Xylem conductivities in grasses

Abstract

Grasses are a main source of carbohydrates for the human populations worldwide. Grasses grow in a variety of environments, from wet rice paddies in Asia to deserts with limited water. Research on water distribution characteristics of grass culms to leaves is important because grasses occur in many habitats and circumstances. The purpose of this study is to understand the characteristics of xylem cells (water-transporting cells), xylem cells within vascular bundles, and xylem conductivity. Xylem conductivity is used to estimate the ability of cells to transport water from culms (stems) to leaf lamina. Data from 12 species of C3 grasses, species mostly confined to temperate climates, and 13 species of C4 grass, species found in tropical climates, were obtained with standard histological procedures. C4 grass culms had twice as many vascular bundles as C3 grasses. Xylem conductivities in culms were two to three times greater for C4 than C3 grasses. Xylem conductivities in leaves were five to six times greater for C4 than C3 grasses. Xylem conductivities of culms to leaf lamina of C4 and C3 grasses species were compared. Leaf conductivities per bundle, relative to culm conductivities per bundle, were almost five times greater for C4 than for C3 grasses. In a similar manner, xylem conductivities in entire leaves, relative to xylem conductivities of entire culms, were two times greater for C4 than for C3 grasses. Overall, the data show that xylem conductivities were higher in both leaves and culms of C4 grasses compared with C3 grasses, and that the contribution of xylem conductivities from culms to leaves were greater for C4 than for C3 grasses. The higher conductivities in C4 grasses was attributed to higher numbers of bundles per culm/ per leaf and not to vessel radii or numbers of vessels per bundle.