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Published online 3 May 2006
Published in Agron J 98:766-773 (2006)
DOI: 10.2134/agronj2005.0203
© 2006 American Society of Agronomy
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Water-Stress-Induced Changes in Resin and Rubber Concentration and Distribution in Greenhouse-Grown Guayule

Maren E. Veatch-Blohm*, Dennis T. Ray and William B. McCloskey

Dep. of Plant Sciences, Univ. of Arizona, Forbes 303, Tucson, AZ 85721

* Corresponding author (dtray{at}u.arizona.edu)

Received for publication July 5, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Guayule (Parthenium argentatum A. Gray) is naturally subject to periods of water stress in its native habitat. It has been shown that, under cultivation, rubber yields increase with increasing irrigation, but rubber concentration per plant decreases. The effect of irrigation on resin concentration is unclear. The purpose of this study was to understand how resin concentration is affected by water stress, and why water stress increases rubber concentration. Greenhouse-grown guayule plants were subjected to water stress in four experiments, each of 3 mo duration. Two experiments were conducted in the summer, the active growth period, and two experiments were conducted in the winter. The water-stressed plants were irrigated when the average soil water potential reached either –0.6 MPa (first summer experiment) or –0.3 MPa (all subsequent experiments). Water-stress effects were monitored by measuring growth, C exchange, biomass, and resin and rubber production. Water-stressed plants had lower C exchange, growth, and leaf-to-stem ratio than well-watered plants. Resin concentration did not respond consistently to water stress. Rubber concentration was generally higher in the water-stressed plants than in the well-watered plants as a result of decreased leaf biomass in both the summer (33 vs. 45 g kg–1) and winter (36 vs. 52 g kg–1) and decreased stem diameter in the summer (8.1 vs. 11.0 mm). Rubber is deposited mainly in the bark; therefore, reduced leaf biomass and stem diameter contribute to higher rubber concentration in the water-stressed plants by increasing the relative amount of bark.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
GUAYULE is a native of the Chihuahuan Desert with a natural history of exposure to water stress (Nakayama et al., 1991). In its native habitat, guayule receives between 17 and 38 cm of precipitation annually, occurring mainly during the summer months (McCallum, 1926; Benzioni et al., 1989). Guayule is currently being investigated as a source of hypoallergenic latex (Siler and Cornish, 1994; Cornish et al., 2001) and as a potential source of natural rubber in the event that rubber supplies from Hevea brasiliensis (Willd. ex A. Juss.) Müll. Arg. are unable to meet natural rubber demands (Mooibroek and Cornish, 2000).

Latex is a liquid suspension of rubber, which is a cis-1-4-polyisoprene (Cornish and Siler, 1995). Resin, a trans-isoprenoid, is a composite of acetone-extractable compounds that contribute to plant growth and development (Thompson and Ray, 1989; Oh et al., 2000). Resin comprises 6 to 11% of the mature plant biomass (Teetor et al., 2003; Veatch et al., 2005), and is currently being used in coproduct development as an antitermitic and an antifungal compound (Maatooq et al., 1996; Nakayama et al., 2001; Nakayama, 2005).

Guayule is typically grown from transplants in a 2-yr cycle, with active growth occurring between April and October and the bulk of new growth completed at the end of the second summer when the plant is 17 to 18 mo old. Guayule is a highly branched shrub, which at maturity ranges from 0.3 to 1.0 m in height and 0.6 to 1.2 m in width (Foster et al., 2002; Foster and Coffelt, 2005; Veatch et al., 2005). Resin is produced throughout the year, and production is usually highest during periods of active growth (Schloman et al., 1986). Guayule produces the majority of its rubber during the winter months, a time in which the plant appears to be dormant and encounters the lowest temperatures and amount of rainfall (Fangmeier et al., 1985; Ji et al., 1993). There is evidence that rubber production in guayule is triggered by either cold temperatures or water stress or both (Schloman et al., 1986; Appleton and van Staden, 1991).

