J For Res (2006) 11:319–328 DOI 10.1007/s10310-006-0214-y
© The Japanese Forest Society and Springer 2006
ORIGINAL ARTICLE Jiaojun Zhu · Hongzhang Kang · Hui Tan · Meiling Xu
Effects of drought stresses induced by polyethylene glycol on germination of Pinus sylvestris var. mongolica seeds from natural and plantation forests on sandy land
Received: June 7, 2005 / Accepted: April 10, 2006
Abstract Effects of drought stresses induced by polyethylene glycol (PEG) (0.0%, 10%, 15%, 20%, 25%, and 30%, with four replicates) on germination of Mongolian pine (Pinus sylvestris var. mongolica) seeds produced in plantations (southern Keerqin sandy land) and natural forests (Hulunbeier sandy plain) were observed. The results indicated that the seeds from both provenances did not germinate when PEG concentration was more than 25%. The time of initial germination and that of its completion of stressed seeds from both provenances were delayed when compared with the unstressed seeds. The germination capacity and germination rate of natural seeds were significantly higher than those of plantation seeds for all treatment levels (P < 0.05). The mean growth rates of radicle and hypocotyl from natural seeds were significantly higher than those from plantation seeds at all treatment levels below 20% PEG treatment (P < 0.05). The ratios of radicle to hypocotyl of 20% PEG treatment were significant higher than those of the corresponding controls for both provenances (P < 0.05). These results suggested that Mongolian pine seeds/seedlings had stronger resistance to PEG drought stresses; 10% PGE stress did not significantly influence germination. Natural seeds exhibited more resistance to PEG stress than plantation seeds. It was concluded that drought stress on seed germination might be one cause of obstructed natural regeneration of Mongolian pine plantations on sandy land. It is recommended that natural seeds be used for afforestation, and light drought stress (e.g., 10% PEG stress) may be useful in improving seed germination and the growth of radicles and hypocotyls.
J. Zhu (*) · H. Kang · H. Tan · M. Xu Institute of Applied Ecology, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China Tel. +86-24-8397-0342; Fax +86-24-8397-0342 e-mail:
[email protected] H. Kang · H. Tan Graduate School, Chinese Academy of Sciences, Beijing, PR China
Key words Water stress · Sandy land · Pinus sylvestris var. mongolica seed · Germination · Provenance
Introduction Mongolian pine (Pinus sylvestris var. mongolica), a geographical variety species of Scots pine (Pinus sylvestris), is naturally distributed in Honghuaerji, on the Hulunbeier sandy plain of China (47°35′–48°36′ N, 118°58′–120°32′ E), and is an important tree species of afforestation in the “three north” areas of China (the north, northeast, and northwest of China), especially in sandy areas (Zheng 1983; Jiao 1989; Wang and Huang 1996; Zhu et al. 2003a; Kang et al. 2004). After successful introduction of the tree species on sandy land in the 1950s, the area of Mongolian pine plantation forests on sandy land reached more than 30 000 ha in northern China (Kang et al. 2004). However, there have been many problems such as withered top, low growth rate, and no regeneration from the earliest Mongolian pine plantations on sandy land. These problems have been considered as decline of the plantations (Chang and Zhao 1990; Jiao 2001; Liu et al. 2002; Zeng et al. 2002a; Zhu et al. 2003a). There have been many assumptions introduced to explain the causes of the decline (Jiao 1989, 2001; Zeng et al. 1996; Wang et al. 1999; Liu et al. 2002; Zhu et al. 2005), but until now no specific theories could successfully interpret it (Jiao 2001; Chen et al. 2004). The lack of natural regeneration in plantations is one of the most important features that characterized the decline. Natural Mongolian pine forest on sandy land has exhibited very strong regeneration in the same stage of the plantations (Jiao 2001; Zeng et al. 2002b; Zhu et al. 2005). In general, seed banks with vitality, appropriate conditions for seed germination, and seedling emergence, survival, and establishment are the most important factors that potentially affect the natural regeneration of forests (Telewski and Jaffe 1981; Ogasawara 1988; Futai and Nakai 1993; Zeng et al. 1996; Gong et al. 1999; Meer et al. 1999; Vickers and Palmer 2000; Bailey and Covington 2002;
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Garcia and Jurado 2003; Zhu et al. 2003b). Research reports indicate that Mongolian pine plantations on sandy land are not short of vital seeds (Jiao 2001; Zeng et al. 2002a, b). Factors such as forest cover, litter, vegetation, and low snow coverage influence the natural regeneration of Mongolian pine plantations (Zeng et al. 2002a, b). However, the most direct factor may be the effects of severe drought stresses on plants in the sandy land, especially poor germination and seedling survival and establishment are regarded as the major causes of a lack of regeneration (Close and Wilson 2002; Kaufmann et al. 2003; McLaren and McDonald 2003; Zhu et al. 2005). Therefore, successful determination of the causes of no regeneration or the decline of Mongolian pine plantations on sandy land depends on our ability to understand the relationships between seed germination and drought stress, because drought stress is the limiting factor for sandy land ecosystems (Han et al. 1999; Zhu et al. 2002; Zeng et al. 2002b; Jiang et al. 2003). In order to explore the relationships between drought stresses and seed germination of Mongolian pine trees on sandy land, we designed an experiment to simulate drought stresses using polyethylene glycol (PEG). The objectives of this study were: (1) to identify the response patterns of Mongolian pine seed germination to drought stresses for two provenances from plantations and natural forests on sandy land; (2) to examine the response differences of seed germination between the seeds from the two provenances to drought stresses; and (3) to discuss whether seed germination is associated with natural regeneration or the decline of Mongolian pine plantations.
