BUD BREAK AND VIGILANCE
As the sun passes through the celestial equator, the vernal equinox marks the point in earth’s orbit where the northern hemisphere is slowly tilted closer to the sun’s rays. With this celestial realignment the days get longer and average daily temperatures gradually increase—Spring has arrived! The species Vitis vinifera L., the domesticated European grapevine, is classified into the genus Vitis within the Vitaceae family of woody perennial deciduous plants. In the temperate zones of the northern hemisphere, Vitis vinifera vines follow a predictable developmental pattern that changes with the seasons. Being deciduous plants, they shed their summer foliage in late autumn as the average daily temperature cools and enter a period of dormancy that will last through the winter. As winter fades, the increasing daily spring temperatures begin to warm the vineyard soil and coax the vine’s root system out of dormancy causing the osmotic pressure within the xylem vasculature to rise. This restarts the flow of water, hormones, and nutrients such as, organic acids and sugars carried in the xylem sap from the roots to the aerial parts of the vines. The hormones transported in the xylem sap provide a molecular signal to structures called compound latent buds (see figure 1) to activate growth and differentiation pathways, while the organic acids and sugars provide the building blocks and energy necessary to support cell division and growth. The result—BUD BREAK! While its seems like this is could be the end of our story, we will have to go back in time a ways to understand the science behind one of Keith’s recent quotes, “Be vigilant people”. So let’s press on shall we. Bud burst is only the resumption of a developmental program that begun in the spring of 2009. The Vitis vinifera reproductive cycle occurs over two consecutive growing seasons and culminates in year two, after bud burst, with fruit and seed development. The first organs to expand and develop on new shoots in the spring are leaves. Within the axil of each leaf develops a bud, known as the prompt bud, which produces a lateral shoot, called a summer lateral (see figure 2). The summer lateral rarely develops inflorescences and in most cases its growth is slow. Subsequently, a new bud is produced in the axil between the summer lateral and leaf (See figure 2) and this bud will develop into a compound bud with a primary, secondary, and tertiary bud, which is protected by a prophyll (a modified leaf, figure 2). At the apex of the primary bud is the shoot apical meristem (SAM), a specialized tissue where stem cells differentiate into organ primordia. During compound bud development the SAM will first produce 3-8 leaf primordia before it divides into two unequal parts. The smaller of the two parts will develop as anlagen (or uncommitted primordia), while the larger remains the shoot apical meristem and continues to produce additional anlagen and leaf primordia. Anlagen have two primary developmental fates, either as inflorescences, which will produce flowers and eventually fruit, or tendrils, modified leaf structures used for climbing. The anlagen that differentiate into inflorescences will produce flower primordia in mid summer before the compound bud goes dormant. During this period, both the current seasons fruit and the developing inflorescence primordia are influenced by the same set of environmental conditions and viticultural practices. Therefore, the potential of any vintage is actually determined by the conditions over two successive growing seasons, not just by the current year. In late summer, the compound bud will enter dormancy and if conditions have been favorable, flower primordia on inflorescences will have finished their initial developmental stages and will wait until next spring to finish there transformation into flowers and eventually grape berries. This brings us back to Keith’s call to, “Be vigilant people”. As latent buds have resumed growth over the past few weeks, the SAM from now till the end of summer (or until it’s tipped) will primarily produce leaves and tendrils. Recall, the inflorescences that will produce this year fruit were born last year when the compound bud was formed. Right now these delicate structures are very susceptible to deleterious environmental changes, especially cold temperature and frost. Prolonged frost exposure (and it doesn’t have to be all that prolonged) will cause irreversible cell damage (in extreme cases death) to developing flower primordia on inflorescence shoots, resulting in reduced or non-reproductive flowering. If this occurs, it is disastrous for fruit development, which will be of low quality, if it takes place at all. This happened in the valley in 2008 where the majority of Grenache Blanc vines suffered extreme frost damage and almost no fruit developed that year (if you don’t believe try to find a 2008 Grenache Blanc from the Santa Ynez valley). So what can we do to remain vigilant and ensure this does not happen to our beloved Saarloos and Sons vineyards? Support Keith and Larry during these trying times people! No gesture is too big or too small. For example, sending a gift basket of Cuban cigars and homemade cookies, which will help Keith and Larry pass those long nights in the vineyard protecting the young buds from frost, is a fine way to show you care.
So stay vigilant people and there will be wine next year!
Signing off until the next time we apply some nerdery to wine,
Dr. Paul Tarr, Ph.D.