Photosynthesis, Water Relations, Temperature and Canopy Structure as Factors Influencing the Growth of Sainfoin (Onobrychis viciifolia Scop.) and Lucerne (Medicago sativa L.)

Abstract

Measurements of the growth of sainfoin and lucerne were made in the field after cutting on 31 May 1977. Sainfoin reached a total above-ground dry weight of 408 g m⁻² over the growing period of 48 days compared with 598 g m⁻² in lucerne. Final leaf area indices (LAIs) were 2.8 in sainfoin and 6.1 in lucerne. The specific leaf areas (SLAs) for sainfoin were approximately half those of lucerne throughout the regrowth period. The maximum rates of leaf appearance were 0.12 leaves per day in sainfoin and 0.85 leaves per day in lucerne. The maximum mean rate of plant extension growth for lucerne of 2.12 mm h⁻¹ occurred during the night, whereas, in sainfoin the maximum rate of 1.72 mm h⁻¹ occurred during the day. Measurements of extinction coefficients for PAR ranged from 0.45 to 0.89 in sainfoin and from 0 42 to 0.57 in lucerne. As the lucerne crop increased in size leaf water potentials and solute potentials became more negative. In sainfoin leaf water potentials remained remarkably high throughout the growth period, solute potentials decreased and turgor potentials increased. The stomatal conductances of the two species were similar. The photosynthetic capacities and rates of dark respiration per unit leaf area in both species were similar. The rate of canopy ‘gross’ photosynthesis at 295 W m⁻² was always greater in lucerne than in sainfoin. This was largely a matter of differences between the species in LAI, although at higher LAIs the more erect structure of lucerne leads to a better utilization of photosynthetically active radiation.