Growth of C3 and C4 Perennial Grasses under Reduced Irradiance

Abstract

Several studies have compared C 3 and C 4 species for response to water and temperature regimes. Little comparative information exists, however, on growth responses of C 3 and C 4 species to irradiance regime. The objective of this study was to determine adaptive responses of C 3 and C 4 grasses to irradiance regime. Three C 3 and two C 4 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 ⊄21‐d intervals each year for 2 yr. Net leaf CO 2 exchange rate (CER) was measured for one C 3 , and one C 4 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 C 4 grasses than for C 3 grasses. Responses of CER to irradiance were greater for the C4 grass than for the C 3 grass. Morphological adaptive responses were similar for C 3 and C 4 grasses, but responses closely related to photosynthesis (e.g., CER, growth rate, and herbage yield) were affected more in C 4 than in C 3 species.