Proc. of Second World Avocado Congress 1992 pp. 191-198

 

 

Properties of Persea indica, an Ornamental for Southern California

 

Ursula K. Schuch, Rainer W. Scora, and Eugene A. Nothnagel

Department of Botany and Plant Sciences, University of California, Riverside, CA 92521, USA

 

Steven D. Campbell

Department of Plant Pathology, University of California, Riverside, CA 92521, USA

 

Abstract. Persea indica L. is the only member of the genus Persea surviving the Laurasian Mediterranean flora in Europe and Africa. Its evergreen foliage and small fruit make P. indica an attractive ornamental and a useful shade tree that improves the microclimate throughout the year. The concentrations of macro- and micronutrients in leaf tissue are comparable to the recommended levels for mature avocado trees. Analysis of essential oils shows high amounts of sesquiterpenes, of which caryophyllene and delta-cadinene are the major constituents. Chemical components of the leaf oils of the commercial Hass and Fuerte cultivars show many quantitative and qualitative differences compared to P. indica. Immature fruit of P. indica can be used in a bioassay to detect Phytophthora cinnamomi, P. citricola, and P. parasitica in soil.

 

 

Persea indica (L.) Spreng is a living fossil, surviving the rich flora of the middle and late Pliocene vegetation (Axelrod, 1978). As the only member of the genus Persea in the once widespread Laurasian Mediterranean flora, P. indica currently survives in the maritime climate of the Canary Islands, Madeira, and the Azores. P. indica evolved in the African Gondwanaland flora and dispersed to Europe where fossils were found in the Miocene flora of Southern France and Spain (Axelrod, 1978). According to fossils identified in California and dated back to the Miocene, this plant must have migrated west at least 50 million years ago.

Plants of the Lauraceae are characterized by prominent oil cells in the leaves, wood and fruit. These oils are mostly aromatic and find wide use as medicine (sassafras tea, camphor), spices (cinnamon), and flavors (bay leaf) (Schroeder, 1976). P. indica is classified in the genus Persea, and the subgenus Eriodaphne. Although P. indica is a relative of the commercial avocado, P. americana, these two species are not graft compatible (Schroeder and Frolich, 1955).

Ornamental properties. Plants considered for landscape use enhance the environment in a variety of ways: physical, aesthetic, economic, and psychological (Harris, 1983). Landscape trees in populated areas are valued mainly for their improvement of the microclimate. Shade from trees reduces solar radiation and reflection from soil. Trees also modify wind patterns and reduce wind velocity. To a certain extent, plants can remove pollutants from the air, and can absorb noise. Visual benefits play an increasing role in densely populated areas, and in this regard trees fulfill many different functions, such as providing color, form, texture, and patterns in the landscape. Economic benefits of trees in the landscape range from monetary value for the wood to an increase in real estate value. In addition, trees can provide a potential wildlife habitat in urban settings.

Because Persea indica exhibits many of these desired features as an ornamental tree, it is frequently planted in Florida and Southern California. The smooth and silver-colored bark provides a striking contrast to the shiny dark green foliage. The tree grows with a straight trunk and has been used as a source of timber in its native habitat. The leaves are oblong, glabrous, and 6 to 13 cm long. Fruit are scarcely fleshy, grow to a length of under 2 cm and are borne in clusters (Fig. 1). Immature fruit are green and turn black when ripening. The evergreen foliage and the small fruit make P. indica an attractive ornamental and a useful tree that ameliorates the microclimate throughout the year.

 

 

Figure 1. Leaves and immature fruit of Persea indica.

 

Leaf nutrient analysis. The nutritional status of mature P. indica leaves was determined. Samples were collected in February from the distal end of branches, oven dried at 70C, and analyzed for macro- and micronutrient concentrations.

