Photosynthesis,water use and growth of a C4 grass stand at high CO2 concentration

Leaf photosynthesis rate of the C₄ species Paspalum plicatulum Michx was virtually CO₂-saturated at normal atmospheric CO₂ concentration but transpiration decreased as CO₂ was increased above normal concentrations thereby increasing transpiration efficiency. To test whether this leaf response led growth to be CO₂-sensitive when water supply was restricted, plants were grown in sealed pots of soil as miniature swards. Water was supplied either daily to maintain a constant water table, or at three growth restricting levels on a 5-day drying cycle. Plants were either in a cabinet with normal air (340 mol (CO₂) mol^-1 (air)) or with 250 mol mol^-1 enrichment. Harvesting was by several cycles of defoliation.With abundant water supply high CO₂ concentration did not cause increased growth, but it did not cause an increase in growth over a wide range of growth-limiting water supplies either. Only when water supply was less than 30–50% of the amount used by the stand with a water-table was there evidence that dry weight growth was enhanced by high CO₂. In addition, with successive regrowth, the enhancing effect under a regime of minimal water allocations, became attenuated. Examination of leaf gas exchange, growth and water use data showed that in the long term stomatal conductance responses were of little significance in matching plant water use to low water allocation; regulation of leaf area was the mechanism through which consumption matched supply. Since high CO₂ effects operate principally via stomatal conductance in C₄ species, we postulate that for this species higher CO₂ concentrations expected globally in future will not have much effect on long term growth.     

Past and present vegetation in relation to solifluction on Ben Arkle,Sutherland

Background: Forest succession in tropical pastures usually starts from woody vegetation patches. Patches may arise within the grass matrix at microsites with favourable soil conditions or through facilitation by established nurse plants.

Aims: We report the formation of woody vegetation patches in tropical pastures after investigating whether patch formation was associated with micro-scale terrain features and whether facilitation was important for patch initiation.

Methods: The study was conducted in three pasture sites in the Atlantic forest domain of Brazil. We compared soil, terrain and species abundance patterns among pairs of woody patch and open pasture plots.

Results: The effect of variation in soil physical and chemical attributes was limited. Some species were able to establish in the grass matrix and survive disturbance from grazing and fire, while other species only established in patches, under other already established trees or shrubs. Some of these species were exotics, which are commonly eliminated in restoration efforts.

Conclusions: Allowing the establishment of species capable of withstanding pasture environments, including exotics, can accelerate succession. Furthermore, the abilities to endure competition from grasses and survive fire are key features of species suitable for the initial stages of forest restoration in tropical pastures.     

不同钙浓度对宽叶雀稗幼苗的生长和抗性生理的影响

研究不同钙浓度对宽叶雀稗(Paspalum wettsteinii)幼苗生长和生理的影响, 对于揭示宽叶雀稗对不同钙浓度环境的适应机理至关重要。该研究采用盆栽砂培试验, 研究不同钙浓度(0、5、25、50、100和200 mmol·L^-1 CaCl₂)和不同处理时间(7、14、21和28天)对宽叶雀稗幼苗生长、渗透调节物质含量、抗氧化酶活性、叶绿素含量和光合参数的影响。结果表明, 随着CaCl₂浓度的增加和处理时间的延长, 宽叶雀稗幼苗株高等形态指标、生物量、渗透调节物质含量、抗氧化酶活性、叶绿素含量和光合参数呈先增后减的趋势, 低钙浓度(5-50 mmol·L^-1)环境下, 株高、叶长、叶宽、根长和生物量与对照(0 mmol·L^-1)相比均升高, 脯氨酸、可溶性蛋白和可溶性糖含量、过氧化物酶、过氧化氢酶和超氧化物歧化酶活性提高, 丙二醛含量和胞间CO₂浓度降低、叶绿素含量增加以及净光合速率、蒸腾速率和气孔导度增强; 高钙浓度(200 mmol·L^-1)环境下, 脯氨酸、可溶性蛋白和可溶性糖含量、过氧化物酶、过氧化氢酶和超氧化物歧化酶活性降低, 丙二醛含量和胞间CO₂浓度增加, 叶绿素含量减少以及净光合速率、蒸腾速率和气孔导度减弱。结合隶属函数分析, 低钙盐浓度(5-50 mmol·L^-1)处理对宽叶雀稗幼苗无抑制作用, 说明宽叶雀稗对低钙浓度具有一定的耐受性; 而在高钙浓度(200 mmol·L^-1)下, 宽叶雀稗幼苗通过提高自身有机渗透调节物质含量、增强酶活性、增加叶绿素含量以及增强光合作用等方式来快速调节植物生理代谢功能, 进而适应高钙浓度环境条件。     

