Ammonium concentration in solution affects chlorophyll meter readings in tomato leaves

Combinations of NH4-N:NO3-N usually result in higher tomato (Lycopersicon e sculentum Mill.) yields than when either form of nitrogen (N) was used alon e. Leaf chlorophyll content is closely related to leaf N content, but the e ffect of the NH4-N:NO3-N ratio on leaf greenness was not clear. The objecti ve of this study was to determine the influence of NH4-N:NO3-N ratios on ch lorophyll meter (SPAD) readings, and evaluate the meter as a N status estim ator arid tomato yield predictor in greenhouse production systems. Fruit yi eld and SPAD readings increased as the amount of NH4-N in solution increase d up to 25%, while higher ratios of NH4-N resulted in a decline in both. Th e N concentration in tomato leaves increased as concentration of NH4-N in s olution 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 gr owth had the highest correlation (r(2)=0.54) with N concentration in leaves , but this could not be used as a reliable estimate of N status and fruit y ield. Lack of correspondence between high N concentration values and fruit yield indicated a detrimental effect of NH4-N on chlorophyll molecules or c hloroplast structure. The SPAD readings, however, may be used to determine the optimum NH4-N concentration in solution to maximize fruit yield.