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SOIL REMI COHEN & CELLAR Giberellin levels increase at bud break. Photo: Thinkstock aspects of berry growth and development from set through veraison. GA also plays a role in bud break, its levels increasing as buds are released from dormancy. GA is also involved in seed germination, by activating hydrolytic enzymes involved in nutrient mobilization for the growth of new cells. Giberellin has also been shown to promote flowering and cellular division, and reverse the inhibition of shoot growth and dormancy induced by ABA. Exogenous GA applications can increase grape size, particularly useful for table grape growers, where the sizing effects are more pronounced in seedless grapes than seeded grapes; negative effects on cluster counts and shoot elongation have been reported with seeded grapes. Auxins also positively influence growth and cell enlargement. The high levels of auxin found in early stages of berry development and the first rapid growth phase are responsible for enhancing cell division and enlargement, and it is speculated that one role is in the 36 V I N E YA R D & WIN E RY MANAGEM ENT | regulation of assimilate portioning within the berry. As well, auxins control the growth of shoots and roots in conjunction with cytokinins. They are involved in apical dominance, and also promote lateral shoots and inherent root growth. Shoot meristematic locations that cease to produce auxins will initiate the process of leaf abscission. Auxin application can stimulate root growth of cuttings, but large concentrations of auxins can be toxic to plants. Cytokinins influence cell division and shoot formation and affect internode length. Cytokinins are often correlated with auxins as they are responsible for mediating auxin transport throughout the plant. Cytokinins also temper the apical dominance effect of auxins. Salicylic acid (SA) is also considered to be a plant growth regulator. SA is involved in plant defense and is known to activate genes that produce chemicals that help protect plants against pathogens and heat stress. It is thought that SA activates gene expression of phenylalanine ammonia-lyase (PAL), a precursor involved in phenylpropanoid metabolism of the grape berry. Exposure to heat or pathogen stress induces the production of SA, which activates PAL and leads to the accumulation of phenolics in grapes. endogenous plant hormones, so that we can understand how environmental factors and viticultural practices affect hormonal regulation of plant physiological processes. If growers choose to apply plant growth regulators to their vines, they should know that they are regulated as pesticides, and that they should seek guidance from a qualified local pest control advisor before applying any PGRs. Remi Cohen is the director of wine- making & viticulture for Cliff Lede Vineyards in Napa Valley. Her primary areas of expertise include viticulture, sustainable vineyard management, winemaking, grower and winery relations, and strategic business development. She also writes a wine industry blog, which can be found at blog.vinestowine.com. Comments? Please e-mail us at feedback@vwmmedia.com. MUCH TO BE LEARNED Plant hormones and their effects in stimulating or suppressing enzymatic activity are complex and interrelated, and much still remains unknown about how hormones control plant physiological processes, and the potential impacts of exogenous applications in commercial viticulture. I remain skeptical about using PGRs for small-scale, high-quality wine grape growing, yet I look forward to learning more about the benefits and risks of externally applied hormones in different viticultural settings, as ongoing research focuses on these complex effects. I encourage more research about the complex interactions of Nov - Dec 2013 w w w. v wm m e d i a . c o m