雀稗属花粉形态及其多萌发孔现象

在广东广泛收集禾本科雀稗属的基础上,观察了雀稗属（Paspalum)9个种及其类群的花粉形态。其花粉一般为球型,平均直径在34－50um之间,花粉表面纹饰一般为疣状突起或皱波状。在四倍体的台湾雀稗（P.formosanum),双穗雀稗（P.paspaloides),雀稗（P.thunbergii),两耳草（P.conjugatum),圆果雀稗（P.orbiculare),长叶雀稗（P.longifolium)和一个十倍体的南雀稗（P.commersonii)的一个类群上发现了禾本科罕见的花粉多萌发孔现象。     

Post‐dispersal granivory in a tall‐tussock grassland: A positive feedback mechanism of dominance?

Questions: Is post‐dispersal seed predation a factor contributing to a positive feedback mechanism for dominance of tall‐tussock grasslands? Do seeds dispersed into neighbouring microhabitats of contrasting dominance differ in their probabilities of being predated? Does predation rate vary among predator groups? Do seed eaters selectively forage among the available seeds? Location: The southern and flooding sub‐regions of the Argentinian Pampas. Methods: We examined seed predation by vertebrate and invertebrate predators within two microhabitats of grassland mosaics (highly dominated tall‐grass patches vs. scarcely dominated short‐grass matrix) for different seed species in semi‐natural grasslands. Proportion of seeds eaten by different predator groups was estimated through exclusion experiments and analysed using ANOVA for split‐split‐plot designs. Experiments were performed during the autumn of two consecutive years in both Pampa sub‐regions. Results: Removal of seeds after a five day trial was two to four times higher in the tall‐grass patches than in the short‐grass matrix. During the same period, vertebrate predation was six times higher than invertebrate predation in the tall‐grass phase of the Southern Pampa, but it did not differ in the short‐grass matrix. Relative predation among phases showed the same pattern in the Hooding Pampa, where preferences by seed species also varied according to phase. Conclusions: The highest predation intensity shown by vertebrates in the tall‐grass patches indicates that they are the main seed predators in these systems, possibly because this microhabitat grants them refuge against carnivorous predators. This could significantly reduce the available seeds for recruitment of subordinate species after different disturbance events (fires, trampling by large herbivores, burrowing), representing an active filter to the floristic composition of the patch and contributing with other mechanisms to the dominance of tall‐grass species.     

A genetic map of tetraploid <Emphasis Type="Italic">Paspalum notatum</Emphasis> Flügge (bahiagrass) based on single-dose molecular markers

Paspalum notatum Flügge is a warm-season forage grass with mainly diploid (2n = 20) and autotetraploid (2n = 40) representatives. Diploid races reproduce sexually and require crosspollination due to a self-incompatible mating system, while autotetraploids reproduce by aposporous apomixis. The objectives of this work were to develop a genetic linkage map of Paspalum notatum Flügge at the tetraploid level, identify the linkage/s group/s associated with apomixis and carry out a general characterization of its mode of inheritance. A pseudo test-cross F₁ family of 113 individuals segregating for the mode of reproduction was obtained by crossing a synthetic completely sexual tetraploid plant (Q4188) as female parent with a natural aposporous individual (Q4117) as pollen donor. Map construction was based on single-dose markers (SDAFs) segregating from both parents. Two linkage maps (female and male) were constructed. Within each map, homologous groups were assembled by detecting repulsion-phase linked SDAFs. Putative Q4188 and Q4117 homolog groups were identified by mapping shared single dose markers (BSDF). The Q4188 map consisted of 263 markers distributed on 26 co-segregation groups over a total genetic distance of 1.590.6 cM, while the Q4117 map contained 216 loci dispersed on 39 co-segregation groups along 2.265.7 cM, giving an estimated genome coverage of 88% and 83%, respectively. Seven and 12 putative homologous chromosomes were detected within Q4188 and Q4117 maps, respectively. Afterward, ten female and male homologous chromosomes were identified by mapping BSDFs. In the Q4117 map, a single linkage group was associated with apospory. It was characterized by restriction in recombination and preferential chromosome pairing. A BPSD marker mapping within this group allowed the detection of the female homolog and the putative four male groups of the set carrying apospory.     

