Original Research Paper
12
Light level does not alter ethylene sensitivity in radish or pea
34
Joseph F. Romagnano
5
Utah State University 6
Crop Physiology Laboratory 7
4820 Old Main Hill 8 Logan, UT 84322-4820 9 Phone: 01 508 868 7832 10 Email: [email protected] 11 12 Bruce Bugbee 13
Utah State University 14
Crop Physiology Laboratory 15
4820 Old Main Hill 16 Logan, UT 84322-4820 17 Phone: 01 435 797 2765 18 Email: [email protected] 19 20
Keywords: ethylene-PPF interaction, digital image quantification, image analysis,
21biphasic dose-response 22
Abstract
23Ethylene accumulation occurs in many plant growth environments. In some 24
instances, low photosynthetic photon flux (PPF) is also a stress factor. Ethylene helps 25
regulate the shade-avoidance mechanism and synthesis rates can be altered by light. We 26
thus hypothesized that ethylene sensitivity in whole plants may be altered in low light. 27
Radish (Raphanus sativus) and pea (Pisum sativum) plants were selected as models due 28
to their rapid growth, use in previous studies and difference in growth habit. We first 29
characterized radish and pea sensitivity to ethylene. Radish vegetation was less sensitive 30
to ethylene than pea vegetation. Pea reproductive yield was highly sensitive. Plants 31
grown under low light levels are typically etiolated and less robust than plants grown 32
under higher light. In a second series of studies we examined the interaction of ethylene 33
(50 ppb pea, 200 ppb radish) with PPFs from 50 to 400 µmol m-2 s-1. There was no 1
statistically significant interaction between ethylene sensitivity and PPF, indicating that 2
high PPF does not mitigate the detrimental effects of chronic low-level ethylene 3
exposure. This also suggests there is no crosstalk between the shade avoidance pathway 4
and the primary ethylene signaling pathway. 5
Introduction
6Elevated levels of atmospheric ethylene cause a variety of abnormal responses 7
including inhibited root and hypocotyl elongation, leaf epinasty, reduced growth, 8
premature leaf senescence, and sterility (Abeles et al., 1992; Klassen and Bugbee, 2002, 9
2004; Mattoo and Suttle, 1991; Morison and Gifford, 1984; Smalle and Van Der Straeten, 10
1997). Plants are the primary source of elevated ethylene in sealed plant growth chambers 11
and other controlled environments with inadequate air exchange (Wheeler, 1996, 2004). 12
Elevated ethylene has caused several unique problems both on the International Space 13
Station and in other spaceflight experiments (Perry and Peterson, 2003, Campbell et al. 14
2001). It is now clear that ethylene has significant potential to interfere with the 15
development of plant based advanced life support systems for long duration spaceflight 16
Also, ethylene is generated in greenhouse environments as a byproduct from combustion 17
powered equipment such as heaters and forklifts (Sargent, 2001). 18
The sensitivity of flowers to ethylene at levels as low as 20 nmol mol-1 (ppb) 19
during anthesis has been well documented and is a significant cause of yield loss in 20
reproduction-dependent crop plants (Payton et al., 1996; Oráez, Blay, and Granell, 1999; 21
Klassen and Bugbee, 2002; Hudelson, 2006). 22
Eraso et al. (2002) demonstrated that ethylene greater than 50 ppb was required 1
to reduce leaf area and total biomass in radish. Klassen and Bugbee (2002) found that 2
biomass of wheat and rice was not significantly decreased at 1000 ppb whereas yield of 3
both crops was significantly reduced by 200 ppb, and suggested that reproductive organs 4
are more sensitive to elevated ethylene than vegetative growth. 5
Elevated ethylene reduces leaf expansion rate and increases leaf epinasty (Abeles, 6
1992), which decreases radiation capture. Woodrow et al. (1988, 1989, 1993) 7
demonstrated that photosynthesis was not affected by ethylene when epinastic leaves 8
were straightened to allow for original rates of radiation capture. Taylor and Gunderson 9
(1988) found that acute exposure to extremely high ethylene concentrations (10,000 ppb, 10
1% in air) reduced quantum yield in soybean leaves, but this high level is not 11
representative of the chronic low-levels that accumulate in a contaminated environment. 12
The general consensus is that low chronic exposure to ethylene has a minimal effect on 13
quantum yield and the photosynthetic apparatus (Abeles, 1992). 14
Although most of our research has focused on the detrimental effects of ethylene, 15
contrary to expectations, some studies suggest it is possible for long-term exogenous 16
ethylene exposure to have a beneficial effect on whole plants or plant communities. 17
