Seed germination inAcaciaspecies and their relation to altitudinal gradient in south-western Saudi Arabia

Journal of Arid Environments (1995) 31: 171–178

Seed germination in Acacia species and their relation to altitudinal gradient in south-western Saudi Arabia

H. A. Abulfatih King Saud University, Abha Branch, College of Education, Department of Biology, Abha, P.O. Box 9032, Saudi Arabia (Received 12 January 1994, accepted 7 February 1994) Seed germination of Acacia species found in south-western Saudi Arabia responded in three different ways to temperature: (1) Germination over a broad range of temperature (between 10 and 40°C) was recorded in Acacia asak, A. ehrenbergiana, A. iraqensis, A. hamulosa and A. tortilis ssp. spirocarpa. (2) Cold-evading germination (between 20 and 40°C) was recorded in A. oerfota. (3) Heat-evading germination (between 10 and 35°C) was recorded in A. origena. Plants of the first two types were found in desert to semi-desert environments between sea level and 2000 m, whereas plants of the third type occur in a cool temperature environment between 1750–2500 m. A strong correlation exists between seed germination, temperature, water and thickness of seed coat. ©1995 Academic Press Limited Keywords: Acacia; germination; altitude; temperature; Saudi Arabia

Introduction The south-western region of Saudi Arabia is characterized by a complex topography and environment, with average annual rainfall ranging approximately between 65 mm in the coastal plains of the Red Sea to 500 mm in the high mountains of Asir. Consequently, such diverse environments leads to the formation of diverse vegetation types (Abulfatih, 1979, 1981, 1991). Along the vegetation gradient between sea level and 3000 m altitudes the climate changes gradually from warm dry to cool temperatures. A vegetation zonation is also distinguished along the elevation gradient (Abulfatih, 1979, 1983a, 1984, 1991, 1992). It is well known that Acacia spp. are widely distributed in the Middle East and can survive under harsh environments (Andrews, 1952; Halvey & Orsham, 1972; T¨ackholm, 1974; Mahmoud, 1977; Migahid, 1978; Chaudhary, 1983; Al-Hubaishi & Muller-Hohenstein, 1984; El-Ghonemy, 1985; Batanouny, 1986). The south-western region of Saudi Arabia is considered as one of the places where a high density and diversity of such species exists when compared with other places in Saudi Arabia, and even with other parts of the Middle East (Vesey-Fitzgerald, 1955; Mandaville, 1973; Abulfatih, 1981; Zahran, 1982; Abulfatih, 1983a; Brooks & Mandil, 1983; Chaudhary, 1983; Abulfatih, 1984; Collenette, 1985; Chaudhary & El-Sheikh, 1988; Abulfatih & Emara, 1988; Abulfatih et al., 1988; Hassan & Al-Farraj, 1989; Al-Zoghet, 1990; Mandaville, 1990; Abulfatih, 1991; Abulfatih, 1992). 0140–1963/95/020171 + 08 $12.00/0

© 1995 Academic Press Limited

172

H. A. ABULFATIH

It was shown by Abulfatih & Bazzaz (1985) that germination on a broad range of temperatures was common among such widely distributed plants as Acacia tortilis (Forssk.)Hayne ssp. spirocarpa (Hochst. ex Rich.)Brenan and A. ehrenbergiana Hayne. Germination tests on Acacia seeds, which were collected from a limited area across their range of distribution and scarified by a file, showed that A. tortilis seeds can germinate between 15 and 40°C, A. ehrenbergiana seeds germinated between 15 and 40°C and A. asak (Forssk.) Willd. between 18 and 40°C. Seed germination of some representative wild species from the lowlands of southwestern Saudi Arabia showed that the optimum temperature for germination was relatively high (30–36°C), as in the case of Senna italica Miller [syn. Cassia italica (Miller) Spreng.], Salvadora persica L. and Calotropis procera (Ait.) Ait. f. For plants coming from the highlands the optimum temperature was relatively low (15–21°C), as in the case of Nepeta deflersiana Schweinf. ex Hedge, Verbascum nubicum Murb. and Jasminum grandiflorum L. var. flouribundum (R. Br. ex Fresen.) P.S.Green (Abulfatih & Bazzaz, 1985). Seeds of highland plants such as Rosa abyssinica R.Br. and Solanum incanum L. did not germinate even when supplied with various water, light and temperature regimes, but successfully germinated when planted in the soil. Seeds of Datura innoxia Miller and Cucumis prophytarum L. var. prophytarum germinated in the dark (Abulfatih, 1983b). Perennial plants growing at high elevations in the Asir mountains form a relatively high root/shoot ratio during their seedling stages, while seedlings of lowland plants show the opposite (Abulfatih & Bazzaz, 1985). In the present study seed germination of seven common Acacia species, which are naturally distributed along the elevation gradient between sea level and 2500 m, were examined along a temperature gradient bar in the laboratory. An approximate correlation was established between the temperature requirement for germination, ambient temperature and elevation.

