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 (Pn), stomatal conductance (gs), 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 Pn 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 F3 lines, Pn at 14 d after R5 in 2005 varied from 22.1 to 34.2 μmol CO2 m-2 s-1 and was more strongly and significantly correlated with gs (P < 0.01) than with LMA (P < 0.05) and LNC (P = 0.11). In 2006, Pn of eight selected F4 genotypes and the parents varied from 27.9 to 35.0 μmol CO2 m-2 s-1. Leaf gas exchange activity of the selected lines was represented by two attributes; the maximum of gs (gmax), and functional leaf lifespan (LL). Values of gmax 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 gmax 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.