Germination of Stenocereus thurberi seeds was evaluated under different conditions of solar irradiation and humidity. Seed germination increased under higher ...
Journal of Arid Environments (1997) 36: 123–132
Seed germination of Stenocereus thurberi (Cactaceae) under different solar irradiation levels
H. Nolasco*, F. Vega-Villasante* & A. Diaz-Rondero† *Laboratory of Comparative Biochemistry and Physiology, Divisi´on de Biolog´ıa Experimental, Centro de Investigaciones Biol´ogicas del Noroeste S.C., Apdo. postal 128, La Paz, B.C.S., 23000, M´exico †INTERCACTI Group, Guasinapi #180, La Paz, B.C.S., 23060, M´exico (Received 8 November 1995, accepted 2 April 1996) Germination of Stenocereus thurberi seeds was evaluated under different conditions of solar irradiation and humidity. Seed germination increased under higher humidity and low solar irradiation which provided cooler temperatures and higher water availability. Seedlings were also greener and more turgid under these conditions. Increased solar irradiation reduced seed germination and decreased seedling size and water content. The results of this study support the importance of natural shelter systems in the arid zones in providing better conditions for S. thurberi seed germination and seedling establishment, particularly in the desert of Baja California. ©1997 Academic Press Limited Keywords: Stenocereus thurberi; seed germination; solar irradiation; temperature; humidity; nurse plant system
Introduction The Cactaceae family (except Rhipsalis) is endemic to America and widely distributed throughout Mexico (Bravo-Hollis, 1978). The Baja California peninsula (Hastings et al., 1972; Wiggins, 1980) is considered a region with a particularly high diversity of cacti with considerable ecological importance and economic potential (Cullman et al., 1986). A typical representative of the Baja California peninsula and Sonora Desert is the columnar cactus Stenocereus thurberi (Engelm.) Buxb. (organ pipe cactus), commonly called ‘pitahaya dulce’ (Fig. 1) in Mexico (Hasting et al., 1972). This species usually has a short trunk before branching into ascending columns (15–20 cm diameter) to 8 m tall (Wiggins, 1980). Stenocereus thurberi blooms during May and July with cream coloured flowers, is pollinated by insects and bats, and produces fruit which ripen in late summer and autumn. The flesh of this fruit is coveted for human and animal consumption because of its sweet and delicate flavour (Roberts, 1989). In the extreme heat of this arid zone, only well adapted seeds and seedlings can germinate and survive. Natural sheltering structures, especially the nurse plant system, 0140–1963/97/010123 + 10 $25.00/0/ae960199
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is recognized as an important promotor of seed germination and seedling establishment in arid environments (Franco & Nobel, 1989; Valiente-Banuet & Ezcurra, 1991; Valiente-Banuet et al., 1991a,b). Observations in the wild suggest that this particular system favours the propagation of cacti. In Baja California Sur there are several plants that may participate as nurse plants. In the wild we have observed seedlings of Stenocereus thurberi and Pachycereus pecten-aboriginum (Engelm.) Britton & Rose, mainly under the canopy of Prosopis juliflora (Swartz) DC., but also under Jatropha cinerea (Ortega) Muell. Arg., Cassia emarginata (L.) Juss., and Tecoma stans (L.) Kunth (unpublished observations), similar to other reports of nurse plant systems (Jordan & Nobel, 1979; Nobel, 1980; Nabhan, 1986; Franco & Nobel, 1989; Valiente-Banuet et al., 1991a,b). This study presents the experimental results of exposing Stenocereus thurberi seeds to different levels of solar irradiation (and therefore temperatures) and humidity to measure their effect on seed germination and seedling quality. The results indicate the need for precise conditions of temperature and humidity, and support the importance of natural sheltering structures, in particular the nurse plant system in arid environments.
Figure 1. Stenocereus thurberi, organ pipe cactus (‘pitaya dulce’), 2·3 m in height.
