Canopy microclimate and its influence on quality of grapes and wine
Dr. J. Satisha* and Dr. R.G. Somkuwar
Many of the earlier observations among several viticulture scientists all over the world indicated a strong effect of climate on both yield and quality of grapes. To fully understand these effects, we need to differentiate between three levels of the climate based on the ideas of the famous German climatologist, Geiger.
Macroclimate (Regional climate): This is the climate of a particular region, and a weather stations located in that region usually describe the general pattern of the macroclimate.
Mesocliamte (Site climate): The mesocliamte of a particular vineyard varies from macroclimate of region because of differences in elevation, slope, aspect or distance from moderating factors like lakes or oceans. Mesocliamte can be very important for success of vineyard especially where climatic factors are limiting.
Microclimate (Canopy climate): It is the climate within and immediately surrounding the vine canopy. Measurement of climate shows differences between and within canopy values and those immediately above it. Microclimate differences can occur over a few centimeters also.
Important microclimatic factorsSome of the important microclimatic factors, which determine the grape quality, are temperature prevailing in the climate, sunlight (both quality and quantity), humidity, wind speed and evaporation. Although most of these factors major role in producing good quality grapes (for table purpose) and wine (in wine grapes), temperature and light are the two major factors playing vital role in determining fruit quality. However, the other factors such as wind speed, relative humidity and evaporation indirectly influence fruit quality by either favoring or controlling incidence of pests and diseases. The role of sunlight (intensity and quality) and temperature on influence of fruit bud formation and fruit quality is discussed below:
Intensity of sunlight: The amount of sunlight falling on a vineyard varies with latitude, season, time of day and cloud cover. Sunlight intensity is commonly measured in units that correspond to the ability of plants to use sunlight in photosynthesis. Consequently, the intensity is often termed “photosynthetically active radiation (PAR). The units are amounts of energy per unit area per unit time. Values of the sunlight intensity measured in the centre of denser canopies can be less than 300µ moles/m2/sec although above canopy values are above 2000 µ moles/m2/sec.Leaves absorb approximately 90% of solar radiation striking individual leaves. So if there are more number of leaf layers, interior leaves are shaded and are not photosynthetically active and will senesce and abscise. Trellis types, row orientation and vine density plays a major role in efficiency of light interception by vineyards.
Quality of sunlight: Not only the amount of sunlight altered in the canopy, but also so is the color spectrum that makes up sunlight. Plant leaves absorb only a part of the sunlight especially in the so-called visible range (400-700 nm wavelength). This is the part of the sunlight spectrum we can see. So as sunlight passes through the canopy, there is less sunlight passes through the canopy, there is less sunlight in the visible range relative to the remaining wavelengths of the spectrum. An important consequence is that the ratio of red light (660 nm) to far red light (730nm) declines in the canopy. Plants like grapevines respond to the red: far-red ratio via their phytochrome system and this is important in affecting, for example, fruit color development. Apart from ratio of red to far-red ratio, ratio of UV-A/UV-B also plays important role in determining fruit chemistry.
Temperature: Temperature of grapevine parts is generally at or near air temperature. This applies unless they are warmed by absorbed sunlight, or cooled by evaporation of water, as takes place with transpiration from leaves.Elevated tissue temperatures due to warming by sunlight are most obvious on sunny and calm days. Exterior leaves and berries are heated up by short wave radiation absorption (leaves about 5 0F higher than ambient and berries about 10-15 0F higher than ambient temperature). It also depends on crop load, wind velocity and berry color. Temperature of shaded berries is lesser than air temperature due to transpiration cooling. At night, exposed leaves and fruits on exterior of canopy may be 1-30F lower than air temperature due to long wave emission.
Humidity: Transpiration by leaves can lead to slight build up of humidity inside dense canopies. If the canopy is open, then the ventilation effect of even a slight breeze can reduce the humidity difference from the canopy exterior to interior. However, even small differences in humidity can be important for the establishment of many fungal diseases.
