Influence of time of day on measurement with chlorophyll meters and canopy reflectance sensors of different crop N status

Abstract

Optical sensors are a promising approach for assessing nitrogen (N) status of vegetable crops. However, their potential may be undermined if time of day influences measurements. This study evaluated the effects of time of day and N addition on measurements, made with two chlorophyll meters, SPAD-502 and MC-100, and two active canopy reflectance sensors, GreenSeeker handheld and Crop Circle ACS-470. Three treatments (N1, deficient, N2, conventional, and N3, excessive N application) of N concentration in the nutrient solution were applied by fertigation throughout a sweet pepper crop grown in soil in a greenhouse. Time of day of 12:00 and 15:00 h had an effect on measurements made with the SPAD-502, but only in the N1 treatment, suggesting that the effects of time of day were related to crop N status. This effect was slight, being 1.7 ± 0.02 SPAD units lower at 12:00 and 15:00 h compared to at 9:00 h (relative decrease of 3.6%). For the MC-100, a slight increase in Chlorophyll Content Index (CCI) values of 3.3 ± 0.1 units (relative increase of 6.3%) was observed at 15:00 and 18:00 h, relative to CCI values at 9:00 h, regardless of N treatment. The time of day effect on chlorophyll meters appears to be negligible in relation to the wide range of values measured in greenhouse-grown sweet pepper. Normalized Difference Vegetation Index, measured both with the GreenSeeker and Crop Circle, and Green Normalized Difference Vegetation Index, measured with the Crop Circle, were not affected by time of day in any of the N treatments, showing that these sensors and indices can be used with confidence at any time of the day.

Assessment of Portable Chlorophyll Meters for Measuring Crop Leaf Chlorophyll Concentration

“…Strong relationships between the Chl a /Chl b ratio and SPAD-502 readings were observed by Netto et al [67] and Li et al [63]; however, we did not find similar relationships between the Chl a /Chl b ratio and PCM readings in the present study, similar to the result obtained by Parry et al [39]. The large difference in light conditions between the field and the greenhouse for canola was revealed by the Flav index of Dualex-4 measurements, which was another indicator of light intensity [36,47,56]. In addition, the chloroplast movement is closely linked with the combined effects of light scattering (detour effect) and non-uniform chlorophyll distribution (sieve effect) [40,43,66].…”

“…Nauš et al [40] found that the movement of chloroplast from the cell walls perpendicular to the incident light (face position) to the cell walls parallel to the incident light (the side position) could induce approximately 35% of the difference in SPAD-502 readings. Padilla et al [36] reported that measurement time in day determines light intensity and can have a strong impact on SPAD-502 and CCM-200 readings. The ratio of Chl a /Chl b was considered to be an indicator of light intensity that has strong influence on chloroplast movement [56,67].…”

“…Similar differences can be found in the Flav index of Dualex-4 between the greenhouse canola (Flav = 0.3) and the field canola (Flav = 1.7) (Table A1). This may reflect the great difference in light conditions between field and greenhouse, as the Flav index of Dualex-4 is an indicator of light intensity [36,47,56].…”

“…It should be noted that factors other than LChl may also influence the light transmittance of a leaf, such as leaf structure, water content and leaf pigment distribution [2,34,35]. Environmental factors such as light intensity can also affect light transmittance of a leaf, resulting in measurement errors of LChl [36,37]. Influences on light transmittance can be categorized into two groups [38][39][40].…”

Climate Changes Challenges to the Management of Mediterranean Montado Ecosystem: Perspectives for Use of Precision Agriculture Technologies

“…Finally, the use of the AOS OptRx has shown great potential for pasture monitoring. According to Padilla et al [46], proximal optical sensors are a promising approach for rapid and periodic assessment of crop status during the growth cycle or at certain phenological stages. They are sensitive to chlorophyll content per unit of leaf area.…”