Tomato leaf chlorophyll meter readings as affected by variety, nitrogen form, and nighttime nutrient solution strength

Leaf chlorophyll content is closely related to leaf nitrogen (N) content, so it is reasonable to assume that ammonium-N (NH4-N): nitrate-N (NO3-N) ratio in the nutrient solution used to grow tomatoes (Lycopersicon esculentum Mill.) hydroponically may affect leaf greenness, and consequently chlorophyll meter (SPAD) readings. It has also been shown that increasing nutrient solution strength (NSS) increases tomato productivity, but there are no reports regarding how NSS affects SPAD readings under greenhouse conditions. Genotype may also influence SPAD readings, and standardization for cultivar and sampling time may be needed. The objective of this study was to characterize SPAD readings for five tomato cultivars and SPAD reading response to a combination of two NSS (1X and 4X Steiner solution strength daily applied 18 days after transplanting at 7 p.m.) and two concentration of NH4-N in solution (0 and 25%) in order to evaluate the potential of SPAD readings as a tomato yield predictor in greenhouse production systems. The SPAD readings were not uniform across tomato varieties tested, being consistently higher for 'Max' and lower for the other varieties. Initially, SPAD readings for tomato varieties used in this study were low at the vegetative stage, and increased up to 40 DAT, but subsequently decreased at 49 DAT, or the fruit set of the first and second clusters. After this time, SPAD readings showed no variation. Chlorophyll meter readings for 'Max' were higher in the top plant layers, but decreased in the top plant layer of the other tomato varieties. The SPAD readings were higher for plants supplied with 25% NH4-N than those without NH4-N in solution, but the use of a nighttime nutrient solution did not affect SPAD readings. None of the possible interactions among tomato variety, NH4-N:NO3-N ratio, and NSS were consistently significant.