Comparative Growth Analyses of Panicum Species with Differing Rates of Photorespiration

Panicum milioides, a naturally occurring species with reduced photorespiration, P. bisulcatum, a C(3) species, and P. miliaceum, a C(4) species, were grown for 4 weeks at altered pO(2) and pCO(2) and several vegetative growth parameters were determined at weekly intervals. Compared to a pO(2) of 10%, a greater O(2) inhibition of the relative growth rate and dry matter production was observed for P. bisulcatum than for P. milioides at both 21% and 40% O(2), whereas little effect of O(2) was noted for P. miliaceum. Similarly, exposures to elevated pCO(2) of 500 and 1000 mu1 CO(2)/liter resulted in a greater stimulation of vegetative growth for P. bisulcatum than for P. milioides, with little effect on P. miliaceum. The CO(2) compensation concentration of P. milioides was less than that of P. bisulcatum over a pO(2) range of 5 to 40%. At 5% O(2), the compensation concentration was relatively O(2)-insensitive, whereas above 5% it increased with increasing pO(2). It is concluded that P. milioides represents the first well documented example of a C(3) plant with reduced photorespiration, based on both leaf CO(2) exchange parameters and growth analyses of dry matter production.