Fiorani, et al. (2002) and Konings and Jackson (1979) reported a correlative effect in 18
which those plants with lower endogenous ethylene production rates had greater 19
tolerance to higher ethylene concentrations and greater beneficial growth effects. For 20
instance, rice with a low endogenous ethylene production rate benefited more from a 20 21
ppb ethylene exposure than white mustard, which has a high endogenous ethylene 22
production rate (Konings and Jackson, 1979). They go on to propose a bi-phasic dose-23
response for ethylene sensitivity. In summarizing research on ethylene insensitive 1
Arabidopsis mutants and other plants, Pierik et al. (2006) built on the notion of Konings
2
and Jackson (1979) and proposed four potential model curves for different classes of 3
ethylene response labeled as Type I, Type II, Type III and Type IV. Type I plants are 4
sensitive to ethylene and have a negative response at all concentrations, following an 5
exponential decrease downwards. Type II and III plants exhibit an initial stimulation in 6
growth, Type III at a higher and broader concentration than the lower and sharper Type 7
II, followed by an exponential decrease similar to Type I. Type IV plants are either 8
unaffected or have a stimulatory response at extremely high concentrations with no 9
subsequent inhibitory response. These curves represent a refinement to the hypothesis 10
that beneficial effects of ethylene exposure are probably limited to a narrow 11
concentration range whose ideal is likely species, organ, and environmental condition 12
dependent. 13
Endogenous ethylene in unstressed terrestrial plants does not appear to inhibit leaf 14
expansion. Endogenous levels are typically one to two orders of magnitude lower than 15
those needed for autocatalytic ethylene synthesis. Bleeker et al. (1988) found that leaves 16
of ethylene insensitive Arabidopsis plants were larger than their wildtype counterparts. 17
However, when Tholen et al. (2004) replicated the study by Bleeker et al. and controlled 18
for ethylene build-up in the atmosphere of the petri dishes, they found that wildtype and 19
ethylene-insensitive mutants had equal leaf expansion rates. Thus, they concluded that 20
endogenous ethylene levels do not affect leaf expansion in unstressed plants. This agrees 21
with previously reported data that shows an ethylene threshold for reduction in leaf 22
expansion (Klassen and Bugbee, 2004). Altered ethylene synthesis is typically thought to 23
be a signal of stress conditions. Indeed, ethylene plays an active role in mediating the 1
shade-avoidance response. 2
Light quantity and quality have been found to alter ethylene synthesis. Etiolated 3
pea seedlings have long served as a model to study the effect of ethylene on internode 4
elongation (see review by Eisinger, 1983). Jiao, et al. (1987) appears to be among the 5
first research group to show interactions between light quality and ethylene synthesis. 6
They observed that dark grown wheat leaves had decreased ethylene synthesis after 7
exposure to white light. Their results also showed that red and far-red light (quantities 8
and ratios of which are altered in the shade of a plant canopy) altered ethylene synthesis, 9
suggesting that phytochrome may regulate ethylene synthesis. Subsequent work using 10
leaf discs of Begonia (Rudnicki, et al., 1993) demonstrated that white, blue, green and 11
red light inhibited ethylene synthesis, but far-red light stimulated production. 12
Vandenbussche et al. (2003) studied shade-avoidance in Arabidopsis and reported 13
a decrease in ethylene synthesis with increased light in short-term studies (hours). The 14
uptake of CO2 was higher in the light, but ethylene synthesis was less. In their subsequent
15
review, Vandenbussche et al., (2005) summarized the current knowledge of ethylene 16
interactions with the shade avoidance mechanism noting that low light increases ethylene 17
production, an overproduction of ethylene in Arabidopsis leads to an exaggerated 18
response to low light, and that ethylene via ethephon can stimulate leaf movement in 19
Arabidopsis similar to that in low light. This agrees with Pierik et al. (2004) who
20
demonstrated that ethylene insensitive tobacco plants were unresponsive to reduced 21
levels of blue light despite shade-avoidance inducing concentrations of ethylene in the 22
canopy. Pierik et al. (2004) state that ethylene functions as a neighbor signal, providing a 23
cue other than light that a plant is in a community and may be subject to shading. Foo et 1
al. (2006) further demonstrated phytochrome A and B regulation of ethylene in pea plants
2
