Responses of Trifolium repens L. root structure and function to shading and phosphorus shortage: limits to adaptative plasticity during establishment?

Abstract

Shading and low soil phosphorus availability may limit root growth of forage species in humid-temperate silvopastoral systems. However, plants are able to cope with such constraints by modifying root structure to improve the establishment and survival. The aim of this work was to evaluate the plasticity of different types of roots of Trifolium repens L. and its functional impact in the first two years of the species. A pot trial designed in 3 randomized complete block was carried out in sub-subdivided plots: main plot was the shading treatment (4 levels: full sun = 0% and 30%, 60% and 90% of photosynthetically active radiation (PAR) reduction), sub-plots were 2 cultivars of large-leafed (cv. Junín and cv. El Lucero) and sub-sub-plots were 2 phosphorous conditions (without P− and with added P+). Whole plants were harvested and the root system was divided into seminal taproot and fibrous roots: coarse roots (1 to ≤ 2 mm of diameter) and fine roots (≤ 1 mm of diameter). Even under 60% of shading, both cultivars were able to maintain root soil penetration and water and nutrients acquisition, regardless of the level of P. These functions were associated with the length and taproot diameter, specific taproot length and fine root biomass during establishment. Instead 90% of shading was a clear limit to plasticity and survival of the plants during the second year. Under mean total daily PAR radiation > 15.3 mol m−2 day−1 and P shortage, it is expected that the enrichment with large-leafed cultivars could be successful in these systems.