Hydraulic failure due to xylem embolism is a key factor contributing to drought-induced mortality in trees. Studying Pinus canariensis inhabiting an archipelago where migration is limited, López et al. examine traits related to hydraulics and growth in populations under contrasting environments, and measure genetic variability. They find that the ability of P. canariensis to inhabit a wide range of ecosystems seems to be associated with high phenotypic plasticity and some degree of local adaptations of xylem and leaf traits. They infer that divergent selection must have acted in the past on xylem vulnerability to cavitation more strongly than on other traits sensitive to water deficit, such as growth or hydraulic efficiency.
Timing of flowering is an adaptive trait regulated by environmental cues and has been intensively studied in annual plants, but in perennials it is currently not well characterized. Quilot-Turion et al. apply cold treatments to two locally adapted populations of the perennial Arabidopsis lyrata from central Europe and Scandinavia and study responses to vernalization. They find that the population from central Europe flowers more rapidly and invests more in inflorescence growth than the Scandinavian population, which has longer leaves and higher leaf rosettes. QTL mapping in the F2 population reveals genomic regions governing these traits and some QTL × vernalization interactions. The results indicate that the two populations have diverged in their plastic responses to vernalization in traits closely connected to fitness through changes in many genomic areas.
Studies of the chemical interactions between plants and insects have been particularly useful in showing the breadth of traits that may be involved in the evolution of interactions. Many of the thousands of chemicals produced by plants are thought to have evolved as defences against insects, microbes, or a combination of these enemies. These compounds can act as defences, repellents or attractants as plants evolve suites of chemicals that attract mutualistic taxa such as pollinators while also repelling others. Whether as defences or attractants, the chemical compounds affecting interactions with other species usually occur as chemical cocktails derived from multiple chemical pathways rather than as isolated chemicals. Proliferation of these compounds has contributed to specialization and diversification on both sides of these interactions.
The interactions between plants and pollinating floral parasites provide a particularly intriguing problem for the evolution of chemical traits in plants. These insects lay their eggs in the same flowers they pollinate, which means that their relationship to the plants has both antagonistic and mutualistic components. Close interactions with pollinating floral parasites have been suggested to favour the evolution of a highly specific plant–insect communication system, in the form of unique compounds that represent ‘private channels’ for exclusive attraction of specific pollinators. If so, then plant species pollinated by a single or a few obligate pollinators should include unique signal compounds or blends that diverge strongly from those produced by plants in more generalized pollination systems and, potentially, also from closely related plant species involved in specific interactions with other species of pollinating floral parasites. So far, however, there is limited evidence for the use of private channels in plants attracting these kinds of pollinator. For example, in the plant genus Yucca, which is involved in an obligate interaction with yucca moths, the three allopatric species analysed thus far show very similar floral scent profiles. Pollinating yucca moths are attracted to yucca floral scent, but the specific role of the novel floral volatiles in pollinator attraction awaits further examination.
Alternatively, growing evidence suggests that flower-pollinator specificity might be accomplished by way of specific blends of otherwise generic floral volatiles. Among the species that have been studied within Glochidion, Breynia and Ficus (for which bioassays have demonstrated olfactory attraction of pollinators), most show strong biosynthetic conservatism among related species, and emit floral scent cocktails composed of common floral compounds. In these cases, floral scent, combined with physical barriers (e.g. size-limiting ostioles in figs), limited visual display and finely tuned temporal dynamics of scent emission may constitute multi-modal ‘floral filters’ whose net result is pollinator-specificity. In other cases, differences in the relative contributions of the different compounds between close relatives, or potential hidden chiral variation in certain compounds, are sufficient to allow the pollinators to discriminate among these species in experimental trials.
A new paper in Annals of Botany describes striking floral scent variation within and between plant lineages, and determines whether these differences are genetically or environmentally based, then investigates the extent to which floral scent is temporally variable at the single flower level and also at the whole plant level as the plant ages, and tests whether visits by the pollinating floral parasite Greya politella alters floral scent production.
