<p>Nitrogen (N) is one of the major nutrient elements that affects growth, development, and quality of floriculture crops. Both sub-optimal and supra-optimal levels of N can negatively affect crop growth. In addition, over- fertilization may cause run-off and leaching of the N fertilizer leading to environmental pollution. Therefore, it is crucial to maintain optimal N level in plant tissue to produce good quality crops and increase productivity. This requires regular monitoring and measurement of plant N status. Laboratory analysis, the only direct method available to measure tissue N content, is destructive of plant tissue and expensive. Other available indirect methods are laborious, expensive, and/ or less reliable. In addition to measuring plant N status, it is crucial to understand acclimation responses at biochemical, leaf, and whole-plant levels in floriculture crops to N-deficit conditions. This will aid in developing a mechanistic model of plant responses to sub-optimal levels of N, proper fertilizer guidelines during production, and screening tools for identifying new varieties with tolerance to low-N level in the root zone. Unfortunately, there is limited research on floriculture crops that is simultaneously focused on plant responses at different scales to N-deficit conditions. The objectives of this research were to (i) assess the feasibility of image-based reflectance ratios for estimating tissue N content in poinsettia (Expt. 1), (ii) develop an affordable, remote sensor that can accurately and non-destructively estimate tissue N <a>content</a> in poinsettia (Expt. 2), (iii) study the physiological acclimation at whole-plant, leaf, and biochemical scales in poinsettia cultivars to N-deficit conditions (Expt. 3).</p> <p>In Expt. 1, we compared several spectral ratios based on the ratio of reflectance of near infrared <i>(R₈₇₀)</i> to reflectance of blue (<i>R</i><i>₈₇₀/R₄₅₀</i>), green (<i>R₈₇₀/R₅₂₁</i>), yellow (<i>R₈₇₀/R₅₉₃</i>), red (<i>R₈₇₀/R₆₂₅</i>), hyper-red (<i>R₈₇₀/R₆₆₀</i>), and far-red(<i>R₈₇₀/R₇₃₀</i>) wavelengths from plants<i></i>to measure whole-plant tissue N content in<i></i>four cultivars of poinsettia (<i>Euphorbia pulcherrima</i>) using a multispectral image station. Results indicated the reflectance ratio <i>R₈₇₀/R₆₂₅</i> was most suitable for assessing tissue N content in plants. In Expt. 2, a low-cost remote sensor was developed based on the findings of Expt. 1 that captured red and near-infrared images of plants, from which a reflectance ratio (<i>R_ratio</i>) was developed. The ratio was linearly related to tissue N content in all poinsettia cultivars. Furthermore, <i>R_ratio</i>was found to be more specific to N than to other elements in the tissue and related to the chlorophyll concentration of the plant. In Expt. 3, poinsettia cultivars ‘Jubilee Red’ (‘JR’) and ‘Peterstar Red’ (‘PSR’) displayed different acclimation strategies for physiology and growth under N-deficit conditions. Significantly higher growth was observed in ‘JR’ than in ‘PSR’ in the sub-optimal treatment, which indicates that ‘JR’ is more tolerant to N stress compared to ‘PSR’. Further analyses indicated that N uptake was higher in ‘JR’ than in ‘PSR’ under N-deficit conditions, without any changes in root morphology or growth. This is possible when higher levels of energy are available to transport nitrate and/or ammonia from the substrate into the root cells. Supporting this, significantly higher photosynthesis and carboxylation efficiency were observed in ‘JR’ than ‘PSR’ under N-deficit condition. These results shows that higher growth of ‘JR’ than ‘PSR’ under N-deficit conditions was likely due to increased N uptake (likely due to increased energy-driven transporter activity), which increased tissue N and chlorophyll levels. Further, these increases resulted in higher carboxylation efficiency and photosynthesis by ‘JR’ than ‘PSR’. Increased carbohydrate synthesis supported leaf growth and provided required energy in the fine root cells for N uptake from the substrate.</p>