Water requirements for guayule have been studied extensively during every major period of guayule research (Ray et al., 2005), from the Emergency Rubber Project (Addicott and Pankhurst, 1944; Kelley et al., 1945; Hunter and Kelley, 1946; Benedict et al., 1947) to studies in the 1980s (Bucks et al., 1985a, 1985b, 1985c; Miyamoto and Bucks, 1985). The relationship of resin concentration to the amount of irrigation is unclear. Some field studies have shown that the amount of irrigation does not affect resin concentration (Benedict et al., 1947; Allen et al., 1987), while others have shown that resin concentration increases with increasing irrigation (Bucks et al., 1985b; Miyamoto and Bucks, 1985; Benzioni et al., 1989); however, resin yield is directly related to plant biomass and increases with increasing irrigation (Bucks et al., 1985b; Ehrler et al., 1985). Field studies have shown that, as the application of irrigation water increases, rubber concentration within the plant decreases but rubber yields increase (Fangmeier et al., 1985; Miyamoto and Bucks, 1985; Mills et al., 1990). On the other hand, rubber concentration within the plant increases as the plants are subjected to water stress, but with a rather large loss in rubber yield (Nakayama and Bucks, 1984; Ehrler et al., 1985). The decrease in rubber yield in water-stressed plants is attributed to reduced biomass production (Ehrler et al., 1985; Nakayama et al., 1991).

Water stress initiates a number of changes within the plant, many of which contribute to guayule's drought tolerance. Water-stressed plants have less leaf lobing, smaller leaves, and smaller leaf area than well-watered plants (Benedict et al., 1947; Bucks et al., 1985a, 1985b; Benzioni et al., 1989). Carbon exchange (often reported as photosynthesis) measurements taken about 2 h before solar noon on whole plants showed no difference in the C exchange rate between well-watered and water-stressed plants (Allen et al., 1987; Allen and Nakayama, 1988). Other researchers have shown that more photosynthates, such as sucrose, are allocated into growth in actively growing plants compared with dormant plants, and that water stress reduces photosynthate translocation in the phloem (Benzioni and Mills, 1991; Kelly and van Staden, 1993). Guayule also undergoes osmotic adjustment in response to water stress, which some researchers believe is related to the rapid recovery of water-stressed leaves in response to even small amounts of added water (Ehrler and Nakayama, 1984; Nakayama and Bucks, 1984; Bucks et al., 1985a; Ehrler et al., 1985; Allen and Nakayama, 1988).

Previous research has focused on water stress under field conditions. Examining water stress under greenhouse conditions allows us to more easily examine the response of individual plants to different irrigation treatments. The purpose of this study was to understand how resin concentration is affected by water stress, and why water stress increases rubber concentration. To answer these questions we investigated growth, C exchange, and resin and rubber production and distribution in the leaves and stems of both well-watered and water-stressed plants.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Experiments were conducted in an evaporatively cooled, temperature-controlled greenhouse at the Campus Agricultural Center of the University of Arizona, Tucson. The experiment was conducted four times for 3 mo at a time. Two experiments ran from June through August in two consecutive summers (2003 and 2004). Two experiments ran from December through February in two consecutive winters (2003–2004 and 2004–2005). Separate 1-yr-old plants were used in all four experiments. For the summer 2003 experiment, plants were produced via tissue culture and included nontransgenic lines AZ 101 and AZ-2 and three transgenic lines. Two of the transgenic lines were derived from AZ 101, one carrying the farnesyl pyrophosphate (FPP) synthase gene (EC 2.5.10) and the other acting as an empty vector control (carrying only kanamycin resistance), and the third transgenic line was derived from AZ-2 and carried the FPP synthase gene. For the winter 2003–2004 experiment, the plants were also produced via tissue culture and included a nontransgenic AZ-2 line, and transgenics of AZ 101 and N6–5, both carrying only kanamycin resistance. For the summer 2004 experiment, all of the plants used were seed derived and were nontransgenic lines AZ 101 and AZ-2. For the winter 2004–2005 experiment, only seed-derived nontransgenic AZ 101 plants were used in the experiment. Lines used were based on plant availability.

