Leaf Greenness and Photosynthetic Rates in Soybean

Leaf photosynthesis is a complex process. Determination of photosynthetic rate is too time‐consuming to be used as a field selection criterion for cultivar development. Two field experiments were conducted in 1994 to quantify the relationships of leaf photosynthetic rates and other leaf characteristics (greenness, area, specific weight) of soybean [Glycine max (L.) Merr.] at several growth stages to identify possible indirect indicators of soybean leaf photosynthesis. Our results showed that leaf greenness, as non‐destructively measured by a handheld portable chlorophyll meter (SPAD‐502) was positively correlated with leaf photosynthesis at growth stages R4 to 3 to 10 d after R5 in 16 genotypes. In one experiment involving two genotypes with similar backgrounds, the correlation coefficient was 0.77 at R4, 0.91 at R5 + 5 d, and non‐significant (0.22) at R6 + 10 d, respectively. In a second experiment with 14 genotypes developed during 50 yr of seed yield improvement in Ontario, leaf photosynthesis at R4 + 2 d and R5 + 7 d was consistently correlated with SPAD‐502 chlorophyll meter readings (r ≥ 0.64). Partial correlation coefficients having the same sign and similar magnitude of significance confirmed the positive correlation between the two variables. Relationships between photosynthesis and leaf area or specific leaf weight were not significant (P >0.05) or inconsistent across growth stages. These results suggest that chlorophyll readings are quick, reliable, and repeatable indicators of leaf photosynthetic rate that can be used for screening soybean genotypes.