2019 ASHS Annual Conference

Optimal level of tissue nitrogen (N) is crucial for growth and quality of ornamental plants. Laboratory analysis is the only direct method available to growers to accurately measure tissue N. However, this method is time-consuming. Other equipment like SPAD or chlorophyll meter are expensive. The objective of this study is to develop easy and affordable technology for measuring N status of single or group of plants. Tissue N affects chlorophyll content in plants and thereby influences plant absorption/reflectance of light. A relatively inexpensive (approx. $150) N-sensor that can be connected to a smartphone or computer was built using a micro-controller, two miniature cameras and light filters to measure reflectance of red (R) and near infrared (NIR) light from plants. An experiment was conducted to test the efficacy of the sensor to measure tissue N in four poinsettia (Euphorbia pulcherrima) cultivars (Christmas Glory White, Christmas Tradition, Christmas Beauty Marble and Wintersun White) by growing them under five levels of fertilizer concentration (112.5, 225, 375, 525 and 675 mg·L^-1 N). Software was developed for capturing, processing and extracting reflectance from images using Matlab and C++ languages. Measurements included ratio of reflectance from R and NIR images (R_ratio), shoot dry weight (g), and elemental analysis of plant tissue including tissue N content. Results indicated a curvilinear relationship between SDW and fertilizer concentration with maximum SDW at 375 mg·L^-1 N. There was a linear relationship between tissue N obtained from the laboratory and R_ratio for different varieties (0.66 < r² < 0.78). The fitted stepwise selection model (r² =0.70) indicated that, among other elements, N explained most of the variation in R_ratio (partial r² = 0.60), while other significant elements including S, Na and Ca had minimal contribution to the ratio (combined R² = 0.1). To further understand basis for R_ratio, a separate study was conducted using three varieties (Christmas Glory White, Christmas Tradition and Wintersun White) under three different fertilizer concentrations (112.5, 375 and 675 mg·L^-1 N) to measure the relationship between chlorophyll concentration (using spectrophotometer) and R_ratio. A linear relationship was observed between chlorophyll concentration and R_ratio (r² = 0.68) for all cultivars. Based on these results, it is concluded that R_ratio measured using the developed sensor is related to chlorophyll concentration in plants and mostly influenced by differences in tissue N content. Moreover, R_ratio can potentially measure tissue N content of whole-plants in several poinsettia varieties.