The experiments were set up as a randomized complete block with four replications except in the winter 2003–2004 experiment, which had five replications. Two to five plants per line were assigned randomly to each treatment within each replication. Consequently, there were ~30 plants per replication in summer 2003, 20 plants per replication in winter 2003–2004, 20 plants per replication in summer 2004, and 10 plants per replication in winter 2004–2005. The plants were grown in pots for a year in the greenhouse, with irrigation every day during the summer and irrigation every 2 d during the winter. The plants were then transplanted into the experimental pots, containing a sandy loam soil, at least 1 mo before the initiation of the irrigation treatments. The plants within a row were placed 36 cm apart, which is the same planting distance used under field conditions; however, due to space limitations within the greenhouse, there were only 57 cm between rows of plants, compared with 102 cm under field conditions. During the experiments, the plants were grown in pots that were 0.61 or 0.76 m tall, with a total pot volume of 18 or 30 L, respectively. The pots were taller than conventional greenhouse pots to provide a deeper root zone, such that water stress symptoms took longer to develop (Earl, 2003). The 0.76-m pots were only available for use in the first experiment (summer 2003).

Irrigation frequency was determined by soil water potential, measured by a combination of predawn leaf water potential measurements, using a pressure chamber, and soil water potential measurements, using Watermark Soil Matric Potential Blocks (Campbell Scientific, Logan, UT) placed at a depth of 20 cm. The well-watered treatment was irrigated daily for 10 min with a drip irrigation system that delivered approximately 500 mL of water to the soil. In summer 2003, the water-stressed plants were watered for 10 min when the soil water potential was approximately –0.6 MPa, about halfway to the permanent wilting point. Although the water-stressed plants survived, their growth in summer 2003 was so reduced that in all subsequent experiments the water-stressed plants were irrigated when the soil water potential was approximately –0.3 MPa. The average irrigation frequency of the water-stressed plants was every 5 d during summer 2003, every 3 d during summer 2004, every 7 d during winter 2003–2004, and every 8 d during winter 2004–2005. For all treatments, the plants were fertilized every seventh irrigation with a one-half strength solution of Peter's 20–20–20 with micronutrients (20% N as 2.1% NO3 and 17.9% urea; 20% P as P2O5; 20% K as K2O; 0.05% each of chelated Cu and chelated Mn; 0.02% chelated B; 0.10% chelated Fe; and 0.0009% Mo). High and low temperatures were recorded and the average daily high and low temperatures ± standard deviation were 32 ± 1.2 and 23 ± 1.7°C, respectively, in summer 2003; 28 ± 1.9 and 16 ± 2.0°C in winter 2003–2004; 32 ± 3.2 and 22 ± 1.9°C in summer 2004; and 29 ± 3.6 and 17 ± 1.9°C in winter 2004–2005.

Plant height, width, and stem diameter were recorded at the start of irrigation treatments and growth was measured as the change from these initial measurements. Plant width was measured at the widest point of the plant in two directions perpendicular to each other, and these two measurements were averaged. Changes in height and width were measured for all experiments except in winter 2003–2004, when only height was measured. Changes in stem diameter were measured during the summer 2004 and the winter 2004–2005 experiments only. Changes in leaf osmotic potential were measured on a subset of plants with a Wescor Vapor Pressure Osmometer (Wescor, Logan, UT) on leaves sampled from the top of the plant the day after irrigation for both treatments.

