THE ECOPHYSIOLOGY OF VERONICA-CHAMAEDRYS, V-MONTANA AND V-OFFICINALIS .4. EFFECTS OF SHADING ON NUTRIENT ALLOCATIONS - A FIELD EXPERIMENT

1. Veronica chamaedrys, V. montana and V. officinalis were grown outside, in a loam/grit soil, for 18 months either in full light or subject to canopy shading. V. officinalis failed to grow in shade. Vegetative and reproductive organs were analysed for nitrogen, phosphorus, potassium, calcium, magnesium and ash. The proportion of the total amount of a particular element in a plant that is allocated to a specific organ can be expressed as a quotient of the proportion of total plant biomass (i.e. carbon) allocated to that organ. The allocation index (AI) was calculated for each element and each organ. 2. Shading increased nitrogen and potassium concentrations in all organs, which is suggested to be symptomatic of an imbalance between nitrogen metabolism and carbon fixation. Leaf nitrogen AIs, however, were lower in shaded plants, indicating that a constrained photosynthetic activity reduces the sink strength of leaves for nitrogen. 3. Whilst phosphorus increased in leaves of shaded V. montana, it increased in stems, pedicels and peduncles of V. chamaedrys. These particular organs of V. chamaedrys exhibit a pronounced elongation response in shade. 4. Shading increased leaf magnesium concentrations and Als. Species means for both these quantities are inversely proportional to the availability of light in natural habitats occupied by each, which is ranked V. officinalis > V. chamaedrys > V. montana. 5. The percentage of ash in seeds was ranked V. officinalis > V. chamaedrys > V. montana: a particular balance of mineral nutrients and energy equips each species with proportionately more of the resources that are most likely to limit seedling establishment. Shading decreased the percentage of ash in V. chamaedrys seeds: seeds produced, and probably destined to germinate, in low light have a higher ratio of energy to mineral nutrients than seeds produced in full light. The allocation of ash to seeds was not plastic in V. montana. 6. Of the three species, roots of V. officinalis contained the highest concentrations of phosphorus and ash (total minerals), and had higher root AIs for all elements but nitrogen. The mineral nutrient characteristics of both established plants and seeds of V. officinalis are consistent with its autecology: the acquisition of nutrients and adequate provision of nutrients to seeds are vital in dry, infertile soils. 7. Genotypic differences elucidated by this experiment, such as leaf magnesium concentration, show a gradation between the three species that accords with their individual autecologies and morphological responses to changes in irradiance.