SHADE EFFECTS ON GROWTH OF TALL FESCUE .1. LEAF ANATOMY AND DRY-MATTER PARTITIONING

Irradiance influences plant growth through photosynthetic activity and developmental responses. Our objectives were to determine the effect of irradiance on leaf anatomy and dry matter partitioning of tall fescue (Festuca arundinacea Schreb.). Plants were grown outdoors for 10 wk during summer 1986 and autumn 1987 with or without screencloth to give irradiances of 30% (low), 60% (medium), or full (high) sun. Data were similar for both years, although daily air temperatures averaged 25-degrees-C in 1986 and 12-degrees-C in 1987. Production of total plant dry matter was reduced at low irradiance, primarily due to fewer tillers per plant. Shoot/root ratio and leaf area ratio (leaf area per plant dry weight) were higher for plants grown at low irradiance than for those grown at high irradiance. Recently developed leaf blades on plants at low irradiance were 54 or 65% longer and had 56 or 77% more leaf area, but were 12% thinner and had 18 or 25% lower specific leaf weight than did those grown at high irradiance. Blade width was not affected. Total stomatal density at low irradiance was 17 or 24% lower than at high irradiance, with a larger reduction on the abaxial than on the adaxial epidermis. Proportional cross-sectional areas of epidermis and bulliform cells, vascular bundle and fiber cap cells, and mesophyll cells were similar over treatments. Blades grown at low irradiance had almost-equal-to 25% more air space than did those at high irradiance. At low irradiance, partitioning of dry matter among plant parts and developmental processes within the leaf blade were shifted to favor production of leaf blade area per unit of dry matter with no influence on forage quality.