That there are wide cultivar differences in these relationships leaves open the possibility our chosen cultivar, Cabernet Sauvignon, has naturally low concentrations.Despite the expectation that the products of membrane lysis would track with its causative reactive oxygen species, MDA concentrations did not commence at cell death onset or follow H2O2 concentrations and similarly saw no differences caused by either irrigation pulses. Like H2O2, MDA is a signal and damaging reactive compound subject to regulatory networks addressing biotic and abiotic stress, thus, accounting for its presence is not straightforward. In its most reactive state and at high levels, MDA can damage proteins, nucleic acids, and photosynthesis-related proteins, while at lower levels it can serve as a signal activating alcohol dehydrogenase . This activation helps achieve redox homeostasis by producing NADPH, which in turn oxidizes aldehydic compounds like MDA to prevent their build up to toxic levels . Post veraison, cellular conditions continue to be more favorable for the proliferation of ROS as scavenging and antioxidant capacity diminishes, whereas MDA may be more favored for degradation as ADH is more active then due to increased ethanolic fermentation in hypoxic conditions. Considering these processes we would have expected our MDA values to spike during cell death and then decline as hypoxic conditions maintained, drainage gutter but they ended the experiment at roughly the initial levels .Our experiment attempted to establish significant differences in water stress before and during cell death onset to test whether late-season-dehydration-related processes respond to irrigation pulses at those timepoints.
Despite a 40% increase in volume in subsequent two-week periods, vines under different irrigation regimes showed no treatment effects across all water stress parameters. The only difference was that the late pulse vines began the experiment with significantly higher stomatal conductance and photosynthesis rates , yet by the time the late pulse occurred these differences were erased. Other experiments investigating the relationship between water stress and cell death maintained consistent differences in watering throughout the growing season and found significant effects on both ψmd and cell death dynamics . We sought a strategy fit for implementing late in the season when root systems and canopies are fixed, and irrigation treatments would only affect post-veraison processes related to cell death. Root growth after veraison is largely inactive as the vine expends more energy into reproductive growth, thus our water treatments would not have spurred much new root growth but rather provided more water availability to fixed root systems. This water would be largely available because of the sizable portion of clay in our soils, though soil moisture probes would be needed to confirm the late treatment had significantly more water available. The low-vigor rootstock used in this study, 420A, could have limited the efficacy of our treatments due to its lower root hydraulic conductivity and aquaporin activity, whereas high-vigor, drought-avoidant rootstocks such as 140Ru, could have rendered the treatments more effective .
For example, after irrigating minimally pre-veraison, Netzer et al. imposed an augmented irrigation regime of .55in/week during a 45 day period post veraison for Cabernet Sauvignon on 140Ru in a Mediterranean climate with sandy soils and found continuously improving ψmd. A higher-vigor rootstock could have enabled our vines to transpire all of the water that was available to them, reduce ψmd, potentially prompting a different response. Given Netzer et al.’s success with alleviating water stress in late ripening, to achieve clarity on the effectiveness of late season irrigation pulses on water stress and cell death, it may be advantageous to use a high vigor rootstock under stronger drought conditions pre-veraison before irrigation is augmented. Of additional consideration regarding the similarity of ψmd across our treatments is that the vines used in this study were pruned by students, not an experienced crew, which does not guarantee uniformity in sap flow or canopy environment. Pruning without respect to sap flow—that is, year-after-year pruning cuts are made adjacent to each other to minimize the redirection of sap flow—has been shown to effect negative changes in xylem hydraulic conductivity in grapevines . The strong relationship found between xylem hydraulic conductivity and photosynthetic capacity of the canopy indicates this would have also affected our gas exchange measurements . Without a consistent approach to pruning among our vines in this respect, variance in xylem hydraulic conductivity could have created enough statistical noise to mute differences in gas exchange and water potential.This study demonstrated that increasing irrigation at the onset of cell death in winegrapes can delay the rate of cell death and maintain greater membrane integrity. When cell death is reduced, xylem backflow, which is difficult to compensate for as cell death progresses, is held off and turgidity is better preserved. Our results also confirm both the approximate range of onset date for Cabernet Sauvignon and the direct relationship between reactive oxygen species and cell death.
