Anatomical significance of the hygrochastic movement in Anastatica hierochuntica

Background and Aims Unlike the dispersal mechanisms of many desert plants, the whole dead skeleton of Anastatica hierochuntica is involved in seed dispersal and preservation. This process depends on the hygrochastic nature of the lignified conducting tissue that bends when dry and straightens under wet conditions. An anatomical interpretation of this mechanical movement was investigated. Methods An anatomical study of the stem was conducted on the juvenile plants raised under different water treatments and on the branch-orders of adult A. hierochuntica size-classes. Key Results In the juvenile stem of A. hierochuntica, the area of cortex, conducting tissue and pith increased with water availability. However, the hydraulic conductance decreased, resulting in a better withdrawal of water in waterstressed plants. The anatomical investigation of the hygrochastic mechanism revealed an asymmetric distribution of the cortical tissues, with the conducting tissues of the stem of juvenile and adult plants being larger in the lower side. The hydraulic conductance was better in the basal and middle branch-orders than the terminal ones, permitting better conductance of water to the subsequent branch-orders. Conclusions The lignified conducting tissue of the whole stem, having a hygrochastic nature, controls the movement of the branches. The greater amount of conducting tissue associated with a higher density of wide xylem vessels was observed in the lower side of the stem as compared with the upper side. Consequently, the conducting tissue in the lower side of the stem was suggested to be more effective in the opening process of the curled dry branches through better and more rapid conductance of water. Alternatively, due to the few narrow xylem vessels in the upper side of the stem, it was likely that the conducting tissue in the upper side is more effective in the closing process by providing more rapid drying. The mechanical rise of water and the related hygrochastic efficiency were maximized in the basal and middle branch-orders that are mostly involved in the mechanical movement.