Ammonium concentration in solution affects chlorophyll meter readings in tomato leaves

Abstract

Combinations of NH4‐N:NO3‐N usually result in higher tomato (Lycopersicon esculentum Mill.) yields than when either form of nitrogen (N) was used alone. Leaf chlorophyll content is closely related to leaf N content, but the effect of the NH4‐N:NO3‐N ratio on leaf greenness was not clear. The objective of this study was to determine the influence of NH4‐N:NO3‐N ratios on chlorophyll meter (SPAD) readings, and evaluate the meter as a N status estimator and tomato yield predictor in greenhouse production systems. Fruit yield and SPAD readings increased as the amount of NH4‐N in solution increased up to 25%, while higher ratios of NH4‐N resulted in a decline in both. The N concentration in tomato leaves increased as concentration of NH4‐N in solution increased. Fruit yield increased as chlorophyll readings increased. SPAD readings, total N in leaves, fresh weight of shoots, and fruit yield all showed a quadratic response to NH4‐N, reaching a peak at 25 or 50% of N as NH4‐N. SPAD readings taken at the vegetative and flowering stages of growth had the highest correlation (r2=0.54) with N concentration in leaves, but this could not be used as a reliable estimate of N status and fruit yield. Lack of correspondence between high N concentration values and fruit yield indicated a detrimental effect of NH4‐N on chlorophyll molecules or chloroplast structure. The SPAD readings, however, may be used to determine the optimum NH4‐N concentration in solution to maximize fruit yield.