The exact mechanism of cell death remains opaque and many questions remain as to how cell death ought to be managed, especially in drought conditions. It is ultimately unclear why the late treatment’s cell death rate was positively affected by the late pulse, as we would expectxylem backflow to be too strong to overcome once cell lysis proliferates, considering there was no difference in water stress across treatments. Nonetheless, we believe this opens the door to more paths of inquiry in the mechanism of cell death. To investigate why a pulse of irrigation near cell death can effectively delay cell death without altering gas exchange or stem water potentials will require monitoring additional parameters, such as xylem/phloem conductivity in the pedicel or peduncle, dissolved oxygen ingress into the berry, berry respiration/fermentation rate, and soil moisture. On the other hand, establishing firm differences in water stress prior to onset would clarify whether the activity of programmed-cell-death-related genes does in fact respond to increased water stress and account for increased ROS and cell death. Establishing such differences would also clarify how far in advance a preemptive transcriptional response occurs, which would inform the timing of proposed irrigation strategies to postpone or delay cell death onset. A framework for an efficient irrigation regime that postpones cell death onset is needed to address future water scarcity, and our treatments and the levels of irrigation applied throughout this experiment are far from optimal to this end.Increasing chemical use, in conjunction with growing weed resistance and limited options for chemical weed control, has raised costs and depleted the bottom line for many rice producers in California. Many of the restrictions on farm chemical use can be traced to growing recognition of environmental externalities from chemicals used on the land and political pressure from environmental groups. For example, a recent district-court ruling banned the application of 38 pesticides along Northwest salmon streams, plastic gutter and estimates of the economic impact of the decision vary wildly . Environmental groups such as Greenpeace oppose the adoption and diffusion of genetically modified food crops such as GM2 rice. This opposition is largely based on the uncertainty of potentially adverse health and environmental impacts of GM rice and the lack of labeling requirements for GM foods. This is a potentially ironic position for environmental groups to take, given the possible environmental advantages of GM crops over more conventional varieties that depend heavily on the use of multiple chemicals and applications that may prove more damaging than the corresponding GM regime. This issue is critical in California, where agriculture is intensive and a relatively heavy user of chemicals. The economic impact on growers from chemical use regulations depends critically on the number of substitution possibilities available for cost-effective weed control. The more options individual rice growers have to control weeds, the less severe will be the adverse impact of the regulations on grower profits. However, environmental activists, regulators, and the courts view a wide range of available chemicals that have varied environmental risks as undesirable. In recent years, widespread adoption of GM crops such as herbicide-tolerant soybeans and canola and pest-resistant [e.g., Bacillus thuringiensis ] corn and cotton has provided growers with new production alternatives that reduce chemical usage. But the new technologies are not without controversy as some consumers have expressed resistance to purchasing foods made from transgenic materials. In California, environmental groups and organic-rice farmers are also opposed to any cultivation of GM rice in the state. This report examines these issues in the context of California rice production. In particular, we estimate the potential economic impacts of one alternative weed-management strategy, namely, cultivation of HT transgenic rice. Potential grower benefits, measured by net returns over operating costs per acre of first-year adoption, are calculated using a partial-budgeting approach3 based on a representative cost structure.
Sensitivity analysis is then utilized to account for the heterogeneity in growing conditions across the state as well as uncertainty regarding yields, technology fees, and government assessments on transgenic seed. To augment these results, the partial-budgeting approach is applied to data from an independent three-year field trial designed to evaluate alternative herbicide regimes, including one transgenic rice cultivar. The report proceeds as follows: The next section reviews available information on transgenic rice and describes the potential impacts of grower adoption in California, including market-acceptance issues. We then describe our methodology and present results for a typical Californiarice producer. Next, a range of estimated impacts based on alternative yield differentials and technology fees is presented, followed by a Monte Carlo analysis. The subsequent section provides an economic analysis corresponding to the three-year field study. Environmental regulations for rice production and potential environmental impacts of the new technology are then evaluated, and the final section discusses the limitations of our analysis and concludes.In 2003, California rice growers harvested 495,000 acres of rice, which yielded 39.6 million hundredweight , constituting about 16.5 percent of acreage and 20 percent of total rice production in the United States . The vast majority of California’s rice is of the medium-grain variety while the southern U.S. states primarily produce long-grain varieties. Over the last several years, there has been no discernible trend in California acreage planted or in total volume of production. World rice prices, on average, have been on a decreasing trend4 and, simultaneously, California growers have faced increasing production costs, especially in the area of weed management [U.S. Department of Agriculture , Economic Research Service 2002]. The top three weeds in California rice production are barnyardgrass, watergrass, and sprangletop while various other broadleaf plants, grasses, sedges, and cattails affect production [Gianessi et al.; California Rice Commission 2003]. Interestingly, red rice, a weed of the same genus and species as domesticated rice, is not a major problem in California despite being the number one weed in Louisiana, Arkansas, and Missouri . The combined effect of lower prices and higher production costs has put downward pressure on California rice grower returns and led to considerable research efforts to improve overall weed management through cultural, chemical, and other management means. In California, both chemical and non-chemical techniques are used for weed control . Recently, however, California rice production has experienced what has been called an “epidemic” of herbicide resistance, especially from watergrass, which has resulted in herbicide costs increasing to close to $200 per acre for some growers .5As such, technologies that allow for a small number of applications of chemicals where efficacy is not affected by the resistance problem, as would most likely be the case for HT rice, have the potential to significantly lower this component of rice production costs. There are currently no commercialized GM rice varieties anywhere in the world. However, many transgenic varieties are in the “development pipeline,” including HT, insect resistant , bacterial and fungal resistant, and nutrient-enhancing “Golden Rice,” which produces beta-carotene, a substance that the body can convert to Vitamin A. A non-transgenic but genetically altered variety called Clearfield® IMI by BASF, a mutated HT variety, was released in the United States in 2002 . Approximately 200,000 acres of Clearfield® were planted across the Southeast in the 2003 growing season, accounting for about 8 percent of the seeded area in that region . Countries that are major rice producers and consumers, including China and Japan, are rapidly developing and testing GM rice varieties . For instance, China has approved for environmental release three insect-resistant rice varieties and four disease-resistant varieties and is developing HT, salt-tolerant, and nitrogen-fixing cultivars . Many of these varieties have the potential to be of value to producers through reduced disease or pest-control costs and to the environment through reduced use of chemicals, thereby reducing runoff and water pollution.