Materials and methods Germination of seven well-known Acacia species were tested in the laboratory on a temperature gradient bar between 10 and 40°C. Seeds of these species were collected directly from the trees, at various places along the elevation gradient between sea level and 2500 m. Seeds were collected from the altitudes where Acacia trees were relatively more common, and such altitudes were as follows: A. tortilis ssp. spirocarpa, 0–1500 m; A. ehrenbergiana, 250–1500 m; A. asak, 250–2000 m; A. oerfota (Forssk.) Schweinf., 250–750 m; A. hamulosa Benth., 500–1000 m; A. iraqensis Rech. f. (syn. A. gerrardii sensu auct., 1750–2000 m; A. origena A. Hinde (syn. A. negrii sensu Hepper), 2000–2500 m. Seeds were collected in 1990, directly from the trees, and stored at room temperature for 1 year before the start of the germination test. A temperature gradient was established using an aluminum bar 20 cm wide, 120 cm long, and 0·3 cm thick. One end of the bar was immersed in water at 65°C and the other in melting ice at 4°C (Fig. 1). The surface of the bar was divided into seven temperature zones, 10, 15, 20, 25, 30, 35 and 40°C. For each species, seven batches of 50 healthy seeds were selected. Each batch of seeds was placed on filter paper, on a given temperature zone, fenced in with a wall of modelling clay 1 cm high and covered by a glass plate to reduce evaporation. The seeds were watered daily with distilled water and kept wet throughout the duration of the experiment. Thermocouples were attached to the gradient bar under each batch of seeds to monitor the temperature. Light was provided by four, 40W, cool, white fluorescent tubes, switched on throughout the experiment. The design of the temperature gradient bar was based on that reported by Abulfatih & Bazzaz (1979).

ACACIA GERMINATION AND ALTITUDE

173

Fluorescent light Hot electric water bath

Modelling clay Seeds on filter paper Glass sheet Ice box

Aluminum sheet Wooden board

Figure 1. Temperature gradient instrument, used in testing seed germination.

To make the process of germination easier all seeds were pretreated with concentrated sulphuric acid for 15 min and then washed with distilled water. Such treatment with sulphuric acid was assumed to remove or digest equal parts of the seed coat, and accordingly, when the germination experiment started, the time required to get to the emergence stage was used as an indicator of hardness of the remaining seeds coat. The thickness of the seed coat was measured by vernier calipers before treatment with sulphuric acid. For each species, 20 seeds were peeled and the thickness of their seed coats were measured. Seed germination was recorded when the epicotyle reached 1 mm long. Seed germination was plotted for each species against temperature gradient. The cumulative seed germination percentage was plotted three times, the first 2 days after the start of the experiment, the second after 4 days, and the third on the day which showed maximum germination. Also, a correlation between elevation, temperature required for 50% germination (the level of less germination variability among species) and ambient minimum and maximum temperature, was assessed. The climatic information recorded in this article was taken from the Climate Atlas of Saudi Arabia (Anon, 1988).

Results Seed germination of the seven common Acacia species, tested on a temperature gradient bar, in the laboratory, showed three types of responses to temperature. The first type was germination over a broad range of temperature: germination between 10 and 40°C was recorded for Acacia asak, (Fig. 2(c)), A. ehrenbergiana (Fig. 2(d)), A. iraqensis (Fig. 2(b)), A. hamulosa (Fig. 2(e)), and A. tortilis (Fig. 2(g)). The second type was cold-evading germination: germination between 20 and 40°C was recorded for A. oerfota (Fig. 2(f)). The third type was heat-evading germination: germination between 10 and 35°C was recorded for A. origena (Fig. 2(a)). The seeds of each species germinated within a specific temperature range (Fig. 3). Acacia origena at 10–35°C, A. tortilis, A. ehrenbergiana, A. asak, A. hamulosa, and A. iraqensis at 10–40°C, A. oerfota at 20–40°C.

