Starch and nitrogen status in soybean during shading and nutrient deficiency

Abstract

Plant mineral requirements are based on nutrient prediction models, but plant energy is seldom taken into consideration. Starch, a potential indicator of plant energy reserve, was assessed by first investigating the association between leaf starch and nitrogen (N) during and after shading, and secondly by quantifying leaf phosphorus (P) and potassium (K) content to the starch‐N relationship. Soybeans [Glycine max (L.) Merr] were field‐grown in 1992, 1993 and 1994 at Fayetteville, Arkansas, and shaded during vegetative and reproductive growth (V7 to R4). The 1994 shading study also incorporated a treatment of K and P fertilizer. Starch and N levels (% leaf DW) were predicted from calibration models based on spectra produced by near infra‐red spectroscopy. Starch measured at 1,800 h was reduced by shading, declined after flowering, and was more strongly associated with radiation and leaf N as growth stage R4 was reached. The range of starch concentration in leaf tissue was from zero to 18% leaf DW, and nitrogen varied from 3.5 to 5.5% leaf DW. Starch and N levels were inversely related for leaves located at or near the top of the plant canopy. From covariant analysis, we did not find the starch‐N relationship to change with differing K content of leaves from the 1994 study. Leaf samples with low and high P content showed differences in the slopes of the starch‐N curves due to the mild nutrient stress. The starch‐N relationship may be useful in identifying plants affected by P deficiency.