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Materials and methods Seeds Fifty fruits of Stenocereus thurberi were randomly collected in July 1993 from plants near ‘El Rosario’, 50 km from the city of La Paz, in Baja California Sur, Mexico (23°06' N, 110°20' W). The seeds were separated from the mature fruit by dissection, then washed with tap water and distilled water to remove any remaining mucilage. The seeds were disinfected by immersion in 10% sodium hypochlorite for 5 min, washed with distilled water and finally dried on absorbent paper before placing them in paper bags for storage in a fresh, dry place until required.
Study site and experimental design Experiments were carried out in an open, shade-free site near La Paz, B.C.S. during May and June 1994. Twelve seeds were placed in glass chambers (2·5 3 2·5 3 4·5 cm) on 5 g of sieved, non-sterilized soil on a No. 20 mesh (the soil was collected in the field, at 5 cm deep in a zone of Stenocereus thurberi forest at ‘El Rosario’, B.C.S.). The soil was wetted with distilled water to two levels. Sets of 20 glass chambers were watered once with either 2·0 ml (low (L)) of 2·5 ml (high (H)) water. The soil pH (6·8) was determined by pH meter (Beckman model 44) with a contact electrode (VWR). Sets of glass chambers (for each water level) were placed on a white polyurethane surface (60 3 30 3 5 cm) under two different shade cloths (60 cm above the glass containers), and under full solar irradiation providing maximum solar irradiation of 43·8 µE s–1 m–2 (H shade), 225 µE s–1 m–2 (L shade), and 1683 µE s–1 m–2 (no shade). The glass chambers were kept closed during the experiment with plastic caps, but they were not sealed allowing gas exchange to occur. The greenhouse effect in glass chambers could be possible due to the presence of gases and water vapour, however this was not considered.
Determination of solar irradiation The solar irradiation during the experiment was determined with an integrating quantum/radiometer/photometer (LI-COR Inc, Mod. LI-188B). Readings were expressed as µE s–1 m–2 and were determined ever 2 h from 0600 to 1800h, and the last reading was taken at 1930h. Because S. thurberi seeds do not require light to germinate (Nolasco et al., unpublished data) light quality was not considered in this experiment.
Determination of temperature Temperature was measured with a mercury thermometer (Taylor, Environmental Instruments) every 2 h from 0600 to 2000h. Soil temperature was measured by introducing the thermometer tip 7 mm into the soil and readings were recorded after 1 min of contact. Ambient temperature was recorded after 3 min of exposing the thermometer to the environment.
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Seed germination Germination was followed for 30 days. Emergence of the seed and radicle were considered to be positive germination. Determination of size, dry weight and water content of the seedlings Seedling size at day 30 was determined by the wet weight of 15 randomly selected seedlings from each treatment, using an analytical scale (Mettler, Mod. AE 163). Three sets of five randomly selected seedlings from each treatment were weighed using the analytical scale, then placed in an open aluminum bag in an oven at 80°C for 36 h. The bags were put into a desiccator chamber for 2 h, then weighed. Water content of the seedlings was calculated by subtracting dry weight from wet weight. Statistics Results were analysed by one-way analysis of variance (ANOVA). Differences between
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means were analysed by the 95% LSD method. Statistical difference between treatments is indicated by letters on top of bars.
Results Solar irradiation Solar irradiation during the experiment is shown in Fig. 2. The results represent the average of readings at day 1, 15 and 25. During the period of the experiment, there was a stable climate with sunny days.
Temperature The ambient and soil temperatures in the chambers are shown in Fig. 3(a) and 3(b), respectively. Results represent the average of readings at day 1, 15 and 25. As expected, the temperature increased proportionately to the solar irradiation. The maximum ambient temperature was 42, 39 and 37°C for full sunlight, low shade and high shade, respectively. In contrast soil temperature was 47, 40 and 38°C, respectively.
Seed germination Seed germination of Stenocereus thurberi at day 7, 15 and 30 is shown in Fig. 4. Seedlings and ungerminated seeds were kept in the chambers without further irrigation until the germination experiment was finished (day 30). No seed germinated in the chambers exposed to full sunlight. In general, seed germination was enhanced by lower levels of solar irradiation and temperature, and by higher moisture levels.