Photosynthesis was estimated by measuring C exchange on a subset of plants for single leaves at the top of the plant, with a LI-6200 (Li-Cor Biosciences, Lincoln, NE). Due to instrument availability, the majority of the C exchange measurements were taken during summer 2003 and winter 2003–2004. During summer 2003, the C exchange measurements were taken the day before and the day after irrigation of the water-stressed treatment. Measurements were taken on five plants in each irrigation treatment in each of the four replications, with one plant missing in each treatment in one replication. During winter 2003–2004, irrigation of the water-stressed plants occurred at different times for the different replications, which dried out at different rates; therefore, C exchange measurements were taken twice a week, without reference to irrigation, but were usually within 3 to 4 d after irrigation of the water-stressed plants. Measurements were taken on three plants in each irrigation treatment in each of four replications. The C exchange measurements were taken within 1 h of solar noon on sunny days using an air flow rate of 185 to 190 µmol s–1, ambient CO2 between 380 and 410 µmol mol–1, an ambient average photosynthetically active radiation of 1300 µmol m–2 s–1, and ambient humidity, which ranged from 25 to 40% relative humidity during the summer and 11 to 30% relative humidity during the winter. Relative humidity measurements under field conditions were similar to relative humidity measurements in the greenhouse throughout the majority of the experiment, with a slightly higher relative humidity in the greenhouse on very dry days and slightly lower humidity in the greenhouse on extremely humid days. Once a month during summer 2003, diurnal C exchange measurements were also taken every 2 h from 0800 to 1600 h approximately 2 d after irrigation of the water-stressed plants on five plants in each irrigation treatment in each of two replications. Diurnal measurements were taken once during winter 2003–2004 every 2 h from 0700 to 1700 h approximately 4 d after irrigation of the water-stressed plants on two plants in each irrigation treatment in each of four replications.

Three months after the start of the experiment, when the plants were 15 mo old, the aboveground biomass was harvested at the soil surface. In addition to shoot biomass, the root ball was also harvested for all experiments except summer 2003. A subset of plants was defoliated, leaf area and leaf weight measured, and the proportion of leaf fresh weight of the overall shoot fresh weight was calculated. The subset included one plant per line per irrigation treatment per replication, except for winter 2003, which included defoliated plants from only three of the five replications. All plant samples were dried for 2 d at 80°C, weighed, and ground in a coffee grinder, which was wiped clean between samples. Resin and rubber were extracted sequentially from the dried, ground samples with acetone and cyclohexane, respectively, using a common extraction method, which is a variation of the gravimetric analysis method developed by Black et al. (1983). A 0.5-g sample of the ground plant material was homogenized in 20 mL of acetone for 30 s with a Polytron homogenizer (Kinematica, Newark, NJ) at a speed of 27 000 rotations per minute (rpm) to extract resin. The homogenizer was rinsed with an additional 10 mL of acetone and the sample was centrifuged at 3500 rpm for 12 min. The supernatant was poured through filter paper into a preweighed aluminum dish. The procedure was repeated twice more, and the supernatant from all three extractions was combined. The acetone in the aluminum dish evaporated overnight and any residual liquid was removed in a vacuum oven for 30 min at 60°C. After the acetone also evaporated from the sample residue overnight, rubber was extracted from it with cyclohexane, using the procedure described above for acetone. Resin and rubber concentrations were calculated with the following formula: ([final pan weight – initial pan weight]/sample weight)100. For all experiments except summer 2003, resin and rubber were also extracted from the roots, and separately from the stems and leaves of the defoliated plants. Resin and rubber yields per plant were calculated by multiplying the plant dry weight by the resin and rubber concentrations of each plant.

