ГЕНЕТИКА, 2006, том 42, № 2, с. 226-232
^=ГЕНЕТИКА ^^^^^^^^^^^^^^^^
РАСТЕНИЙ
УДК 575.17
GENETIC DIFFERENCES BETWEEN POPULATIONS AND HABITATS IN Lamium purpureum PLASTIC RESPONSE TO CONSPECIFIC DENSITY
© 2006 r. N. BariSiC KlisariC1, B. StojkoviC2, A. Tarasjev1
1 Department of Evolutionary Biology, Institute for Biological Research, University of Belgrade, Belgrade 11000, Serbia and Montenegro; fax: (38111) 276-14-33; e-mail: tarasjev@ibiss.bg.ac.yu 2 Department of Genetics and Evolution, Faculty of Biology, University of Belgrade, Belgrade 11000, Serbia and Montenegro
Received October 03, 2005
Plants from Open and Shade habitats in two natural populations (Vrsac and Avala) were grown in two densities (High and Low). As expected, density had significant effect on most of measured traits and that effect was concordant with Shade avoidance syndrome predictions. Genetic differences between populations both in mean trait values and in plastic responses to density were also detected. Number of leaves and flowers showed plasticity in Avala population only, while shoot weight was plastic in both populations but with greater plasticity in Avala population. Differences between habitats for plant height and number of internodes were present in Vrsac population only. Habitat difference in response to density was revealed for seed weight and it was due to lack of response in plants originated from Shade habitat in Vrsac population. This study showed that not only populations, but also subpopulations occupying different habitats can differ genetically in their plastic response to density, and that between habitat differences can be population-specific.
In natural conditions plants are continuously exposed to variable surroundings including both abiotic and biotic components of environment. Plants growing in crowded conditions inevitably compete for light, water, mineral nutrients, for space to grow and to acquire resources [1]. Perhaps the most interesting resource for which plants compete is light. Unlike any other resource, light is available in unlimited supply, but for individual plant quantity and quality of light energy directly depends on size and proximity of neighboring plants, which create shade and reduce plant ability to photosynthesize. Rather for light itself, plants are competing for access to light [1]. There are two main strategies to provide survival under shaded conditions -plants may tolerate shade or try to avoid it [2, 3]. Shade avoidance represents one of the most important competitive strategies for plants since density effects and their variation in space and time are almost inevitable [4]. Plants perceive the presence of the other plants through change in spectral quality, the ratio of red to far-red (R : FR) light [5]. Because chlorophyll absorbs more red than far-red light the R : FR ratio is lowered by the presence of other plants [6]. This way the R : FR ratio, changes with alteration of proximity and number of neighbor plants [7]. Therefore, plants are able to detect future competitors through phytochrome mediated system [5, 8, 9], and to induce morphological response in order to acquire more radiant energy for photosynthesis [2, 6, 10, 11]. The expected multiple plastic response in crowded conditions ("shade avoidance syndrome") for most plant species include stem elongation, branching reduction, increased petiole length, increased chlorophyll concentration and acceleration of flowering [2, 5]. The smallest difference in elongation rate can deter-
mine whether a plant has its uppermost leaves in full sun or shaded by other plants. The elongated stems are often thin and fragile but in high densities they support each other [12]. Contrary to that in low densities an erect phenotype may be maladaptive because of greater risk of breakage [13, 14].
Plants growing under forest canopies also have the problem of reduced light availability, but in this case the optimal strategy wouldn't be the elongation but increase of the light capturing ability through larger leaves and increased chlorophyll concentration [15, 16]. It is expected that plants from woodlands should be less responsive to R : FR cue then species, populations and genotypes originated from open habitats [17-19]. Since different populations can occupy different types of environments we should expect local differentiation between them, as long as there is a genetic variation present and gene flow is restricted [20]. There are many examples of differences in plastic responses between populations [19-21].
In this work we compared samples of Lamium purpureum from Open and Shade habitats in Avala mountain with corresponding ones from Vrsac mountain exposing them to two contrasting density treatments -High and Low. Goals of this experiment were to: 1) estimate the effects of major environmental factor-planting density on measured traits in two L. purpureum populations; 2) evaluate differences among populations and habitats of L. purpureum in ecologically important traits; 3) estimate genetic differentiation among populations and habitats in their plastic responses to con-specific density.
