Consequences of phenotypic plasticity vs. interspecific differences in leaf and root traits for acquisition of aboveground and belowground resources

Trade-offs between acquisition capacities for aboveground and belowground r esources were investigated by studying the phenotypic plasticity of Leaf an d root traits in response to different irradiance levels at low nutrient su pply. Two congeneric grasses with contrasting Light requirements, Dactylis glomerata and D. polygama. were used. The aim was to analyze phenotypic cov ariation in components of Leaf area and root length in response to above- a nd belowground resource limitation and the consequences of this variation f or resource acquisition and plant growth. At intermediate shading (30 and 2 0% of full sunlight) the plants were able to maintain their total root leng th, despite a strongly increased total leaf area and a reduced biomass allo cation to roots. This was associated with an unaltered or slightly increase d nutrient uptake and growth. At 5.5% relative irradiance, growth was sever ely reduced, especially in the shade-tolerant D. polygama. The results show that constraints on acquisition capacities for aboveground and belowground resources. caused by biomass allocation, may be alleviated by plasticity i n other traits such as tissue-mass density and thickness of roots and leave s. The results also suggest different adaptive constraints for phenotypic p lasticity and for genetically determined interspecific variation. Phenotypi c plasticity tends to maximize resource acquisition and growth rate in the short term, whereas the higher tissue-mass density and the longer leaf life -span of shade-tolerant species indicate reduced loss rates as a more advan tageous species-specific adaptation to shade in the long term.