California agriculture has developed considerably more water-efficient rice production than the worldwide average. Because of shallower water submergence and better management, California rice is estimated to require 1080 liters of water per kilogram of rice . Incentives to increase water use efficiency must be considered. California’s water system will adapt to future population growth and increased demand, however, it may be at the expense of agriculture, with Central Valley agricultural users being most susceptible to climate change related impacts upon water resources.There are several predicted environmental changes of pertinence to crop development in California as a consequence of global climate change. Changing climate variables will impact crop and animal physiology with respect to yields and quality of cropping and animal systems. These climate variables include increases in temperature, atmospheric CO2 and O3 concentrations, changes in the amount and seasonality of precipitation, the availability of water resources and climate uncertainty . Major physiological impacts of these anticipated climatic changes include diminished yields from increased temperatures during crop and animal development , shorter periods of crop development , reduced product quality from unseasonal precipitation or adverse temperatures during fruit development , shifts in growing regions suitable for specialty crops ,grow hydroponic and decreased incidence of frost damage . This section will review our current understanding of crop and animal physiological responses to the predicted changes in climate, highlighting California agricultural commodities of greatest economic importance.

California’s unique environment allows for the production of a diverse range of crops, many of which are high value specialty crops for which California is the sole producer in the United States. We selected several of California’s top economically important crops to review individual crop physiological responses to increased temperature . Temperature influences crop growth and development through its impact on enzyme- and membrane-controlled processes. Carbon acquisition by photosynthesis typically has a temperature optimum close to the normal growth temperature for a given crop, while the carbon loss via respiration increases with temperature . Therefore, crop growth will be indirectly controlled by temperature due to the balance between photosynthesis and respiration rates. Temperature also serves as a controlling factor for developmental processes, and the accumulation of low or high temperatures often serves as cues for flowering and fruit maturation stages . For lettuce, an increase in temperature results in shorter crop duration, and thus, a reduction of intercepted radiation and decreased biomass at harvest. For example, the final weight of heads of lettuce and the time from transplanting to marketable fresh weight will likely decrease under elevated temperatures. An increase of 1.2°C to 5.6°C compared to a control resulted in significant decrease in time for lettuce to reach a fresh weight greater than 200 g . Growers may benefit from such circumstances since the effects in final yield are projected to be relatively small. However, extreme temperatures, especially if the onset of such temperatures is rapid, may present greater challenges to growers in controlling bolting and tipburn . The incidence of tipburn appears to be higher when plants are suddenly exposed to stress, such as high heat, because higher levels of physiologically active gibberellins under prior stress-free growth raise the permeability of cell membranes and disturb calcium transport into rapidly growing tissues .

Bolting is a day-length response, but high heat mediates this response . Not surprisingly, most of summer lettuce production is in the cool, foggy valleys of the Central Coast, with fall and winter production in the desert and Central Valley. Climate change may thus reduce summer production of this important crop.Increased temperature can accelerate the development of rice resulting in lower yields, which tends to offset any increase in yield stimulated by increased atmospheric CO2 . For field production in the Philippines, grain yields were found to decline 10% for each 1°C increase in minimum temperature . Additionally, high temperatures induce spikelet sterility in rice and elevated CO2 levels may exacerbate this effect . An elevated atmospheric CO2 concentration of 300 µl l-1 CO2 above ambient decreased the critical air temperature for sterility by 1°C . This interactive effect was hypothesized to be due to higher canopy temperatures that resulted from decreased stomatal opening and transpirational cooling under elevated CO2. Rice cultivars currently grown in the United States may be more sensitive to temperature stress than Asian cultivars, with upper temperature thresholds of 32°C -35°C .Temperature influences the development of stone fruits and nuts with regards to chilling requirements, heat accumulation and heat stress damage. Chilling is generally defined as the accumulation of hours between 0°C-7°C during the period of bud dormancy in fruit and nut crops, which is from November to March in California; California is currently classified as a moderate to high chill region. Decreased chilling can result in late or straggled bloom, decreased fruit set and poor fruit quality, which will decrease the marketable yield of these commodities . Heat accumulation refers to the summation of hours between 7°C-35 °C, and length of time of fruit development has been strongly correlated with the units of heat accumulation during 30 days after bloom . Enhanced rate of fruit development due to increased temperatures at this time can result in decreased fruit size. For example, warm spring temperatures in 2004 caused early fruit development and harvest, and small fruit size for peaches and nectarines, reducing the quality categories of these fruits . Heat stress can cause fruit damage and reduced quality during bud and fruit development leading to double fruits, fruit sutures, and heat burn . The effects of temperature on yield and quality of citrus are comparable to other perennial tree crops in that citrus require a cool period for dormancy, but are also subject to freezing or heat stress losses during critical fruit development periods .

