Functional traits related to seedling performance in the Mediterranean leguminous shrub Retama sphaerocarpa: Insights from a provenance, fertilization, and rhizobial inoculation study

We studied functional traits related to survival and growth in seedlings of the Mediterranean leguminous shrub Retama sphaerocarpa (L.) Boiss. Plants from five provenances were grown at two contrasting fertilization rates. In one of the provenances, low-fertilized plants were also inoculated with a Bradyrhizobium strain to assess the influence of nodulation on seedling performance. Seedlings were transplanted onto an abandoned cropland and their growth and survival rates were measured for two years. Additionally, the persistence of the inoculated strain in the field was tracked using genetic methods. Fertilization produced two phenotypes of contrasting performance. High fertilization produced large plants with high nutrient concentration, photosynthetic rate, and root growth capacity. Plants with this phenotype had higher transplanting survival, growth, and water use efficiency than the plants of the low-fertilized phenotype, which were small and had low nutrient concentration, photosynthetic rate and root growth capacity. Provenances differed in relative growth rate (RGR) under optimal growing conditions and these differences were negatively related to the length of the growing season and positively related to the precipitation at the places of origin of seeds. Across provenances, transplanting survival and growth was positively related to the shoot carotenoid concentration. However, this relationship was only observed among low-fertilized seedlings. Among low-fertilized plants, nodulation did not increase either transplanting survival or growth significantly. The Bradyrhizobium strain used to inoculate seedlings survived at least two years in transplanted inoculated plants in spite of the presence of other native rhizobial strains in the field. In conclusion, high transplanting performance of R. sphaerocarpa seedlings is linked to a suite of attributes that promote fast seedling establishment during the wet season, which probably enhances drought avoidance during the dry season and helps avoid photoinhibition during the summer drought.