Nishanta Rajakaruna

The Edaphic Factor in the Origin of Plant Species

Although speciation has been a central focus in evolutionary biology for more than a century, there are very few case studies where we have a good understanding of the exact forces that may have acted in the diversification of a group of organisms. In order to examine such forces, botanists have often focused on closely related plants that are found under contrasting soil conditions. The study of such edaphically differentiated plants has provided valuable insight to the role of natural selection in evolution. This paper discusses several key studies that have appeared in the literature in the last half century emphasizing the role unusual soil conditions—such as those found on serpentinite outcrops, mine tailings, guano deposits, and salt flats—can play in the diversification of plant species. Many of these studies have not only shown adaptive differentiation in response to various edaphic features, but have also attempted to examine the link between adaptive traits and traits that are directly responsible for reproductive isolation between the divergent taxa. With the advent of novel genetic techniques and an increased understanding of the genetic architecture of various adaptive traits dealing with substrate tolerance, it will soon be possible to demonstrate the central role of the edaphic factor in plant evolution.

Antimicrobial Activity of Plants Collected from Serpentine Outcrops in Sri Lanka

Thirty-two plant species collected from serpentine (ultramafic) soils in Sri Lanka were screened for antimicrobial properties against three Gram-positive and two Gramnegative bacteria, a non-acid fast bacterium, and the yeast, Candida albicans. Methanol extracts of 29 species belonging to 12 families were active against at least one microorganism. Activity against the Gram-positive and non-acid fast bacteria was common, however, only two taxa, Lantana camara L. (Verbenaceae) and a species of Phyllanthus L. (Euphorbiaceae), were active against the Gram-negative bacterium Pseudomonas aeruginosa. None of the species was active against the other Gram-negative bacterium, Escherichia coli, or C. albicans. Photoactivity was observed from extracts of 10 species belonging to 10 families, including Convolvulaceae, Lamiaceae, and Rhamnaceae where photoactivity has not been previously reported. Interestingly, Leucas zeylanica (L.) R. Br. (Lamiaceae), one of only three species collected from more than one site, showed population-level variation in photoactivity. This is the first study where plants from highly stressful serpentine environments have been tested for antimicrobial activity. Our findings suggest that plants from serpentine environments may have altered antimicrobial activities when compared to their relatives from non-serpentine environments, urging the need to pay attention to substrate, habitat, etc., when collecting plants to test for antimicrobial properties.

Transfer of heavy metals through terrestrial food webs: a review

Heavy metals are released into the environment by both anthropogenic and natural sources. Highly reactive and often toxic at low concentrations, they may enter soils and groundwater, bioaccumulate in food webs, and adversely affect biota. Heavy metals also may remain in the environment for years, posing long-term risks to life well after point sources of heavy metal pollution have been removed. In this review, we compile studies of the community-level effects of heavy metal pollution, including heavy metal transfer from soils to plants, microbes, invertebrates, and to both small and large mammals (including humans). Many factors contribute to heavy metal accumulation in animals including behavior, physiology, and diet. Biotic effects of heavy metals are often quite different for essential and non-essential heavy metals, and vary depending on the specific metal involved. They also differ for adapted organisms, including metallophyte plants and heavy metal-tolerant insects, which occur in naturally high-metal habitats (such as serpentine soils) and have adaptations that allow them to tolerate exposure to relatively high concentrations of some heavy metals. Some metallophyte plants are hyperaccumulators of certain heavy metals and new technologies using them to clean metal-contaminated soil (phytoextraction) may offer economically attractive solutions to some metal pollution challenges. These new technologies provide incentive to catalog and protect the unique biodiversity of habitats that have naturally high levels of heavy metals.

Immobilization and phytotoxicity reduction of heavy metals in serpentine soil using biochar.

PurposeSerpentine soils derived from ultramafic rocks release elevated concentrations of toxic heavy metals into the environment. Hence, crop plants cultivated in or adjacent to serpentine soil may experience reduced growth due to phytotoxicity as well as accumulate toxic heavy metals in edible tissues. We investigated the potential of biochar (BC), a waste byproduct of bioenergy industry in Sri Lanka, as a soil amendment to immobilize Ni, Cr, and Mn in serpentine soil and minimize their phytotoxicity.Materials and methodsThe BC used in this study was a waste byproduct obtained from a Dendro bioenergy industry in Sri Lanka. This BC was produced by pyrolyzing Gliricidia sepium biomass at 900xa0°C in a closed reactor. A pot experiment was conducted using tomato plants (Lycopersicon esculentum L.) by adding 1, 2.5, and 5xa0% (w/w) BC applications to evaluate the bioavailability and uptake of metals in serpentine soil. Sequential extractions were utilized to evaluate the effects of BC on bioavailable concentrations of Ni, Cr, and Mn as well as different metal fractionations in BC-amended and BC-unamended soil. Postharvest soil in each pot was subjected to a microbial analysis to evaluate the total bacterial and fungal count in BC-amended and BC-unamended serpentine soil.Results and discussionTomato plants grown in 5xa0% BC-amended soil showed approximately 40-fold higher biomass than that of BC-unamended soil, whereas highly favorable microbial growth was observed in the 2.5xa0% BC-amended soil. Bioaccumulation of Cr, Ni, and Mn decreased by 93–97xa0% in tomato plants grown in 5xa0% BC-amended soil compared to the BC-unamended soil. Sequentially extracted metals in the exchangeable fraction revealed that the bioavailabile concentrations of Cr, Ni, and Mn decreased by 99, 61, and 42xa0%, respectively, in the 5xa0% BC-amended soil.ConclusionsResults suggested that the addition of BC to serpentine soil as a soil amendment immobilizes Cr, Ni, and Mn in serpentine soil and reduces metal-induced toxicities in tomato plants.

