Establishing new guayule lines by direct seeding

Industrial Crops and Products 9 (1999) 93 – 100

Establishing new guayule lines by direct seeding M.A. Foster a,*, J.L. Fowler b, L.G. Kleine a, N. Puppala b b

a Texas Agricultural Experiment Station, Box 1549, Pecos, TX 79772, USA Department of Agronomy and Horticulture, New Mexico State Uni6ersity, Las Cruces, NM 88003, USA

Received 13 March 1998; accepted 6 July 1998

Abstract Direct seeding of guayule (Parthenium argentatum Gray) in west Texas has been successful. However, studies have involved only USDA and ‘Mexican Bulk’ lines. New selections developed by plant breeders must be screened for their adaptability to direct seeding. The objective of this study was to evaluate the standard establishment and production parameters (biomass, rubber and resin content, rubber and resin yield) of six direct seeded guayule selections (AZ-R1, AZ-R2, CAL-6, CAL-7, UC-101, and UC-104). An experiment was initiated on a Delnorte gravelly loam on 23 July 1993 at the Texas Agricultural Experiment Station near Fort Stockton, TX. Conditioned and raw seed of the six guayule selections were planted 10 mm deep on raised beds spaced 1 m apart. Laboratory seed germination ranged from 34–92%. Seedling density was greatest with conditioned seed (61 seedlings/m) versus raw seed (37 seedlings/m) when averaged across all lines 30 days after planting. Individually, seedling density in plots established by conditioned seed was greatest with CAL-6 (68 seedlings/m), UC-104 (66 seedlings/m), AZ-R1 (65 seedlings/m), UC-101 (64 seedlings/m), and CAL-7 (61 seedlings/m), and were significantly greater than the seedling density in plots established with raw seed. Both conditioned and raw seed produced adequate guayule stands. Nonetheless, conditioning improved germination, and in the field, conditioned seed was observed to germinate and emerge quicker resulting in more efficient irrigation management. The AZ-R1 seedlings were the most vigorous and were tolerant to flea beetles (Epitrix sp.). Plants were thinned to a 0.36 m spacing in April 1994. Six plants from each selection in the conditioned seed treatments were harvested on 23 February 1995 and 6 March 1996. Each plant was divided into branch (clipped 10 cm above the soil surface) and root sections. Biomass and rubber and resin yields were greatest in the branch versus root fractions. Total rubber yield (branches plus roots) at both harvests was greatest in selections AZ-R2 and UC-104. © 1999 Elsevier Science B.V. All rights reserved. Keywords: New crops; Seed conditioning; Guayule; Direct seeding; Natural rubber; Parthenium argentatum

* Corresponding author. Tel.: +1 915 4455050; fax: +1 915 4459231; e-mail: [email protected] 0926-6690/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0926-6690(98)00019-3

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1. Introduction A primary factor restricting the commercialization of guayule as a source of natural rubber has been the high cost of stand establishment. Previously, transplanting greenhouse-grown seedlings has been the only reliable method of stand establishment. Bucks et al. (1986) estimated that development of direct seeding methods could reduce the cost of establishment below US$400/ha versus US$900–1200/ha for transplanting. Advances in seed conditioning techniques have improved germination and seedling development over a broad temperature range (Chandra et al., 1983; Chandra and Bucks, 1986). The conditioning process was designed to break dormancy and promote the onset of germination under conditions of acute osmotic stress. Bucks et al. (1986) reported that in field trials, seed germination was greater with conditioned seed versus raw seed. Recent studies have shown that guayule can be successfully direct seeded in west Texas with conditioned seed and precision planting (Foster and Moore, 1992; Foster et al., 1996a,b). Planting was conducted throughout the growing season, and the authors concluded that direct seeding in west Texas could occur during the late spring and early summer when soil temperature exceeds 15°C. Early planting allowed seedlings to develop sufficiently to become hardened before frost. Direct seeded guayule has shown adequate tolerance to dimethyl 2,3,5,6-tetrachloro-1,4,-benzenedicarboxylate, a pre-emergence herbicide applied at 4.5 and 9.0 kg ai/ha) to control annual grass and broadleaf weeds (Foster et al., 1993). Guayule seedlings grow slowly, producing only about 1 cm top growth and 5 cm root growth by 2 weeks after emergence (Miyamoto and Bucks, 1985). Therefore, they cannot compete effectively against weeds. An effective preplant or pre-emergence herbicide will be required for optimum stand establishment. Guayule has been direct seeded in New Mexico with the aid of polyacrylamide soil conditioners (Fowler and Tinguely, 1993). Germplasms tested were either USDA lines or ‘Mexican Bulk’ germplasm (Thompson and Ray, 1989). Plant breeders have developed new germplasm which, when coupled with improved

