Reorientation in Tilted Stems of Papaya by Differential Growth

Abstract

Premise of research. Papaya (Carica papaya L.) is a fast-growing plant that is able to reorient quickly from a tilted position due to a local growth in the basal stem region. This seems to be a necessary adaptation in the struggle for light in the plant’s natural habitat, natural forest gaps. Interestingly, the stem anatomy of papaya consists exclusively of parenchymatous wood exhibiting substantial turgor pressure. The only reinforcing tissue is found in the bark, consisting of a lattice-like mesh of lignified fibers filled with parenchyma. This peculiar mechanism of reorientation raised questions concerning the role of turgor pressure during growth and the mechanical interactions between fiber mesh and parenchyma as a consequence of the unusual anatomy.Methodology. A set of mechanical measurements of sections of secondary phloem and xylem were conducted, comparing Young’s modulus and turgor pressure of the upper and lower sides of reerecting stems. Additionally, tissues from both sides were microscopically analyzed.Pivotal results. We found that Young’s moduli differed significantly between tissues from both sides of reerecting stems due to morphological adaptations. Experimental data showed that turgor pressure remained constant during cell expansion throughout the erecting process. Thus, cell growth and turgor pressure sustaining water inflow are in a state of equilibrium. Microscopic analyses revealed pronounced radially directed cell growth in the upper side of the xylem in comparison to the lower side.Conclusions. The stem bends upward via a coupling effect due to local secondary growth of the xylem affecting the peculiar phloem fiber arrangement. The xylem extends by directed radial growth on the upper side of leaning stems and generates an outward-pointing force. The phloem fiber mesh expands in a tangential direction. As a result, the structure contracts in an axial direction and eventually causes a pulling force on the upper side.