Influence of stratification on germination of Pinus leucodermis

ESTRATTO DA

National Research   Council Canada

Conseil national de recherches Canada

Reprinted from 

 Canadian          Journal of  Forest Research

Volume 16 - N. 4 - 1986

Pagine 867-869

Pinus leucodermis Antoine is a coniferous species occurring  in Italy as relic populations growing on mountains in Calabria  and Basilicata (Corbetta 1974). There is disagreement concerning its taxonomy. In fact, P. leucodermis is considered either as a separated entity (De Bazac 1964; Gaussen et al. 1964) or as a Variety of Pinus heldreichii Christ. (Mirov 1967). In any case, the Italian populations represent the western natural limit of both species.                                                                       This tree could play an important role in afforesting arid slopes in mountains because it grows on very poor limestone soils and  is tolerant of frost and drought (Brogi   1960).

 However, very little is known about the ecology of germination of  P. leucodermis. Therefore, the present study was undertaken to investigate the effects oold-moist stratification on seed germination of three seed lots collected at different altitudes germination of three seed loots collected at different altitudes along the southern slope of Monte Pollino, Calabria, Italy.

 Materials and methodss

Cones were collected at the beginning of natural dispersal in the autumn of 1983 from three stands located at different altitudes on the Calabrian slope of Monte Pollino (39° 54’ N  16°  11’ E) ( Table 1). Thirty-five threes, at least 50 m apart, per stand and 25 cones per tree were sampled. Following extraction, seeds were combined into single lots representing each stand. Damaged and undersized seeds were removed by hand from each seed lot. Seeds were dried to a moisture content of 7-8% and stored in sealed containers at 2°C. At the beginning of the experiment (January 1985) non significant variations in seed moisture content were observed.

Eight 50-seed replicates from each seed lot were assigned to each of the following seven treatments: O, 30, 35, 40, 45, 50, and 60 days of cold-moist stratification. Cold-moist stratification was accomplished by keeping seeds in the dark at 2-3°C in 14-cm glass Petri dishes containing moist filler paper.

Afterwards, each 50-seed replicate was transferred into portable plastic germinators (Magini 1962), which were covered and aerated, where seeds were germinated on continously moistened filler paper. In each germinator, the relative humidity was approximately 100%. For each seed lot, 28 germinators (four 50-seed replicates for each of the seven treatments) were placed in a germination cabinet on a 30:20°C (8:16h) temperature regime and the other 28 were kepi at a Constant temperature of 20°C. Germination temperatures were maintained within ± I°C of the set point. An 8 h light: 16 h dark photo period was applied using cool-white fluorescent lamps; 8 h of light corresponds to the 30°C temperature. The photosynthetic photon flux density (400-700 nm), measured with a LiCor 1600 quantum sensor at the level of the plastic germinators, was 20-30 µnol m~²s~'.

Germination was initially recorded on the 7th day and, subsequently, every 3 days until the 37th day. Seeds were considered germinated when radicals were 3 times seed length. Seeds exhibiting abnormal germination were excluded from germination counts. At the end of the test, non germinated seeds were examined to exclude empty seeds. Cumulative percent germination on a full-seed basis (CPG) was calculated. Mean germination lime (MGT) was computed as follows: MGT = 2 gt/N, where g is the number of seeds germinated during each 3-day period, t is the central value of each 3-day period measured by days from the beginning of the test, and N is the total number of seeds germinated during the test (Yousheng and Sziklai 1985).

CPG, after are sine transformation, and MGT were subjected to a three-way ANOVA in which effects of stratification, seed lot, and temperature were evaluated.

Fig. 1. Influence of stratification on cumulative percent germination (CPG) and mean germination lime (MGT) at different germination temperatures.             30:20°C (8:16h); O, 20°C.

Results and discussion

CPG and MGT show steady improvement by stratification prolonged up to 40 days (Fig. 1, Table 2). CPG (final count) for  non stratified seeds was 43.8, 35.3, and 33.9% of that for seeds stratified for 30, 40, and 60 days, respectively. CPG reached 50% after 21,13, 11, and 10 days for seeds stratified for O, 30, 40, and 60 days, respectively. MGT after 40 days of chilling was about half of MGT for non stratified seeds. An analogous period of chilling (30-42 days) is recommended for seeds of P. heldreichii (Anonymous 1966; Anonymous 1974).

The favorable effect of prechilling on germination allows speculation about a rather high degree of dormancy (without stratification, over 60% of filled seeds did not germinate at favorable temperature), which can be interrupted by stratifica­tion (Bewley and Black 1982; Villiers 1972). Since seeds were germinated on an 8h light: 16h dark photoperiod, an effect of light in breaking dormancy can not be ruled out. In a later experiment (no stratifìcation, 30:20°C), no differences were observed between an 8h light: 16 h dark photoperiod and continous light, while a lower CPG was noted in the dark (A. Veneziano, personal communication).

Under natural conditions, internal dormancy probably inhibits germination and is broken after the seeds have experienced some winter low temperatures.

Because cone maturation and seed dispersal normally occur on Monte Pollino in the second half of September and in the first half of October, the significance of dormancy might be to prevent or at least limit germination before the onset of winter. In fact, during this period (October and beginning of November) daytime air temperatures can exceed 18, 16, and 15°C at altitudes of 800-1000, 1000-1250, and 1500 m, respectively (Mennella 1973), which could promote germination. After this time, germination is probably inhibited by low temperatures until spring, when the weather again becomes suitable for seedling growth. This behavior is common to several species indigenous to climates where cold winters can be fatal to established seedlings (Bewley and Black 1982).

CPG was always higher at 30:20°C, while MGT differences were small between 20°C and 30:20°C. CPG was on average higher in seed lots collected at lower elevations. A clear pattern, however, is lacking for MGT as well for the percent of dormant seeds.

A negative correlation between germination capacity and altitude has been reported for other species (e.g., Norway spruce (Picea abies Karst.) (Anderson 1965; Piussi 1967) and yellow poplar (Liriodendron tulipifera L.) (Barnett and Farmer 1978)).

A significant interaction was found between temperature and stratification. In fact, averaging data over ali seed lots, differences between CPG at 30:20°C and at 20°C were 20.1, 11.9, 9.3, and 6.4 with O, 30, 40, and 60 days of chilling, respectively. Similar interactions have been reported for red maple (Acer rubrum L.) (Farmer and Goelz 1984) and Fraser fir (Abies fraseri (Pursh) Poir.) (Adkins et al. 1984). A greater effect of stratification on seed lots A and B can be observed if data at 30:20°C are considered. This might indicate a deeper dormancy in seed lots from higher elevations, but this hypothesis must be supported by further studies.

Acknowledgements

This research was supported by grants from National Research Council and Ministry of Agriculture and Forestry of Italy. The technical assistance of Mr. F. Bandini is gratefully acknowledged.

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