William DeBoer
Bill DeBoer is a research scientist at Indiana-based steadyGROWpro. A master gardener intern, Bill is responsible for overseeing the company’s laboratory operation, including the design and execution of research projects, plant propagation, seed germination and overall plant care. Bill has a B.S. and M.S. from
Off-season tomatoes are usually dull, tasteless and way too firm. Thankfully, however, there is now research being conducted on how to breed that summer flavor in those popular fruits.
Most people have tasted, or rather not tasted, hydroponically-grown tomatoes during the “off-season.” Dampened by the cold of winter, consumers are looking for that taste of warmer times only to be rewarded with a shell of the summer tomato. Color, taste, firmness and selection are usually severally lacking. There is still much experimentation that is needed until we produce a great tomato in the winter, but research is truly a work in progress. While breeding programs have brought us closer to a better-flavored tomato, the scope of this article will focus on the cultural practices that should enhance the palate response. With that said, genetics should focus not only on storage qualities (shelf life, skin thickness, disease resistance for plants, etc.) but on promoting those factors associated with superior taste, which include but are not limited to soluble solids (measured by a refractometer in Brix, %), pH and organic acids. Now let’s dive into the chemicals and the factors influencing tomato flavor.
With plant genetics aside, let’s focus on certain compounds that impact flavor. Tomatoes are roughly 90% water with the solids representing 5% to 7% of the dry weight. Tomato fruit flavor is generally created from the ratio of carbohydrates (glucose, fructose, sucrose) to organic acids (citric and malic acid). Generally a good-flavored tomato possesses both a high amount of sugars and a high amount of acids. Likewise, bland tomatoes contain a high amount of sugars to a low amount of acids. As the tomato matures, the amount of sucrose (imparts a sweet flavor) decreases as starch (not so sweet) increases. Starch consists of many linked units of glucose acting as an energy storage reserve. From this reserve, the readily metabolizable glucose units can be cleaved off for the plant’s use.
When a traditional red tomato is changing from the orange to red color, the acidity drops, which increases the fruit’s pH. This has important ramifications to harvesting as tomatoes that are fully vine ripened may have a reduced blend of acid to sugar. However, the ripening process is ethylene dependent. This plant hormone gives rise to lycopene (antioxidant/carotenoid) production, the causative agent in red color change. Therefore, the implication of early harvest may slightly reduce the rising pH, but will impact the flavor attributed to lycopene which is maximized during the deep red stage of ripening. In the book Tomato Plant Culture, it is stated that the recent genetic breeding programs of tomatoes have favored increased yields, shelf life and skin thickness with the negative side effect of an increase in fruit pH. Further stating, this is why so many consumers are shifting back to the heirloom varieties as the pH is generally lower than the recent hybrid cousins and therefore generally regarded as better tasting.
So now you may be asking, “How can I produce tomatoes that are high in both sugar and acid?” While some of these subsequent points below are not directly tied to flavor, they do promote healthier, more productive plants, which coupled with the right cultivar, should produce better tasting tomatoes.
Maintain ideal temperature range of 77oF daytime to 650F nighttime
Ideal and constant temperatures will influence the biochemical reactions related to the water, carbohydrate and organic acid content of the ripened fruit. Proper growing temperatures will also improve yields by increasing pollination capability and decreasing fruit drop.
Expose plant to high/full light
Saturation of light by leaf receptors will maximize photosynthesis and subsequent carbohydrates that will be used for a myriad of biochemical reactions including fruit production.
Utilize a nutrient formula geared toward optimal tomato production
The nitrogen ratio should be derived from nitrates more than ammonia. In addition, keeping the ratio of potassium to nitrogen high will promote thicker skin that should reduce skin cracking.
Fertilization with a slightly higher EC at 3.5 dS/m
Ensuring a higher level of fertilizer is probably beneficial in a two-fold manner. In one regard, maintaining adequate levels of nutrients, like calcium, will not only improve plant vigor, but in the case of high cytosolic calcium, reduce the prevalence of blossom end rot. Secondly, the increased fertilizer (salt content) may contribute to the benefits of slight saline stress as detailed below.