174

H. A. ABULFATIH

Germination (%)

100 90 80 70 60 50 40 30 20 10 0

Germination (%)

100 90 80 70 60 50 40 30 20 10 0

Germination (%)

All species showed a very high percentage of germination at their respective optimum temperature (85% to 95%), with only one exception, the hard-coated seeds of A. origena, which recorded only 58% germination (Fig. 3). Seeds of different species required different periods to show the first sign of germination (when the epicotyle is 1 mm long) across the temperature gradient. A.

100 90 80 70 60 50 40 30 20 10 0

100 90 80 70 60 50 40 30 20 10 0

(a)

10

15

20

25

30

35

40

100 90 80 70 60 50 40 30 20 10 0

(c)

10 15 20 25 30 35 40

100 90 80 70 60 50 40 30 20 10 0

(e)

10 15 20 25 30 35 40

Germination (%)

Temperature (°C)

100 90 80 70 60 50 40 30 20 10 0

(b)

10 15 20 25 30 35 40

(d)

10 15 20 25 30 35 40

(f)

10 15 20 25 30 35 40

(g)

10 15 20 25 30 35 40

Temperature (°C)

Figure 2. Seed germination of Acacia species along temperature gradient. Readings were taken after 2 days(– + –), 4 days(–m–) and the maximum days required to reach the level of maximum germination(–j–). Species and maximum days: (a) Acacia origena, 18 days; (b) Acacia iraqensis, 10 days; (c) Acacia asak, 6 days; (d)Acacia ehrenbergiana, 14 days; (e) Acacia hamulosa, 6 days; (f) Acacia oerfota, 8 days; (g) Acacia tortilis 10 days.

ACACIA GERMINATION AND ALTITUDE

175

asak, A. hamulosa and A. ehrenbergiana showed the first signs of germination after 0·5 days at 25°C, A. iraqensis after 1 day at 25°C, A. tortilis after 1 day at 30°C, A. oerfota after 1·5 days at 30°C, and A. origena after 2 days at 20°C. The time required to reach maximum germination varied among species (Fig. 2), recording 6, 6, 8, 10, 10, 14, and 18 days respectively for Acacia asak, A. hamulosa, A. oerfota, A. iraqensis, A. tortilis, A. ehrenbergiana, and A. origena. Discussion The distribution of the seven common Acacia species, along the altitudinal gradient between sea level and 2500 m in south-western region of Saudi Arabia, is greatly affected by the process of seed germination and its relation to temperature and water. Seeds of A. origena from a cool temperate environment, at elevations ranging between 1750 and 2500 m, germinated between 10 and 35°C and produced peak germination between 15 and 25°C. Seeds of A. oerfota collected from a desert to semidesert environment, between 250 and 750 m, were able to germinate between 20 to 40°C, with peak germination at 30°C. Seeds of the latter species showed an exceptional response by not responding to temperature below 20°C, indicating that this species might be restricted to a very distinctive environment. Seeds of the other species from desert to semi-desert environments, between sea level and 2000 m, were able to germinate between 10 and 40°C, with peak germination between 20 and 25°C. From Figs 2, 3 and 4, and trying to relate the ambient average monthly minimum and maximum temperature of December and July with the temperature required to reach 50% germination (the level of least germination variability among species), one 100 90

(d) 80

(e)

Germination (%)

70 60

(a) 50

(f) (c)

40

(b) 30

(g) 20 10 0

10

15

20

25

30

35

40

Temperature (°C)

Figure 3. Maximum seed germination of Acacia species along temperature gradient. (a) Acacia origena, (b) A. iraqensis, (c) A. asak, (d) A. ehrenbergiana, (e) A. hamulosa, (f) A. oerfota, (g) A. tortilis.

176

H. A. ABULFATIH

can hypothesize that seeds would germinate best during the summer season provided water is available within their respective ranges of distribution. However, more field studies on soil moisture and temperature regimes are required to know more precisely what is the appropriate time for seed germination in nature. Minimum and maximum temperatures are the most limiting factors in the distribution of the Acacia spp. in south-western Saudi Arabia. Seeds of Acacia origena, for example, which are normally found at high elevations, where ambient temperatures are relatively low, did not germinate at 40°C and recorded the first 50% germination of its seed population at temperatures ranging between 15 and 25°C. On the other hand, all other Acacia species, which are normally found at lower elevations, where the ambient temperature is relatively higher, recorded 50% germination at temperatures ranging between 25 and 35°C. Such results could roughly explain why these species are confined to specific altitudes along the elevation gradient. The length of time required to show the first signs of germination (when the primary root was 1 mm long) could very well be related to the hardness of the seed coat. Accordingly, seeds can be arranged with respect to the hardness of their seed coats (after treating them similarly with concentrated sulphuric acid) from thin to thick in the following order: A. asak, A. hamulosa and A. ehrenbergiana 0·5 day; A. iraqensis, and A. tortilis, 1 day; A. oerfota, 1·5 day; and A. origena, 2 days. The thickness of the seed coat plays a significant role in the process of germination. The mechanism of seed coat disintegration is greatly affected by water availability, temperature, and the thickness of seed coat, in addition to many other biological, chemical and physical factors. It seems that there is a good relationship between the thickness of the Acacia seed coat and water availability in nature. Generally speaking, 3000