All data were analyzed across and within seasons using multiple regression in the fit model platform of the JMP 4.0 statistical software, academic version (Sall et al., 2001), for a randomized complete block with four replications (five replications in winter 2003–2004). Line, irrigation treatment, season, and their interactions were treated as fixed effects. All values within a block were used to test for differences between seasons. Fresh weight, dry weight, leaf area, resin and rubber concentrations, and resin and rubber yields were log10 transformed for analysis. Specific comparisons were done using orthogonal contrast (Sall et al., 2001). A P value of <0.05 was considered significant throughout.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
There were significant differences between seasons and between irrigation treatments, and a significant interaction between season and irrigation treatment for almost all parameters measured. There were no significant differences between lines, and no significant interactions between any of the lines and irrigation treatments or seasons were found. Therefore, all data presented are for irrigation treatments within seasons. Differences from the combined result within a season for a year are noted. The water-stressed plants had significantly smaller changes in height and width than the well-watered plants in all experiments (Table 1). The water-stressed plants also had a significantly smaller change in stem diameter than the well-watered plants in summer 2004 and in winter 2004–2005 (Table 1). Both fresh and dry weights were significantly reduced in the water-stressed plants (Table 2). The water-stressed plants also had reduced leaf area (Table 2) and a significantly smaller proportion of leaf fresh weight in the overall shoot fresh weight than the well-watered plants (Table 2).


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  		Table 1. Mean plant height, width, and stem diameter at the start of irrigation treatments, and mean change in height, width, and stem diameter ± standard error in well-watered and water-stressed guayule in the summer (2003 and 2004) and the winter (2003–2004 and 2004–2005).

 

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  		Table 2. Mean shoot (leaves and stems) fresh and dry weight, proportion of leaf fresh weight of shoot fresh weight, and leaf area ± standard error of well-watered and water-stressed guayule in the summer (2003 and 2004) and the winter (2003–2004 and 2004–2005).

 
The osmotic potential of the leaves of both well-watered and water-stressed plants decreased during much of the experimental period, and was significantly lower in the water-stressed plants than the well-watered plants in both summer and winter (Fig. 1 ). There were also significant differences between treatments during midday C exchange measurements. The day before irrigation, during summer 2003, the water-stressed plants had much lower C exchange than the well-watered plants (Fig. 2 ). Carbon exchange rates of the water-stressed plants increased significantly the day after irrigation, but the water-stressed plants still had significantly lower C exchange than the well-watered plants (Fig. 2). Carbon exchange was also significantly reduced in the water-stressed plants during winter 2003–2004, but reduction in C exchange took longer to develop than it did in the summer (Fig. 3 ). No significant differences in C exchange were found between treatments in December, but C exchange was significantly lower in the water-stressed plants than the well-watered plants in January and February (Fig. 3). The diurnal C exchange measurements during summer 2003 were significantly higher in the well-watered plants than the water-stressed plants only at solar noon and 1400 h, with no significant differences between treatments observed at other times during the day (Fig. 4 ). In contrast, during the winter 2003–2004 diurnal measurements, the water-stressed plants had significantly lower C exchange than the well-watered plants at all times except 0700 and 1700 h (Fig. 5 ).


Figure 1
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  		Fig. 1. Mean leaf osmotic potential of well-watered (W) and water-stressed (D) guayule in the summer and winter the day after irrigation. The osmotic potential was measured on 19 plants in each irrigation treatment in summer 2003 (five plants in each of four replications, with one plant missing in one replication), on eight plants in each irrigation treatment in summer 2004 (two plants in each of four replications), on nine plants in each irrigation treatment in winter 2003–2004 (three plants in each of three replications), and on four plants in each irrigation treatment in winter 2004–2005 (one plant in each of four replications). The vertical bar indicates ± standard error.

 

Figure 2
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  		Fig. 2. Mean midday C exchange rate of well-watered (W) and water-stressed (D) guayule the day before and the day after irrigation in June, July, and August 2003. Carbon exchange measurements were recorded within 1 h of solar noon. Measurements were taken on 19 plants in each irrigation treatment (five plants in each of four replications, with one plant missing in one replication). The vertical bar indicates ± standard error.

