2017 ASHS Annual Conference

It has been widely demonstrated that the best tool to improve and increase water-use efficiency is to adopt soil-moisture sensor (SMS) based irrigation. SMS provide information on a continuous basis, reflecting soil moisture content during, before and after an irrigation event. However, for many specialty crops, including many valuable ornamental flowering plants, the relationship between soil-water status and plant water-status has not been clearly demonstrated. This lack of understanding of basic physiological requirements for growth and yield represents a fundamental gap in knowledge for growers of specialty crops who wish to use technology to maximize crop water management. Poinsettia (Euphorbia pulcherrima cv. Prestige Red) was selected as an ideal species for study, because it is one of the highest valued greenhouse plants in the USA. Yet, other than photoperiodism, very little information has been published on the physiological requirements for growth for this species. We established an experiment in a natural lit greenhouse (Davis, CA, USA) where poinsettia plants were grown in 3 gal. black, plastic, nursery pots, filled with potting media composed of equal parts peat, sand, and redwood compost. Each pot was watered automatically by drip emitters. We applied three different watering frequencies to test the relationship between moisture availability and growth physiology. The three watering treatments were weekly, bi-weekly (i.e., every two weeks), and tri-weekly (i.e., every three weeks). Available soil moisture was measured with volumetric SMS (Decagon EC-5) placed in each pot; and also by using the amount of water applied and physical properties of the soil-less media. Plant water use was monitored with external heat-pulse sap-flow sensors, leaf porometer (Decagon SC-1), and leaf water potential (Scholander pressure chamber), as well as traditional growth measurements (e.g., biomass, height, etc.). The results show a clear relationship between available soil moisture and plant growth. Plants in the weekly watered treatment group were on average more than 2.5x larger than bi-weekly water plants; and more than 3x larger than tri-weekly watered plants. This information provides growers with first known reported water-use-efficiencies for Poinsettia ‘Prestige Red’. Additionally, the results also present the first reported data correlating soil water content with Poinsettia sap-flow, stomatal conductance, and leaf water potential. These types of physiological indicators provide growers with new information about critical thresholds, which can become guidelines for irrigation management.