Regulated deficit irrigation (RDI) is a common and useful practice for water conservation and improving grape quality. To attain better grape characteristics and wine quality, a substantial degree of irrigation stress is intentionally allowed to occur during the first part of berry formation and can continue until later into veraison. Understanding the effects of deficit irrigation on photosynthetic rates could be helpful in determining at what degree and duration a grower should perform this irrigation practice. The focus of this study was to determine the effects of using differing degrees of RDI in a vineyard located in Paso Robles, California (central coast region) on gas exchange of the Syrah variety. The target irrigation levels were set for each season at 75, 60, 45, and 30% ETc of a fully irrigated vine (100% ET). The 60% replication was considered as the control for this study, as it is the standard target ET rate for the vineyard where this study was conducted. A gas analyzer (LICOR 6200) was used to measure the overall rate of photosynthesis during two successive growing seasons (2004 & 2005). Measurements were taken from bloom through harvest and were compared among the four different irrigation levels. Along with overall photosynthetic rate, the leaf temperature, stomatal conductance, light level, and relative humidity were also measured. The results of the two year study were statistically compared through an analysis of variance (ANOVA) and analyzed for their significance. The results of the study showed that minor differences in the mean photosynthetic rates were found to occur during brief periods of the growing season. These differences ranged from 1-4 weeks and did not occur at similar times of the growing season. However, no statistical significant differences were found to exist when compared among the four irrigation levels for the entire growing season. Observed differences in canopy sizes indicated that irrigation amounts had affected the overall growth to some degree during this two year study. Several plant physiological measurements showed a significant difference in the measured gas exchange rates between sun exposed leaves and the shade leaves within the treatment area. A significant correlation of the effect of leaf temperature on stomatal conductance was observed to exist in one of the irrigation treatments (45% ET) during this study. Other plant physiological measurements indicated that highly significant differences existed between the photosynthesis rate and leaf temperature. Photosynthetic rates were highly significantly correlated to leaf conductance, air temperature, and relative humidity. A significant difference of photosynthetic rates was identified to occur between stomatal conductance and air temperature. This study concludes that differential irrigation amounts on Syrah in the Central Coast region, specifically Paso Robles, have minimal effect on overall photosynthetic rate and does not fully support the anisohydric stomatal reaction that has recently been studied by plant physiologists working with this variety.

This paper examines vineyard perceptions and adoption of climate change adaptation and water conservation measures in the Paso Robles American Viticultural Area (AVA). A survey was distributed to all 220 vineyards and vineyard management companies that operate in the AVA, with a 53.64% response rate. The objective of the survey was to determine vineyard manager and owner attitudes towards climate change and adaptation, as well as their perceptions of how these threats would impact their operation. A second objective was to document the current level of water conservation and climate adaptation while identifying the barriers and opportunities for further adoption of these practices. The third objective was to share a climate projection to assess perceived impacts, attitudes, and perceptions of the projection and capacity to adapt. The final objective was to develop a typology to assist targeted outreach of vineyards. Our results showed that the most important current climate impact is heat, with water regulations and supply being the biggest concern in the future. Vineyards were also found to be relatively unsure about their capability to further implement adaptation measures, with relatively low adoption of most practices already. Decision support tools, like the climate projection, were found to be useful and desired by vineyards for management. Results of our study showed that trust in the information source can be a barrier to use of these tools. We identified further barriers to adoption of practices and identified an outreach strategy using a typology of vineyards, which focused on smaller vineyards and those without wineries. These results can be used to increase efficacy of government and NGO programs that aim to support climate adaptation and water conservation in the region.