Why do grape leaves fall off the vine in fall to early winter? What happens physiologically? What triggers dormancy?
The book Oregon Viticulture (Oregon State University, 2003) describes dormancy, acclimation, and cold hardiness:
“In autumn, the vine enters dormancy–the stage with no leaves or growth activity, which extends until budburst the following spring. Despite the apparent inactivity of this stage, it can be a critical time for grapevines when they may be exposed to potentially damaging low temperatures […] There are three stages of the dormant season: acclimation, the period of transition from the non-hardy to the fully hardy condition; midwinter, the period of most severe cold and greatest cold hardiness; and deacclimation, the period of transition from fully hardy to the non-hardy condition and active growth.”
This book does not address leaf drop (abscission) nor does it explain the physiological reasons a vine enters dormancy. The Grape Grower by Lon Rombough (Chelsea Green, 2002) says only that leaf drop follows harvest time, and is part of the vine’s hardening off process: “The leaves drop, the shoots become woody to the tips, and the vine gets ready for winter. This is when the vine undergoes deactivation and reenters dormancy.” To find the scientific explanation you are seeking, it might make sense to contact a plant physiologist who specializes in vines. There are specialists at University of California, Davis’s department of Viticulture and Enology.
I consulted Plant Physiology (4th ed.) by Taiz and Zeiger (Sinauer Associates, 2006), and here is what it says about leaf abscission in general:
“These parts [leaves, flowers, fruits] abscise in a region called the abscission zone, which is located near the base of the petiole of leaves. In most plants, leaf abscission is preceded by the differentiation of a distinct layer of cells, the abscission layer, within the abscission zone. During leaf senescence, the walls of the cells in the abscission layer are digested, which causes them to become soft and weak. The leaf eventually breaks off at the abscission layer as a result of stress on the weakened cell walls.
Auxin [plant growth hormone] levels are high in young leaves, progressively decrease in maturing leaves, and are relatively low in senescing leaves when the abscission process begins.”
Here is a little information about vine dormancy, from U.C. Davis.
Excerpt:
“In late autumn, triggered by short days, petioles detach from the shoot and leaf drop occurs. The vines can no longer manufacture carbohydrates through the process of photosynthesis and are storing carbohydrates in the form of starch. This dormant state will continue until daylight hours and temperatures increase in spring, when axial buds that were formed before dormancy become activated to break.”