Five of these path ways cycle, glyoxylate and dicarboxylate metabolism, starch and sucrose metabolism are related to carbohydrate metabolism.In addition, alpha-linolenic acid metabolism, isoquinoline alkaloid bio synthesis, and methane metabolism were also disturbed; these pathways are related to lipid metabolism, biosynthesis of other secondary metabolites, and energy metabolism, re spectively. These results indicate that accumulated nCu may have a significant impact on carbohydrate and amino acid metabolism for cucumber plants.Galactose metabolism is the most significantly disrupted pathway by nCu, with 5 metabo lites affected in this pathway including galactinol, glycerol, myo-inositol, galactose-6-phosphate, and fructose. All of these metabolites were down-regulated except fructose; this indicates galactose metabolism was inhibited by nCu. The TCA cycle is the second most significantly disrupted pathway . Succinic acid, fumaric acid, citric acid and pyruvic acids, which are TCA cycle upstream intermediates, were significantly changed by exposure to nCu at all levels. Maleic acid, a trans-isomer of fumaric acid, was also increased. In the TCA cycle, acetyl-coenzyme A is a key to the conversion of oxaloacetate to citric acid in the mitochondria. The down-regulated citric acid is likely evidence that nCu influences the availability of acetyl CoA.Interestingly, most of the metabolites related to carbohydrate metabolism are reported to be a response to iron deficiency in the leaves and xylem sap of several plant species, including peach, tomato, and sugar beet.It has been demonstrated that the response of leaves to iron deficiency is up-regulation of carbohydrates and TCA cycle metabolites and N-related metabolites.Combining the present results and a previous study,46 this may indicate that iron deficiency induced by nCu contributes in part to changes in cucumber fruit metabolite profiles.

Arginine and proline metabolism is apparently the most disrupted pathway . There are eight metabolites implicated in this pathway and three of them are significantly changed,nft growing system including proline, ornithine, and citrulline. Ornithine and citrulline are also involved in the urea cycle, which suggests a perturbed urea cycle. The second disrupted pathway is glycine, serine, and threonine metabolism, with five metabolites affected in this pathway: homoserine, threonine, serine, glycine and pyruvic acid. Among them, glycine and pyruvic acid are significantly increased due to plant exposure to nCu at all levels. This may indicate this pathway was activated. Isoleucine, valine, and leucine, which are branched-chain amino acids , were up-regulated due to nCu exposure . It has been reported that these amino acids share several common enzymes.The up-regulation may indicate that the related enzymes have been activated. Pyruvate, a precursor of valine and leucine, was also up-regulated. It is reported that BCAA may serve as an oxidative phosphoryla tion energy source during plant stress.66 Therefore, the up regulation of BCAA may indicate an adaptation process of cu cumber plants to stress induced by nCu.As mentioned before, Cu was present in cucumber fruit at an average concentration of 21.3, 27.4, 28.9, and 31.5 mg kg−1 for control, 200, 400 and 800 mg kg−1 treatments. This indicates that Cu uptake did not increase linearly with exposure. It is likely that the binding of Cu ions to soil clay minerals decreased their bio*availability. In addition, most of the Cu was sequestered in root tissues. Therefore, the likelihood of Cu over-accumulation in fruit is low. According to US Department of Agriculture, annual per capita consumption of fresh cucumbers in the United States is 3.0 kg in 2013, which means average daily cucumber con sumption is ∼8.2 g per person-day . The aver age cucumber water content is 95%, so the daily consumption is 0.41 g dry weight per person-day. Thus, daily personal Cu intake from cucumber used in this study would be 10.0, 11.2, 11.8, and 12.9 μg from control, low, medium and high treatments.

According to the Food and Nutrition Board at the U.S. Institute of Medicine of the National Academies, the recommended average requirement for Cu is 700 μg per per son-day, with a tolerable upper intake level of 10 mg per per son-day.67 Therefore, Cu intake from consumption of cucumber exposed to nCu enriched soil would be within the recommended Cu levels, even at the higher application level. Hence, consumption of nCu treated cucumbers, even at the high level, represents no significant added risk to consumers. Fruit quality is affected by, among others, sugars and fatty, amino, and carboxylic acids. The profile alteration of these nutrients may result in flavor and nutritional supply changes induced by exposure to nCu. Organoleptic analysis was beyond the scope of this work.In 2003, nearly 106 million acres of transgenic or genetically engineered crops was planted in the United States, part of 167 million acres of such crops grown worldwide . Despite the fact that the first commercialized transgenic food crop was the Flavr Savr tomato, four agronomic crops account for virtually all of the current acreage. Last year, only four horticultural crops developed using recombinant DNA technology were available in the United States: papaya, sweet corn, squash and a carnation. Except for transgenic papaya, which accounts for approximately 50% of the Hawaiian crop , the fraction of the total horticultural commodities represented by transgenic varieties is miniscule. The absence of significant commercialization of transgenic varieties in horticulture is not due to lack of potential products or value . The basic techniques of molecular biology have become routine, and considerable research is being conducted on horticultural crops. For example, herbicide resistance has been transferred into bent grass and bluegrass to make weed control in municipal and highly managed turf environments such as golf courses more efficient. However, they have not been commercialized. Similarly, some horticultural crops, including lettuce and tomato, have been engineered with herbicide resistance and tested in field trials but remain uncommercialized. Disease resistance, particularly to viruses, can be developed using biotechnology, and potato and papaya cultivars engineered for virus resistance have been commercialized, but many potential applications are currently underutilized. Improving traits that directly benefit consumers, such as nutritional or aesthetic quality, is also technically feasible now in many horticultural crops, but only a few products have reached the market.