by showing that plants lacking both phytochromes overproduced ethylene. 3
Vandenbussche et al. (2007) found that blue light triggered cryptocrome signaling played 4
a role in Arabidopsis hypocotyl response to ethylene dependent on a base rate but 5
independent from the gibberellic acid pathway. In addition to re-stating that ethylene 6
works as a primary neighbor detection signal through atmospheric accumulation, Pierik et 7
al. (2009) reconfirmed the linkage between light responses and ethylene signaling. In
8
Arabidopsis seedlings, they demonstrated increased ethylene evolution under low R:FR
9
ratios and that an intact ethylene signaling pathway was required for petiole elongation. 10
The effects of light on ethylene synthesis can be variable. Kurepin et al. (2010) 11
studied ethylene evolution from 7 d old Helianthus annuus shoot tissues at PPF levels of 12
10, 100 and 1000 µmol m-2 s-1. They found that as PPF increased, ethylene synthesis 13
increased in hypocotyls and decreased in cotyledons and leaves. They reconcile their 14
findings with those of Vandenbussche et al. (2003) by remarking that the Arabidopsis 15
seedlings tested by Vandenbussche et al. (2003) consist largely of cotyledon and leaf 16
tissue whose subsequent increase in ethylene production masks the decrease in 17
production from hypocotyls. Kurepin et al. (2010) go on to demonstrate that ethephon 18
application decreased hypocotyl, cotyledon-leaf mass at the higher PPF levels tested 19
(100, 1000 µmol m-2 s-1) whereas application of the ethylene action inhibitor AVG had no 20
effect at any PPF level. 21
Given the sensitivity of etiolated plants to ethylene and the effect of light quantity 22
and quality on ethylene synthesis, we hypothesized that PPF level would alter ethylene 23
sensitivity. This hypothesis is particularly important for the closed plant growth chambers 1
on the space station, where ethylene routinely accumulates and where the light levels are 2
low. It also provides further insight into the mechanisms behind shade avoidance. 3
Materials and Methods
4Radish Ethylene Sensitivity
5
Radishes (Raphanus sativus, cv. Cherry belle) were grown in six polycarbonate,
6
30-cm diameter, 60-cm tall chambers with a root-zone depth of 21-cm filled with 1:1 7
peat:perlite media (Fig. 1). Each chamber was continuously and independently supplied 8
with air or an air/ethylene mixture at 15 L min-1 with each chamber maintaining a 9
positive air pressure. Klassen and Bugbee (2002), provide a complete description of the 10
ethylene dilution and distribution system used in these studies. Air for the zero ppb 11
controls was filtered through potassium permanganate impregnated beads (Purafil™), 12
which maintained ethylene concentrations below the limits of detection (5 ppb). Each 13
chamber was maintained at a 25/20˚C day/night temperature. Nutrients were provided by 14
watering 3x daily with dilute (120 mS m-1; 1.2 dS m-1) Peters 5-11-26 Hydrosol 15
supplemented with 10 µM Fe EDDHA, 1.4 mM CaNO3, and 10 µM Na2SiO3. Radishes
16
were grown at a PPF of 400 µmol m-2 s-1 from high pressure sodium (HPS) lamps with a 17
16-h photoperiod. Radishes were grown at 0, 20, 40, 80, 120 and 160 ppb ethylene in 18
independent chambers for at least two replicate studies. Leaves and storage roots 19
(bulbous tap root without lateral roots) were harvested at 20 days post emergence (DPE). 20
Pea Ethylene Sensitivity Studies
21
53 Day Study. Peas (Pisum sativum cv. Earligreen) were planted in replicate
22
chambers in a greenhouse using a randomized complete block design and a density of 40 23
plants m-2 (8 plants per chamber; Fig. 2). Supplemental HPS lamps provided a PPF of 1
600 µmol m-2 s-1 for a 16 h photoperiod. Plants were watered with the same nutrient 2
solution described for radish. Ethylene concentrations were maintained at 0, 10, 20, 40, 3
70 and 120 ppb in two replicate chambers. Plants were harvested at maturity with full 4
seeds in full pods before senesence (53 DPE). Subsequent ANOVA analysis (SPSS v. 15) 5
indicated no block effect and data from both blocks were combined. 6
33 Day Study. Individual plants were grown in replicate chambers identical to
7
those described for pea (Fig. 2). Nutrient solution was provided as described for radishes. 8
Ethylene levels were 0, 30, 60, 120 and 200 ppb. Plants were harvested 33 days post 9
planting before pods in the controls (< 1 cm long) could fill. Dry mass was measured for 10
the vegetative portion of the plants, including unfilled pods. 11
Ethylene-PPF Interaction Studies
12
Radish and pea plants were grown in the chambers described above for radish 13
