LEAF GREENNESS AND PHOTOSYNTHETIC RATES IN SOYBEAN

Leaf photosynthesis is a complex process. Determination of photosynthe tic rate is too time-consuming to be used as a field selection criteri on for cultivar development. Two field experiments were conducted in 1 994 to quantify the relationships of leaf photosynthetic rates and oth er leaf characteristics (greenness, area, specific weight) of soybean [Glycine mat CL.) Merr.] at several growth stages to identify possible indirect indicators of soybean leaf photosynthesis. Our results showe d that leaf greenness, as non-destructively measured by a hand-held po rtable chlorophyll meter (SPAD-502) was positively correlated with lea f photosynthesis at growth stages R4 to 3 to 10 d after R5 in 16 genot ypes. In one experiment involving two genotypes with similar backgroun ds, 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 experim ent with 14 genotypes developed during 50 yr of seed yield improvement in Ontario, leaf photosynthesis at R4 + 2 d and R5 + 7 d was consiste ntly correlated with SPAD-502 chlorophyll meter readings (r greater th an or equal to 0.64). Partial correlation coefficients having the same sign and similar magnitude of significance confirmed the positive cor relation between the two variables. Relationships between photosynthes is and leaf area or specific leaf weight were not significant (P > 0.0 5) or inconsistent across growth stages. These results suggest that ch lorophyll readings are quick, reliable, and repeatable indicators of l eaf photosynthetic rate that can be used for screening soybean genotyp es.