Crop models have been used to simulate citrus responses to increasing temperatures and atmospheric concentrations of CO2 in the United States . Simulated citrus yields increased 20- 50% nationwide depending on the climate change scenario, with a 65% reduction in crop loss due to fewer periods with freezing temperatures . In response to temperature increases of up to 5°C, simulated yields increased for northern regions of California, but decreased in southern growing areas of the state . Including the effect of elevated atmospheric CO2 concentrations on citrus production counteracted the yield decreases in southern locations. The production region for citrus may be able to expand northward under predicted climate change conditions .The potential response of grapevines to increased temperatures due to climate change is relatively unknown. Increasing temperatures may result in premature ripening and thus decreases in quality for some cultivars in some major California wine grape growing regions based on modeling approaches that calculated threshold temperature impacts by using down scaled temperature projections for key counties relative to monthly average temperatures . Additionally,growing lettuce hydroponically modeling of climate change scenarios has shown that grapevine yields may become more variable, which will increase economic risk for growers . Climate change and its physiological impacts upon grapes may also influence terroir, an important quality and marketing characteristic that is especially important in this industry .The predicted increases in photosynthesis for most C3 species due to elevated CO2 have been widely accepted . However, the direct outcome of increased photosynthetic rates is uncertain in terms of increasing crop growth and allocation to harvestable yield . For example, wheat yields of 156 experiments under elevated CO2 were highly variable , probably because of interactions of elevated CO2 with other environmental factors, such as temperature, water and nutrient availability . Concentrations of mineral nutrients in plant tissues grown under elevated CO2 decrease, even when nutrient supply is not restricted . Of particular interest is the decrease in the concentration of nitrogen because N plays a central role in plant metabolism and bio-geochemical cycles. In a meta-analysis comparing 69 different C3, C4 and N-fixing species grown under elevated CO2, C3 species showed an average decrease in N concentration of 16% , compared to a decrease of only 7% in C4 and N-fixing plants. There is a close relationship between photosynthetic capacity and concentrations of leaf N, soluble protein, and the carboxylating enzyme, Rubisco .

Photosynthesis down-regulation after initial stimulation by elevated CO2 occurs with a simultaneous decrease in the concentration of Rubisco . Several hypotheses to explain the decrease in shoot N concentration and photosynthetic acclimation have been put forth. First, nutrient limitation could be the result of increased N immobilization by soil microorganisms receiving more plant-derived C under elevated CO2, or by increased sequestration of N into long-lived biomass . Second, plant N demand of C3 species may be lower because the decrease in photosynthesis under elevated CO2 leads to higher efficiency of the photosynthetic apparatus, thus requiring less Rubisco and diversion of N into enzymes of the photo respiratory pathway . Acclimation of photosynthesis might occur because a build-up of carbohydrates triggers a feedback regulation of transcription factors for enzymes involved in photosynthesis Third, elevated CO2 decreases nitrate assimilation in C3 but not in C4 species . This may contribute to decreased Rubisco concentration and carbon gain when the plant N supply is in the form of NO3 – , since NO3 – reduction to NH4 + appears to compete with photosynthesis for reductant . Conversely, N fixation by symbionts and by free-living rhizobia, is stimulated by elevated CO2 . In short, elevated CO2 interacts with environmental variables and plant physiological responses that differ among species, making it difficult to predict plant productivity, species composition and nutrient cycling, especially in natural ecosystems and rangelands. In California grasslands, for example, elevated CO2 decreased net plant productivity, when temperatures, precipitation or soil N in the form of NO3 – addition were increased compared to ambient levels . In a grazed pasture on the west coast of the North Island of New Zealand, species composition changed as a consequence of elevated CO2. The proportion of legumes increased, while the N concentration of the other species decreased . Total N intake by ruminants was not changed, but the N recycled by the animals was more susceptible to losses due to ammonia volatilization because the higher proportion of legumes in the diet of the grazers leads to a greater proportion of N excreted as urea . Thus, a change in forage quality and a shift in the relative abundance of plant species induced dietary changes that may enhance loss of nitrogen and increase release of N trace gases. The consequences of elevated CO2 for crop plants are decreases in grain protein and in the case of wheat, bread making quality . Elevated CO2 has been shown to increase grape yield without altering wine quality, but more research is needed to consider the interaction between increased CO2 and temperature . Other crops, such as strawberries , will likely benefit from elevated CO2 concentration. The fruit flavor of strawberries was superior in those grown in elevated CO2 during a 3-year open top chamber experiment . Fruit dry matter, fructose, glucose, and total sugar contents, as well as volatile aroma compounds and antioxidant properties were increased. Malic and citric acid contents were decreased on the other hand, compared to those in strawberries grown under ambient CO2 .Many crop species are sensitive to elevated O3, including cotton , watermelon , alfalfa and plum . In a meta-analysis of 53 studies of soybeans exposed to O3 concentrations of >60 ppb, an decrease in biomass of 34% and seed yield by 24% was observed . Elevated CO2 ameliorated damage caused by O3 at 60 ppb, probably because elevated CO2 decreases stomatal conductance and thus lowers O3 concentrations inside the leaf . However, growth of paper birch and sugar maple was severely reduced after long-term exposure to elevated O3 under both ambient and elevated atmospheric CO2 concentrations . Therefore, effects of elevated O3, in combination with elevated CO2, may only be fully assessed after long term exposure of perennials to the altered atmospheric conditions. This may be of particular significance to tree crops, which is some cases, California is the primary or sole producer in the United States.In a report to the California Energy Commission, Adams et al. used climate and crop data to model the impacts of changing climate, increasing CO2, and technology on California crops .