sensitivity (Fig. 1). For radish, a PPF regime of 50, 200 and 400 µmol m-2 s-1 was 14
imposed with steady-state ethylene at 0 or 200 ppb. Radish plants were harvested at 22 15
DPE. For pea, a PPF regime of 70, 200 and 400 µmol m-2 s-1 was imposed with steady-16
state ethylene at 0 or 50 ppb. Pea plants were harvested at 14 DPE. 17
Non-Destructive Quantification of Plant Size via Digital Photography
18
Klassen et al. (2003) showed the high correlation of pixel area with growth. 19
Digital images of plants were taken with the fixed focal length lens height kept at a 20
constant height above the media surface. Images were imported into Adobe Photoshop 21
CS2®. The extract filter was used to separate the plants from the background. Once plants 22
were extracted from the original background, 15% grey was placed as the new 23
background while maintaining the pixel dimensions of the original image. The “magic 1
wand” tool with the tolerance set in the range from 1-10 and set to highlight contiguous 2
pixels only was used to select the grey background. The “inverse selection” command 3
was then used to select for the plants. The histogram palette was used to obtain the total 4
number of pixels for all plants in the container. The number of pixels per plant was then 5
calculated as an average of all plants in a chamber. 6
Ethylene Measurement
7
Ethylene was automatically measured in each chamber every 30 minutes using an 8
automated Shimadzu GC17a v. 3.4 equipped with a flame ionization detector. An 1/8 in 9
diameter x 2 m Porapak® Q column at 120˚C oven temperature and 70 ml min-1 helium 10
carrier flow was used to separate ethylene contained in samples loaded via 5 ml sample 11
loop. Ethylene was retained for approximately 0.83 min with a 5 ppb detection threshold. 12
0 ppb control chambers showed no ethylene present within the constraints of this 13
detection limit (less than 5 ppb). The system was equipped with two common-outlet 16-14
port sample valves (VICI Valves, Houston, TX) which allowed for the continuous cyclic 15
monitoring of ethylene from 31 separate locations. 16
Results
17Non-Destructive Measurement of Ethylene Sensitivity
18
Elevated ethylene decreased green pixel area in both radish and pea for all days 19
measured (Fig. 3). Treated plants that were small at day of emergence remained 20
comparably small throughout the life cycle. By 10 DPE when the radish canopy started to 21
close, leaf expansion of plants grown at 160 ppb were 35-40% the size of controls (Fig. 3, 22
radish inset). 23
The effect of ethylene on pea was greater than radish (Fig. 3). Similar to radish 1
plants, the effect on plant size was apparent at emergence and remained throughout the 2
life cycle (Fig. 3, pea inset). Plant size was reduced by 30% at 10 ppb; this is a 3
significantly lower sensitivity threshold than radish (Fig. 3, inset). Subsequent 4
measurements of vegetative dry mass of both species confirmed the pixel data (Fig. 4, 5
Top). 6
The data for radish and pea ethylene sensitivity were placed in context with data 7
originally published in Klassen and Bugbee (2004) (Fig. 4). Vegetative radish root was 8
less sensitive to ethylene than reproductive pea yields. Peas were among the most 9
sensitive of the crops tested (Fig. 4). 10
Carbon Partitioning and Yield
11
Both storage root and shoot dry mass of radish decreased in response to ethylene
12
(Fig. 4, Top). As predicted by digital pixel counts, shoot and root dry masses were also 13
35-40% of controls at 160 ppb ethylene. Both shoot and root percent dry mass showed a 14
slight increase with increasing ethylene but it was not statistically significant (data not 15
shown). Harvest index was not affected by ethylene treatment (data not shown), 16
indicating that carbon partitioning into the radish root is not altered by ethylene. 17
Pea yield, defined as seed dry mass, exponentially decreased with increasing 18
ethylene (Fig. 4, bottom). Yield decreased ~37% at 10 ppb ethylene, similar to pixel 19
count predictions. Shoot fresh and dry mass, pod fresh and dry mass, number of seeds per 20
pod, shoot height, internode length, and number of pods per plant all followed trends 21
similar to yield (data not shown). Harvest index decreased, demonstrating a profound 22
effect on reproductive growth (Fig. 5). 23
Ethylene-PPF Interaction
1
As expected, reduced PPF decreased plant size and caused etiolation in both 2
radish and pea plants (Figs. 6 & 7). At 50 µmol m-2 s-1 PPF, 200 ppb of ethylene reduced 3
the epinastic response of radish shoots (Fig. 6). Ethylene at 200 ppb decreased radish root 4
and shoot fresh mass 55 to 65% (data not shown). Ethylene at 50 ppb decreased pea shoot 5