Essential oils and alkanes in leaves. For essential oil analysis, 500 g of healthy leaves were randomly harvested from a 32-year-old P. indica seedling tree at the University of California Research and Extension Center, Irvine, California. The leaves were macerated and steam distilled in a Clevenger apparatus for 2 h. The resulting oil was kept under nitrogen until injected into a Shimadzu Mini-3 gas chromatograph with a flame ionization detector and 50:1 split ratio. A J&W DB-5 fused silica capillary column was used with 0.25 mm i.d., 60 m length and 0.25 micrometer coating thickness. The injector temperature was 200C. The oven temperature was programmed to 65C for 30 min and then increased at a rate of 1C/min. to 200C, where it remained for 25 min. N-octane and n-eicosane were added to some samples for standardization of retention times.

For mass spectra identification a Hewlett-Packard 5890II gas chromatograph, a 5971A mass selective detector and G1030A MS computer station were used. A capillary direct interphase heated at 280C delivered column output into the quadruple mass spectrometer with electrons of 70 eV energy for electron impact ionization. The appropriately diluted oil in hexane (1 //L) was injected in splitless mode. Injector temperature was 270C and helium flow was 22 cm/sec. All other parameters were the same as for the Shimadzu gas chromatograph. Some mass spectra were identified by the NIST/EPA/MSDC data base (49,000 spectra) stored in the computer.

Indicator for Phvtophthora. Phytophthora root rot is a serious disease causing substantial losses in avocado production (Zentmyer, 1980). In the seedling stage P. indica is very susceptible to Phytophthora cinnamomi, the major pathogen of commercial avocado, and has been used to test for the presence of the fungus (Zentmyer and Ohr, 1978). For this test, young seedlings are placed in soil suspension, and within 3 to 5 days black stripe cankers develop on the stem and root system. Other species of Phytophthora, such as P. citricola Sawada and P. parasitica, are reported to attack P. indica seedlings (Zentmyer, 1980). Another method to test for the presence of P. cinnamomi uses whole firm fruit of Persea americana that are placed in soil and flooded with water (Zentmyer et al., 1960, 1967; Zentmyer and Ohr, 1978). Purplish brown spots develop at the water line within 4 to 6 days. Recently, however, seedlings of P. indica have been discovered in the Canary Islands that show resistance to Phytophthora cinnamomi (Schroeder, 1989).

An experiment was conducted to determine whether fruit of Persea indica can be used to test for the presence of Phytophthora in soil. Green, immature P. indica fruit were collected in February and March, 1991 from a tree at the University of California Research and Extension Center, Irvine, California. Field soil, known to be infested with Phytophthora cinnamomi and P. parasitica, and autoclaved soil for control was placed in Styrofoam cups and Petri dishes. The suspension in the cups consisted of 20 g soil with 200 mL of distilled water, while the slurry in the Petri dishes was prepared with 10 g of soil and sufficient water to immerse half of the fruit. The root system of ten-week-old P. indica seedlings was placed in the soil suspension, and green, immature P. indica fruit, collected two days before the experiment started, were placed in the soil slurry. The experiment was conducted in a greenhouse at 19/29C average day/night temperatures and natural photoperiod. Development of canker on stems, blackening of roots, discoloration of fruit, and wilting symptoms of seedlings were monitored. Once symptoms were visible, affected plant parts and comparable tissue of controls were plated on selective PVP medium (Tsao and Ocana, 1969) to confirm the presence of the Phytophthora fungi. A second experiment was conducted in the same manner as described above, using autoclaved soil as control, P. cinnamomi-infested field soil, and autoclaved soil that was inoculated with 1 mg per container Phytophthora citricola (P1273 strain), grown on millet.

Results

Leaf nutrient analysis. Concentrations of macronutrients in Persea indica leaves were mostly within the ranges considered adequate for leaves of mature P. americana trees (Table 1). Boron levels of P. indica were slightly low, and iron levels were threefold above the highest recommended concentration for P. americana. Leaves appeared healthy and showed no signs of mineral deficiencies or toxicities.