Addition of sodium chloride at 35 ppm
Research studies have shown a correlation between slight salt stress and higher accumulation of soluble solids in particular sugars, organic acids and amino acids. At first one might be inclined to think this is merely a function of salt removing water from the fruit and thereby concentrating the sugar, organic acids, etc. However, one study showed that during initial fruit development, salt stress caused a doubling of starch in the fruit that subsequently increased the soluble sugars in the ripened fruit. It appears that slight saline stress at the time of initial fruit development may be advantageous, but constant saline stress likely would lead to decreased vigor of the plant and decreased yield and size of the fruit.
During cultivation, avoid periods of drought stress followed by excess watering
Drought stress has been linked to fruit cracking and blossom end rot (BER). Proper irrigation will go a long way in reducing both.
Store ripening tomatoes in warm temperatures
Temperatures below 400F will cease enzymatic activity necessary for maturity.
In conclusion, both genetics and cultural practices play a pivotal role in the overall taste of the tomato fruit. In general the smaller cherry and grape tomatoes have a higher total soluble solids (Brix %) relative to the larger ones like beefsteak. Experimentation by growers with new and emerging cultivars coupled with ideal cultural practices will continue to improve the tomato flavor closer to its summer relative. For more information on growing tomatoes in the greenhouse or field, refer to the book Tomato Plant Culture in the literature cited.
References
- Jones Jr., J. B. Tomato Plant Culture: In the Field, Greenhouse, and Home Garden 2nd Edition. CRC Press, 2008; pg. 71,74, 77, 101, 103, 105-106, 109, 111, 114-117.
- Peet, M. M.(Ed.) Sustainable Practices for Vegetable Production in the South. Focus Publishing, Newburyport, MA, 1996; pg. 149-157.
- Helyes, L., Dimeny, J., Pek, Z., and A. Lugasi. 2006. “Effect of Maturity Stage on Content, Color, and Quality of Tomato Fruit.” International Journal of Horticultural Science. 12(1): 41-44.
- Helyes, L., and Z. Pek.2006. “Tomato fruit Quality and Content Depend on Stage of Maturity.”HortScience. 41(6):1400-1401
- Fraser, P.D., Enfissi, E. M.A., Halket, J. M., Trusdale, M.R., Yu, D., Gerrish, C., and P. M. Bramley. 2007. “Manipulation of Phytoene Levels in Tomato Fruit: Effects on Isoprenoids, Plastids, and Intermediary Metabolism.” The Plant Cell. 19:3194-3211.
- Morgan, L. 1997. “Organic Fertilizers for Hydroponics.” Growing Edge, 9(2):32-39.
- Yin, YG, Kobayashi, Y., Sanuki, A., Kondo, S., Fukuda., Ezura, H., Sugaya, S., and C. Matsukura. 2010. “Salinity Induces Carbohydrate Accumulation and sugar-regulated Starch Biosynthetic Genes in Tomato Fruits in an ABA-and osmotic stress-independent Manner.” Journal of Experimental Biology. 61:563-574.
Organic is an often misunderstood term. Here’s Bill DeBoer to help clear a few things up.
“I don’t use chemicals, I grow organically!” This is a reoccurring message continuously articulated by a certain sect of organic growers or consumers of organic products. It also shows that there is a preponderance of misinformation and a lack of understanding. Everything is a chemical, including those organics derived from natural origins. What these people are trying to say they choose a more natural choice when growing organically (as opposed to the negatively-perceived synthetic counterpart). However, the point of this article is not to draw negative criticism toward organics, but rather address misconceptions in an effort to help you make sounder, more-informed decisions. So, whether you are grounded in either organics or synthetics, you need to internally digest the following concepts. (Note: when I use the term organic in the article, I am referring to chemicals derived from natural origins.)
