Our goals for 2001 were to design and build a properly engineered chamber for measuring photosynthesis on mature grapevines in the vineyard, and to adequately test the prototype. This preparatory effort was necessary to ensure the success of a large field experiment during 2002 and 2003 in which several large chambers will be used to measure photosynthesis in deficit-irrigated ‘Cabernet Sauvignon’ vines in a commercial vineyard.
Photosynthesis is the process by which plants use energy from sunlight to transform carbon dioxide (CO2) into simple sugar molecules for subsequent use in growth, development, and energy storage. Sugars that accumulate in grape berries are products of photosynthesis. Carbon dioxide, the substrate for photosynthesis, diffuses into the leaf through stomates, tiny pores on the underside of the leaf. Water molecules simultaneously diffuse out of the stomate into the atmosphere as water vapor, a process called transpiration. When a vineyard manager adopts ‘regulated deficit irrigation,’ he or she attempts to withhold water from the vine to the point that the vine becomes water stressed. Generally this results in some stomates closing, thereby reducing transpiration. Because CO2 diffuses into the leaf through the same pores that water vapor diffuses out, water stress may limit CO2 for photosynthesis. The most appropriate way to collect dynamic measurements of photosynthesis that reflect the integrated response of the entire vine canopy is through the use of a whole-vine chamber.
A whole-vine chamber must accommodate the trellis, be lightweight, robust enough to withstand high wind, and easy to remove for vineyard operations. Because the vineyard does not have electricity, the system must be run by generator. In 2001, prototypes of large, open-top chambers were constructed and tested. Horticultural, agronomic, and ecological literature were consulted for guidelines on design and operation so that our measurements would be valid. The final design incorporates an aluminum frame in a circular chamber built in halves (see diagram under “Research Accomplishments”). The air delivery system also is modular to facilitate rapid setup. Because photosynthesis is highly dependent on temperature, the chamber was designed to minimize the difference in temperature between inside and outside without artificially raising transpiration. The chamber’s covering material maximizes transmission of both sunlight and the radiant energy emitted by the vine. The vertical temperature distribution inside the chamber closely matched that of a vine outside, with the largest difference between inside and outside at 2 °C (4 °F) on sunny days. Air blown into the chamber was delivered through perforated tubes to increase mixing and to avoid pockets of CO2-depleted air. Tests with smoke bombs showed that air entering the chamber mixed in less than 10 seconds. Humidity in the chamber (vine included) was close to that outside. High winds destroyed the first prototype, so structural changes were made and the new chamber survived 50-mph gusts. Construction of 6 chambers currently is underway for the 2002-2003 experiment. Other necessary equipment was secured during 2001 by purchase or on loan from USDA-ARS: CO2 analyzer and solenoid unit; laptop PC and data logging hardware; travel trailer for housing computer-controlled equipment and a 5000?W gas generator; intake fans, pressure sensors, radiation sensors, and humidity probes.