2017 ASHS Annual Conference

Greenhouse-grown cut roses (Rosa x spp. L.) are intensively managed, receiving very large inputs of water, fertilizers, agrochemicals and labor. While the crop is typically fertigated (at almost every irrigation) with complete nutrient solutions, significant attention is given to the applied nitrogen (N) concentrations, often ranging from 150 to 250 mg/L. Our research indicates that N concentrations as low as 90-100 mg/L could satisfactorily sustain flower yields, albeit often having leaf N concentrations (status) below 3% (dry weight basis), historically set as the lower N sufficiency level for greenhouse rose crops. Leaf N status expressed on a dry weight basis was not found to be a dependable indicator of cut flower productivity over the course of a growing season. Conversely, chlorophyll levels and color (hue, chroma and value) attributes in leaves of harvested flowers correlated linearly with leaf N status. The literature indicates that photosynthetic rate and leaf N expressed on a leaf dry weight basis correlate well for many cultivated and wild plant species, declining to zero at about 0.84% leaf N (considered extreme N deficiency). While this reflects a relationship that is independent of species and growth conditions, it has been argued that it could be hiding variation that may be of significance for plants that are growing in certain specialized environments, perhaps including intensively managed greenhouses. This variation has been reported to be readily apparent when expressing photosynthesis and leaf N content in units of leaf area. An experiment has been initiated to re-evaluate the relationships of rose leaf N status, expressed both on leaf weight and area basis, and the flower productivity and quality of cut flower yields. The effect of the rootstock is also under evaluation by comparing the response of plants growing on their own roots versus the same cultivar grafted on ‘Natal Briar’, the predominant rootstock in use today by the greenhouse rose industry.