Divergent selection for mesophyll cell size (mean cross-sectional area) was made within a population of ‘Veya’ perennial ryegrass. Selection for small mesophyll cells generally resulted in smaller mesophyll cells, heavier seed, and greater yield of shoot dry matter than selection for large cells. Between the third and fifth leaf stage, families from parents with small cells exhibited greater net assimilation rate and lower leaf area ratio than those from parents with large cells. Rate of light-saturated photosynthesis of young leaves was negatively related to cell size in the F1 generation.
Greater shoot dry matter in the small, compared with the large, cell selections was accounted for in terms of greater initial seed reserves (heavier seed) and also greater net assimilation rate at some stage prior to full expansion of the third leaf. It was concluded that mesophyll cell size is a useful criterion for selecting for light-saturated photosynthesis in Lolium perenne.