The edaphic factor and patterns of variation in Lasthenia californica (Asteraceae)

Transectional studies of Lasthenia californica in the Jasper Ridge Biological Preserve (Stanford University) have documented the existence of two races (A and C) based upon flavonoid chemistry, achene morphology, allozymes, and flowering time differences. The two races coexist on a serpentine outcrop and have maintained a sharply defined pattern of distribution for a period of at least 15 yr. The present study has revealed significant differences in the physical and chemical features of the soils harboring the two races. Soils at the lower ends of the transects, where race A plants grow, have higher pH, cation exchange capacity, relative water content, total ionic strength, percentage clay, and sodium and magnesium concentrations than do soils harboring race C plants at the upper ends of the transects. Soils supporting race C plants have higher calcium, potassium, and nickel concentrations and higher calcium:magnesium ratios. Plant tissue concentrations of ions were also significantly different in the two races. Race A plants accumulated sodium to concentrations three times those observed with race C plants. Plants from an additional 22 sites gave very similar results. Greenhouse studies indicated that the two races from Jasper Ridge show differential responses to ridge-top and ridge-bottom soils. Race A achenes germinated, grew to maturity, and set seed about equally in the two soils. Race C achenes germinated in both types of soils but showed significantly poorer growth and absolutely no flowering when found in the soils of race A plants. Differential responses to edaphic conditions on the ridge may contribute to the pattern of distribution observed over the years. It is suggested that race A plants are more tolerant of edaphic stress than race C plants and that physiological specialization may contribute to the present distribution of the two races throughout the species range. It is not yet possible to state which is the more significant factor in driving this specialization, the chemistry of the soil or its physical characteristics, or whether there is interaction between the two. This is the first study to present evidence for soil/plant variation within a serpentine site. The linking of sodium levels to racial differentiation within the serpentine habitat is also a new discovery.

Edaphic races and phylogenetic taxa in the Lasthenia californica complex (Asteraceae: Heliantheae): an hypothesis of parallel evolution

Lasthenia californica sensu Ornduff consists of two races that differ in their flavonoid pigments and edaphic tolerances. Recent phylogenetic studies of Lasthenia have revealed that members of L. californica sensu Ornduff belong to two phylogenetic species. The relationship of the edaphic races to these new species and to each other is the focus of this study. Characterization of flavonoid profiles and phylogenetic placement of 33 populations demonstrates that races and phylogenetic taxa are not concordant, suggesting that one or both edaphic races evolved in parallel in the two clades. We hypothesize an edaphically linked ecological role for flavonoid differences that first revealed the existence of two races.

Serpentine Geoecology of Eastern North America: A Review

Abstract Serpentine outcrops are model habitats for geoecological studies. While much attention has been paid to serpentine outcrops worldwide, the literature on eastern North American serpentine and associated biota is scant. This review examines the available literature, published and unpublished, on geoecological studies conducted on serpentine in eastern North America, from Newfoundland through Québec and New England south to Alabama. Most serpentine outcrops in the region have been mapped, but there have been few intensive mineralogical and pedological investigations. The limited soil analyses available suggest elevated levels of heavy metals such as Ni, near-neutral pH values, and Ca∶Mg ratios < 1, characteristic of serpentine soils worldwide. Botanical studies to date have largely focused on floristic surveys and the influence of fire exclusion and grazing on indigenous vegetation. To date, 751 taxa of vascular plants belonging to 92 families have been reported from serpentine outcrops in the region. Two taxa, Agalinis acuta and Schwalbea americana, are federally endangered in the United States while many others are listed as rare, endangered, or imperiled in one or more states or provinces. Globally, six species, Adiantum viridimontanum, Minuartia marcescens, Pycnanthemum torrei, S. americana, Scirpus longii, and Symphyotrichum depauperatum are listed as imperiled (G2) while one species, Agalinis acuta, is listed as critically imperiled (G1). Cerastium velutinum var. villosissimum is the only recognized serpentine endemic plant for eastern North America while Adiantum viridimontanum, Aspidotis densa, M. marcescens, and S. depauperatum are largely restricted to the substrate. Based on current distributions, we propose that A. viridimontanum and M. marcescens be considered endemic to serpentine substrates in eastern North America. Studies on cryptogams list 165 species of lichens and 146 species of bryophytes for the region. None of the species found appear to be restricted to the substrate. Compared to other regions of the world, ecophysiological and evolutionary investigations are scant. Biosystematic investigations are restricted to the taxa Adiantum aleuticum, C. velutinum var. villosissimum, and S. depauperatum. Plant-soil relations, especially the capacity to hyperaccumulate metals such as Ni and the ecological consequences of metal accumulation, are also under explored. One report from eastern Canada lists Arenaria humifusa, M. marcescens, Packera paupercula, and Solidago hispida as hyperaccumulating Ni although the findings have yet to be confirmed by subsequent investigations. Overall, serpentine geoecology in eastern North America remains largely unexplored.

