GROWTH OF C3 AND C4 PERENNIAL GRASSES UNDER REDUCED IRRADIANCE

Several studies have compared C3 and C4 species for response to water and temperature regimes. Little comparative information exists, however, on growth responses of C3 and C4 species to irradiance regime. The objective of this study was to determine adaptive responses of C3 and C4 grasses to irradiance regime. Three C3 and two C4 perennial forage grasses were field-established near Ames, IA, and grown under 37, 70, and 100% of full sunlight by use of polypropylene shade cloths. Morphology and growth measurements were conducted three times at almost-equal-to 21-d intervals each year for 2 yr. Net leaf CO, exchange rate (CER) was measured for one C3 and one C4 grass. Specific leaf weight increased and leaf-area ratio decreased with increasing irradiance similarly in all five species. Responses of herbage yield, shoot dry weight, and crop growth rate to irradiance were two to three times greater for C4 grasses than for C3 grasses. Responses of CER to irradiance were greater for the C4 grass than for the C3 grass. Morphological adaptive responses were similar for C3 and C4 grasses, but responses closely related to photosynthesis (e.g., CER, growth rate, and herbage yield) were affected more in C4 than in C3 species.