Issue link: http://read.dmtmag.com/i/197178
SOIL REMI COHEN & CELLAR spring. As ABA levels decline, and giberellic acid increases, buds are released from dormancy and shoot elongation begins. ABA is also present during seed maturation, and seed germination will not be triggered until ABA levels decline, although this is less relevant to commercial viticulture since most vines are vegetatively propagated through cuttings. Although ABA levels in vines decline after bud break, once shoots have elongated enough to produce mature leaves, ABA concentration begins to increase again, slowing cellular and meristematic growth. In fact, exogenous applications of ABA inhibited shoot growth and advanced growth cessation and leaf senescence, but the effects vary depending on leaf age, with more pronounced effects on older leaves. This suggests that late-season application of ABA could possibly help induce dormancy and protect 34 V I N E YA R D & WIN E RY MANAGEM ENT | late-season cultivars in regions that have early fall frost events. Plant response to water stress is also regulated by ABA, as ABA precursors are generated in the roots during periods of water stress. The ABA then travels through the xylem back to the chloroplasts where it closes the stomates by regulating potassium and sodium uptake in the guard cells, causing them to lose turgidity and thereby close the stomates. ABA is also involved with the complex process of grape ripening. Grapes are considered nonclimacteric fruit, which means that they have a slow decline in respiratory function during ripening and no peak of ethylene, another plant hormone. This is in contrast to climacteric fruit, such as tomatoes and bananas, which have a peak in respiratory function and a corresponding or sequential peak in ethylene levels. Nov - Dec 2013 MYRIAD VARIABLES Since grapes have low and declining levels of ethylene during ripening, it is difficult for scientists to ascertain if ethylene plays a significant role in grape ripening. Metabolic production of different ethylene receptors changes throughout berry development, so berry ripening likely responds to ethylene differently, depending on the stage of berry development. This correlates well to experiments where applied ethylene has been shown to either accelerate or delay ripening, depending on the timing of application. Some scientists have reported a slight increase in ethylene just prior to the better-documented increase in ABA just before veraison in grapes. However, ABA is at its peak concentration in berries during early veraison. ABA levels are low during early berry development, increase at the onset of veraison, and w w w. v wm m e d i a . c o m