Seedling size The size (expressed as a wet weight) of the seedlings at day 30 is shown in Fig. 5. The results represent the mean weight of seedlings of each treatment. Fifty randomly selected seedlings were used to obtain size distribution, except for the low shade/low water treatment where only 33 seedlings were used. The main peak for seedling size, seedling water content and dry weight is shown in Table 1. Figure 6 is a photograph of representative seedlings. No germination occurred in full sunlight: thus, no wet and dry weights, water content or photographs of seedlings were possible.
Discussion and conclusions In this study we demonstrated the importance of shade for germination and establishment of seedlings. Shade, decreased solar irradiation, temperature and water evaporation, generally favour seed germination and seedling growth (Valiente-Banuet et al., 1991a,b). The importance of the nurse plant system for seed germination of desert plants has been observed in field studies (Nobel, 1980; Jordan & Nobel, 1981; Franco & Nobel, 1989; Valiente-Banuet et al., 1991a,b). In nature, the initial development of seedlings is enhanced beneath the canopy of nurse plants, which provide shade (including a different quality of light), higher nutrients and humidity, and the result of this
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interaction will depend on competition for water (Nobel, 1980; Franco & Nobel, 1989); in some cases, seedlings succumb to the nurse plant in the competition for water (McAuliffe, 1984; Valiente-Banuet et al., 1991a,b). The solar irradiation in an area rich in nurse plants near La Paz, B.C.S. (23°06' N, 110°20' W, September 20 1994, 1200h) had values from 1000 to 20 µE s–1 m–2 (taking readings every 20 m, along four parallel lines 200 m long), while the values under the canopy of nurse plants were from 490 to 30 µE s–1 m–2 (readings taken at 30 cm from the perimeter of the trunk, under the canopy of three 5–6 m Prosopis juliflora nurse trees). In contrast, the values of solar irradiation in an unsheltered area without any nurse plant was from 1580 to 1560 µE s–1 m–2 (unpublished results). In our experiments, Stenocereus thurberi was unable to germinate under exposure to full solar irradiation; but was enhanced when solar irradiation was decreased. The maximum solar irradiation in sheltered experiments was up to 225 µE s–1 m–2 under the low shade (L shade) and 43 µE s–1 m–2 under high shade (H shade), similar to conditions
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Figure 3. (a) Ambient and (b) soil temperatures in the germination chambers during the experiment. Plots were constructed from the average of readings on day 1, 15 and 25.
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Figure 4. Seed germination of Stenocereus thurberi at day 7, 15 and 30 of incubation in the germination chambers. No germination occurred in full sunlight. Statistical difference between treatments is indicated by different letters above bars.
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Figure 5. Size, expressed as wet weight, of the seedlings at day 30 of incubation in the germination chambers. Because there was no germination at full sunlight, there are also no results for seedling weight. Statistical difference between treatments is indicated by different letters above bars.
Table 1. Size distribution, water content and dry weight of seedlings
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95·62±0·12 93·49±2·46 94·97±0·13 86·79±1·99
0·95±0·04 0·88±0·04 0·88±0·08 1·09±0·26
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Figure 6. Relative size of seedlings under different germination conditions. Because there was no germination at full sunlight, seedlings were not available (A) H water/H shade; (A) H water/L shade; (B) L water/H shade; (B) L water/L shade.
