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54 WINES&VINES June 2017 GRAPEGROWING PRACTICAL WINERY & VINEYARD mize damage to vines caused by pests and diseases, waterlogging, frost etc. While inadequate inor- ganic nutrition (nitrogen) and/or water stress will slow growth, ex- cessive water stress will slow pho- tosynthesis. A degree of mild water stress and a slowing of growth may be an advantage by redirecting photosynthates from vegetative growth to fruit development later in the growing season. Additional aspects in determin- ing vine balance include the mech- anisms that control the distribution of photosynthates to the different sinks (predominantly shoots, roots and fruit, etc.) within the vine. Distribution of the sugars from sources (generally the leaves, ex- cept in early spring when remobili- zation from overwintering reserves are the predominant source) de- pends on the demand from various sinks in the vine. Photosynthates are conducted to the various sinks through phloem vessels. The majority view is that trans- port is through osmotically pow- ered pressure-driven flow, as proposed by Ernst Munch. 17,23 The direction and rate of flow and relative contribution that a source provides to a sink depends on the proximity of the source to that sink and the concentration differ- ence between the two. In practice, sinks will be sup- plied from sources close to them, although this may be modified by the relative demand by the sink (e.g. stronger sinks will accumulate photosynthates from more distal leaves), and the direction of flow may change during the growing season. For example, fruit becomes the predominant sink during ripen- ing, while reserves in the trunk and root system are the main sources until an adequate leaf area has developed in the spring. Grapevine triangle The grapevine triangle integrates two important concepts of bal- ance common to all plants. The triangle represents the functional equilibrium between the three predominant sinks (shoot growth, root growth and fruit yield), and the strength of each sink is repre- sented by the shape of the trian- gle. The capacity of the plant to accumulate photosynthates is re- flected in the area of the triangle. The larger the triangle, the greater the capacity. (See "Concept of Vine Capacity and Carbohydrate Partitioning" on this page.) In 1927, W.H. Pearsall showed that plants, like animals, exhibit allometric growth. 20 He harvested http://www.angelfire.com/ca5/RandDGlass CONCEPT OF VINE CAPACITY AND CARBOHYDRATE PARTITIONING High-capacity site Low-capacity site Shoot The area of the triangle reflects the dry matter (carbohydrate) accumulation (capacity) of the vine. The base and height of the triangle are the partitioning of x and y axis and the partitioning of carbohydrate between the major organs. Fruit Root