Are organics always safer than synthetics? No. People have bought into the premise that anything natural is safer. The natural state of a chemical inherently does not dictate safety. For example, imagine organically growing the castor plant (Ricinus communis). From this you could exact a very powerful compound—ricin—which is highly toxic to mammals. While this example is meant to be impractical, the concept is not: natural chemicals can be just as deadly as synthetic ones. Have you ever looked at the chemical compounds found in organic pesticides or the LD50 (lethal dose inducing mortality in 50% of test population) value? Remember, toxicity is a function of the exposure time as well as the dose/concentration. Simple chemicals that are regarded as harmless can be very toxic if the concentration is high or the exposure time is long—for example, water is toxic if consumption continuously exceeds 1.5 L (0.4 gal.) per hour.
Safety is at the forefront for organic growers and consumers of organic products, but have you ever looked at the signal word (i.e. caution, warning or danger) on the chemical label? By human nature, we will believe something without questioning the validity if enough people say it is true. There is a reason marketing people can receive hefty salaries: thanks to them, safety has become unanimous with organics. Funny, since federal regulation uses the same wording for organic pesticides?
It is important to note that organics generally have a significant advantage to synthetic counterparts. The half-life, or breakdown, of organic compounds tends to be on average quicker than synthetic chemicals, whose half-life can be long and breakdown is slow (thus the persistence in the environment is longer). However, while organic pesticides can breakdown quickly, their effect is often short lived and frequent applications are more necessary in comparison to synthetic pesticides. Thus, an individual must always understand the type of chemical, application, frequency, concentration and relative persistence. After all, at face value, what appears to be more toxic: compound A applied once or compound B applied six times in the same time span? Obviously, more information is needed.
Another critical consideration is dismissing linear thought processes. Just because a compound is organic and targets one type of pests, it does mean it will not cause alternative problems. For instance rotenone, a very effective organic pesticide for certain beetles and caterpillars, is also highly toxic to aquatic life. Therefore, you must avoid spraying around any body of water. Another example is nicotine, which causes paralysis to pest insects and is readily absorbed by the skin of mammals—and it is quite toxic! Lastly, pyrethrins are highly effective at eradicating a wide range of pests, but they are also toxic to helpful pollinators like honeybees. As growers and gardeners, we cannot think our actions are singular or linear. When using both synthetic and organic chemicals, our actions have direct and sometimes irreversible consequences. Always research the active ingredient prior to use, as well as proper protective equipment, relative toxicity and susceptible population (which can range from helpful insects to people).
A study published in 2006 proposed an interesting hypothesis that microbial (bacterial and fungal) contamination, not pesticide residue, is of larger concern to public health. It begs the question, are organically-raised fruits and vegetables less likely or more likely to have microbial growth due to pesticide practices? I don’t have the answer, but it is an intriguing point nonetheless. Another important component to this review was the difference in detection of synthetic and organic pesticide residue. A ten-year trend line by Baker et al. showed a significant increase in the detection of synthetic pesticide residue relative to that of organic pesticide. One of the main points was organically treated fruits and vegetables still had detectable pesticide (albeit organic) residue. Ultimately, there is not enough information to make definitive statements on overall safety, which is compounded by the fact that sampling methods are not always accurate. However, it does show we cannot think of most organic produce as “chemical free.”
In conclusion, practices involving chemicals that have a low environmental persistence, are effective toward the target pest and have a low risk factor towards the consumer should be our future goal. If human and environmental health is the chief concern for organic growers, then knowledge is the strongest ally. I have been and will always be an advocate for the safest and healthiest option of growing produce. This article should not be viewed as anti-organic, but rather as pro-education. Consumers should have all the facts so they can make an informed decision. For more information on mode of action and relative toxicity of organic pesticides, refer to the Oregon State Extension Publication on Least Toxic Organic Pesticides for Gardeners. If you remember nothing of this article, retain these points to ponder:
- Everything is a chemical and everything can be toxic if the right dose or exposure time is met.
- Read the label, and ask questions when you are unsure.
- Organic does not equate to safe just as synthetic doesn’t represent unsafe.
- Actions have consequences and application of pesticides, even organic ones, can have a negative impact on organisms from fish to bees to humans.
- There are pros and cons that an individual must weigh when selecting a pesticide, organic or synthetic (i.e. cost, effectiveness, relative toxicity, etc).
