Comparative digestibility and anatomy of some sympatric C3 and C4 arid zone grasses

The importance of C, (panicoid) and C3 (festucoid) leaf anatomy to differences in in vitro dry matter digestibility (DMD) of grasses was assessed using two Australian summer-growing arid zone C4 grasses, Cymbopogon obtectus and Digitaria brownii, and two C3 grasses, Thyridolepis mitcheiliana and Monochather paradoxa. Seed of these sympatric species was collected from a 400 mZ area near Charleville, Qld. Thus the usual problem of confounding C4 and C3 type with species of different environmental adaptation was avoided in this study. Proportions of tissue types in leaf cross sections and DMD were measured for plants grown in a glasshouse under summer (k water stress) or winter ( - stress) conditions. Differences in tissue proportions due to C, and C3 leaf anatomy were greater than those induced by growth conditions. C4 leaves consistently had less mesophyll and more of the less-digestible epidermis, bundle sheath, sclerenchyma and vascular tissues. Water-stress resulted in leaves with an increased proportion of mesophyll, but differences in tissue proportions between summer- and winter-grown leaves were small. Species were divided into fast (D. brownii and 7, mitchelliana) and slow (C. obtectus and M. paradoxa) leaf development groups for comparison of DMD. Within each group the C3 species consistently had the highest leaf DMD, but across groups the difference of 18-27 days in time to develop a new leaf sometimes overrode the advantage to DMD of the Cj anatomy. Stem DMD did not differ consistently between C4 and C3 species. The DMD was highest for water-stressed material, and higher in winter- than summer-grown plants. The anatomy associated with either C4 tropical or C3 temperate grass genera clearly contributes to difference in DMD between leaves, but variation in DMD associated with leaf age or environment was only in part attributable to differences in tissue proporxions.