PRESOWING TREATMENTS TO BREAKING SEED DOR- MANCY OF Pterocarya stenoPtera C. DC. AS AN INDICATOR OF POTENTIAL INVASIVENESS

The Small-winged wingnut is a 12 to 20 metres tall, deciduous and fast growing tree with large substantial branches that spread as wide as the tree is tall. The 15 to 30 cm long winged fruit catkins, which turn brown in the autumn, are suspended below the branches. The species is not described by domestic authors in Serbia. The seed was introduced from Sofia (from the Arboretum of the Forestry University of Sofia, Bulgaria Лесотехнически университет, София), and subjected to classical stratification and naked stratification during the period of 1 and 2 months. Germination capacity varied among the treatments. The largest number of germinated seeds was observed in the shorter stratifications, (1 month) the classical (56.5%) and the naked (51.0%) with no significant difference. The values of real germination of all treatments were between 69-88%. High value of RG, compared to GC, indicated that more than 40% of the seeds were empty. However, the results of the control as well as other parameters of seed germination clearly point to the fact that practically no deep embryo dormancy was observed, but the seed is recalcitrant and microbiotic. The results indicate a non-invasive character of the species in terms of reproductive potential.


INTRODUCTION
The introduction of plants into a new environment is related to a potential risk of its invasiveness.The introduced plant species adjusts to the new environment in varying degrees by overcoming a few barriers, ranging from the extent that overcoming geographical barriers to become non-indigenous to the extent that overcoming the spread barrier becomes invasive (R i c h a r d s o n et al., 2000).Invasive alien species cause significant global changes and influence losses in the economy, biodiversity and functioning of affected ecosystems.Habitat destruction and invasive species are now recognized as the two most important factors that threaten biodiversity at the global level.The Curitiba Declaration on Cities and Biodiversity is the basis for studies of invasive species in cities as the primary centres of their introduction and distribution.The greatest number of invasive species is recorded as a category of ornamental plants.According to research in the U.S.A, where they have had a long-term monitoring (150 years), 85% of the invasive plants are ornamental (R e i c h a r d et al., 2001).EEA (European Environment Agency) considered invasive species as one of the main threats to biodiversity (set up homogocene -homogeneous ecosystem with few dominant species) (S c a l e r a , 2008).
In order to prevent possible harm to biodiversity by invasive species, it is important to examine the mechanisms of invasion and ecological and biological characteristics of invasive species.Knowledge of these mechanisms and characteristics of invasive species allow finding the right ways to control them.It is quite understandable that the study of biological properties taken before the introduction has a special value.There are regulations that provide guidance on the introduction of neophytes in many countries.For example, in Slovenia the Regulations on the nature of risk assessment and license (The Official Gazette of the Republic of Slovenia, No. 43/02), which contains regulations considering invasive species, defines the conditions and methods for assessing the risk to the environment before the introduction of alien plants and animal species to the environment.
Biological properties that are related to reproduction are of primary consideration.Abundance and frequency of fruiting, the beginning of the generative stage of ontogeny, sexual expression of the species, self-pollination, types of seed dispersal, dormancy and seed longevity, are only some of the qualities crucial for the emergence of the next generation and the rate of spread of the introduced plant.
Small-winged wingnut or Chinese wingnut, Feng Yang (Pterocarya stenoptera C. DC.) is a species that has not been introduced in Serbia until now.It is a tree up to 30 m high that naturally occurs in China.The flowering is in April-May, with leaf emergence.The pendulous catkins remain on the branches during the winter.
Caucasian wingnut (Pterocarya fraxinifolia (Lam.)Spach), is a species that has been successfully grown in parks in Serbia (Pe t r o v i ć , 1952, Vu k i ć e v i ć , 1974, J o v a n o v i ć , 1985) for over 100 years as an ornamental and it belongs to the same genus.P r o s t r a n et al., (2010) reported that the age of five trees of Pterocarya fraxinifolia in the Zemun Park is 115 years.Pterocarya fraxinifolia do not show signs of invasiveness in Serbia.Despite regular fruiting, it very rarely gives spontaneous seedlings.
The dominant method of propagation of the genus Pterocarya is by seed, although the Pterocarya fraxinifolia could be propagated by layering as well.Green cuttings of Pterocarya × rehderiana C.K. Schneid.(Pterocarya fraxinifolia × Pterocarya stenoptera) root well in May and June when the young shoots are treated by IBA powder (1%), while later in July the concentration of IBA powder is reduced to 0.1%.The rooting substrate contains peat and sand (2:1); rooting takes place in greenhouse with mist system, and in 5 weeks there were 75% of rooted cuttings (G r b i ć , 2004).
The aim of this paper is to investigate some of the seed traits, in fact dormancy (type and intensity), of Pterocarya stenoptera, the species that is present in neighbouring countries and whose seed was collected in Sofia, by different presowing methods.

