Trade-offs between vegetative growth and acorn production in Quercus lobata during a mast year: the relevance of crop size and hierarchical level within the canopy

The concept of trade-offs between reproduction and other fitness traits is a fundamental principle of life history theory. For many plant species, the cost of sexual reproduction affects vegetative growth in years of high seed production through the allocation of resources to reproduction at different hierarchical levels of canopy organization. We have examined these tradeoffs at the shoot and branch level in an endemic California oak, Quercus lobata, during a mast year. To determine whether acorn production caused a reduction in vegetative growth, we studied trees that were high and low acorn producers, respectively. We observed that in both low and high acorn producers, shoots without acorns located adjacent to reproductive shoots showed reduced vegetative growth but that reduced branch-level growth on acorn-bearing branches occurred only in low acorn producers. The availability of local resources, measured as previous year growth, was the main factor determining acorn biomass. These findings show that the costs of reproduction varied among hierarchical levels, suggesting some degree of physiological autonomy of shoots in terms of acorn production. Costs also differed among trees with different acorn crops, suggesting that trees with large acorn crops had more available resources to allocate for growth and acorn production and to compensate for immediate local costs of seed production. These findings provide new insight into the proximate mechanisms for mast-seeding as a reproductive strategy

A method for screening soybean seedlings for resistance to northern stem canker caused by Diaporthe phaseolorum var. caulivora

Five soybean varieties, Williams, Harosoy, AC Hime, Coles and Leo were inoculated 2 and 4 wk after planting with Diaporthe phaseolorum var. caulivora to determine if differences in resistance to stem canker could be detected in the early stages of plant development. Seedling stems were severed above the unifoliolate leaves and inoculum was applied to the cut surface. Stem lesion lengths were measured at 1, 2 and 3 wk following inoculation. The mean lesion length on seedlings inoculated 2 wk after planting (27.2 mm) was significantly less than seedlings inoculated 4 wk after planting (44.5 mm). Based on mean lesion length, Williams was the most susceptible variety followed by Harosoy. AC Hime, Coles and Leo were equally resistant. Stem canker development measured at 2 and 3 wk following inoculation was negatively correlated with stem moisture at the time of inoculation suggesting that stem moisture is important in the development and severity of stem canker