Leaflet expansion rates for fifteen soybean cultivars

Abstract

Abstract In determining soybean (Glycine max (L.) Merr.) yield, the rate of leaf area expansion may be more important than the rate of apparent photosynthesis per unit leaf area. Fifteen soybean cultivars from two Maturity Groups were surveyed for the rate at which terminal leaflets of the sixth, eighth, and tenth trifoliate leaf expanded under field conditions. Leaflets lenght (LL) and width (LW) were measured every day from when the leaflets were 10–15 mm long until full expansion, a period of 11 or 12 days. Leaflet lenght expansion (LLE) and leaflet width expansion (LWE) rates were calculated from these data. After full expansion, leaflet area (LA) and specific leaf weight (SLW) were determined. Maturity Group III cultivars differed for: LA, LW, and LWE of the sixth, eighth, and tenth leaves; SLW of the sixth and eighth leaves; LLE of the eighth and tenth leaves; and LL of the tenth leaf. Maturity Group IV cultivars differed for SLW, LA, LL, LW, LLE, and LWE for all three leaves. The tenth leaf of most cultivars was smaller and expanded at a slower rate than the sixth leaf. Highly significant correlations between LA and leaflet expansion rates were found (r = 0.55 to 0.90). For the eighth and tenth leaves of Maturity Group IV, high values for SLW were associated with slow rates of leaflet expansion. Specific leaf weight of Maturity Group III was not correlated with any other leaflet characteristic. Because single leaflet expansion and total leaf area expansion rates are negatively related to SLW or apparent photosynthesis, it may be difficult to identify cultivars that combine a fast rate of leaf expansion with a fast rate of apparent photosynthesis.