These products pass through an elaborate biochemical cycle that eventually forms one molecule of a 6-carbon sugar and regenerates RuBP. The reaction of RuBP with oxygen oxidizes the RuBP, splits it into one molecule of a 3-carbon compound and one molecule of a 2-carbon compound , and subsequently releases carbon dioxide, hence the names C2 pathway or, more commonly, photo respiration. In total, photo respiration consumes biochemical energy, but does not result in any net production of sugar . Thus, photo respiration has been viewed as a wasteful process, a vestige of the high carbon dioxide atmospheres under which plants evolved . The balance between C3 carbon fixation and photo respiration depends on the relative amounts of carbon dioxide and oxygen entering the active site of rubisco and the affinity of the enzyme for each gas . At current atmospheric levels of carbon dioxide and oxygen , photo respiration in most crops dissipates over a quarter of the organic carbon produced during carbon dioxide assimilation . In contrast, C4 crops , which have a metabolic carbon dioxide pump that increases the concentration of this compound at the catalytic site of rubisco, minimize photo respiration at the expense of the additional energy required for pumping. Elevated levels of atmospheric carbon dioxide inhibit photo respiration in C3 plants,vertical greenhouse making photosynthesis more efficient. Initially, this accelerates both their photosynthetic carbon dioxide assimilation and their growth by about a third.
After a few days or weeks, however, carbon dioxide assimilation and growth both slow down until they are accelerated in the long term by only about 12% and 8%, respectively . Moreover, leaf nitrogen and protein concentrations ultimately decrease more than 12% under carbon dioxide enrichment . Such a loss of nitrogen and protein significantly diminishes the value of this plant material as food for animals and humans. Fig. 6. Differences in leaf carbon fixation capacity versus total nitrogen concentration between C3 plants grown at elevated and ambient carbon dioxide concentrations. Each symbol designates the mean ratio for a species. Shown are the regression line and 1:1 line . This data suggests that changes in photosynthesis from carbon dioxide enrichment derive from changes in plant nitrogen levels under carbon dioxide enrichment . Together these trends are known as carbon dioxide acclimation.According to this hypothesis, plants under carbon dioxide enrichment initially assimilate more carbon dioxide into carbohydrates than they can incorporate into their growing tissues. In response, they diminish carbon dioxide assimilation by decreasing their rubisco levels . This change in rubisco levels, however, is not necessarily selective; the decrease may instead just be part of the overall decline in protein and nitrogen concentrations .Another hypothesis for carbon dioxide acclimation is that shoots accumulate carbohydrates faster than roots can absorb nitrogen from soils, making leaf nitrogen concentrations decrease .
As these leaves senesce and drop to the ground, plant litter quality declines, microbial immobilization of soil nitrogen increases because of the high carbon-to-nitrogen ratios in the litter, soil nitrogen availability to plants further diminishes because more soil nitrogen is tied up in microorganisms, plants become even more nitrogen limited, plant protein levels decline and plant processes including photosynthesis slow down . This hypothesis, however, has difficulty in explaining the variation in carbon dioxide acclimation among sites and among methods of carbon dioxide enrichment .We have discovered another explanation for carbon dioxide acclimation: in C3 plants, shoot assimilation of nitrate into organic nitrogen compounds depends on photo respiration, so any condition that inhibits photo respiration also inhibits shoot nitrate assimilation . Thus, at elevated carbon dioxide concentrations, C3 plants that rely on nitrate as a nitrogen source suffer severe deprivation of organic nitrogen compounds such as proteins. The resulting decline in organic nitrogen compounds reduces the plants’ yield and biomass production. While high applications of nitrogen fertilizer may partially compensate for this, the plants’ nitrogen and protein concentrations still diminish . Ammonium and nitrate are the two main sources of nitrogen that are accessible to plants from the soil. Plants show a wide range of responses to carbon dioxide enrichment because the balance between nitrate and ammonium availability varies over seasons, years, locations and plant species. In an annual California grassland where nitrate was the predominant nitrogen source, net primary productivity diminished under carbon dioxide enrichment . This was presumably because elevated carbon dioxide inhibited plant nitrate assimilation , and the grasses became deprived of organic nitrogen.
