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w i n e G R O WIN G Friend or foe of RED WINE QUALITY PhotoS M. Keller N ITROGEN ? BY Markus Keller, Washington State University, Prosser, Wash. Y ield is determined by the amount of carbohydrates (sugar) partitioned to the fruit rather than to other organs of grapevines. The classical view of the relationship between grape yield and quality is that of a linear decrease in quality as yield increases. Moreover, many grapegrowers believe that the more they stress their vines, the better the resulting fruit quality will be. However, reality is not that simple. A more scientific view is that of an optimum curve with initially increasing quality, followed by a plateau, and then a reduction in quality when yield is further increased. Under changing external conditions, this curve can be shifted upward or downward. Rather than setting a specific, inflexible target yield, it should be the goal of every economically thinking vineyard manager to achieve the highest yield possible without compromising quality. Roles of nitrogen Nitrogen is only one of a whole range of tools available to vignerons to manage the balance between yield and quality. Other tools include vineyard design (row orientation, planting distance, trellis design, training system), pruning (node number and position), canopy management and soil and water management. However, of all mineral nutrients, nitrogen is the most potent in terms of influencing vine growth, morphology and tissue composition. This is primarily because nitrogen is a chemical component of many critically important plant constituents. Nucleic acids contain it, and they in turn make up the genetic information contained in the vine's DNA. Nitrogen is also in amino acids, which when linked together make up the proteins and enzymes that drive all biochemical reactions. It is an integral part of chlorophyll, responsible for intercepting and capturing sunlight, of hormones used for communication between differThis text edited from first publication: Sept./Oct. 2005. 40 p r acti c al w i ne ry & v i n e yard JANUARY 20 14 ent plant organs, and of certain secondary metabolites, some of which also contribute to wine flavor. Nitrogen is present in the soil solution in the form of nitrate (NO3–) and ammonium (NH4+) ions. Nitrate is the vine roots' nitrogen uptake form of choice. It is reduced to ammonium in the roots and metabolized (assimilated) into the amino acids glutamine and glutamic acid. Glutamine is the main Sunburn on Syrah berries; a common problem with overexposed nitrogen transport form grapes and excess nitrogen supply. in the transpiration stream (xylem), but with increasing nitroenhances photosynthesis, which means gen supply in the root zone, nitrate is that more sugar is available for growth increasingly transported to the shoots in and fruit ripening. However, when addition to glutamine.2,5 This is because nitrogen uptake and assimilation in the excess nitrate arrives in the leaves, it roots are expensive in terms of carbohycompetes for carbohydrates and may drate requirements (both to fuel energy result in a shift of the vine's priorities generation and as backbones for amino from fruit ripening to shoot growth.9,10 acids). Therefore, if nitrogen supply Nitrogen and vigor exceeds demand, the excess nitrate must Excessive vegetative growth (vigor) can then be assimilated in the leaves and fruit, lead to shaded canopies and, along with which for them constitutes a conflict of interests. Nitrogen (in chlorophyll and enzymes) stimulates photosynthesis in the leaves. Photosynthesis is the process by which the energy from sunlight is transformed into biochemical energy (ATP) and used to fix carbon dioxide (CO2) and water (H2O) to produce sugar (glucose). This glucose is stored as starch or made into sucrose, which is then exported in the Figure I: Grapes grown on high-nitrogen soils are much more phloem to various plant susceptible to sunburn (right), than grapes grown with low parts. nitrogen availability (left). Increasing nitrogen supply