Role of Nitrogen in Compensatory Response of Hybrid Poplar to Ozone-Induced Accelerated Foliar Senescence

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9631030 Pell When plants are grow in an environment with elevated ground levels of the air pollutant, ozone, a common response is accelerated aging of foliage. During the normal aging process, foliage exhibits a gradual decline in rate of net photosynthesis accompanied by a decrease in a key enzyme known as Rubisco. This enzyme is responsible for fixation of carbon dioxide; Rubisco is also the most ubiquitous protein in the leaf accounting for up to 70% of soluble leaf protein. As leaves at the base of a plant age and eventually abscise, protein breakdown products are transported elsewhere in the plant for synthesis of new protein. One result is that in some species, including poplar, young tissue of ozone-stressed plants may actually exhibit a transitory, relative increase in Rubisco concentration and associated net photosynthesis. This response is thought to provide some compensation to the air pollution injury. Plants grow in a complex environment, often with other limitations. Since Rubisco, as a protein, provides an internal source of nitrogen, the investigators are interested in determining whether nitrogen limitation in the soil might enhance these ozone responses. The question has relevance since many plants growing in the natural ecosystem are found in nutrient limited soils. Models are being developed to try and predict plant responses to multiple stresses. Inherent in these models is an understanding of how plants minimize stress and what conditions will influence the way plants respond. The results of these studies will aid in the refinement of these models. In these studies hybrid poplar plants will be treated with chronic doses of ozone. We will follow the development and aging of leaves at the base and crown of the canopy. We will focus on the changing status of Rubisco and relate synthesis and degradation of the protein to availability of nitrogen.