutrient solution three times a day as previously described (Reuveni and Bugbee, 1997). Plants in each chamber were exposed to one of three CO₂ levels (350, 1200, and 5000 µmol mol^-1) and one of two C₂H₄ levels (0 and 50 nmol mol^-1). Plants growing between chambers provided guard rows to minimize the effects of side lighting. Plants were harvested and processed following the same procedures as in the growth chamber studies. Data were analyzed as a randomized split block design including two cultivars, two C₂H₄ levels, three CO₂ levels, and two replicate treatment chambers per study by a standard ANOVA procedure (Tables 2, 3).

View larger version (47K): [in this window] [in a new window]   Fig. 2. Diagram of one of 12 rectangular chambers in the greenhouse system. Two cultivars were grown within each chamber.

View larger version (142K): [in this window] [in a new window]   Fig. 3. Picture of the 12-chamber greenhouse system.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

Seed yield was reduced by 37% in the 50 nmol mol^-1 C₂H₄ treatment and plants in the 1000 nmol mol^-1 treatment were almost completely sterile (Fig. 4) . A similar trend was observed in harvest index (HI). Although C₂H₄ increased tillering as indicated by an increased number of total heads (sterile + fertile), seed set per head was significantly reduced. Seed mass was not significantly different among treatments. Yield and HI were only slightly reduced in the preanthesis treatment verifying our previous finding that C₂H₄ induced male sterility is most significant around the time of anthesis (Klassen et al., 1999).

View larger version (31K): [in this window] [in a new window]   Fig. 4. Ethylene response curves for yield components of USU-Apogee wheat grown at different photosynthetic photon fluxes (PPF). The short-term exposure pre-anthesis treatment is represented by a triangle.

Growth Chamber Trial: Super Dwarf Rice
The effects of C₂H₄ on rice were unique. Rice plants did not exhibit rolled leaves, shortened leaf length, or shortened plant height, but did exhibit chlorosis, premature leaf senescence, and delayed heading in all C₂H₄ treatments.

Similar to wheat, a C₂H₄ level of 50 nmol mol^-1 significantly reduced yield (Fig. 5) . Seed yield and HI were reduced by more than 50% in the 50 nmol mol^-1 treatment and plants exposed to 300 and 1000 nmol mol^-1 were almost completely sterile. The yield of the 1000 nmol mol^-1 preanthesis treatment was not determined because of the long delay in heading (38 d > control). Similar to wheat, yield reductions were associated with poor seed set. Seeds per head were reduced in the 50 and 100 nmol mol^-1 treatments but the number of heads was not affected. Both seeds per head and head number were reduced in the 300 nmol mol^-1 treatment, and there were no heads in the 1000 nmol mol^-1 treatment.

View larger version (17K): [in this window] [in a new window]   Fig. 5. Ethylene response curves for yield components of Super Dwarf rice. Data for the short-term exposure, pre-anthesis treatment is not included since this treatment failed to reach maturity.

Greenhouse Trials: Wheat Cultivar x CO₂ Interactions
USU-Apogee was more tolerant of elevated C₂H₄ levels than Super Dwarf. Yield was not affected by 50 nmol mol^-1 C₂H₄ for USU-Apogee, but was reduced by 60% in Super Dwarf (Fig. 6) . The insensitivity of USU-Apogee to 50 nmol mol^-1 C₂H₄ was surprising since this C₂H₄ level reduced the yield of USU-Apogee by 35% in the growth chamber (Trial 2). This difference suggests an interaction between C₂H₄ and environmental factors that were different between the greenhouse and growth chamber trials.

View larger version (25K): [in this window] [in a new window]   Fig. 6. Mean yield and harvest index (HI) of USU-Apogee and Super Dwarf wheat with and without 50 nmol mol-1 C2H4 at three levels of CO2. Yields are normalized to percent of the control at 1200 µmol mol-1 CO2 for ease of comparison. HI was calculated assuming the root mass to be 10% of the total above ground biomass.

Environmental differences between the growth chamber and greenhouse trials that may have contributed to the observed difference in the C₂H₄ response of wheat include temperature, light, and rooting medium. Although the temperature set points were the same for both the greenhouse and growth chamber studies, it was not possible to maintain constant temperatures in the greenhouse. Temperatures in the greenhouse followed a diurnal pattern with cool mornings and gradual warming until reaching a maximum at midday. Thus, when the daily average temperature of the greenhouse was maintained to be 22°C, instantaneous values typically ranged from 16 to 28°C. In contrast, temperatures in the growth chamber were stable (±1°C from set point) throughout the trial.

We hypothesize that temperature may interact with C₂H₄ sensitivity in wheat and rice. We know that high temperature inhibits fertilization in both wheat and rice and that fertilization is most sensitive to high temperature around the time of anthesis (Evans et al., 1975; Gusta and Chen, 1987; Mackill et al., 1982). Satake and Yoshida (1978) found that high temperature interferes with anther dehiscence and causes poor pollination in rice. We previously determined that C₂H₄ inhibits anther dehiscence, causing failed seed production in wheat (Klassen et al., 1999). Thus, high temperature affects yield in both wheat and rice in ways similar to that of C₂H₄. We are now preparing to examine the effects of temperature on C₂H₄ sensitivity.

Like temperature, light levels were more variable in the greenhouse than in the growth chamber. While the average light intensity was similar in the second growth chamber study and the greenhouse studies (47.5 vs. 50 mol m^-2 d^-1), instantaneous levels in the greenhouse typically ranged from 400 to 800 mol m^-2 s^-1. Mortensen (1989) studied interactions between light (50, 100, 150 mol^-2 s^-1) and C₂H₄ (55 and 112 nmol mol^-1) on lettuce growth. His data indicated that the effect of C₂H₄ was minimized at the highest light level examined. The effect of light intensity on C₂H₄ sensitivity warrants further investigation since high light intensity has also been shown to promote seed set in wheat (Wardlaw, 1970).

Finally, although the greenhouse and growth chamber rooting media were different (peat:Perlite media vs. hydroponics), the root zone environments were similar since the soilless media was watered three times a day with a nutrient solution. Both systems thus provided ample water, oxygen, and nutrients. Growth rates and yields of plants grown hydroponically and in soilless media have been comparable in several previous studies (Bugbee et al., 1994; Reuveni and Bugbee, 1997).

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
A C₂H₄ level of 50 nmol mol^-1 significantly reduced yields in both wheat and rice. Although this level is higher than typical field levels, it is sufficiently low to be of concern in poorly ventilated greenhouses and growth chambers, and in urban areas where atmospheric levels may exceed 50 nmol mol^-1. Significant variability in C₂H₄ sensitivity was observed between species and cultivars. USU-Apogee is less sensitive to C₂H₄ than either Super Dwarf wheat or Super Dwarf rice. Although C₂H₄ sensitivity varied with environment, elevated CO₂ (up to 5000 µmol mol^-1) did not affect C₂H₄ sensitivity in wheat.

Our results suggest yield reductions associated with elevated C₂H₄ can be minimized through a combination of screening for C₂H₄ tolerant genotypes and the identification and control of environmental factors affecting C₂H₄ sensitivity.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Work supported by the National Aeronautics and Space Administration Advanced Life Support Program administered by the Johnson Space Center, and by the Utah State Agricultural Experiment Station, Utah State Univ. Approved as journal paper No. 7404.

Received for publication April 9, 2001.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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