In contrast, ammonium is the major form of nitrogen available to plants in marshes because wet, anaerobic soils promote denitrification and nitrate leaching . For example, the dominant C3 plant in the Chesapeake Bay marsh showed little carbon dioxide acclimation ; even after a decade of treatment, photosynthesis and growth remained about 35% greater under carbon dioxide enrichment , with little change in nitrogen concentrations . In wheat, another C3 plant, elevated carbon dioxide atmospheres stimulated less growth under nitrate than under ammonium nutrition .Several physiological mechanisms appear to be responsible for the dependency of nitrate assimilation on photo respiration. First, the initial biochemical step of nitrate assimilation is the conversion of nitrate to nitrite in leaves. This step is powered by the high-energy compound NADH , and photo respiration increases the availability of this compound . In contrast, C4 plants generate ample amounts of NADH in leaves via a different biochemical pathway. This explains why shoot nitrate assimilation is relatively independent of carbon dioxide concentrations in C4 plants . Second, the subsequent biochemical step of nitrate assimilation is the conversion of nitrite to ammonium in the chloroplasts of leaf cells, which requires the transport of nitrite into the chloroplast. Elevated carbon dioxide inhibits this transport . Third, this subsequent step also requires chemical energy from the oxidation of a different high-energy compound called ferredoxin. Several other processes — in particular, carbon dioxide assimilation — depend on the same energy source and seem to have priority in using it. Ferredoxin becomes involved in nitrate assimilation only when carbon dioxide availability limits C3 photosynthesis .Many crops in California depend on nitrate as their primary nitrogen source. As atmospheric carbon dioxide concentrations rise and nitrate assimilation diminishes, these crops will be depleted of organic nitrogen, including protein, and food quality will suffer . Wheat, rice and potato provide 21%, 14% and 2%, respectively, of protein in the human diet . At elevated carbon dioxide and standard fertilizer levels, wheat had 10% less grain protein . Similarly, grain protein in rice and tuber nitrogen in potato declined by about 10% at elevated carbon dioxide concentrations. Several approaches could mitigate these declines in food quality under carbon dioxide enrichment. Increased yields may compensate to some degree for total protein harvested . Several-fold increases in nitrogen fertilization could eliminate declines in food quality , but such fertilization rates would not be economically or environmentally feasible given the anticipated higher fertilizer prices and stricter regulations on nitrate leaching and nitrous oxide emissions. Greater reliance on ammonium fertilizers and inhibitors of nitrification might counteract food quality decreases. Nevertheless, the widespread adoption of such practices would require sophisticated management to avoid ammonium toxicity,vertical grow towers which occurs when plants absorb more of this compound than they can assimilate into amino acids and free ammonium then accumulates in their tissues . Several of these issues might be simultaneously addressed by fertigation, or frequent additions of small amounts of ammonium-based fertilizers in water delivered through micro-irrigation. These findings have broad implications for the future of plant distributions and food production. Enriched carbon dioxide atmospheres will not enhance the performance of C3 plants to the extent originally envisioned. A 10% decline in food protein content will further burden regions of the world already affected by hunger. With a better understanding of ammonium and nitrate use by crops and careful nitrogen management, we can turn these phenomena to our advantage.With its Mediterranean climate of moist, mild winters and dry moderate summers, a broad range of fruit and vegetable crops can be grown year round on the central coast of California. Monterey and Santa Cruz counties combined produced $912 million gross value of strawberries and over $2.7 billion worth of vegetables in 2011.
As the interest in organic farming and the demand for organic produce has increased during the last decade, organic farming on the central coast has also greatly increased. There were over 9,300 certified organic hectares in Monterey and Santa Cruz Counties in 2011, five times the number recorded in 1998 . The total farm gate revenue from organic farming in these counties was over $197 million in 2011, representing a dramatic 12-fold increase in 13 years . This trend is also true for organic strawberry production. In 2000, 77 ha of organic strawberries were grown in central coastal California, but by 2012 this had increased to 509 ha, representing 8.3% of the total strawberry production in the area . Continued growth of organic strawberry production in this area, however, faces the challenge of managing soil-borne diseases without the use of synthetic fumigants and fungicides. Verticillium wilt is a soil-borne disease caused by Verticillium dahliae that can damage a wide range of important crops in California. Host crops include lettuce, tomatoes, potatoes, apples, cotton, artichokes, and strawberries . Due to its resilient overwintering structure , this pathogen can survive many years in soil even without host plants . In the premethyl bromide era, Verticillium wilt was a major limiting factor to strawberry production in California . Today, Verticillium wilt is one of several key soil-borne diseases facing California strawberry production and poses a long-term threat for organic strawberry production in the state.To avoid Verticillium wilt and other soil-borne diseases, as well as meet the requirements of the USDA National Organic Program , organic strawberry growers must implement crop rotation. Due to its high sensitivity to the disease, several years between strawberry plantings are necessary . For specialized strawberry growers in California, establishing a crop rotation system implies a major change in the design and management of the farming system. Due to the high costs of production and the high leasing fees of crop lands , specialized organic strawberry growers need to minimize the break time between strawberry crops as much as possible to stay in business. The following biological and cultural approaches to soil-borne disease management in strawberries have been tested: host resistance ; small cell transplants ; organic amendments such as compost ; high nitrogen organic fertilizers ; broccoli residues ; mustard residues , Sudan grass , and other cover crops ; microbial amendments including vesicular arbuscular mycorrhizal fungi ; plant growth promoting rhizobacteria ; crop rotations with broccoli, lettuce or Brussels sprouts; mustard seed meal ; soil-less trough production ; and anaerobic soil disinfestation . Further, a minimum of a three-year rotation is recommended for strawberries that do not use chemical fumigants in Europe , the Northeast and Midwest United States, and in eastern Canada . However, no research has yet integrated multiple biological and cultural practices for different rotation periods of organic strawberries in California. The objective of this project was to demonstrate the effects of strawberry planting frequency in organic strawberry/vegetable rotations and combined biological and cultural practices on fruit yield and disease level. We hypothesized that the use of non-host rotation crops for Verticillium wilt plus bio-fumigation with broccoli, mustard cover crop residues, relatively resistant strawberry cultivars, and compost application would suppress disease sufficiently to grow strawberries in rotation every two or three years. To test the above hypothesis, in 2001, we initiated a five year organic strawberry/vegetable rotation experiment in a commercial California field.The loss of soil productivity when crops are grown repeatedly on the same land resulting in poor plant growth and reduced yields is called “yield decline” , “soil sickness” , or “replant problem” . Such losses, called here “yield decline,” have been reported in many crops worldwide including strawberries . Biotic and abiotic factors can cause yield decline. Further, although one factor may possibly be responsible for yield decline, it is more likely that a combination of factors interact to cause the effect . This study also demonstrated the challenges researchers face when using a participatory process where farmer involvement is a key part of the experiment.