Evaluation of crop reflectance indices for greenhouse irrigation scheduling

Abstract

Development of direct, non-contact, non-destructive methods for irrigation control purposes is of great interest. The present work deals with the evaluation of reflectance indices for irrigation control in greenhouse environment. On this purpose, measurements were carried out on a gerbera cultivation (Gerbera jamesonii) grown hydroponically in a greenhouse located at Arta, Greece. Radiation reflected by the plant canopy - in 8 narrow wavelength bands, centered at 460, 510, 560, 610, 660, 710, 760 and 810 nm - was monitored under different irrigation regimes at selected days during one year. Additionally, greenhouse microclimate variables, plant water use, substrate moisture, leaf temperature and stomatal resistance, were measured. Crop Water Stress Index (CWSI) and leaf stomatal resistance (rl) were used as reference indicators of plant water status in order to evaluate reflectance indices. The difficulties to apply remote sensing methods inside a greenhouse are related to noise removal such as interferences from greenhouse structure and machinery shadows, effects from heating systems, floor and substrate reflectance, distinction between different types of stress and definition of threshold values for irrigation scheduling. The presented results are expected to assist at the design of protocols for further research as they are promising regarding the development of reflectance indices for determination of irrigation events timing.