Leaf phenology mediates provenance differences in herbivore populations on valley oaks in a common garden

1. Plants from different populations often display a variation in herbivore resistance. However, it is rarely understood what plant traits mediate such differences. 2. It was tested how leaf phenology affects herbivore populations in a 15‐year‐old common garden of valley oaks (Quercus lobata Née) with different populations and maternal parents from throughout the Q. lobata range. 3. The abundance of leaf miners (Stigmella sp. Shrank) and leaf phenology of oaks in the common garden was measured. 4. Leaf miner abundance varied among provenance locations (population), but not among maternal parents within populations. Leaf phenology varied by provenance location and maternal parent, and trees that leafed out earlier accrued higher leaf‐miner abundance. Path analysis indicated that leaf phenology was the likely driver of provenance and parental differences in resistance to leaf miners. 5. Understanding population differences is particularly important when considering transport of genotypes for ornamental or restoration purposes. The present study suggests that similarity in leaf phenology may be one factor that could be used to find genotypes with a similar herbivore resistance to local genotypes.     

Density‐dependent polyphenism and geographic variation in size among two populations of lubber grasshoppers (Romalea microptera)

1. Density‐dependent phase polyphenism occurs when changes in density during the juvenile stages result in a developmental shift from one phenotype to another. Density‐dependent phase polyphenism is common among locusts (Orthoptera: Acrididae). 2. Previously, we demonstrated a longitudinal geographic cline in adult body size (western populations = small adults; eastern populations = large adults) in the eastern lubber grasshopper (Romalea microptera) in south Florida. As lubbers are confamilial with locusts, we hypothesised that the longitudinal size cline was partly due to density‐dependent phase polyphenism. 3. We tested the effect of density, population, and density×population interaction on life‐history traits (pronotum length, mass, cumulative development time, growth rate) of, and proportion surviving to, each of the five instars and the adult stage in a 2 × 3 factorial laboratory experiment with two lubber populations, each reared from hatchling to adult at three different densities. 4. The effect of density on life history and survival was independent of the effects of population on life history and survival. Higher densities led to larger adult sizes (pronotum, mass) and lower survivorship. The western population had smaller adult masses, fewer cumulative days to the adult stage, and higher survivorship than the eastern population. 5. Our data suggest that lubber grasshoppers exhibit density‐dependent phase polyphenism initiated by the physical presence of conspecifics. However, the plastic response of adult size to density observed in the laboratory is not consistent with the relationship between phenotypes and adult density in the field. Genetic differences between populations observed in the laboratory could contribute to size and life‐history differences among lubber populations in the field.     

A gnotobiotic culture system with oak microcuttings to study specific effects of mycobionts on plant morphology before, and in the early phase of, ectomycorrhiza formation by Paxillus involutus and Piloderma croceum

Homogeneously developed oak ( Quercus robur L.) microcuttings were challenged in a Petri-dish system with the mycobionts Piloderma croceum J. Erikss. & Hjortst. and Paxillus involutus (Batsch) Fr. Non-destructive observations over 10 wk followed by d. wt measurements at the end of the assays served to precisely characterize root and shoot development, dynamics of mycorrhizal colonization and morphological ratio. In the system, plant development, and especially root morphogenesis, had more similarities to those of stump cuttings or of older seedlings than to those of 3-month-old seedlings. Whereas Paxillus involutus displayed early mycorrhizal colonization and had no significant morphological effects on the host Piloderma croceum modified markedly the entire plant development before a delayed mycorrhiza formation. The latter mycobiont stimulated elongation and production of the lateral root system and also increased the leaf surface. However, no corresponding weight increases were noted, which was reflected by significant increase of both specific root length and specific leaf area. These differential effects are discussed in relation to data concerning carbon requirement and auxin production of the mycobionts. The developed system was shown to be highly suitable for comparative studies with diverse mycobionts on recognition and physiological balance between partners before, and in the early stage of, formation of mycorrhizas.