Exotic invasive herbivores can overwhelm poorly defended native hosts, and result in reduced growth and survival. Severe damage may also alter the biomechanics of the attacked plant. Soltis et al. study the impact of feeding by hemlock woolly adelgid, Adelges tsugae, on the biomechanics of eastern hemlock trees, Tsuga canadensis, and find evidence of weakness and brittleness in attacked twigs and needles. Changes in resource allocation may contribute to these mechanical effects, and can increase plant susceptibility to subsequent mechanical stresses, such as wind or snow load. The interaction between herbivory and physical environmental stresses is probably accelerating the decline of eastern hemlock in North America.
There is considerable evidence for the presence of positive species diversity–productivity relationships in plant populations, but the parameters determining the type and strength of the relationship are poorly defined. Collet et al. study a tree plantation that mixes beech (Fagus sylvatica) and sycamore (Acer pseudoplatanus) according to a double gradient of density and species’ proportion, and find that density and tree size are the primary factors determining individual growth and stand productivity. Mixtures of these two functionally similar species have highest production at maximum evenness, indicating a complementary effect between them. The presence of a mixture combines both stabilizing mechanisms (individuals from both species show higher growth when surrounded by individuals from the other species) and equalizing mechanisms (the two species have very similar growth curves) that, in turn, determine the species’ relative dominance.
Efforts to cryopreserve germplasm of recalcitrant-seeded species are hampered by potentially lethal intracellular freezing events. Wesley-Smith et al. study embryonic axes of silver maple, Acer saccharinum, subjected to various drying, cooling and warming treatments and find that intracellular ice formation is not necessarily lethal. In fully hydrated axes cooled at an intermediate rate, the interiors of many organelles are apparently ice-free and this may prevent the disruption of vital intracellular machinery. The findings challenge the accepted paradigm that intracellular ice formation is always lethal, as the results show that cells can survive intracellular ice if crystals are small and localized in the cytoplasm.
Brachypodium distachyon is considered a powerful model system to study the response of temperate cereals to adverse environmental conditions. Colton-Gagnon et al. examine cold acclimation and freezing tolerance in seven diploid accessions, and find that cold treatment accelerates the transition from the vegetative to the reproductive phase in all of them. This is associated with the gradual accumulation of BradiVRN1 transcripts, and the accessions exhibit a clear cold acclimation response by progressively accumulating proline, sugars and COR gene transcripts. However, whole-plant freezing tests show that the accessions only have a limited capacity to develop freezing tolerance when compared to winter varieties of temperate cereals such as wheat and barley. Furthermore, little difference in terms of survival is observed among the accessions tested despite their previous classification as either spring or winter genotypes.
Analysis of cellular patterns in plant organs provides information about the orientation of cell divisions and predominant growth directions. Raczyńska-Szajgin and Nakielski study patterns in the epidermis of asymmetrical wild-type dorsal petals and symmetrical dorsalized petals of the backpetals mutant of Antirrhinum majus (snapdragon) to determine how growth in initially symmetrical petal primordia leads to the development of mature petals differing in their symmetry. They find that during primordia development a characteristic fountain-like cellular pattern is maintained with only slight modifications, and petal cells divide in non-random directions. These features of the cellular pattern are presumably related to principal directions of growth. Two scenarios are considered to explain how gradual modifications in these directions may contribute to the transition from a symmetric to an asymmetric cellular pattern in the wild type petal.
Shoot characteristics differ depending on the meristem tissue that they originate from and the environmental conditions during their development. Negrón et al. observe and model the effects of plant water status on axillary meristem fate and flowering patterns along proleptic and epicormic shoots of almond trees, Prunus dulcis. They find that the two shoot types differ in their patterns of axillary meristem fates along the shoot, and in their axillary meristem fate responses to water stress. The structure of proleptic shoots is more sensitive to water stress than epicormic shoots and reflects differences in their ontogenetic status as well as growth rate patterns during the season.
Clonal spread influences genetic structure and diversity in plant populations as well as their realized outcrossing rate. Using microsatellite markers, Somme et al. investigate genetic diversity and the effect of clone distribution, structure and size on the mating of bee-pollinated marsh cinquefoil, Comarum palustre (Rosaceae), which is a rare, self-compatible species that grows in endangered European wetlands.
They find that clones are spatially clumped, with intermediate to no intermingling of the ramets, and large clones show lower outcrossing rates than small clones. Pollen dispersal mainly occurs within patches with very few pollination events occurring between patches of more than 25 m separation.These factors need to be taken into account in management strategies for ensuring population persistence.
Heterodichogamy, a dimorphism in which two morphs coexist in a population and undergo synchronous reciprocal sex changes, is an extremely rare and poorly understood sexual system. Fukuhara and Tokumaru study Platycarya strobilacea (Juglandaceae) and demonstrate that it is heterodichogamous based on observations of inflorescence architecture, sexual expression and pollination biology. Pollination by thrips is suggested by their frequent presence with attached pollen grains, the scarcity of other insect visitors, the synchronicity of thrips number in male spikes with the maturation of female flowers, and the morphological characters shared with previously reported thrips-pollinated plants.
Many aquatic species with stylar polymorphisms have the capacity for clonal and sexual reproduction. Haddadchi et al. study differences between a monomorphic population of Nymphoides montana and polymorphic populations. They find that very few seeds are produced in the monomorphic population due to dysfunctional pollen and ovules, and that stigma–anther separation is minimal. ISSR results show that the monomorphic population is one large, single-ramet genotype, unlike the multi-genotypic fertile polymorphic populations. Evolutionary loss of sex in a clonal population in which a mating morph is absent is evident, and under these conditions clonal growth may assure reproduction and expand the population via spreading stolons.
Traits affecting the form and function of fine roots in woody plants show complex phenotypic variation. Lee et al. manipulate root segments of 2-year-old Acer rubrum and Quercus rubra seedlings in order to compare functional traits and trait plasticities in fine root tissues with natural and reduced levels of colonization by microbial symbionts. They find negligible plasticity for root diameter, branching intensity and nitrogen concentration across both species between levels of colonization. Roots with reduced colonization have decreased tissue density and increased specific root length, but species differences are significant and greater than treatment effects in traits other than tissue density. If common, such a result would greatly simplify and strengthen ecosystem- and community-level investigations that require information about the costs and benefits of constructing and maintaining fine root tissues.