Leaf Gas Exchange Activity in Soybean as Related to Leaf Traits and Stem Growth Habit

Understanding the relationships between leaf photosynthetic activity and physiological and morphological traits of soybeans [ Glycine max . (L.) Merr.] with different stem growth habits could be exploited for yield improvement. Leaf photosynthetic rate (P n ), stomatal conductance (g s ), leaf mass per unit area (LMA), and leaf nitrogen content per unit area (LNC) were measured for field‐grown progeny of a cross between an indeterminate type, ‘Stressland’, and a determinate type, ‘Tachinagaha’, soybean. The P n of the uppermost fully expanded leaves of Stressland was 19% greater than Tachinagaha during the first 20 d after seed filling (growth stage R5) in 2005. Among 18 F 3 lines, P n at 14 d after R5 in 2005 varied from 22.1 to 34.2 μmol CO 2 m −2 s −1 and was more strongly and significantly correlated with g s ( P < 0.01) than with LMA ( P < 0.05) and LNC ( P = 0.11). In 2006, P n of eight selected F 4 genotypes and the parents varied from 27.9 to 35.0 μmol CO 2 m −2 s −1 Leaf gas exchange activity of the selected lines was represented by two attributes; the maximum of g s (g max ), and functional leaf lifespan (LL). Values of g max were positively correlated with stomatal density ( r = 0.91, P < 0.01), and LL was positively correlated with LMA ( r = 0.89, P < 0.01). Some indeterminate types had very high g max but exhibited shorter LL. Genetic improvement of maximum leaf photosynthesis appears possible by increasing stomatal density, and the Dt1 locus for stem habit has a positive effect on soybean leaf photosynthesis, both in its maximum and duration.