agronomic practices, has significantly increased rubber yields as compared to the standard USDA or Mexican germplasm (Estilai and Dierig, 1996). The University of California has released seven cultivars (CAL-1–CAL-7) (Tysdal et al., 1983; Estilai, 1985, 1986). The Arizona Agricultural Experiment Station and the USDA/ARS jointly released six lines (AZ-1–AZ-6), which were bred for their ability to regenerate after cutting to allow for multiple harvests, vigor of the regrowth, and bulk rubber yield (Ray et al., 1998). Rubber yields of these lines were 58–101% greater when compared to the USDA cultivars. Table 1 Supplemental irrigation and rainfall during 1993, 1994, and 1995 at the Texas Agricultural Experiment Station near Fort Stockton, TX Year

Days after planting

Irrigation (mm)

Rainfall (mm)

Total water (mm)

1993

30 60 90 150 Total

470 70 60 0 600 160 360

10 15 0 25 50 200 225

480 85 60 25 650 360 585

1994 1995

Table 2 Percent laboratory seed germination of six guayule lines Linea

AZ-R1 AZ-R2 UC-101 UC-104 CAL-6 CAL-7 Meanb

Seed treatment Conditioned

Raw

92 56 92 72 74 68 76a

76 34 66 46 60 54 56b

a The AZ lines were obtained from D.T. Ray (University of Arizona), and the UC and CAL lines from A. Estilai (University of California, Riverside, CA). Seed was increased at the New Mexico State University Leyendecker Plant Science Research Center. b The mean conditioned seed germination was significantly greater than the raw seed germination different according to a t-test at the 5% level of probability.

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Table 3 Average number of seedlings/m of row in a guayule direct seeding study planted 23 July 1993 at Fort Stockton, TX

Irrigation and rainfall were recorded throughout the study (Table 1).

Line

2.2. Seed treatment

Evaluation date (1993) 30 July

5 August

13 August

Conditioned AZ-R1 64b AZ-R2 44c UC-101 64b UC-104 65b CAL-6 78a CAL-7 64b Mean 63

71ab 46de 68b 79a 76ab 71ab 69

64b 43de 66b 72ab 76a 69b 65

65a 40c 64a 66a 68a 61a 61

Raw AZ-R1 AZ-R2 UC-101 UC-104 CAL-6 CAL-7 Mean

32fg 23g 38ef 44e 57c 54cd 54

31fg 23g 36ef 51cd 54c 51cd 41

27d 23d 33cd 44b 47b 48b 37

25ef 16f 31de 33de 40cd 46c 32

20 August

a

Means within columns followed by the same letter are not different according to Fisher’s protected least significant difference at the 5% level of significance.

The improved lines have not been tested for their adaptability to direct seeding. This study was designed to determine the feasibility of establishing improved guayule lines by direct seeding, and evaluate each selection for biomass production, rubber and resin content, and rubber and resin yield.

2. Materials and methods

2.1. Study site The study was established in 1993 at the Texas Agricultural Experiment Station, 20 km west of Fort Stockton in Pecos County, TX. The soil type was a Delnorte, very gravelly loam (loamy-skeletal, mixed, thermic family of shallow Typic Paleorthids) with 1.7% organic matter and a pH of 8.4. Mean annual temperatures range from a minimum of − 1°C in January to a maximum of 35°C in July. Annual precipitation averages 300 mm with 65% occurring from April to September.

Seed used in the experiment included two unreleased breeding lines, AZ-R1 and AZ-R2, from D.T. Ray (University of Arizona); two released lines, CAL-6 and CAL-7, and two unreleased breeding lines, UC-101 and UC-104, from A. Estilai (University of California, Riverside, CA). The seed was increased at the New Mexico State University Leyendecker Plant Science Research Center near Las Cruces, NM, and conditioned by the process outlined by Chandra and Bucks (1986). The conditioning process involved imbibing seeds under aerobic conditions in a medium containing polyethylene glycol, gibberellic acid, potassium nitrate, and thiram (tetramethylthiuram disulfide) fungicide. Laboratory germination was determined by placing 100 seeds of each line (both conditioned and raw) in separate petri dishes maintained at room temperature and under constant fluorescent light of 6500 lm/m2 intensity for 11 days.

