Effects of Irradiance on Relative Growth Rates, Net Assimilation Rates, and Leaf Area Partitioning in Cotton and Three Associated Weeds

Cotton (Gossypium hirsutum L. var. ;Stoneville 213'), velvetleaf (Abutilon theophrasti Medic.), redroot pigweed (Amaranthus retroflexus L.), and hemp sesbania (Sesbania exaltata [Raf.] Cory) were grown in a controlled environment room at 31/25 C day/night temperature and three irradiances: 90, 320, and 750 mueinsteins meter(-2) second(-1). From total dry weights and leaf areas determined at intervals during the first exponential phase of growth, we used mathematical growth analysis techniques to calculate net assimilation rates (NAR), relative growth rates (R(w)), relative leaf area expansion rates (R(a)), leaf area partition coefficients (LAP), and leaf area ratios (LAR). In all four species, R(w), R(a), and NAR decreased with decreasing growth irradiance, while LAP and LAR increased. Within each species, R(w) was positively correlated with NAR but negatively correlated with LAP and LAR. In comparisons among the four species within each growth irradiance, R(w) was positively correlated with LAP. We discuss the relationship between LAP and LAR and show that LAP = (R(a)/R(w)) (LAR).