Extreme divergence in floral scent among woodland star species (Lithophragma spp.) pollinated by floral parasites. (2013) Annals of Botany Volume 111(4): 539-550. doi: 10.1093/aob/mct007
A current challenge in coevolutionary biology is to understand how suites of traits vary as coevolving lineages diverge. Floral scent is often a complex, variable trait that attracts a suite of generalized pollinators, but may be highly specific in plants specialized on attracting coevolved pollinating floral parasites. In this study, floral scent variation was investigated in four species of woodland stars (Lithophragma spp.) that share the same major pollinator (the moth Greya politella, a floral parasite). Three specific hypotheses were tested: (1) sharing the same specific major pollinator favours conservation of floral scent among close relatives; (2) selection favours ‘private channels’ of rare compounds particularly aimed at the specialist pollinator; or (3) selection from rare, less-specialized co-pollinators mitigates the conservation of floral scent and occurrence of private channels. Dynamic headspace sampling and solid-phase microextraction were applied to greenhouse-grown plants from a common garden as well as to field samples from natural populations in a series of experiments aiming to disentangle the genetic and environmental basis of floral scent variation. Striking floral scent divergence was discovered among species. Only one of 69 compounds was shared among all four species. Scent variation was largely genetically based, because it was consistent across field and greenhouse treatments, and was not affected by visits from the pollinating floral parasite. The strong divergence in floral scents among Lithophragma species contrasts with the pattern of conserved floral scent composition found in other plant genera involved in mutualisms with pollinating floral parasites. Unlike some of these other obligate pollination mutualisms, Lithophragma plants in some populations are occasionally visited by generalist pollinators from other insect taxa. This additional complexity may contribute to the diversification in floral scent found among the Lithophragma species pollinated by Greya moths.
Growing in competition with close relatives may produce modifications in competitive traits to ameliorate competition with kin. Milla et al. study multi-trait phenotypic expression in response to competition with conspecifics of varied degrees of genealogical relatedness in an annual legume, Lupinus angustifolius. In contrast to reports on other species, they find that relatedness to competing neighbours has a negligible impact on the phenotypic expression of individuals and groups of L. angustifolius, calling into question the generality of kin recognition across plant lineages.
Seeds from different populations of the same species often show distinct germination responses to the smoke-derived chemical karrikinolide (KAR1), thus limiting its potential for use in weed management. Gorecki et al. study the effect of the environment during seed development (i.e. the parental envioronment) on the KAR1-response of seeds from different populations of Brassica tournefortii and find that variation in response is reduced when parent plants are grown under the same conditions. Seeds from plants grown under drought stress are more responsive to KAR1 than seeds from well-watered plants. The parental environment is thus one of the key drivers of the KAR1-responses of seeds.
Functional plasticity in a plant’s hydraulic architecture may constitute a mechanistic basis for anticipating the differential success of plant species in response to climate variability. von Arx et al. compare several lifetime functional traits in the root xylem of the long-lived perennial herb Potentilla diversifolia (Rosaceae) growing in a 14-year water supplement experiment, and find that plants receiving additional water show a shift towards wider vessels, greater hydraulic conductivity, reductions in starch storage capacity and increased specific leaf area. These phenotypic changes represent consistent, dynamic responses to increased water availability that should increase plant competitive ability.
Suaeda maritima shows morphologically different forms on high and low areas of the same salt marsh. Wetson et al. demonstrate that roots of this halophyte have a constitutively very high activity of lactate dehydrogenase (LDH) regardless of whether they are growing in aerated or severely hypoxic conditions, and not the inducible increase in activity that has been demonstrated in other plants during hypoxia. This high LDH activity is likely to be a factor in the high phenotypic plasticity observed in reciprocal transplants between high- and low-marsh field sites and in simulated tidal-flow tanks in a glasshouse.
Population differentiation in a variable environment is related to the selection pressures that plants experience. Ward et al. compare differences in growth- and defence-related traits in two isolated populations of Acacia raddiana trees from sites at either end of an extreme environmental gradient in the Negev desert. They find no evidence of trade-offs between physical and chemical defences and plant growth parameters; rather, there appears to be positive correlations between plant size and defence parameters. The great variation in several traits in both populations may result in a diverse potential for responding to selection pressures in different environments.
Studies have suggested that plant sexual reproduction is particularly vulnerable to climate change, and a number of ecologically and evolutionarily relevant genes have recently been identified. Shimizu et al. consider that studying gene functions in naturally fluctuating conditions is very important in order to predict responses to changing environments. For example, modelling has shown that FLC in Arabidopsis halleri acts as a quantitative tracer of the temperature over the preceding 6 weeks, and recent studies of SCR in A. thaliana have identified gene functions in natura that are also unlikely to be found in laboratory experiments.
The successful spread of invasive plants is often linked to a diverse gene pool that facilitates local adaptation to variable environmental conditions: but what about clonal plants? Riis et al. study three non-native, clonally reproducing macrophytes, Egeria densa, Elodea canadensis and Lagarosiphon major, and find that field populations display remarkably little genetic variation and show little interaction between habitat conditions and plant morphological characteristics. The results strongly suggest that invasive clonal aquatic plants adapt to a wide range of habitats in introduced areas by phenotypic plasticity rather than local adaptation.