MATERIAL AND METHOD
The parent tree is located in the Arboretum of Forestry of the University of Sofia (Лесотехнически университет, София) (Fig. 1).The fruits were collected from the tree on September 9 th 2007.The seeds were stored wet in a plastic bag in a refrigerator, until the stratification or the germination test.
The pendulous catkins on the long hairy pedicle 30 cm (10-45), remain on the branches during the winter.There are 36 (28-41) fruits in one group.Nuts are with two narrow and longribbed wings (1.2-2.5×6-7mm) (Fig. 2).The wings are connected to each other at a sharp angle.Small nut 6-7 mm long, slightly hairy or bare is mature in mid-September (Fig. 3).The mass of 1000 cleaned seeds is 40-55g and the percentage of sound seed is 70.Five-month stratification is recommended (M i l e v et al., 2004).
A germination test was carried out in a growth chamber, Тype 1291/ TPC-1/LP-113, on filter paper (4×100 seeds per treatment) at 20°c (±2).Photoperiod was 16/8 (light/dark).The control was tested immediately, October 19 th 2007, and the rest of the seed was stratified before the test.The stratifications were: (1) one-month naked, (2) two-month naked, (3) one-month in perlite and (4) two-month in perlite.The testing lasted for 21 days, while germination (EC) was calculated on the 7 th day.Naked stratification was performed in transparent polyethylene bags.Seeds were pre-soaked in water at room temperature for 48 hours after which they were treated with fungicide (Previcur), surface drained, and then they were put into bags that were closed and placed in a refrigerator at a temperature of 3-5°c.Each week, the seeds were washed and aerated.Classical stratification was carried out in boxes filled with perlite.In order to investigate the longevity and recalcitrancy of the seeds, the rest of the seed had been stored for a year and germinated after that period.
The result was presented by means of nine indicators of germination.Some of them reflect only the quantitative value of germination (germinative capacity (GC), real germination (RG), and germinative energy (GE)), others only the dynamics (mean germination period (MGP), germination intensity (GI), the coefficient of the rate of germination (CRG), and the coefficient of uniformity of germination (CUG), and still others refer to both groups (germination value by Czabator (GV (Cz)) and germination value by Djavanshir and Pourbeik (GV (MC)).

RESULTS
As the seed of small-winged wingnut belongs to the non-albuminous seeds and at the time of full maturity contains only a well-developed embryo, morphological dormancy (type B, N i k o l a e v a , 1977) was excluded.Exogenous dormancy types (A), caused by physical or chemical properties of the outer layer of seeds, are also not recorded, because the seeds absorb moisture and increase weight during soaking in water.All parameters that reflect the germination, neither of qualitative nor of quantitative or combined, indicate endogenous type of dormancy -C dormancy (N i k o l a e v a , 1977), and the differences between the results obtained with control seeds and seeds stratified indicate that this is C 1 moderate (shallow) physiological dormancy (Table 1).
Germinative capacity indicates a significant difference between the control and the treatments.After one month stratification in perlite, the largest number of seeds germinated (56.5%).Seeds from one month naked stratification germinated to a lesser extent (51%), but there was no statistically significant difference between these and the previous treatment.Both types of two-month stratification affected the lower germination but did not show significant difference between them.Real seed germination puts control between the homogeneous group of one-month stratification (with higher values: 88 and 85%) and two-month stratification (lower: 75.6 and 69.8%).The following three parameters (Germinative energy, Mean germinative period, Germinative intensity) indicate that the germination of stratified seeds was more dynamic, particularly of the ones that were in perlite.Combined indicators of germination, which mainly indicate the quality and dynamics of germination, singled out stratified seeds as better with the exception of the coefficient of uniformity of germination that is best for control.Seed that had been stored for one year and germinated after that period gave negative results as the germination did not occur.This result is indicating the microbiotic character of small-winged wingnut seed and its recalcitrant nature.

DISCUSSION
Stratification of seeds (in perlite or naked) resulted in the highest germination percentage and germination rate after 30 days.This definite response to stratification is typical for the dormant embryo.Similar results were revealed in a study of C i c e k ( 2008) with Pterocaria fraxinifolia -Caucasian wingnut.After a five-week cold naked stratification (without a medium), seed of Pterocaria fraxinifolia had the highest germination rate.Yo u n g and Yo u n g (1994) stated that the same species had seed dormancy and seed germinated after 3 months of cold stratification.S h a o (1989) found that Pterocarya stenoptera had more than 70% germination when treated with a temperature varying from 20 o c (or 15 o c) to 30 o c before stratification.Pretreatments and seed germination can vary greatly among seed lots (E d w a r d s , E l -K a s s a b y, 1996, B a s k i n , B a s k i n , 1998, T i l k i , G u n e r, 2007, J u l l , B l a z i c h , 2000).Seeds generally have an inherent high genetic variability, which results in great heterogeneity in their behaviour and particularly in their germinability following stratification procedures.In the present study, real seed germination and distribution of homogeneous groups in all treatments indicated that more than 40% of the seeds were empty -control treatment with 80.5% was between one-month stratifications (with higher values: 88 and 85%) and two-month stratifications 75.5 and 69.8%.Av s a r (2002) similarly stated that most of the Pterocaria fraxinifolia seeds were empty.
In the final study, the germination did not occur in the treatment with seeds that had been stored for one year.This result points out the microbiotic character of the seed and its recalcitrant nature.Recalcitrant seeds cannot survive drying below relatively high moisture content in the seed itself (often in the range 20-50% wet basis) and cannot be successfully stored for long periods.Many of the large-seeded hardwood species with nuts fruit in the temperate zone were recalcitrant e.g.corylus spp., castanea spp., Fagus spp., Quercus spp., Junglans spp.(H a r t m a n n , 1990).According to the research of G o r d o n et al. (1982), species Pterocarya fraxinifolia belongs to this group.When fresh recalcitrant seeds begin to dry, viability is first slightly reduced as moisture is lost, but then it starts being reduced considerably at a certain moisture content in the seed that is determined as the "critical moisture content".If drying continues further, the viability is reduced to zero.

CONCLUSIONS
Small-winged wingnut has been introduced in the Balkans (Sofia, Bulgaria) where it successfully grows and fruits.Similar climatic conditions in Belgrade and Sofia point out that the area of Belgrade (and Serbia) is suitable for the introduction of this species.The properties such as fast growth, ornamental flowers and fruits are valuable attributes for its application in landscape architecture and horticulture.