The aim of this study was to evaluate the extent to which differences in the ploidy level and geographic range within the cosmopolitan grass P. australis are reflected in phenotypic traits, such as plant size and ecophysiological characteristics, and if the predominance of octoploids in Asia and Australia could be explained by a superior phenotype that may have evolved within the highly genetically and cytotypically diverse P. australis species. Hence, we grew clones of different cytotypes collected in Europe (where 2n = 4x is the dominant ploidy level) and in Asia and Australia (where octoploids predominate) under identical environmental conditions to avoid environmental effects on phenotype development. Furthermore, the clones had been grown in a common outdoor environment under similar conditions in terms of soil, water, nutrition and climate for at least 5 years prior to the experiment. It is therefore justified to assume that any difference in growth, morphology and physiological characteristics between the clones is genetically determined.
Previous studies have reported significant variations in stand height, plant morphology and salinity tolerance related to the ploidy level (Hanganu et al. 1999
; Pauca-Comanescu et al. 1999
; Hansen et al. 2007
). In the Danube delta, Romania, tetra- (4x
), hexa- (6x
) and octoploids (8x
) co-exist, and tetra- and octoploids can be identified by their phenotype as shoots of octoploids are longer and thicker with more nodes than those of tetraploids, and panicles and leaves of octoploids are also larger than those of tetraploids (Pauca-Comanescu et al. 1999
). Also, Hansen et al. (2007)
found that different ploidy levels had different morphologies, with Romanian octoploids generally being taller and thicker than other ploidy levels. In the present study, another Romanian octoploid (E624RO8x) was also among the tallest clones, although not significantly taller than a Russian tetraploid, a Japanese octoploid or an Australian decaploid. Hence, it does not seem to be a general characteristic that octoploids are larger than other ploidy levels. Also, our results did not show any statistically significant differences between ploidy levels. For example, the Czech octoploid (E666CZ8x) was shorter than the corresponding Czech tetraploid (E620CZ4x), and in plants from the Danube delta, where different ploidy levels co-exist, there were no significant differences between the different ploidy levels (E646RO4x, E624RO8x, E625RO6x, E656RO6x and E660RO12x). Although European clones appeared very different in size and ecophysiological traits, the Asiatic and Australian clones were very similar. However, similarities were independent of ploidy level and phylogenetic relationships. Not all Asiatic/Australian clones analysed in the present study belonged to the predominantly octoploid group identified by Lambertini et al. (2006)
. The clone A205RU4x was genetically closer to the European clones than to the Asian/Australian octoploids, but it did not cluster far from the Asiatic/Australian clones in the PCA. Although other European clones clustered together with the Asiatic/Australian clones, the Japanese octoploid clone clustered far away from its close relatives in Sakhalin Island and Australia, revealing large variation in ecophysiological traits also within the predominantly octoploid group that evolved in Asia and Australia.
Differences among clones could not be explained by the ploidy level, geographic range or phylogenetic relationships, but appeared to be genotype dependent. However, the variation was remarkably higher within the European group of clones than within the Asiatic/Australian group, considering that European clones were from a restricted geographic area (Romania and the Czech Republic), whereas the Asiatic/Australian clones covered a much larger geographic range represented by Sakhalin Island, Japan and Australia.
It has been suggested that growth and expansion rates are a combined effect of genotype and the native environment (Parker et al. 2003
; Ward et al. 2008
). This interaction underlines the possible features of successful invaders, which correlate with the length of the growing season – one of the opportunistic traits of invasive species (Zedler and Kercher 2004
). The tetraploids analysed in the present study seemed, in general, to invest predominantly in the first stage of the growing season, with intensive growth in spring to reach the maximum height after 2–3 months. In contrast, growth rates of octoploids were moderate in spring, but growth extended over a longer period of time. This difference in growth pattern suggests that the clones are adapted to growing seasons of different length, and also that tetraploids possess more opportunistic traits than octoploids. Other studies also found differences in the length of the growing season among clones that could be related to the geographic origin of the clones (Clevering et al. 2001
The 15 P. australis
clones analysed in this study had high phenotypic variation in Pmax
and WUE. The variations observed were genotype dependent, but they could not be clearly attributed to the ploidy level or to the geographic range of origin. This was due to the high variability between representatives of the same ploidy level and variability between clones from the same geographic range, as well as variation between replicas of the same clone. Although all clones had origins at similar latitudes (lowest 35°, highest 50°) and had been acclimated to the same environment for at least 5 years prior to the experiment, significant differences in Pmax
and WUE were observed between clones. Hence, the clones did not acclimate to the growth environment to the same extent. Physiological processes have earlier been reported to acclimate to new growth environments (Lessmann et al. 2001
). However, the native habitat of the clones and differential expression of photosynthesis-related genes may also be some of the reasons for the observed differences in gas exchange characteristics of the clones we studied. Environmentally induced differences in physiological parameters have been found to be generally larger than the genetically determined differences between populations of P. australis
(Lessmann et al. 2001
). Since in our experiment the premise of similar environmental conditions is ensured, the differences observed between clones can be assumed to be genetic.
The fact that the concentrations of the light-absorbing chlorophylls did not differ among clones is not surprising since the content of chlorophylls is expected to be rather stable within species under common environmental conditions. However, the observed difference in the Chl a
ratio between the clones indicates variations of the functional pigment complex, as well as possible physiological differences in the photosynthetic apparatus. The concentration of total carotenoids as well as the chlorophyll to carotenoid ratio, [(a + b
)], also differed among the clones. These differences may be related to differences in environmental conditions and the length of the growing season in the native habitat of the clones, since these parameters are often associated with senescence (Kurahotta et al. 1987
; Chen and Cao 2008
The concentrations of Cl, Na, K, Ca and Mg in the leaves of the plants also differed significantly among the clones, and the differences could not be related either to the ploidy level or to the geographic origin of the clones. The concentrations of these elements in the leaves may be related to the osmotic balance of the cells, and hence to mechanisms and possible strategies adopted by the clones in stress conditions. A differential ability to regulate ions like Cl−
in the plant tissue may be related to differences in salt tolerance (Lissner and Schierup 1997
; Lissner et al. 1999
; Munns and Tester 2008
; Pagter et al. 2009
) and drought resistance (Pagter et al. 2005
). The significant differences observed here support the hypothesis that stress tolerance in P. australis
is genotype dependent.