2.3. Plot design The experimental design was a randomized complete block with four replications. Plots were single rows (spaced 1 m apart) 9 m in length. Conditioned and raw seed were planted 10 mm deep on raised beds on 26 July 1993 with a Gaspardo SV255 pneumatic planter at 100 seeds/ m. The plots were kept moist during germination and emergence by sprinkler irrigation. Supplemental irrigation was applied by sprinklers throughout the study. Treatments were thinned on 25 April 1994 to a 36 cm spacing between plants.

2.4. E6aluations Stand establishment evaluations were conducted by recording the number of guayule seedlings present in each treatment. Seedlings were counted weekly until maximum emergence was attained, and monthly thereafter.

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Six plants from each selection in the conditioned seed treatments were harvested on 23 February 1995 and 6 March 1996 for rubber and resin analyses. Each plant was divided into branch (clipped 10 cm above the soil surface) and root sections. The samples were air dried (branch sections were defoliated), ground in a Fitzmill Comminutor with a 2.36 mm screen, and stored at − 20°C. At this point, the samples were ground again in a cyclone mill with a 1 mm screen, packed into sample cups, and allowed to cool to room temperature. Rubber and resin content was determined with a NIR Systems Model 6500 scanning infrared spectrophotometer (Kleine and Foster, 1990). A t-test at the 5% level of probability was

performed to compare the mean laboratory germination of conditioned and raw seed using each line as an individual data point. All other data were subjected to analysis of variance and treatment means were separated by Fisher’s protected least significant difference at the 5% level of significance (Snedecor and Cochran, 1980).

3. Results and discussion

3.1. Plant establishment The greatest amount of irrigation water was applied during the 30-day period following planting (Table 1). Guayule seed germination began

Table 4 Branch, root, and total biomass of six direct seeded guayule lines harvested in February 1995 and March 1996 Line

Biomass (kg/ha) 1995

AZ-R1 AZ-R2 CAL-6 CAL-7 UC-101 UC-104

1996

Branch

Root

Total

Branch

Root

Total

3231aa 2280b 1917c 2132bc 2220bc 2289b

2154a 1679bc 1402bc 1361c 1696b 1653bc

5385a 3959b 3319b 3493b 3916b 3942b

6580a 5603ab 4294b 4833b 4470b 4972b

3009a 2462ab 1886bc 2156bc 1839c 2461ab

9589a 8065ab 6181b 6989b 6310b 7433b

a

Means within columns followed by the same letter are not different according to Fisher’s protected least significant difference at the 5% level of significance. Table 5 Resin and rubber content of six direct seeded guayule lines harvested in 1995 and 1996 Line

Resin content (%) 1995

AZ-R1 AZ-R2 CAL-6 CAL-7 UC-101 UC-104 a

Rubber content (%) 1996

1995

1996

Branch

Root

Branch

Root

Branch

Root

Branch

Root

12.4aa 10.6b 10.6b 10.1b 10.0b 11.0b

10.1a 9.0c 9.5abc 9.3bc 9.6abc 9.8ab

12.7a 12.0a 10.6b 9.8b 10.9b 10.5b

9.7ab 9.3b 9.4b 9.4b 9.5b 10.1b

5.7c 11.1a 11.2a 9.6b 11.2a 10.7ab

5.0c 9.8ab 10.3a 8.5b 10.1ab 10.4a

4.6c 9.2ab 9.3ab 8.2b 8.9b 10.4a

4.5c 10.1a 9.4ab 8.4b 9.2ab 10.0a

Means within columns followed by the same letter are not different according to Fisher’s protected least significant difference at the 5% level of significance.

