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Estilai, A. and M.C. Shannon. 1993. Salt tolerance in relation to ploidy level in guayule. p. 349-351. In: J. Janick and J.E. Simon (eds.), New crops. Wiley, New York.

Salt Tolerance in Relation to Ploidy Level in Guayule

Ali Estilai and Michael C. Shannon


  1. METHODOLOGY
  2. RESULTS AND DISCUSSION
  3. FUTURE PROSPECTS
  4. REFERENCES
  5. Table 1

Guayule (Parthenium argentatum Gray) is a promising alternative to rubber tree (Hevea brasiliensis Muel. Arg.) for production of natural rubber in semiarid regions of the world. For the United States, which is totally dependent on foreign sources of natural rubber, developing guayule as a commercial crop should be a high priority. A domestic source of natural rubber is vital to our national defense and helps balance the budget by reducing the one billion dollars spent annually for the imports of Hevea rubber from southeast Asia.

Guayule grows naturally in the semiarid Chihuahuan Desert region of north central Mexico and the Big Bend area of Texas in the southwestern United States. The native stands are restricted to outwash fans and rocky slopes of calcareous soils. In general, guayule is considered to be only slightly tolerant to soil salinity (Hammond and Polhamus 1965). Although guayule seeds germinate successfully in highly saline solutions of up to 22 dS/m, seedling emergence is reduced severely when saline waters are used for irrigation (Miyamoto et al. 1984). Salinity of irrigation water has been reported to decrease rubber yield and water use efficiency (Miyamoto and Bucks 1985). Experiments conducted in the 1940s at Texas indicated that salinity greater than 3.3 dS/m would be unsuitable for guayule culture (McGinnies and Mills 1980). In a more recent study, conducted at Brawley, California, guayule appeared more salt tolerant than many crops that were considered tolerant at the 6 dS/m level (Maas et al. 1986). The apparent disagreement between the findings of the two studies is due to the high sodium content found in the Texas water versus the high calcium content in the California water (Nakayama et al. 1991). Greenhouse sand culture experiments have shown the detrimental effects of high sodium concentration as compared to calcium on guayule growth (Wadleigh and Gauch 1944). No statistically significant interaction effects were found between plant population and salinity (Hoffman et al. 1988).

Salt-tolerant guayule cultivars are needed for economic production of rubber on marginal lands and in areas with low quality saline water. The available guayule germplasm which is being used to develop new guayule cultivars are 2n = 36, 54, or 72 (Bergner 1944, 1946; Stebbins and Kodani 1944). Plants with 2n = 36 are considered to be diploid (Bergner 1944; Estilai et al. 1985; Hashemi et al. 1989). Diploids reproduce sexually and, because of a sporophytic system of self-incompatibility, they produce seed by cross-pollination (Gerstel 1950; Estilai 1984). Plants with 2n = 54 and 72 are polyploid (triploid and tetraploid, respectively), and their mode of reproduction is by facultative apomixis, the simultaneous occurrence of sexual and apomictic modes of reproduction (Esau 1944; Gardner 1947; Powers and Rollins 1945). Information on salt tolerance of guayule plants with different chromosome number is unavailable. The primary objectives of this study were to compare diploid, triploid, and tetraploid guayule germplasm, irrigated with saline water, for important agronomic traits and to identify salt tolerant individuals for development of improved cultivars.

METHODOLOGY

Open-pollinated seeds from diploid, triploid, and tetraploid guayule germplasm were planted in a greenhouse at the University of California, Riverside in January 1989 following procedures described previously (Estilai and Waines 1987; Estilai 1991). Seedlings were hand-transplanted into experimental plots at Brawley, California on May 24, 1989. The experimental design was a randomized complete block with four replications. Each entry in a replicate was planted in a plot consisting of two rows, each 16 m long, with the interrow and interplant spacing of 1 m and 0.45 m, respectively. Approximate population density was 22,200 plants/ha. Experiments were surrounded by a row of border plants.