Wind speed: Wind pattern around the vineyards are complex, and there is an interaction between wind direction and row orientation. Similar to sunlight, wind speed is very low in the centre of the dense canopies. This occurs because leaves slows down air flow.
Evaporation: Evaporation of free moisture from plant surfaces is encouraged by high values of temperature, sunlight, wind speed and low relative humidity. Hence, there can be significant differences in evaporation rates between the canopy exterior and interior. This is very important for fungal disease development, as the rate at which dew or rainfall evaporates will again depend on canopy density.
Influence of canopy microclimate on important physiological process:
Transpiration: Water absorbed by the roots is drawn into the leaves from where it evaporates in a process known as transpiration. This occurs through stomata. The rate of transpiration is closely linked to the climate, being highest under sunny hot, windy and low humidity conditions.
Opening and closing of stomata also depends on intensity of sunlight. They begin to open with very low light levels soon after dawn and are fully open at PAR of about 200-µ moles/m2/sec. As temperature and sunlight increase and humidity fall, transpiration increases. Exterior leaves on a grapevine canopy are exposed to higher sunlight levels and temperature and this transpire more than shaded interior leaves. Interior leaves in dense canopies may be exposed to such low light levels that their stomata do not completely opens.Photosynthesis: It is the process by which energy from the sun is used by the green tissues of plants to convert carbon dioxide to sugars. These sugars are basic building blocks of most chemical materials found in grapevine. These materials include carbohydrates, proteins, phenols, organic acids and others. Photosynthesis depends on sunlight. In grapevines, no photosynthesis occurs at low light levels, below 30-µ moles/m2/sec. Part of this reduction is due to stomata being partly closed reducing inflow of CO2. As light intensity increases so does photosynthesis and at certain level rate of photosynthesis ceases, called as saturation point. Due to low sunlight intensity, interior leaves have low photosynthetic rates, which means they contribute little to the vine. When in deep shade, interior leaves turn yellow and incapable of photosynthesis. Studies have shown that the exterior leaves contribute most photosynthesis in dense canopies. Green berries can also photosynthesize but their contribution is much smaller than that from leaves.
Effect of shade on vine physiology:
Excess shade in the fruit cluster zone has adverse effect on fruit quality. Excess shade is known to reduce berry size and also reduces skin to seed ratio because; shaded berries are known to carry heavier seeds.
Shaded berries also contain higher seed tannins than skin tannins. In wine grapes, tannins play a major role in determining wine astringency and mouth feel. Skin tannins are soft tannins and seed tannins are harsh tannins. If seeds contain more tannins, wine made from such berries is tend to become more astringent and bitter in taste. Shaded berries are known to contain reduced anthocyanins in the skins thereby reducing the fruit color in colored grapes. Both excess light and / or shade have adverse effect on skin color.
Some of the adverse effects of shade on wine quality are shade decreases wine color, reduces sugar level in grape juice thereby affects fermentation, decreased phenols and anthocyanins in colored wines, decreased varietal or fruit characters, increased juice and wine potassium and wine pH, increased ratio of malic to tartaric acid, increased herbaceous or grassy character in wines.
Influence of canopy microclimate on yieldIn grape vines where double pruning and single cropping is practiced, inflorescence primordial is initiated in latent buds at about 40-45 days after foundation pruning which is performed usually between 1st and 2nd week of April. This is called fruit bud differentiation. The fruit buds developed inside the latent buds will be sprouted and produce flower clusters immediately after forward pruning which is usually occurs during last week of October to Mid-November. Two major climatic factors which plays important role for formation of flower primordial are temperature and light.
Temperature: Most of the grape cultivars respond to varied levels of temperature with respect to inflorescence primordial formation. A positive correlation was established between mean air temperature and the percentage of fruiting shoot. Higher temperature at the time of fruit bud differentiation is closely correlated with subsequent fruitfulness of latent buds. Several reports have concluded that a pulse of only four hours per day (or night) of high temperature (300C) was sufficient to induce maximum number of inflorescence primordial. But, higher temperature at the later stages of fruit bud differentiation has adverse effect on inflorescence formation. The temperature requirement for fruit bud differentiation varies with the cultivars.
