The influence of slope on Spartium junceum root system: morphological, anatomical and biomechanical adaptation.

Root systems have a pivotal role in plant anchorage and their mechanical interactions with the soil may contribute to soil reinforcement and stabilization of slide-prone slopes. In order to understand the responses of root system to mechanical stress induced by slope, samples of Spartium junceum L., growing in slope and in plane natural conditions, were compared in their morphology, biomechanical properties and anatomical features. Soils sampled in slope and plane revealed similar characteristics, with the exception of organic matter content and penetrometer resistance, both higher in slope. Slope significantly influenced root morphology and in particular the distribution of lateral roots along the soil depth. Indeed, first-order lateral roots of plants growing on slope condition showed an asymmetric distribution between up- and down-slope. Contrarily, this asymmetric distribution was not observed in plants growing in plane. The tensile strength was higher in lateral roots growing up-slope and in plane conditions than in those growing down-slope. Anatomical investigations revealed that, while roots grown up-slope had higher area covered by xylem fibers, the ratio of xylem and phloem fibers to root diameter did not differ among the three conditions, as also, no differences were found for xylem fiber cell wall thickness. Roots growing up-slope were the main contributors to anchorage properties, which included higher strength and higher number of fibers in the xylematic tissues. Results suggested that a combination of root-specific morphological, anatomical and biomechanical traits, determines anchorage functions in slope conditions.

Flower power: its association with bee power and floral functional morphology in papilionate legumes.

BACKGROUND AND AIMS: A test was made of the hypothesis that papilionate legume flowers filter pollinators according to their ability to exert strength to open flowers to access rewards. In addition, interactions with pollen vectors were expected to explain the structural complexity of the architecture of these flowers since operative flower strength may be determined by a combination of morphological traits which form part of an intrafloral functional module. METHODS: Six papilionate species were studied: Collaea argentina, Desmodium uncinatum, Galactia latisiliqua, Lathyrus odoratus, Spartium junceum and Tipuana tipu. Measurements were made of the strength needed to open keels and the strength that pollinators were capable of exerting. Morphological traits of all petals were also measured to determine which of them could be either mutually correlated or correlated with operative strength and moment of strength and participated in a functional module. KEY RESULTS: It was observed that pollinators were capable in all cases of exerting forces higher and often several times higher than that needed to access floral rewards, and no association could be detected between floral operative strength and strength exerted by the corresponding pollinators. On the other hand, strong and significant correlations were found among morphometric traits and, of these, with operative strength and moment. This was particularly evident among traits of the keel and the wings, presumably involved in the functioning of the floral moveable mechanism. CONCLUSIONS: Though visitors are often many times stronger than the operative strength of the flowers they pollinate, exceptionally weak bees such as Apis mellifera cannot open the strongest flowers. On the other hand, strong correlations among certain petal morphometric traits (particularly between the keel and wings) give support to the idea that an intrafloral module is associated with the functioning of the mechanism of these legume flowers. In addition, the highly significant correlations found across petals support the view of functional phenotypic integration transcending the ontogenetic organization of flower structure.

The response of Spartium junceum roots to slope: anchorage and gene factors.

BACKGROUND AND AIMS: Plant anchorage is governed by complex, finely regulated mechanisms that occur at a morphological, architectural and anatomical level. Spanish broom (Spartium junceum) is a woody plant frequently found on slopes--a condition that affects plant anchorage. This plant grows throughout the Mediterranean area where it plays an important role in preventing landslides. Spanish broom seedlings respond promptly to slope by altering stem and root morphology. The aim of this study was to investigate the mechanisms whereby the root system of Spanish broom seedlings adapts to ensure anchorage to the ground. METHODS: Seedlings were grown in tilted and untilted pots under controlled conditions. The root apparatus was removed at different times of growth and subjected to morphological, biomechanical and molecular analyses. KEY RESULTS: In slope-grown seedlings, changes in root system morphology, pulling strength and chemical lignin content, all features related to plant anchorage in the soil, were related to seedling age. cDNA-AFLP analysis revealed changes in the expression of several genes in root systems of slope-grown plants. BLAST analysis showed that some differentially expressed genes are homologues of genes induced by environmental stresses in other plant species, and/or are involved in the production of strengthening materials. CONCLUSION: Plants use various mechanisms/strategies to respond to slope depending on their developmental stage.