Water and carbon dioxide are combined in the chloroplasts to produce sugars and starch using light energy. The water used in photosynthesis is absorbed from the soil by the vine roots. The carbon dioxide is obtained directly from the air and enters the leaves via the stomata.
6H2 O + 6CO2 + light energy (in the presence of chlorophyll) > C6 H12O6 + 6O2
Six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen.
The photosynthetic parts of plants can usually be recognized by their green color due to the presence of chloroplasts. This pigment is capable of absorbing energy from sunlight and utilizes carbon dioxide and water from within the cell to produce sugar with oxygen as a waste product. Photosynthesis involves many steps and is controlled by the action of selected enzymes. The resulting sugars are basic building blocks of most chemical materials found in the grapevine. These sugars are used in respiration.
What Affects the Rate of Photosynthesis
The photosynthesis rate of grape vines is influenced by factors of the environment and by various characteristics of the vine.
These factors include:
Light; Light intensity, quality (wavelengths) and duration. A single leaf in direct sunlight will absorb about 90% of the sun’s radiation. Stomata open and close in relation to sunlight. They are fully open at PAR of about 200µEm-2S-1. In grapevines no photosynthesis occurs at low light levels, below about 30µEm-2S-1 (about 1.5% of full sunlight). As light intensity increases so does photosynthesis until about ⅓ full sunlight is reached or 700 µEm-2S-1. At this intensity photosynthesis in grapevines is termed ‘light saturated’ and remains at about the same rate with even higher sunlight levels.
Temperature; The ideal temperature for photosynthesis is generally between 20 and 30°C (and optimally at about 24°C). Below 10°C there is little photosynthesis and it declines rapidly above 35°C.
Water Status; Water affects the opening and closing of the stomata and thus the entry of carbon dioxide into the leaves. When the vine’s water supply is limited there is a rapid decline in photosynthesis. The amount of water available for photosynthesis is determined by, the rate of transpiration, humidity, mineral availability, and stomata opening.
Leaf Age – The rate of photosynthesis increases rapidly in a young grapevine leaf during the period of rapid leaf expansion as it gets older and loses its color the rate of photosynthesis declines.
Translocation is the process by which chemical materials and nutrients are moved throughout the vine. Stored foods flow in the phloem from the leaves to other parts of the vine.
Water absorbed by the roots is drawn into the leaves from where it evaporates in a process known as transpiration. The green parts of the grapevine evaporate large amounts of water during the growing season. The grapevine can control the rate of moisture loss (transpiration) via the stomata. The stomata open in the day and close at night. Under some conditions, they may also close during the day particularly on a hot, windy afternoon.
Factors Affecting Transpiration
The most important environmental factors affecting transpiration are:
Humidity; Transpiration decreases as the humidity surrounding the leaves increases. As the vapor in the air increases, this slows down the water loss through the stomates.
Temperature; A rise in leaf temperature will increase transpiration. Sunlight on a leaf causes the temperature of the leaf to become warmer than the surrounding air. The vapor pressure (conversion of a liquid to a gaseous form) inside the leaf will then be higher than the surrounding air resulting in increased water loss as the two pressures attempt to equalize.
Light Intensity; The temperature of the leaf is increased as the light intensity increases and this increases the rate of transpiration. The light level determines whether the stomates are open or closed.
Wind; The wind movement over the leaf takes with it the layer of water vapor accumulated near the surface. This increases the rate of transpiration. Winds of 11 to 14km/hr are sufficient to cause the closure of stomates (reducing photosynthesis and transpiration).
Water content of the soil; Transpiration is influenced by both the water content of the soil and the rate at which roots can absorb water. During daylight, water is often transpired at a greater rate than it is absorbed from the soil. At night, this situation is reversed.
Plants require an internal source of energy to grow and manufacture complex chemical molecules. This chemical energy is produced by respiration where sugars (from photosynthesis) and oxygen along with other compounds interact and produce energy along with water and CO2. During respiration, the energy stored by the vine is released.
C6 H12O6 + 6O2 > 6CO2 + 6H2 O + Energy
Glucose + Oxygen (in the presence of many enzymes) is converted to Carbon Dioxide + Water + Energy
Comparison between photosynthesis and respiration
The process of photosynthesis and respiration may appear but are not the reverse of each other. The series of enzymes used in each process is different and the order of reactions is not the reverse of each other. Photosynthesis and respiration occur at the same time and are interdependent In photosynthesis, energy is stored and in respiration, energy is released.
Mary Retallack - Retallack Viticulture