2500

Ao

2250

Ao

Elevation (m)

2000

Ao, Aj, Aa

1750

At, Ae, Aa LE 15, 21 22, 36

At, Ae, Aa

1250

At, Ae, Aa, Ah

1000

At, Ae, Aa, Ah, Ao

750

FH 18, 24 25, 37

At, Ae, Aa, Ah, Ao

500

0

UE 12, 18 19, 34

Ao, Aj, Aa

1500

250

HM 10, 16 17, 32

At, Ae, Aa, Ao

At 35

30

30

30

25

25

25

25

25

20

15

Min. and max. air temperature (°C)

2750

CP 20, 26 27, 38

Temperature (°C) required for 50% germination

Figure 4. Approximate correlation among the temperature at which the first 50% of the seed population germinated, the elevation from which the Acacia seeds were collected, and the air temperature represented consequently by December average daily minimum and maximum and July average daily minimum and maximum at five distinctive ecological zones along the elevation gradient in southwestern Saudi Arabia. Ecological zones are as follows: (CP) Coastal Plains, (FH) Foot Hills, (LE) Lower Escarpments, (UE) Upper Escarpments, and (HM) High Mountains. Acacia species are: (Ao) A. origena, (Ai) A. iraqensis, (Aa). asak (Ae) A. ehrenbergiana, (At) A. tortilis, (Ah) A. hamulosa, and (Ao) A. oerfota.

ACACIA GERMINATION AND ALTITUDE

177

the more water that is available the thicker the seed coats, and vice versa. The seed coats of A. hamulosa (0·17 ± 0·01 mm) A. asak (0·19 ± 0·01 mm) and A. ehrenbergiana (0·21 ± 0·2 mm) are the softest of all species under examination, and accordingly, they mainly occur in very severe habitats on rugged slopes where soil water is very limited. On the other hand, all the other Acacia species have thick seed coats and are commonly found on low ground, along wadi banks (seasonal water way), or on high mountains where the precipitation is often high. Their seed coat thickness are as follows: Acacia tortilis (0·29 ± 0·4 mm), A. origena (0·4 ± 0·05 m), A. iraqensis (0·45 ± 0·4 mm) and A. oerfota (0·51 ± 0·02 mm). The slight differences in the germination responses of A. tortilis and A. ehrenbergiana recorded in this experiment and those shown by Abulfatih & Bazzaz (1985) from the south-western parts of Saudi Arabia can be explained on the basis of the variation in the seed source and the ecotypic differences among the plants from which the seeds were collected for the germination tests. Such ambiguous results in the germination responses invites ecologists to look in more depth at the ecotypic and taxonomic variations found in each Acacia sp. along its broad range of distribution in the Middle East.