 

Figure 3
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  		Fig. 3. Mean midday C exchange rate of well-watered (W) and water-stressed (D) guayule in December 2003 and January and February 2004, an average of 3 to 4 d after irrigation of the water-stressed plants. Carbon exchange measurements were recorded within 1 h of solar noon. Measurements were taken on 12 plants in each irrigation treatment (three plants in each of four replications). The vertical bar indicates ± standard error.

 

Figure 4
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  		Fig. 4. Mean diurnal C exchange rates of well-watered (W) and water-stressed (D) guayule in July 2003. Diurnal measurements were taken 2 d after irrigation of the water-stressed plants. Carbon exchange measurements were recorded every 2 h beginning at 0800 and ending at 1600 h. Measurements were taken on 10 plants in each irrigation treatment (five plants in each of two replications). The vertical bar indicates ± standard error.

 

Figure 5
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  		Fig. 5. Mean diurnal C exchange rates of well-watered (W) and water-stressed (D) guayule in February 2004. Diurnal measurements were taken an average of 4 d after irrigation of the water-stressed plants. Carbon exchange measurements were recorded every 2 h beginning at 0700 and ending at 1700 h. Measurements were taken on eight plants in each irrigation treatment (two plants in each of four replications). The vertical bar indicates ± standard error.

 
Resin concentration in the shoots (leaves and stems) was significantly higher in the well-watered plants in two of the four experiments; however, resin concentration was not consistently different between treatments in either the summer or the winter (Table 3). Resin yield was significantly higher in the well-watered plants in all experiments (Table 3). Resin concentration in the roots was significantly higher in the water-stressed plants in the summer and significantly higher in the well-watered plants in the winter (Table 4). Leaf resin concentration was significantly higher in the well-watered plants in the winter, but no significant differences between treatments were found in the summer (Table 4). Stem resin concentration was higher in the water-stressed plants in the summer, but not significantly different between treatments in the winter (Table 4). The proportion of the total shoot resin was significantly higher in the stems than in the leaves in both treatments, with the proportion of total shoot resin significantly higher in the leaves and stems of the well-watered and the water-stressed plants, respectively (Table 5).


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  		Table 3. Mean shoot (leaves and stems) resin and rubber concentrations and yields ± standard error of well-watered and water-stressed guayule in the summer (2003 and 2004) and the winter (2003–2004 and 2004–2005).

 

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  		Table 4. Mean resin and rubber concentration ± standard error in the roots, leaves, and stems of well-watered and water-stressed guayule in the summer (2004) and the winter (2003–2004 and 2004–2005).

 

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  		Table 5. Mean proportion of total shoot resin and rubber yields ± standard error from leaves and stems of well-watered and water-stressed guayule in the summer (2004) and the winter (2003–2004 and 2004–2005).

 
Rubber concentration was higher in the water-stressed plants than the well-watered plants in whole shoots (leaves and stems) in three of the four experiments—in both summer experiments and in winter 2003–2004 (Table 3). The water-stressed plants did not have significantly higher rubber concentration in the shoots in the combined winter data, but this was due to the very high rubber concentration in the well-watered plants in winter 2004–2005 (Table 3). Rubber yield was significantly higher in the well-watered plants in all experiments (Table 3). No treatment had consistently higher rubber concentration in the roots, and leaf rubber concentration was not significantly different between treatments (Table 4). Stem rubber concentration was significantly greater in the water-stressed plants in the summer and in the well-watered plants in the winter (Table 4). The high rubber concentration in the stems of well-watered plants in the combined winter data was due to the significantly higher rubber concentration in the well-watered plants compared with the water-stressed plants in winter 2004–2005 (Table 4). In contrast, stem rubber concentration was not significantly different between treatments in winter 2003–2004 (Table 4). The proportion of total plant rubber was significantly greater in the stems than in the leaves, and the proportion of total plant rubber in the stems was significantly greater in the water-stressed plants in both summer and winter (Table 5).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The water-stressed plants, even at the reduced stress level used after summer 2003, had reduced growth compared with the well-watered plants (Table 1). They also had reduced leaf area (Table 2), which decreased the transpirational demand on the plant; however, it had the negative effect of reducing the available photosynthetic area. The reduced photosynthetic area, in combination with the reduced C exchange on an individual leaf basis (Fig. 2 and 3), contributes to a smaller photosynthate pool available for growth. The water-stressed leaves also had lower osmotic potential than the well-watered leaves (Fig. 1) even after irrigation, which may have reduced the ability of the water-stressed plants to recover full C exchange capacity after irrigation, since many of the solutes used for osmotic adjustment, such as soluble sugars, reduce photosynthesis by feedback inhibition (Basu et al., 1998). In contrast, plants from the well-watered treatment had a larger photosynthetic area (Table 2), higher C exchange measurements (Fig. 2 and 3), and higher osmotic potential (i.e., less negative; Fig. 1) than the water-stressed plants. Therefore, these plants maintained steady growth, which is consistent with the observations of Kelly and van Staden (1987, 1993) that photosynthates are allocated to growth and not rubber production in actively growing plants.

