Leaf Senescence: How It Informs Vine Health and Vineyard Management

October 14, 2019 | Written by: Gill

•         Leaf senescence: “planned death”, signals the growing season end for temperate zone plants (https://www.hunker.com/12003940/flowers-plants-found-in-temperate-forests) inclusive of grapevines. Leaves contain the green pigmented compound chlorophyll, that is directly active in photosynthesis. Photosynthesis (Pn), the process that transforms light energy, CO2, and H2O to chemical energy, O2, and H2O is fundamental to life as we know it. Pn uses visible spectrum light waves from about 400 nm (blue) to about 700 nm (red). Grape leaves absorb most of the damaging UV light, but are transparent to visible light. Photosynthesis does not use light wavelengths that are very short (<400 nm, ultraviolet) or very long (>700 nm, infrared).  

•       Age matters. Young and old leaves are not as effective at Pn as are ‘middle aged’ leaves, that is, leaves that are about 40 days old. Younger leaves are not fully functional and older leaves in the process of breaking down, are not as photosynthetically efficient.

o   “So, what?”, you might say, but photosynthesis is important to savvy winegrowers, because Pn, light, and leaves impact the yield and quality of your wine. Berry composition and wine “quality” depends on vine health and canopy management. To effectively manage your vine’s canopy, some knowledge of leaf physiology…or “the life of the leaf” is useful.

•         Light directly affects a grapevine in several ways:

o   Light impacts grape berry composition (color, flavor and aroma),inflorescence initiation and fruit set, incidence of disease and cane maturation (formation of periderm and lignification).

o   The thing to keep in mind, especially in sunlight rich New Mexico, is that a green plant’s leaves use no more than ~5% of the incident sunlight. Five percent would be an amount we would normally dismiss, if not for life’s total dependence on that 5%!  Nonetheless, the other 95% makes trouble, as it can lead to excessive heat (Figure 1).

Figure 1. Chardonnay grapes with browning and damage due to over-exposure to sunlight and consequent buildup of excessive heat.

•         Temperature is important, as 50^o degrees F is the lower limit of activity for temperate zone plants. Photosynthesis works best in the temperature range of 68^o to 95^o degrees F. However, the optimum temperature for grape leaves has not been fully defined.

•          So, speaking of light, what triggers “senescence”?

o   Decreasing daylength triggers leaf senescence. Senescence should begin on the same day for a given cultivar at given latitude. The critical daylength will vary according to cultivar and species.

o   Progression of senescence will be slower in a cool year and faster in warm years.

o   Heat stress can accelerate the process.

•         Leaves turn yellow as chlorophylls are degraded more rapidly than yellow-orange carotenoid pigments and the latter are “unmasked” and become visible.

•         Newly produced red anthocyanins will present themselves after chlorophyll has degraded over half way. Leaves of most grape cultivars, including dark-skinned types, are yellow rather than red. The red colored leaves we sometimes see are likely due to anthocyanins formed by restricted phloem. Restrictions of the phloem can be due to disease infection, wind injury, nutrient deficiency or girdling, (Figures 2 and 3). Feeding by the three-cornered leaf hopper (Figure 3) that constricts the phloem can result in a red leaf or entire shoot. These phenomena are not uncommon in New Mexico.

Figure 2. Grapevine with red leaves, likely a result of infection by Agrobacterium vitis, that results in Crown Gall and constriction of the vine’s vascular system.

Figure 3. Feeding by the three-cornered leaf hopper that has ‘girdled’ the leaf petiole and caused it to turn red due to the formation of red pigmented anthocyanins.
 

•         Anthocyanins in leaves are phenolic compounds that once released into the soil can act to suppress germination of other species (weeds) thus, acting as preemergence bioherbicides!

o   As chlorophyll breaks down it no longer assimilates carbon. All the proteins, amino acids and nutrients in the leaves are then exported to other plant parts. Chlorophyll itself is degraded and broken down because when not used in Pn it is a strong phytotoxin!

o   Some carbon is used to generate the energy for the process itself.

o   Leaves remobilize 50-80% of their N and P, 50% of their sulfur and 20% of their iron before they die and fall to the ground. This might be a reason to retain leaves post-harvest to allow the storage of these mineral nutrients in the vine’s semi-permanent parts: cordons, trunks and roots.

o   After leaves senescence, the phloem is plugged and sealed, the leaves break away at the base of the petiole (abscise).

o   So, the vine invests most of its carbon and nitrogen in leaves. The leaves, in turn, cycle this material into perennial plant parts and organic material or humus that is formed in the soil after the leaves fall and are broken down…
“the circle of life” so to speak.