dry mass by 40% (data not shown). Some leaf curling was observed in pea plants at all 6
light levels but not in radishes. When the mass data was plotted as percent control vs. 7
PPF, there was no significant effect of ethylene on sensitivity; treated plants were 8
decreased in size by the same amount regardless of PPF (Fig. 8). 9
Discussion
10Threshold for Ethylene Sensitivity
11
To prevent loss of yield, Klassen and Bugbee (2004) suggest an ethylene 12
inhibition threshold of 10 ppb for crops dependent on pollination and seed set and 30 ppb 13
for crops dependent on leaf expansion and vegetative growth. Although these separate 14
thresholds were given, the sensitivity data they presented merged both reproductive and 15
vegetative crops. We clearly make the distinction between vegetative and reproductive 16
crops (Fig. 4). This distinction is important since several types of tissues are being 17
compared and contrasted: roots and leaves harvested for vegetative crops are grouped 18
together as similar and are contrasted against reproductive crops that are dependent upon 19
flowering rates, pollination and fruit set all of which have varying, and unknown in many 20
cases, levels of sensitivity to ethylene. 21
At our lowest ethylene concentration for radish (20, 40 ppb) there is some 22
evidence for Type II or Type III (as described by Pierik et al., 2006) growth stimulation 23
in response to low ethylene. This is consistent with the results of Eraso et al. (2002) in 1
which radish vegetation and root mass was not significantly impacted at ethylene 2
concentrations below 50 ppb. A similar effect was also seen for mizuna (Brassica rapa, 3
japonica group) vegetation. All the rest of the data exhibit Type I decreases with no 4
suggestion of improved growth at low ethylene concentrations. These data represent the 5
first steps towards the comprehensive species-wide screening of ethylene concentration 6
dependency called for by Pierik et al. (2006) to demonstrate the generality of the bi-7
phasic model. It appears that if bi-phasic stimulation of ethylene is a valid proposal, it is 8
limited to non-reproductive tissues and at concentrations below 50 ppb. 9
Pea plants provide a unique example of compounded sensitivity in that losses in 10
both vegetative and reproductive tissues affected yield, making them among the most 11
ethylene sensitive crops tested. This is different from the wheat and rice crops reported 12
by Klassen and Bugbee (2002) and flower abortion in tomato reported by Hudelson 13
(2006). In those cases, vegetation was largely unaffected at ethylene concentrations that 14
severely reduced reproductive yield. For these plants, in chronic ethylene exposure 15
situations, application of ethylene perception inhibitors might be beneficial. 16
Potential morphological responses to ethylene
17
Ethylene is well known to induce epinasty and hyponasty in radish and pea 18
seedlings at levels above about 1 ppm (Morgan, 2011; Polko et al. 2011;). We observed 19
some hyponasty in the ethylene-treated peas but the effect was similar at all light levels. 20
For the radish plants our levels of ethylene exposure may have been below the threshold 21
for these morphological effects to be apparent. Klassen and Bugbee (2004) found 22
significant effects of low ethylene levels on cell expansion, but they did not observe 23
significant morphological alterations to the leaves in any of the eight crops studied, 1
including radishes. 2
Non-Destructive Measurement of Growth
3
Pixel counts can accurately predict plant size (Klassen et al. 2003). The accuracy 4
is affected by several factors. Foremost, more vertical leaf angle can lead to an 5
underestimation of plant size. Ethylene can affect leaf angle. If light is provided from a 6
single direction and side lighting is minimized, then a decrease in pixel count due to leaf-7
angle change is representative of decreased radiation capture potential, assuming that 8
actual leaf area has not changed. Neither radish nor pea plants exhibited noticeable 9
changes to leaf angle. Alterations to leaf size caused the greatest differences between 10
treatments. Indeed, in this study pixel counts accurately predicted dry mass at time of 11
harvest. 12
Ethylene-PPF Interaction and Shade Avoidance
13
Although we did not measure biochemical responses for shade-avoidance 14
reactions, prior experiments in young plants such as those of Vandenbussche et al. 15
(2003), Foo (2006), Pierik et al. (2009) and Kurepin et al. (2010) have established the 16
role of ethylene signaling in seedlings and young plants experiencing shade stress. 17