Materials and methods Seed collecting The seeds used in this experiment were obtained from different sources. Seeds of natural Pinus sylvestris var. mongolica forests came from the Honghuaerji, Hulunbeier sandy land of Inner Mongolian Autonomous Region of China (47°35′–48°36′ N, 118°58′–120°32′ E), while seeds of plantation Pinus sylvestris var. mongolica forests came from Zhanggutai, Zhangwu County of Keerqin sandy land of Liaoning Province, China (42°40′–43°45′ N, 122°31′–123°34′ E), which were the earliest Mongolian pine plantations introduced in the 1950s. Both batches of seeds were collected in October of 2003. The seeds were stored in opaque paper bags at a constant temperature (4° ± 0.5°C) in the laboratory (41°51′ N, 123°37′ E, 50 m asl) until the experiment (Humara et al. 2000; Núñez and Calvo 2000). Although exact details of the provenance locations and the parental trees were not recorded, the seedlot characteristics were investigated and are summarized in Table 1. Experimental design In order to eliminate the effects of seed size on germination, similar seeds were selected from the two provenances for
the experiment. The seeds were sterilized by soaking in a 5% solution of potassium permanganate for 30 min. The petri dishes used in the experiment were sterilized at 105°C in an oven for 2 h. The experimental design was completely randomized. A series of drought stresses, i.e., water potentials of −0.20, −0.40, −0.75, −1.35, and −1.53 MPa were produced by different concentrations of polyethylene glycol 6000 (PEG 6000) solution according to Michel and Kaufmann (1973). PEG solutions of 10% (100 g l−1), 15% (150 g l−1), 20% (200 g l−1), 25% (250 g l−1), and 30% (300 g l−1) were used to produce drought stresses (Michel and Kaufmann 1973; Shi and Ding 2000). The water potentials were verified using a Psypro Dew Point microvoltmeter (Wescor, Logan, UT, USA). A control (no PEG stress) was observed together with the drought-stressed ones. Germination conditions Four replicates of 50 seeds from each provenance were used for each treatment, and placed in standard petri dishes (90 mm diameter) on cellulose paper (90 mm diameter) moistened with different concentrations of PEG solutions and distilled water. The PEG solutions and distilled water were added to each petri dish for the loss of water everyday, and were covered to prevent evaporation and maintain the relative humidity close to 100% (Humara et al. 2000). The cellulose paper was changed every 4 days. Germination was carried out in a germinator under constant conditions at 21°C (20°–22°C) under white light for 12 h and at 17°C (16°– 19°C) in darkness for 12 h. All seeds were kept under such conditions for 4 weeks. For the purpose of this study, seeds were considered to have germinated when the radicle emerged from the seed (1–2 mm in length out of the tegument) (Humara et al. 2000; Núñez and Calvo 2000; Ren and Tao 2004; Zhu et al. 2005). Progress of seed germination and analysis Germinated seeds were counted and removed to other petri dishes with the same conditions every 24 h until germination ceased. Preliminary germination profiles indicated that no further germination occurred for the Mongolian pine seeds after 28 days under the test conditions. If any seed germinated in one petri dish for each treatment, the day was determined as the initial germination day. When there were no seeds germinating over six continuous days, the day was considered as the final germination day. Ungerminated seeds were soaked in water for 48 h, and were then crushed using forceps for viability testing. Seeds containing turgid white cotyledon and embryo were considered viable, whereas seeds that were hollow or contained brown, decomposed material were not viable (Close and Wilson 2002). Germination response was expressed as a percentage of viable seeds germinated. Equations 1–3 and the parameters therein were employed to express the germination situation.
GC = SNG SN0 × 100%
(1)
321 Table 1. General site data of original growing areas of natural and plantation forests of Pinus sylvestris var. mongolica on sandy land Item
Provenance
Longitude Latitude Altitude (m asl) Precipitation (mm) Evaporation (mm) Annual mean temperature (°C) More than 10°C accumulated temperature (°C) Minimum temperature (°C) Annual sunshine duration (h) Frost-free period (days) Average wind speed (m·s−1) Origin of sandy land Sand soil type Physical sand particle Soil quick-acting nitrogen (g kg−1) Soil quick-acting phosphate (g kg−1) Vegetation
Weight of 1000 seeds (g)
Honghuaerji, Hulunbeier sandy land, Inner Mongolia (natural forests), China
Zhangwu County, Keerqin sandy land, Liaoning (plantations), China
119°55.955′–120°2.690′ E 48°8.801′–48°12.569′ N 824–928 378 1174 −3.7 2000
122°30.702′–123°33.768′ E 42°39.712′–43°44.620′ N 197–239 496 1700 5.9 3067–3148
−45.0 2500–3000 90 3.8 Alluvial, aeolian, and lacustrine sand fixed sand dune Pine sandy soil and soddy sandy soil ≥0.01 mm, 91.7%;