MATERIALS AND METHODS
Study species and sites. L. purpureum is an annual or biannual member of Mint family (Lamiaceae). This species inhabits areals in Europe, Northern Africa and Asia and most often can be found in open habitats, but it populates forest edges too. For this study flowering plants were collected in two natural L. purpureum populations from Vrsac and Avala mountains during 2001 flowering season. Vrsac mountains are located at the southeastern part of Vojvodina (northern Serbian province), with the heighest point of 641 m. Avala is a mountian close to Serbian capital Belgrade, with an altitude of 511 m. At each habitat (Open and Shade) harvesting area was not less then 900 m2.
Radiation intensity in Open habitat on Avala locality was 909.46 ^mol m-2 s-1 and red/far red ratio (R/FR) was 1.11 (Estimated with LiCor 1000 Datalogger and Lil90SA and Skye SKR-110 sensors) while in Shade habitat radiation intensity was 249.57 ^mol m-2 s-1 and red/far red ratio (R/FR) was 0.85. On Vrsac locality radiation intensities were 1594.11 ^mol m-2 s-1 and 758.61 ^mol m-2 s-1, and R/FR ratios were 0.99 and 0.73 in Open and Shade habitat respectively. Differences between localities and between habitats were statistically significant at P < 0.05 level both for intensity and red/far red ratios, while significant Locality x Habitat interactions were not detected.
Experimental design. Seeds collected from harvested plants were germinated in Petri dishes on filter paper moisturized with distilled water. Petri dishes were kept at 25°C, and juvenile plants (in the two leaf stage) were transferred to containers filled with 1:1 mixture of sand and substrate (Unitas Corporation, Cacak). Each container had 57 cells (4 cm in diameter and 150 cm3). Petri dishes and containers were treated with fungicide (Previcur-N, Aventis and Chemical Agrosava, Belgrade, 1.5 ml/l).
Well established plants were subsequently planted into plastic pots (one plant per 95 mm in diameter and 380 cm3 pot) filled with 1:1 sand/substrate mixture. 21 replicas of each population/habitat combinations were placed in one of two density treatments and one of four blocks in glasshouse of Institute for Biological Research, Belgrade, during March 2002. Plants assigned to Low density treatment were arranged in 12 pots per m2 density, spaced approximately one pot diameter apart, while plants assigned to High density treatment were arranged in 50 pots per m2 density with no space between them and were additionally surrounded with border row. Pot position was assigned at random within block/treatment combinations and pots were rotated every week. Pots were regularly watered and treated with fungicide (Bayleton, Bazer, Leverkuzen, Deutchland, 1 g/1) as required. Every two weeks plants in each pot were additionally fertilized with 200 ml of Floravit solution (Credocommerce, Novi Sad, 5 ml/1, containing N 12%, P 6%, K 6% and microelements Fe, B, Cu, Mn, Zn, Co, Mo). Seeds were collected from mature fruits
two times a week starting from April 2002 untill the end of the experiment (after 13 weeks).
Data collection and analysis. Following traits were recorded after 8 weeks: plant height, leaf length, leaf width and number of leaves, internodes and shoots per plant. Total number of flowers per plant was also recorded. Plants were dried in oven (60°C) and shoot, root and seed weights were measured on Chyo JL-200 high precision weighing balance to the nearest 0.1 mg. Due to very small dimensions as well as low weight (less then 1 mg) of L. purpureum seeds, five grains mass was taken as a measure of seed weight.
Appropriate transformations were determined by the method given by Box et al. [22] and SAS programme given by Fernandez [23]. Square root was applied to plant height, shoot weigth and number of inter-nodes, shoots and flowers, cubic root to number of leaves and root weight, while leaf length, leaf width and seed weight were transformed to natural logarithms.
Each trait was analyzed by multiway type III ANOVA [24] with population, habitat, density (all fixed) and block (random) as main factors. Two way interactions between population, habitat and density were also included in the model. ANOVA was followed by Scheffe test for multiple comparisons [25]. Bonferroni correction for multiple comparisons was also applied [26]. Traits that revealed significant interaction terms were further analyzed by performing multiway ANO-VAs for each population and for each pair of population/habitat separately. All analyses were performed by SAS statistical package [27].
RESULTS
Planting density affected all analyzed traits with exception of internode number (Tables 1 and 2). Most traits had higher values in higher density, while numbers of leaves and shoots were lower in High density treatment in both analyzed p
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