References Abulfatih, H. A. (1979). Vegetation of higher elevation of Asir, Saudi Arabia. Proceedings of Saudi Biological Society, 3: 139–148. Abulfatih, H. A. (1981). Plant ecology of Dalaghan National Park, Asir Province, Saudi Arabia. Proceedings of Saudi Biological Society, 5: 131–141 Abulfatih, H. A. (1983a). Elevationally restricted floral elements of the Asir Mountains, Saudi Arabia. Journal of Arid Environments, 7: 35–41 Abulfatih, H. A. (1983b). Germination of nine species from Abha. Journal of Arid Environments, 6: 247–251. Abulfatih, H. A. (1984). Wild Plants of Abha and the Surrounding Areas (in English and Arabic). Jeddah: Saudi Publishing and Distributing House. xi + 125 pp. Abulfatih, H. A. & Bazzaz, F. A. (1985). Laboratory studies of germination and growth in plant species associated with an elevation gradient in southwestern Saudi Arabia. Arab Gulf Journal for Scientific Research, 3: 449–459. Abulfatih, H. A. & Emara. H. A. (1988). Altitudinal distribution of the hemiparasitic Loranthaceae in south-western Saudi Arabia. Biotropica, 20: 81–83. Abulfatih, H. A., Emara, H. A., & Hashish, A. (1988). The influence of glazing on vegetation and soil of Asir Highlands. Arab Gulf Journal for Scientific Research, Agricultural and Biological Sciences, B7: 69–78. Abulfatih, H. A. (1991). Quantitative assessment of wild trees in southwestern Saudi Arabia. Saudi Biological Society, Biological Sciences, 1: 117–127. Abulfatih, H. A. (1992). Vegetation zonation along an altitudinal gradient between sea level and 3000 meters in southwestern Saudi Arabia. Journal of King Saud University, Vol. 4, Science, No. 1: 57–97. Abulfatih, H. A. & Bazzaz, F. A. (1979). The biology of Ambrosia trifida L. II. Germination, emergence, growth and survival. New Phytologist, 83: 817–827. Al-Hubaishi, A. & Muller-Hohenstein, K. (1984). An Introduction to the Vegetation of Yemen. Deutsche, Gesellschaft fur Technische Zusammenarbeit (GTZ) GmbH, Eschborn. 274 pp. Al-Zoghet, M. (1990). Wild Plants of Jubail and Yanbu. Riyadh: Publications of the Royal Commission for Jubail and Yanbu. 245 pp. Andrews, F. W. (1952). The Flowering Plants of Anglo-Egyptian Sudan, Vol. 2. Abroath: T. Buncle. 484 pp. Anon. (1988). Climate Atlas of Saudi Arabia. Riyadh: Ministry of Agriculture and Water Publications. 11 pp + 118 Plates. Batanouny, K. H. (1986). Environment and Plant Life in Qatar. Qatar: University of Qatar Publications. 245 pp.

178

H. A. ABULFATIH

Brooks, W. H. & Mandil, K. S. D. (1983). Vegetation dynamics in the Asir woodlands of southwestern Saudi Arabia. Journal of Arid Environments, 6: 357–362. Chaudhary, S. A. (1983). Acacia and other genera of Mimosoideae in Saudi Arabia. Riyadh: Regional Agriculture and Water Research Center, Ministry of Agriculture and Water. 87 pp. Chaudhary, S. A. & El-Sheikh, A. (1988). Vegetation of some high altitude areas of Saudi Arabia. Proceedings of Saudi Biological Society, 11: 257–246. Collenette, I. S. (1985). An Illustrated Guide to the Flowers of Saudi Arabia. MEPA, Kingdom of Saudi Arabia, Flora Publication, No.1., London: Scorpion Publishing. 514 pp. El-Ghonemy, A. A. (1985). Ecology and Flora of Al-Ain Region. Abu Dhabi: Al-Wahda Press. 205 pp. Halvey, G. & Orsham, G. (1972). Ecological studies on Acacia species in the Negev and Sinai, 1. Distribution of Acacia raddiana, A. tortilis, and A. gerrardii ssp. negevensis as related to environmental factors. Israel Journal of Botany, 21: 197–208. Hassan, H. M. & Al-Farraj, M. M. (1989). Distribution and seed description of the genus Acacia (Mimosoideae–Leguminosae) in Saudi Arabia. Arab Gulf Journal for Scientific Research, 7: 125–144. Mahmoud, H. (1977). Germination of three desert Acacias in relation to their survival in arid environment. Proceedings of Saudi Biological Society, 1: 74–94. Mandaville, J. P. (1973). A contribution to the flora of Asir, southwestern Saudi Arabia. Coconut Grove, Miami, Field Research Publication, 4: 1–13. Mandaville, J. P. (1990). Flora of Eastern Saudi Arabia. London: Kegan Paul International. 482 pp. Migahid, A. M. (1978). Flora of Saudi Arabia, Vol. 2. Riyadh: Riyadh University Publications. 647 pp. T¨ackholm, V. (1974). Student's Flora of Egypt. Cairo: Cairo University Publications. 888 pp. Vesey-Fitzgerald, D. F. (1955). The vegetation of the Red Sea coast south of Jeddah, Saudi Arabia. Journal of Ecology, 43: 477–489. Zahran, M. A. (1982). Vegetation Types of Saudi Arabia. Jeddah: Faculty of Meteorology and Environmental Studies, King Abdulaziz University. 63 pp.