Localization of the 5S and 45S rDNA sites and cpDNA sequence analysis in species of the Quadrifaria group of Paspalum (Poaceae, Paniceae)

BACKGROUND AND AIMS: The Quadrifaria group of Paspalum (Poaceae, Paniceae) comprises species native to the subtropical and temperate regions of South America. The purpose of this research was to characterize the I genomes in five species of this group and to establish phylogenetic relationships among them. METHODS: Prometaphase chromatin condensation patterns, the physical location of 5S and 45S rDNA sites by fluorescence in situ hybridization (FISH), and sequences of five chloroplast non-coding regions were analysed. KEY RESULTS: The condensation patterns observed were highly conserved among diploid and tetraploid accessions studied and not influenced by the dyes used or by the FISH procedure, allowing the identification of almost all the chromosome pairs that carried the rDNA signals. The FISH analysis of 5S rDNA sites showed the same localization and a correspondence between the number of sites and ploidy level. In contrast, the distribution of 45S rDNA sites was variable. Two general patterns were observed with respect to the location of the 45S rDNA. The species and cytotypes Paspalum haumanii 2x, P. intermedium 2x, P. quadrifarium 4x and P. exaltatum 4x showed proximal sites on chromosome 8 and two to four distal sites in other chromosomes, while P. quarinii 4x and P. quadrifarium 2x showed only distal sites located on a variable number of small chromosomes and on the long arm of chromosome 1. The single most-parsimonious tree found from the phylogenetic analysis showed the Quadrifaria species partitioned in two clades, one of them includes P. haumanii 2x and P. intermedium 2x together with P. quadrifarium 4x and P. exaltatum 4x, while the other contains P. quadrifarium 2x and P. quarinii 4x. CONCLUSIONS: The subdivision found with FISH is consistent with the clades recovered with cpDNA data and both analyses suggest that the Quadrifaria group, as presently defined, is not monophyletic and its species belong in at least two clades.     

Genetic and embryological evidences of apomixis at the diploid level in <Emphasis Type="Italic">Paspalum rufum</Emphasis> support recurrent auto-polyploidization in the species

Gametophytic apomixis is an asexual mode of reproduction by seeds. This trait is present in several plant families and is strongly associated with polyploidy. Paspalum rufum is a forage grass with sexual self-incompatible diploids (2n = 2x = 20) and aposporous-apomictic pseudogamous tetraploids (2n = 4x = 40). In previous work embryological observations of the diploid genotype Q3754 showed 8.8–26.8% of the ovaries having one meiotic plus an aposporous-like embryo sac, suggesting some capability for apomictic reproduction. The objective of this work was to characterize progenies derived from Q3754 to determine if aposporous sacs were functional and generated progenies via apomixis at the diploid level. Re-examination of Q3754 ovaries showed that 12.5% of them contained one sexual plus an aposporous sac confirming previous results. Progeny tests were carried out on two experimental families (H₁ and S₁) employing heterozygous RAPD marker loci. Family H₁ was obtained crossing Q3754 with a natural diploid genotype (Q3861) and S₁ derived from the induced self-pollination of Q3754. Genetic analysis of H₁ showed that all individuals derived from sexual reproduction. However, 5 out of 95 plants from S₁ showed the same heterozygous state as the mother plant for 14 RAPD loci suggesting a clonal origin. Further experiments, designed to test the functionality of aposporous sacs by flow cytometric analyses, were carried out on a third family (M₁) obtained by crossing Q3754 with the tetraploid plant Q3785. Histograms of 20 M₁ plants showed 15 diploids (75%), 4 triploids (20%) and 1 tetraploid (5%). Triploids and the tetraploid may have originated from functional aposporous embryo sacs. Likewise, the reconstruction of the developmental route of 40 individual seeds demonstrated that 11 of them (27.5%) derived from fertilized aposporic sacs. The results presented in this work indicate that gametophytic apomixis is effectively expressed at the diploid level in Paspalum rufum and could be the foundation of a recurrent auto-polyploidization process in the species.