Adaptive Differentiation in Response to Water Stress by Edaphic Races of Lasthenia californica (Asteraceae)

Two edaphic races of Lasthenia californica sensu Ornduff (races A and C) grow in parapatry on a serpentine outcrop at Jasper Ridge Biological Preserve, California. The races occupy distinct edaphic habitats that have different water‐holding capacities. We predict that the two races will show differentiation in reproductive strategies related to their response to water stress. In order to test this hypothesis, we performed a greenhouse experiment to characterize the reaction norms of the two races exposed to a gradient in water availability. We measured the response of five variables to the watering treatments: early survivorship, days to flowering, root/shoot dry mass ratio, total dry mass, and a measure of reproductive fitness, number of flower heads. We found that the races differ in their allocation patterns to roots compared with shoots and in days to flowering, indicating genetic differentiation for these traits. Race A consistently allocates relatively more biomass to roots while race C flowers earlier. However, the reaction norms of the two races for all nonreproductive traits are parallel, indicating that races do not differ in their plastic response to drought stress. The number of flower heads, our measure of reproductive fitness, did, however, exhibit differential response to water availability between the two races. Under low watering treatment, race C plants are able to maintain flower head production, while race A plants show a monotonic decrease in head production as water stress increases. Results indicate that race C plants are better adapted to drought; they are able to maintain a high reproductive output under low water availability. However, as the phenotype of race A is affected by drought, reproductive output decreases, as we would predict for plants that rarely experience drought in their natural environment.

Metal release from serpentine soils in Sri Lanka

Ultramafic rocks and their related soils (i.e., serpentine soils) are non-anthropogenic sources of metal contamination. Elevated concentrations of metals released from these soils into the surrounding areas and groundwater have ecological-, agricultural-, and human health-related consequences. Here we report the geochemistry of four different serpentine soil localities in Sri Lanka by coupling interpretations garnered from physicochemical properties and chemical extractions. Both Ni and Mn demonstrate appreciable release in water from the Ussangoda soils compared to the other three localities, with Ni and Mn metal release increasing with increasing ionic strengths at all sites. Sequential extraction experiments, utilized to identify “elemental pools,” indicate that Mn is mainly associated with oxides/(oxy)hydroxides, whereas Ni and Cr are bound in silicates and spinels. Nickel was the most bioavailable metal compared to Mn and Cr in all four soils, with the highest value observed in the Ussangoda soil at 168u2009±u20096.40xa0mgxa0kg−1 via the 0.01-M CaCl2 extraction. Although Mn is dominantly bound in oxides/(oxy)hydroxides, Mn is widely dispersed with concentrations reaching as high as 391xa0mgxa0kg−1 (Yudhaganawa) in the organic fraction and 49xa0mgxa0kg−1 (Ussangoda) in the exchangeable fraction. Despite Cr being primarily retained in the residual fraction, the second largest pool of Cr was in the organic matter fraction (693xa0mgxa0kg−1 in the Yudhaganawa soil). Overall, our results support that serpentine soils in Sri Lanka offer a highly labile source of metals to the critical zone.

Edaphic adaptation maintains the coexistence of two cryptic species on serpentine soils

PREMISE OF THE STUDYnDivergent edaphic adaptation can contribute to reproductive isolation and coexistence between closely related species, yet we know little about how small-scale continuous edaphic gradients contribute to this phenomenon. We investigated edaphic adaptation between two cryptic species of California wildflower, Lasthenia californica and L. gracilis (Asteraceae), which grow in close parapatry on serpentine soil.nnnMETHODSnWe reciprocally transplanted both species into the center of each species habitat and the transition zone between species. We quantified multiple components of fitness and used aster models to predict fitness based on environmental variables. We sampled soil across the ridge throughout the growing season to document edaphic changes through time. We sampled naturally germinating seedlings to determine whether there was dispersal into the adjacent habitat and to help pinpoint the timing of any selection against migrants.nnnKEY RESULTSnWe documented within-serpentine adaptation contributing to habitat isolation between close relatives. Both species were adapted to the edaphic conditions in their native region and suffered fitness trade-offs when moved outside that region. However, observed fitness values did not perfectly match those predicted by edaphic variables alone, indicating that other factors, such as competition, also contributed to plant fitness. Soil water content and concentrations of calcium, magnesium, sodium, and potassium were likely drivers of differential fitness. Plants either had limited dispersal ability or migrants experienced early-season mortality outside their native region.nnnCONCLUSIONSnDemonstrating that continuous habitats can support differently adapted, yet closely related, taxa is important to a broader understanding of how species are generated and maintained in nature.