However, our results indicate, at least for growth of intact radish and pea, that PPF has no 18
interaction with ethylene sensitivity. 19
Plants in low light should produce minimal ethylene so that leaf and stem 20
expansion are as rapid as possible. Once the plants have adequate light, ethylene 21
synthesis should increase, restricting growth. Since Foo (2006), Pierik et al. (2009) and 22
Kurepin et al. (2010) support the observations of Vandenbussche, et al. (2003), this 23
suggests that, in this case of chronic exposure, neither photoreceptor regulation or shade 1
avoidance pathways affect response to chronic ethylene and that there is no crosstalk 2
between these two possibly independent pathways. This further suggests that the shade-3
avoidance capabilities of the chronically exposed plants might remain intact. It is equally 4
possible, however, that the shade-avoidance signal may be overwhelmed and non 5
responsive in this situation. Studies that examine synthesis-light interactions during long-6
term plant growth should yield greater insight. 7
Acknowledgements
8The National Aeronautics and Space Administration Graduate Student Researchers 9
Program (grant #NNG05GL53H) and the Utah Agriculture Experiment Station at Utah 10
State University (paper #8432) supported this research. We would also like to thank Alec 11
Hay, Julie Chard, and Rob Hyatt and the other members of the Utah State University 12
Crop Physiology Laboratory who assisted with this project. 13 14 15 16 17 18 19 20 21 22
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Figure 1. Radish plants and six flow-through chambers used for radish sensitivity
characterization and both ethylene-PPF interaction trials. The front-center polycarbonate chamber has been removed for the photo. Blended-gas supply lines feed into the top of chamber directly in front of the fan. Photo has been color corrected to remove orange cast of HPS lamps. Reflective surface in back of image creates illusion of more than six chambers.
Figure 2. Pea plants growing in 12-chamber greenhouse system.
Each chamber was individually controlled for temperature and flow-through air/ethylene mix. PPF was equalized in each chamber with window screen. Each row was treated as a block (North or South) in a randomized complete block experiment design. There was no block effect in the final data. Photo has been color corrected.
Radish
Pea
Figure 3. Time course of ethylene effect on radish and pea size measured in pixels. Data points in the
graph are from individual chambers in replicate trials. The equation for a sigmoid growth curve was used to fit regression lines to the data (r2 ≥0.95 for all lines). The inset shows pixel data from days 3 and 10 post emergence for radish (regression lines are identical despite chamber variance) and days 8 and 15 post emergence for pea as a percent of control. 160 ppb reduced radish plant size by 40% whereas 20 ppb reduced pea growth by a similar amount. Canopy closure past day 10 for radishes and day 15 for pea prohibited further analysis via digital imaging. Subsequent harvest data was highly correlated with pixel data.
Figure 4. Ethylene sensitivity of vegetative and reproductive crop plants. Vegetative
crops are less sensitive to elevated ethylene than reproductive crops. Radish plants were less sensitive than lettuce or mustard. Pea plants were one of the most sensitive crops tested. Dotted reference lines indicate a 10% loss in potential yield. Except for pea and radish data, all data are modified from Klassen and Bugbee (2004).
Pea
Figure 5. Effect of ethylene on pea harvest index.
Harvest index decreased as ethylene increased in all chambers above 20 ppb, indicating a decrease in carbon partitioning to reproductive structures. Seed set was zero at 120 ppb for one of the chambers. Regression line is a 2 parameter exponential decay with r2 = 0.75.
Light Levels (PPF; µmol m-2 s-1) 50 200 400 0 200 E th yle n e (p p b )
Figure 6. Radish plants from the first ethylene sensitivity–light interaction trial. Increased
Figure 7. Pea plants from the first ethylene sensitivity–light interaction trial. Increased light
levels did not alter sensitivity to ethylene.
Light Levels (PPF; µmol m-2 s-1)
70 200 400 0 50 E th yle n e (p p b )
Figure 8. Effect of PPF on radish root and shoot
fresh mass and pea shoot dry mass. Demonstrated by nearly horizontal linear regression lines, increased PPF did not alter ethylene sensitivity (r2 < 0.42 for all lines). Shoot dry mass from pea plants grown at 50 ppb ethylene was 58 to 60% of control. Radish root and shoot fresh mass from plants grown at 200 ppb ethylene were 35 to 45% of control.