under Prosopis juliflora nurse plants at ‘El Rosario’, B.C.S., Mexico. Seed germination was higher at the lower levels of solar irradiation at both levels of hydration. The main factor affecting seed germination could be related to the difference in soil temperature, with a maximum temperature of 47°C, 40°C and 38°C in the full sunlight, low shade and high shade treatments, respectively. The increase of temperature in full sunlight and low shade conditions could be also induced by a greenhouse effect inside the chambers. Germination in H shade/L water treatment was not significantly different to L shade/H water treatment at 7 and 15 days incubation, but was significantly different at day 30, under lower solar irradiation (this was apparently more important than the initial water content of the soil). During the summer in the Baja California peninsula, soil temperatures can reach nearly 70°C in open areas (H. Romero, 1993, pers. comm.) as reported in other arid environments (Franco & Nobel, 1989). The ambient temperature (September 20 1994, 1200h) in an unsheltered area was 43°C; under a nurse plant at ‘El Rosario’ it was around 36°C, similar to temperatures which favoured germination in our experiment (see Fig. 3). The experiments carried out in this study support the importance of the nurse plant systems as promoters of a micro-environment with low solar irradiation, lower temperatures and higher humidity than the surrounding fully exposure areas. In this way, the nurse plant system may facilitate seed germination of desert plants and seedling establishment in the wild (Jordan & Nobel, 1979; Nobel, 1980; Franco & Nobel, 1989; Valiente-Banuet et al., 1991a,b). The experimental results found in this study indicate that even if germination was not significantly different between some treatments (i.e. H shade/H water and H shade/L water, day 30) there are important differences in the quality of seedlings. In general, by reducing solar irradiation and increasing water availability, the seedlings were greener and more turgid in appearance. The wet weight and water content were also increased by the above conditions. The size distribution of seedlings (as wet weight) indicated that larger seedlings developed under low solar irradiation and higher water availability. Considering the wide distribution of S. thurberi in the Sonoran Desert (Hastings et
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al., 1972) and the properties of seed germination and seedling quality found in this study, we can assume that the nurse plant system has played an essential role in the current distribution of this cactus. Of course, the participation of birds and other animals in seed dispersal also could play an important role (Olin et al., 1989; Rodr´ıguez et al., 1992; Fleming, 1994). Some studies suggest that zoochory process enhanced seed germination (Barnea et al., 1990; Buchholz & Levey, 1990), we believe that seed dispersal and enhancement of seed germination by ornithochory processes are dependent on the seed characteristics and the digestive tract of the consumers. In conclusion, S. thurberi seed germinated and grew better under conditions of low solar radiation similar to the local nurse plant system in nature. This suggests that these systems must be recognized and protected in order to conserve the biodiversity of Sonoran Desert cacti. We are very grateful to H.D. Homero Avil´es, Reyna Isabel Martinez for technical support as part of INTERCACTI Group, to Mr Sergio Rosas for photography work, Dr Roy Bowers for correcting the English, and to Laura Alzaga-Mayagoitia and Heidi Romero-Schmidt for their suggestions. Thanks to the Centro de Investigaciones Biologicas ´ del Noroeste, S.C., to the Academia de la Investigacion ´ Cient´ıfica of Mexico and to the Royal Society of Great Britain for supporting cactus research of our Group. The authors also wish to express their special appreciation to Dr G.E. Wickens for his advice, suggestions and critique.
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Rodr´ıguez, R., Rivera, L. & Mata, E. (1992). La paloma de alas blancas: ave cineg´etica en Baja California Sur. In: Ortega, A. (Ed.), Uso y Manejo de los recursos naturales en la Sierra de la Laguna, Baja California Sur, pp. 266–292. La Paz: Centro de Investigaciones Biologicas. ´ 237 pp. Valiente-Banuet, A. & Ezcurra, E. (1991). Shade as a cause of the association between the cactus Neobuxbaumia tetetzo and the nurse plant Mimosa luisana in the Tehuacan valley, Mexico. Journal of Ecology, 79: 961–971. Valiente-Banuet, A., Vite, F. & Zavala-Hurtado, J.A. (1991a). Interaction between the cactus Neobuxbaumia tetetzo and the nurse shrub Mimosa luisana. Journal of Vegetation Science, 2: 11–14. Valiente-Banuet, A., Bolongaro-Crevenna, A., Briones, O., Ezcurra, E., Rosas, M., Nunez, ˜ Barnard, G. & Vazquez, E. (1991b). Spatial relationships between cacti and nurse shrubs in a semi-arid environment in central Mexico. Journal of Vegetation Science, 2: 15–20. Wiggins, I.L. (1980). Flora of Baja California. Stanford: Stanford University Press. 1025 pp.