Seedlings were irrigated with normal water until Nov. 6, 1989 when the first measurements were obtained for plant height and width. Plots were then irrigated with saline water of electrical conductivity of 7.5 dS/m. Prior to harvest on Feb. 26, 1991 (when plants were 21 months old), height and width were measured for 10 plants per plot. The 10 plants were cut at 0.05 m above ground, leaves and peduncles removed, and plants weighed and chipped. Immediately after chipping, two samples were taken to determine percent dry weight and rubber and resin contents. Samples used to determine the percent dry weight were dried in a forced air oven at 75°C and reweighed.

Samples used for rubber and resin analyses were stored in a freezer and later were ground in the presence of liquid nitrogen. Resin and rubber were extracted from the finely ground plant materials using acetone and cyclohexane, respectively. Detailed procedures for determination of rubber and resin contents have already been reported (Black et al. 1983; Estilai and Mayhew 1990).

RESULTS AND DISCUSSION

Table 1 compares the three germplasm lines for nine agronomic traits. The polyploid germplasm was significantly superior to the diploids for all traits. The triploid and tetraploid germplasm were similar in performance except for resin content and resin yield. The triploid germplasm had the highest annual resin content of 10.7% and the highest resin yield of 420 kg/ha. Diploid, triploid, and tetraploid germplasm, respectively, produced 60, 64, and 40% more resin than rubber (Table 1). Resin is an important co-product, and may provide an additional revenue for guayule commercialization. Possible applications of resin include use as an adhesion modifier for strippable coatings and as wood preservative.

Annual rubber yield, the most important trait for guayule commercialization, varied from a minimum of 125 kg/ha for the diploid germplasm to a maximum of 256 kg/ha for the triploid germplasm. These levels of productivity are far below the annual rubber yield of 1,000 to 1,500 kg/ha needed to make guayule a successful irrigated crop in prime agricultural lands. However, considering the reduced value of lands in semiarid regions and lower costs for the low quality saline water, annual rubber yield of 500 kg/ha may be acceptable for low input agriculture.

The guayule germplasm showed variation for all traits studied. More than 20 plants with annual rubber yield of 40 g (potential rubber yield of 500 kg/ha) were identified and will be used to develop cultivars with increased rubber production under irrigation with saline water.

FUTURE PROSPECTS

The variation observed for salt tolerance among the germplasm with different chromosome number suggests the possibility of selecting ecotypes for arid, semiarid, and marginal lands. This variability also provides suitable material to study the genetic basis and the physiological nature of salt tolerance in guayule.

REFERENCES


Table 1. Comparison of diploid, triploid, and tetraploid guayule entries for nine agronomic traits.

Guayule entries
Diploid Triploid Tetraploid
Plant traits Range Mean Range Mean Range Mean
Height (cm) 47-66 56.5az 52-64 59.5a 48-67 60.5a
Width (cm) 54-69 61.8a 65-75 69.5b 62-76 70.5 b
Dry weight (kg ha-1 yr-1) 2,029-2,689 2,334a 3,590-4,351 3,856b 3,513-4,338 3,758b
Rubber content (%) 4.9-5.7 5.3a 6.3-7.2 6.6b 6.3-6.8 6.5b
Resin content (%) 7.9-9.4 8.6a 9.2-12.2 10.7b 7.9-10.3 9.2c
Rubber+resin (%) 12.9-14.6 14.0a 15.9-18.5 17.5b 14.4-16.9 15.7c
Rubber yield (kg ha-1 yr-1) 105-147 125 a 224-279 256b 228-272 245b
Resin yield (kg ha-1 yr-1) 178-241 200 a 332-503 420b 277-380 345c
Rubber + resin yield (kg ha-1 yr-1) 296-388 325a 574-781 677b 506-654 590c
zRow means followed by the same letter are not significantly different at the p = 0.05 level as determined by Duncan's new multiple range test.


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