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Table 6 Resin and rubber yield of six direct seeded guayule lines harvested in February 1995 and March 1996 Line

Resin yield (%)

Rubber yield (%)

1995

AZ-R1 AZ-R2 CAL-6 CAL-7 UC-101 UC-104

1996

1995

1996

Branch

Root

Total

Branch

Root

Total

Branch

Root

Total

Branch

Root

Total

395aa 241b 208b 212b 223b 254b

215a 150b 133b 127b 161b 162b

610a 391b 341b 338b 384b 417b

825a 668ab 453c 481bc 486bc 518bc

290a 230b 177c 204bc 172c 248ab

1115a 898ab 630c 685bc 657bc 766bc

185a 250a 220a 203a 242a 245a

108b 164a 146ab 115b 167a 170a

293c 414a 366abc 318bc 409ab 415a

302b 515a 392ab 400ab 386b 513a

134c 248a 174bc 182b 164bc 246a

436b 763a 566b 582b 550b 759a

a

Means within columns followed by the same letter are not different according to Fisher’s protected least significant difference at the 5% level of significance.

3 – 5 days after planting, followed by emergence several days later (Miyamoto and Bucks, 1985). Therefore, frequent irrigation is crucial during the first 30 days after planting to promote seed germination, prevent soil crusting and facilitate seedling emergence, and to protect young seedlings against desiccation. Frequency and amount of irrigation should be reduced to avoid the damping-off diseases associated with the soil-borne fungi Pythium sp., Phytophthora sp., Fusarium sp., and Rhizoctonia sp., prevalent under moist conditions as the seedlings become established (Mihail et al., 1991). Guayule is susceptible to salinity at emergence and during seedling stages (Miyamoto and Bucks, 1985); therefore, the plots were irrigated either in the early morning or at night to avoid excessive evaporation and salt accumulation on the soil surface. Initial seedling establishment requires water of low salinity and the use of irrigation methods (sprinkler irrigation versus furrow) that minimize salt accumulation near the seeded zone. Water at the research site contained 1.3 dS/m dissolved salts. Although sprinkler irrigation was used throughout our study, once plants are established and the susceptibility to salt diminishes, furrow irrigation can be initiated. Miyamoto et al. (1990) concluded that the salt tolerance of established guayule was higher than alfalfa and almost as tolerant as Pima and Upland cotton. However, tolerance at the seedling stage was lower than carrots, one of the most salt-sensitive crops grown in the Southwest. Guayule was successfully established by direct

seeding in the spring of 1995 at the University of Arizona Agricultural Experiment Station near Marana (Ray et al., 1996). Percent laboratory germination of conditioned seed averaged 76% and was significantly greater than raw seed (56%) (Table 2). Previously, Bucks et al. (1986) reported that raw seeds had a lower germination and survival rate than conditioned seeds in a direct seeding trial in Arizona. Chandra and Bucks (1986) found that conditioning enhanced the viability and vigor characteristics of fresh guayule seeds, and the frequency of retarded growth, damping-off and other abnormal growth patterns was much higher in untreated seeds. Guayule seedling establishment in our study was significantly greater with conditioned seed versus raw seed, except with AZ-R2 (Table 3). This confirms earlier results reported by Foster and Moore (1992) and Fowler and Tinguely (1993). The poor AZ-R2 seedling establishment was probably due to the low initial germination rate. There was no significant line by seed treatment interaction at any of the evaluations. By the 20 August 1993 evaluation, there were no significant differences in the average number of seedlings/m, except with AZ-R2. Conditioned seed appeared to germinate and emerge quicker and more uniformly than raw seed, allowing for efficient irrigation management. The AZ-R1 seedlings were the most vigorous and were tolerant to flea beetles (Epitrix sp.). Bucks et al. (1983) observed that conditioned seeds were the first to emerge, and demonstrated the highest rate

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of emergence throughout the experiment. Based on within-row spacing recommendations for transplants, at least three established seedlings/m would be required for an acceptable guayule stand. Both conditioned and raw seed met these requirements. However, conditioned seed improved establishment across a broad temperature range, and provided more uniform germination and emergence. The percentage germination of raw guayule seed is highly variable and can be affected by several factors including genetics, insect damage, nutrient supply, and temperature (Benedict, 1946). Seeding rate should be adjusted to reflect seed quality. Our seeding rate of 100 seeds/m could be reduced substantially which would lower production costs.

