ABSTRACT
Lianas are woody climbers and their stems and leaves deal with different environmental pressures such as resistance to mechanical damage and dehydration. The damage resistance of plants can be biomechanically evaluated by their stiffness, bending and toughness. Despite the well-known relationship between physical resistance and moisture of plant organs in woody plants, this relationship is uncertain and has not been previously evaluated in lianas. Thus, this study investigated experimentally the effect of stems and leaf dehydration on the structural Young's modulus in the stem and fracture toughness in leaves across time in the liana Amphilophium crucigerum (Bignoniaceae). Ten stem and leaf samples were collected and assigned to two distinct conditions: (i) samples kept moist and (ii) samples underwent gradual dehydration with natural moisture loss by air exposition. Successive measures of structural Young's modulus and fracture toughness were taken every 4 hours during a 48-hour period for both conditions. Stem and leaf samples which underwent gradual dehydration showed greater bending stiffness and fracture toughness, respectively, while the samples kept moist presented no changes in any studied biomechanical features during the entire experiment. We concluded that the moisture of both stem and leaf samples are critical factors to estimate the biomechanical properties of lianas stem and leaves.
Lianas são trepadeiras lenhosas e seus caules e folhas lidam com diferentes pressões ambientais, como a resistência aos danos mecânicos e de desidratação. A resistência ao dano das plantas pode ser biomecanicamente avaliada pelas propiedades de dureza, flexão e tenacidade. Apesar da conhecida relação entre resistência física e umidade dos órgãos das plantas em espécies lenhosas, essa relação não foi avaliada anteriormente e é incerta em lianas. Assim, este estudo investigou experimentalmente o efeito da desidratação de caules e folhas na estimativa do módulo estrutural de Young do caule e da tenacidade à fratura da folha ao longo do tempo, na liana Amphilophium crucigerum (Bignoniaceae). Dez amostras de caules e folhas foram coletadas e distribuídas em duas condições distintas: (i) amostras mantidas úmidas e (ii) amostras em processo de desidratação gradativa com perda natural de umidade quando expostas ao ar. Medidas sucessivas do módulo de Young e da resistência à fratura dos órgãos foram tomadas a cada 4 horas durante um período de 48 horas em ambas as condições. Amostras de caule e folhas que sofreram desidratação gradual apresentaram maior rigidez à flexão e tenacidade à fratura, respectivamente, enquanto as amostras mantidas úmidas não alteraram essas características durante o experimento. Concluímos que a umidade das amostras de caules e folhas em lianas também é um fator crítico para estimar as propriedades biomecânicas desses órgãos em seu ambiente natural.
ABSTRACT
Background and Aims Roots are key in the evolution of plants, being in charge of critical functions, such as water and nutrient uptake and anchorage of the plant body. Stems of lianescent Sapindaceae conform to the anatomical patterns typical of climbing plants, having cambial variants in their stems and vessel dimorphism in their wood. The roots of these lianas, however, are largely unexplored, so we do not know whether the plant habit has as strong an impact on their anatomy as on the anatomy of their stems. Our aim was, therefore, to thoroughly explore the anatomy of liana roots, underground organs under selective pressure completely different from that experienced by the stems. Methods We studied mature roots of 14 species belonging to five of the six genera currently recognized in the lianoid tribe Paullinieae (Sapindaceae) using traditional methods for macro- and microscopic analyses, as well as micro-computed tomography (micro-CT) techniques. Key Results Roots were shown to be strongly shaped by the lianescent habit in Paullinieae, exhibiting traits of the lianescent vascular syndrome in terms of both wood and overall anatomy. The only way to distinguish root from stem in secondary growth is by the exarch protoxylem position in the roots, as opposed to the endarch position typical of the stems. The most conspicuous trait of the lianescent vascular syndrome, which is the presence of vessel dimorphism, is evident in all roots, and we hypothesize that it helps to create an efficient, safe pathway for water conduction from this organ towards the stems. Other anatomical features present were parenchyma bands, present in the wood of almost all of the analysed species, except for Thinouia and Urvillea, where parenchyma-like fibre bands alternating with ordinary fibres are present. The majority of the roots showed no cambial variants. However, lobed roots were found in Urvillea rufescens and phloem wedges were observed in Serjania lethalis and Serjania caracasana. Neo-formed peripheral vascular strands and cylinders were common in mature roots of Serjania caracasana, and vascular connections were found uniting the peripheral and central vascular cylinders through phloem wedges, as revealed by anatomical and micro-CT analyses. The vascular connections likely represent another key mechanism to create a network that increases the area of vascular tissue and contributes as an additional conduction pathway within these thick roots. Conclusions Some traits from the lianescent vascular syndrome, such as vessel dimorphism, are present in the roots of lianescent Sapindaceae, while others, such as cambial variants common in the stems, are largely absent.