Different Responses of Various Chlorophyll Meters to Increasing Nitrogen Supply in Sweet Pepper.
Intensive vegetable production is commonly associated with excessive nitrogen (N) fertilization and associated environmental problems. Monitoring of crop N status can enhance crop N management. Chlorophyll meters (CMs) could be used to monitor crop N status because leaf chlorophyll (chl) content is strongly related to crop N status. To monitor crop N status, relationships between CM measurements and leaf chl content require evaluation, particularly when excessive N is supplied. The SPAD-502 meter, atLEAF+ sensor, MC-100 Chlorophyll Concentration Meter, and Multiplex sensor were evaluated in sweet pepper with different N supply, throughout the crop, ranging from very deficient to very excessive. CM measurements of all sensors and indices were strongly and positively related to leaf chlorophyll a + b content with curvilinear relationships over the entire range of chl measured (approximately 0–80 μg cm-2). Measurements with the SPAD-502, and atLEAF+, and of the Multiplex’s Simple Fluorescence Ratio index (SFR) had asymptotic responses to increasing leaf chl. In contrast, the MC-100’s Chlorophyll Content Index (CCI) had a progressively increasing response. At higher chlorophyll a + b contents (e.g. >40 μg cm-2), SPAD-502, atLEAF+ and SFR measurements tended to saturate, which did not occur with CCI. Leaf chl content was most accurately estimated by CCI (R2=0.87), followed by the SPAD-502 meter (R2=0.85). The atLEAF+ sensor was the least accurate (R2=0.76). For leaf chl estimation, CCI measured with the MC-100 meter was the most effective of the four sensors examined because it: (1) most accurately estimated leaf chl content, and (2) had no saturation response at higher leaf chl content. For non-saturating leaf chl content (approximately 0–40 μg cm-2), all indices were sensitive indicators. As excessive applications of N are frequent in intensive vegetable crop production, the capacity of measuring high leaf chl contents without a saturation response is an important consideration for the practical use of chlorophyll meters.