3.2. Production parameters Plants in this study were harvested in 1995 (19 months old) and 1996 (31 months old). The plants were divided into branch (clipped 10 cm above the soil surface) and root sections. The branch section would represent production obtained when guayule is harvested commercially by only clipping the tops rather than harvesting whole plants. The partitioning of rubber yield in these two fractions has been reported for guayule transplants, generally indicating that about 66% occurs in the aboveground growth and 33% in the root (Hammond and Polhamus, 1965; Garrot et al., 1983; Toukdarian and Elder, 1983). These determinations have not been documented for direct seeded plants. Ray et al. (1997) harvested plants of line CAL-6 that were established by direct seeding and transplanting in 1995 at Marana, AZ. Harvests were conducted six times from January – March 1997. Latex content in the direct seeded plants was lower than in transplants in January and until the end of February. However, by March the latex content was essentially equal. Branch biomass was greater than root biomass in each selection at both harvests (Table 4). Total biomass was greatest in AZ-R1 in 1995 (5385 kg/ha) and 1996 (9589 kg/ha); although in 1996, the total was not significantly different from AZR2. Although production parameters of direct seeded shrubs have not been investigated, biomass

in our study could have been greater if additional irrigation water had been applied. Nakayama et al. (1991) reported that with irrigated systems, satisfactory yields could be attained with water applications (irrigation+rainfall) of 1000–1300 mm. Under west Texas conditions, supplemental water applications of 500–800 mm by furrow irrigation may be needed after the plants are established. Also, the study was not initiated until 23 July, well past the late spring optimum planting date. Once establishment was complete, the plants had little time for growth before the growing season concluded. In effect, only one growing season (1994) occurred before the February 1995 harvest. Resin content was generally greater in the shrub branches than the roots (Table 5). This trend has also been reported for transplanted guayule (Garrot et al., 1983). Branch rubber content ranged from 5.7 (AZ-R1) to 11.2% (CAL-6 and UC-101) in 1995, and from 4.6 (AZ-R1) to 10.4% (UC-104) in 1996 (Table 5). Root rubber content varied from 5.0 (AZ-R1) to 10.4% (UC-104) in 1995, and from 4.5 (AZ-R1) to 10.1% (AZ-R2) in 1996. These data show that rubber content has increased dramatically in the improved lines compared to the older USDA selections. Garrot et al. (1983) noted that rubber content in branches of 2-year-old USDA transplants clipped 8 cm above the soil surface varied from 5.1 to 7.0%. Root rubber content ranged from 2.4 to 4.8. Toukdarian and Elder (1983) separated 2-year-old single plant selections into stem and root portions. Rubber content averaged 5.6 and 3.4% in the stems and roots, respectively. Resin and rubber yields were higher in the plant branches versus the roots in 1995 and 1996 (Table 6). Total resin yield was greatest in selection AZ-R1 and averaged 1115 kg/ha in 1996. Total rubber yield at the final 1996 harvest was greatest in AZ-R2 (763 kg/ha) but was not significantly different to the yield of UC-104 (759 kg/ha). In a regional yield trial (1985–1988), the average rubber yield for six, 3-year-old transplanted selections at Fort Stockton, TX, was 1080 kg/ha (Ray et al., 1989), and greater than the direct seeded yields

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from our study. With proper irrigation techniques, direct seeded shrub biomass and rubber yield can be improved.

4. Conclusions Improved guayule germplasm was successfully established by direct seeding in west Texas. Seedling density was greatest with conditioned seed versus raw seed. Although raw seed produced adequate guayule stands, conditioning improved germination, and, in the field, conditioned seed was observed to germinate and emerge quicker allowing for more efficient irrigation management. Seeding rate should be adjusted to reflect seed quality. The AZ-R1 seedlings were observed to be the most vigorous and were tolerant to flea beetles (Epitrix sp.), but rubber content and yield were low. Rubber and resin content (%) was greater in branch segments (harvested 10 cm above the soil surface) than in the remaining roots. These data were greater than that reported for transplants, but biomass and rubber and resin yields were lower. This was probably due to improper irrigation techniques. We recommend that direct seeded stands be sprinkler irrigated until maximum emergence occurs, then maintained by furrow irrigation. Total water requirements (rainfall + irrigation) of 1000 – 1300 mm may be needed.

Acknowledgements This work was supported by USDA-CSREES and we wish to thank Carmela Bailey, Program Director, Agricultural Materials, for her assistance. Special recognition is given to Melissa Grote, Texas A&M Agricultural Research Station, for preparing the manuscript.

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