In summer 2003, resin concentration was higher in the well-watered plants and rubber concentration was higher in the water-stressed plants (Table 3), and the well-watered plants had a much higher leaf-to-stem ratio than the water-stressed plants (Table 2). Teetor et al. (2003) found that green leaves and green stems contribute between 35 and 45% of the total resin in the shoot, with the highest concentrations found in green stems. In the same study, rubber concentration was highest in the stems and lowest in the leaves. Based on their data and our data on leaf-to-stem ratio, we hypothesized that the higher shoot resin concentration in the well-watered treatment was due to the high contribution of leaves to the total resin within the shoot. In contrast, shoot rubber concentration in the well-watered plants would be lower because the leaves, which have very little rubber, would have a diluting effect on the high concentration of rubber in the stems.

To test this hypothesis, resin and rubber were extracted separately from the leaves and stems of the defoliated plants from all subsequent experiments. We hypothesized that if the relative amounts of leaf and stem was the major contributing factor to the differences in resin and rubber concentration between treatments, then the resin and rubber concentrations in the different plant parts would not be different between the treatments. In winter 2003–2004, the hypothesis held true for rubber concentration in the leaves and stems, but the well-watered leaves had higher resin concentration than the water-stressed leaves even though resin concentration within the stems was not significantly different between treatments (Table 4). The contribution of the leaves to the total shoot resin was significantly higher in the well-watered plants than the water-stressed plants (Table 5), although the well-watered plants did not have significantly higher shoot resin concentrations than the water-stressed plants (Table 3). In winter 2004–2005, resin concentration in the leaves and the stems and rubber concentration in the stems were significantly higher in the well-watered plants (Table 4); however, the proportion of total shoot resin and rubber from the leaves and stems was not significantly different between treatments (Table 5). In summer 2004, resin and rubber concentrations in the leaves were not significantly different between treatments, but the stems of the water-stressed plants had significantly higher resin and rubber concentrations than the well-watered plants (Table 4). The higher resin concentration in the stems of the water-stressed plants may be due to the smaller diameter of the stems (8.1 mm) compared with the stems of the well-watered plants (11.0 mm; Table 1), as small stems are where the highest resin concentrations are found (Teetor et al., 2003). Despite the lower resin concentration in the stems of the well-watered plants, the leaves contributed a significantly higher proportion of the total shoot resin in the well-watered plants than the leaves in the water-stressed plants (Table 5), such that resin concentration in the shoots was not significantly different between treatments (Table 3).