The major technologies that have been approved and widely adopted by the industry focus on input traits, or those affecting production of the crop rather than the qualities of the final product. Although most approved genes confer insect resistance and herbicide tolerance, a range of genetic traits has been approved by the U.S. regulatory system .Insect resistance. Insect resistance has been engineered primarily by using two classes of bacterial genes derived from Bacillus thuringiensis ssp. kurstaki and ssp. tenebrionis . These Bt genes control a broad spectrum of lepidopteran and coleopteran insects, respectively. The genes have been approved for use in major row crops and some horticultural crops . Potato and sweet corn varieties engineered for resistance to Colorado potato beetle and corn earworm, respectively,vertical hydroponic nft system were in commercial production for several years and were technically and agronomically successful, allowing significant reductions in insecticide use . However, the transgenic potato varieties were withdrawn from the market after major processors and distributors chose not to purchase and market them. Bt sweet corn, while still available, is not widely grown for the same reason . Herbicide tolerance. Several herbicide-tolerance genes are registered for use. The most widely commercialized gene is a bacterial enzyme conferring tolerance to glyphosate, the active ingredient of Roundup herbicide. Transgenes conferring tolerance to bromoxynil , glufosinate and sulfonylurea herbicides are also approved for use in a wide variety of crops. In addition, crops tolerant to imidizolinone and sulfonylurea herbicides have been developed through nontransgenic methods based on natural or induced mutations. However, no horticultural crops engineered for herbicide resistance have been commercialized, although several have been developed and tested. Virus resistance. The use of viral coat protein genes to confer resistance has been approved for several virus crop combinations . The most commercially successful has been papayas engineered for resistance to the papaya ring spot virus. This product has revived the Hawaiian papaya industry, which was devastated by the virus in the 1990s . Small acreages of transgenic squash resistant to mosaic viruses are also grown. Virus-resistant potato varieties were formerly commercialized but are not currently being marketed. Newer technologies, such as RNA interference or RNA silencing , offer promise for developing resistance to other damaging diseases, such as those caused by geminiviruses . Flavr Savr tomato. Transgenic horticultural crops providing direct benefits to the consumer have also been developed. Calgene’s Flavr Savr tomato silenced the gene encoding polygalacturonase, an enzyme implicated in fruit softening. The expectation was that the tomato would soften and spoil more slowly and could be picked at a later stage of maturity. This later harvest, in principle, would permit greater development of flavor compounds and better taste. This product, first marketed in 1994, was a success with consumers but failed economically for a variety of reasons .

This same gene was also used in a tomato variety processed for paste and marketed by Zeneca in the United Kingdom. The trait reduced processing costs and consumers accepted the clearly labeled product, until the European uproar over biotech foods forced it off the supermarket shelves. U.S. regulatory agencies also approved several other delayed-ripening tomato varieties based on strategies targeted to block the ethylene biosynthetic pathway that is essential for ripening. None of these products is currently marketed, despite their technical feasibility and potential consumer benefits. Rather, they were preempted by a nonbiotech approach utilizing the naturally occurring rin mutant of tomato that delays fruit ripening. Heterozygous plants produce fruits that ripen at a significantly slower rate than normal fruits.A nontransgenic approach achieved essentially the same objective, and aggressive breeding and marketing of the long-shelf-life rin hybrid tomatoes made the transgenic approach redundant. Lengthening post harvest time. Similarly, virtually all bananas marketed in the United States are naturally deficient in their ability to initiate ethylene synthesis, allowing them to be shipped green and ripened by exposure to ethylene gas prior to sale. However, where natural mutants are not available, the general approach of manipulating ethylene synthesis has great potential for application in other climacteric fruits , particularly tropical fruits with short post harvest lives such as mango, papaya and banana. A higher quality fruit bringing tangible value to the consumer could improve the market acceptance of biotech crops. Despite their early introduction and initial market success, and in contrast to what many consumers may believe, the only biotech horticultural commodities currently marketed in the United States are Hawaiian papayas, a small amount of squash and sweet corn, and a few carnation varieties. The commonly cited estimate that as much as 70% of food products in U.S. supermarkets contain ingredients from GE crops is not attributable to fruits and vegetables, but rather to the widespread use of corn, canola and soybean oil, soybean protein, corn starch and related products in virtually all processed foods.As global acreage of biotech agronomic crops sustains its eighth consecutive year of double-digit growth rates , it is paradoxical that the trend in horticultural crops is exactly the opposite, particularly as many of the approved genes fit naturally with the needs of horticultural crops. Fear of consumer rejection on the part of both producers and marketers of horticultural products is a major impediment to wider utilization of biotechnology, even though many consumer polls do not find a majority negative opinion about it . Many food companies are unwilling to risk the consequences of alienating even a small fraction of their potential market . Clearly, growers, distributors and consumers must all see biotech crops as in their best interests for commercialization to be successful. Products offering compelling value could alter the economic forces influencing producer choices and could create consumer demand to pull such products through the marketplace. Loss of pesticides. One factor that may significantly alter grower economics is the loss of currently registered pesticides due to environmental and health concerns. For example, methyl bromide is scheduled to be withdrawn from use in the United States in 2005 because it contributes to depletion of the ozone layer. Methyl bromide is widely used in horticultural crops to control soil borne diseases and weeds and to fumigate harvested crops to eliminate insects.