Light intensity: Several studies all over the grape growing regions of the world on the effect of light intensity on fruitfulness have revealed that shading reduces fruitfulness. A mean of about 10 hour’s sunshine per day during inflorescence formation is needed for an acceptable level of fertility in Thompson Seedless grapes. Shading for four weeks during fruit bud differentiation reduced the fruitfulness of latent buds to a grater extent than shading treatment applied earlier that stage. Light intensity is also determined b the training system adopted for grape cultivation. Vertically trained shoots are more fruitful than horizontally trained shoots. Direct exposure of latent buds to high light intensity improves the fruitfulness of buds, but no significant difference was observed with respect to effect of light quality on inflorescence formation. It is also observed that buds situated inside the canopy of field grown vines are less fruitful than those at the exterior where buds are more strongly illuminated. The use of trellis and split canopies such as GDC gives improved fruitfulness of buds and an overall increase in productivity by 50-90%.
Climatic factors have a significant effect on fruit set. Due to inhibition of pollen tube growth and ovule fertilization, fruit set is greatly reduced when temperatures fall below 65°F (18.3°C) or exceed 100°F (37.8°C) during set. Cold temperatures are often associated with incomplete detachment of the calyptras, while both cold and hot temperatures may reduce fruit set by preventing the growth of pollen tubes and ovule development. Rainfall or high humidity may reduce fruit set, hindering pollination by impeding the complete detachment of the calyptras. Rain can also dilute the stigmatic fluid and thus interfere with the germination of pollen grains.
Influence of canopy microclimate on grape and wine quality:
Table grapes: Canopy microclimate plays a major role in determining the quality of table grapes. In table grapes appearance of berries in terms of color and free from sun scorch determines the consumer acceptance. Exposure of clusters to direct afternoon sunlight not only results in berry scorching, but elevated temperature of berries results in berry shriveling due to more moisture loss. On the other hand clusters developed in shaded regions of the canopy develops poor sugar acid blend with increased acid content and less sugars. Hence it is necessary to decide appropriate canopy management practices such as training system, shoot orientation, shoot positioning, leaf removal to expose clusters to optimum sunlight or shoot positioning to protect clusters from afternoon western sunlight etc to improve quality of fruits.
Wine grapes: Differences in temperature between ambient air and exposed fruit increased as solar radiation increased and wind speed decreased, as one might expect from heat transfer principles. It is also understood that solar radiation and wind velocity were the two most important determinants of fruit temperature wherein during the day shortwave radiation is the primary source of fruit warming and convection was the primary source of heat transfer away from the cluster. Temperature is a key factor controlling berry acid content. During the initial stages of fruit development the optimum temperature for acid synthesis ranges between 68° and 77°F (20° and 25°C). It is also well established that fruit acidity at harvest is negatively correlated with temperature during the ripening period. In general, fruits ripened at low temperatures have greater total acidity (particularly of malic acid) than fruits ripened at high temperatures. Similarly fruits developed in shaded regions have less sugar, which adversely affects alcohol content of the finished vines. Canopy microclimate also determines the flavanol content, color and aroma of the wine.
Manipulation of canopy microclimate:
- Vigor control either through mechanical / chemical means (Shoot pinching or application of growth retardants)
- Training system – Open canopy or closed canopy
- Canopy management practices such as training system, shoot orientation, shoot thinning, shoot positioning, leaf removal, cluster thinning etc
- Benefits of better canopy management practices:
- Improving the fruit yield and quality in table grapes and quality of wine in wine grapes
- Reducing the incidence of some important diseases
- Decreasing the production cost and thereby increasing benefit cost ratio
Dr. J. Satisha* and Dr. R.G. Somkuwar
Senior Scientist (Horticulture)
National Research Center for Grapes, Pune – 412 307