The increased rubber concentration in the water-stressed plants in all experiments except winter 2004–2005 might be due to two factors. First, rubber deposition occurs mainly in the bark parenchyma of guayule stems and to a much lesser extent in other parts of the plant (Gilliland et al., 1984). The greater contribution of the stems to the total shoot biomass, with a smaller contribution of the leaves (Table 2), could increase the overall rubber concentration in the water-stressed plants. This hypothesis is supported by examining the proportion of the total shoot rubber contributed by the stems in each treatment. The treatment with the highest proportion of the total shoot rubber from the stems also had the highest rubber concentration in the shoot (Tables 3 and 5).

Second, differences in stem diameter between treatments contributed to the significant differences in rubber concentration between well-watered and water-stressed plants, especially during summer 2004. Smaller stems tend to have higher rubber concentrations than larger stems (Dierig et al., 1989) and the greatest number of rubber-bearing parenchyma cells (Latigo et al., 1996). Lloyd (1911) and Mitchell et al. (1945) found that, as stem diameter increased, the bark-to-wood ratio decreased. Very little rubber is accumulated in the woody portion of the stems (Gilliland et al., 1984), with one estimate of 82% of the stem rubber located in the bark and only 18% of the stem rubber in the wood (Kuruvadi et al., 1997). Therefore, plants with a greater bark-to-wood ratio would have higher rubber concentrations within the stem, which would contribute more to the overall rubber concentration within the plant. During summer 2004, the water-stressed plants had a smaller change in stem diameter (Table 1) and smaller actual stem diameter (8.1 mm) than the well-watered plants (11.0 mm) at harvest, resulting in a greater bark-to-wood ratio. In winter 2003–2004, the actual stem diameter of the well-watered plants (11.9 mm) at harvest was significantly greater than the water-stressed plants (11.1 mm), but rubber concentration in the stem was not significantly different between treatments (Table 4). With a difference in stem diameter between treatments of only 0.8 mm, the water-stressed plants may not have had a large enough increase in the bark-to-wood ratio to increase the stem rubber concentration significantly in the water-stressed plants compared with the well-watered plants. Despite the large difference in stem diameter between the well-watered (13.2 mm) and the water-stressed plants (10.7 mm) in winter 2004–2005, rubber concentrations within the shoots and the stems were not what we had anticipated based on the results of the three previous experiments (Tables 3 and 4). These unanticipated findings are thought to be the result of two factors. First, there were only a small number of plants available for this experiment, so that any extreme value in either treatment could strongly affect the results. Second, in contrast to the water-stressed plants in the other experiments, the water-stressed plants in this experiment had a much more rapid and severe response to water stress, from which they did not significantly recover until a couple of weeks before harvest.

The plants grown during the winter had lower resin and rubber concentrations than the plants grown during the summer (Tables 3 and 4). The lower resin concentration was probably related to the lower growth rates in the winter (Table 1), since resin is generally higher in actively growing plants (Schloman et al., 1986). The soil water potentials in these experiments did not decrease as rapidly in the winter as they did in the summer, with almost twice as much time between irrigations. Therefore, the winter-grown plants were not exposed to severe water stress as often as the summer-grown plants. Winter is the time when most rubber deposition occurs in guayule (Ji et al., 1993); therefore, the low C exchange observed in the water-stressed plants during the winter (Fig. 3) would have reduced the production of rubber precursors, limiting rubber production. The water-stressed plants may not have had the time needed to accumulate substantial quantities of rubber before water stress altered the leaf-to-stem ratio and reduced changes in stem diameter. Also, the low night temperatures in the greenhouse during the winter experiments were warmer than the temperatures thought to trigger high rubber production in the field (Ji et al., 1993), which may have contributed to the low rubber concentrations observed in both winter experiments.

The main purpose of this study was to gain a better understanding of why rubber concentration in guayule is increased by water stress, by examining individual plants within a greenhouse. Our data indicate roles for both the relative amounts of leaf and stem and the relative amounts of bark and wood within the stem. Although guayule is unlikely to encounter these exact conditions in the field, the results presented here could be used as a starting point for further investigation and understanding of guayule's responses to water stress under field conditions.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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