 

 

 

 

Essential oils and alkanes in leaves. The occurrence of essential oils for P. indica and P. americana cvs. Hass and Fuerte are shown in Figure 2. The ratio of monoterpenes, sesquiterpenes, and sesquiterpene esters are shown in Table 2. Of the monoterpenes, cis-ocimene was the most abundant with 1.1%, and of the sesquiterpenes, beta-caryophyllene (24.0%) was followed by germacrene D (9.1%) and delta cadinene (9.1%). Ratios were similar for P. indica and the Hass cultivar, where the majority of oils were sesquiterpenes and sesquiterpene esters. In contrast, oils from leaves of the cv. Fuerte were mainly comprised of monoterpenes and contained no sesquiterpene esters at all.

The order of elution of MS-identified monoterpenes was dimethyl benzene, alpha-pinene, beta-myrcene, decane, beta-phellandrene, cis-ocimene and trans-ocimene. That of sesquiterpenes was delta-elemene, alpha-cubebene, alpha-copaene, beta-bourbonene, beta-caryophyllene, beta-gurjunene, alpha cadinene, gamma gurjunene, alpha-caryophyllene, gamma-muurolene, germacrene D, valencene, alpha-muurole, gamma-cadinene, delta-cadinene, beta-calacorene, alpha-cadinol, and santalol. In 'Hass' and 'Fuerte', beta-pinene, +-limonene, 1,8-cineole, estragole and anethole were additionally identified with the MS. Hass spectra of alpha-cadinene, caryophyllene, sesquiterpene esters, and germacrene D that were isolated from essential oils of P. indica are shown in Figure 3.

Alkanes of Persea have been analyzed earlier (Scora et a/., 1975) and are shown in Table 3. P. indica has an alkane composition characteristic of the subgenus Eriodaphne with high levels of C₃₃ and low levels of C₂₉. Some of the cultivars of the subgenus Persea that are hybrids between guatemalensis and drymifolia are characterized by leaf alkanes comprised of one third of C₂₉ and nearly one third of C₃₃.

 

 

 

 

 

 

 

 

Support for purchase of the mass selective detector used in this study was provided by National Science Foundation grant DIR-8914574.

Literature cited

Axelrod D.I. 1973. History of the Mediterranean ecosystem in California. In: Mediterranean Type Ecosystems, p. 225-277. F. di Castri and H.A. Mooney (eds.) Springer Verlag, New York.

Harris, R.W. 1983. Arboriculture: Care of trees, shrubs, and vines in the landscape. Prentice-Hall, Englewood Cliffs, New Jersey.

Scora, R.W., B.O. Bergh, and J.A. Hopfinger. 1975. Leaf alkanes in Persea and related Taxa. Biochem. System, and Ecol. 3:215-218.

Schroeder, C.A. and E.F. Frolich. 1955. Avocado rootstock-scion studies. Calif. Agri. 9:11-12.

Schroeder, C.A. 1976. Some useful plants of the botanical family Lauraceae. Calif. Avocado Soc. Yrbk. 59:30-34.

Schroeder, C.A. 1989. The Laurel or Bay Forest of the Canary Islands. Calif. Avocado Soc. Yrbk. 73:145-147.

Tsao, P.M. and G. Ocana. 1969. Selective isolation of species of Phytophthora from natural soils on an improved antibiotic medium. Nature 223:636-638.

Zentmyer, G.A., J.D. Gilpatrick, and W.A. Thorn. 1960. Methods of isolating Phytophthora cinnamomi from soil and from host tissue (Abstr.). Phytopathology 50:87.

Zentmyer, G.A. and H.D. Ohr. 1978. Avocado root rot. Univ. Calif. Div. Agric. Sci. Leafl. 2440. 15pp.

Zentmyer, G.A., A.O. Paulus, and R.M. Burns. 1967. Avocado root rot. Calif. Agr. Exp. Stn. Ext. Serv. Circ. 511. Revised. 16pp.

Zentmyer, G.A. 1980. Phytophthora cinnamomi and the diseases it causes. Amer. Phytopath. Soc. Monograph 10, 96 pp.

 

Figure 2.  Gas chromatograph analysis of essential oils from leaves of Persea indica and Persea americana cultivars Hass and Fuerte.

 

Figure 3. Mass spectrum of compounds isolated from essential oils of Persea indica leaves.