Literature Cited:
- Baker, P.B., Benbrook, C.M., Groth, E., 3rd, and Benbrook, K. L. (2002). Pesticide Residue in Conventional, Integrated Pest Management (IPM-Grown and Organic Foods: Insights from Three US Data Sets. Food Addit. Contam., 19, 427-446.
- Grubinger, V. Pesticides for Organic Growers. Integrated Pest Management. Retrieved from http://www.hort.uconn.edu/ipm/general/htms/orgpest.htm
- Least Toxic and Organic Pesticides for Gardeners. Oregon State Master Gardener Program. Retrieved from http://extension.oregonstate.edu/lincoln/sites/default/files/Least_Toxic_Pesticiddes_for_Gardeners.pdf
- MacMillan, Annie. (Unknown date). Do Organic Pesticides Pose Any Hazards to Growers? [PDF Presentation] Retrieved from http://extension.unh.edu/Agric/AGPMP/documents/macmillan2.pdf
- Magkos, F., Arvaniti, F., and Zampelas, A. (2006). Organic Food: More Safety or Just Peace of Mind? A Crical Review of the Literature. Critical Review in Food Science and Nutrition, 46, 23-56. http://spot.colorado.edu/~carpenh/Magkos.pdf
Providing nutrients to an aquaponics system is a potentially fatal balancing act. Here, William DeBoer discusses how growers can supply adequate nourishment for their plants while not killing their fish.
How can you ensure plants have the right balance of nutrients in your aquaponics system? When growing plants hydroponically, providing essential elements—such as nitrogen, phosphorous, potassium, calcium—is relatively simple; just follow the recommendations of a given fertilizer formulation based on the water volume of your reservoir. With aquaponics, however, the addition of fish adds a big problem. Most synthetic plant fertilizers are no longer a safe way to deliver nutrients to the plants as these salt compounds can be toxic to the fish.
Instead, the grower relies on the essential elements found in the mineral premix in fish food, as well as the nitrogenous compounds found in fish waste. While this union of aquaculture to hydroponics is a natural fit, it is not without problems. This article will discuss these problems by explaining nutrient monitoring and the synergistic and divert requirements, and how to maximize this give and take between what is right for aquaculture and what is right for hydroponics.
I have often viewed aquaponics as more of an art than hard-core science. While it is true there are a myriad of measureable parameters, there is a certain obscurity when it comes to maintaining optimal nutrient concentrations. Could you measure all essential macro- and micronutrients? Yes, all pertinent elemental concentrations can be quantified and recorded using a sophisticated spectrophotometer. Is this practical for the everyday grower? No; this machine, coupled with all the needed reagents, can cost a couple thousand dollars. Finding simple aquarium titration kits is much more economical albeit they have a reduced degree of accuracy and precision.
Measuring temperature, pH, alkalinity (initially and before any pH adjustments) and levels of ammonia, nitrite and nitrate are a minimum requirement. Other growers might also assess hardness, electrical conductivity and total dissolved solids, coupled with various other elemental tests. It is important to note that most aquarium kits record total ammonia nitrogen (TAN). In water, the TAN exists in equilibrium between unionized ammonia (NH3) and an ionized ammonium ion (NH4+)—the former is considered most toxic. Any grower can easily use a table to calculate the ratio of NH3/NH4+ given temperature and pH. Also, if you are interested in calculating the total nitrogen content of the system, you must calculate the percentage of nitrogen, nitrite and nitrate in TAN.
One of the most integral water-quality parameters that affect the availability of nutrients to the plants, as well as the health of fish, is pH. The ideal range of pH for the nitrifying bacteria (Nitrosomonas spp. and Nitrobacter spp.) is slightly alkaline (7.0 to 9.0), whereas the ideal pH for micronutrient availability is slightly acidic (5.5 to 6.5). While the lower pH will not eliminate the colonizing bacteria, it can impact the efficiency of detoxifying ammonia to nitrite to nitrate. Therefore, stabilization of pH is paramount in managing the health of plants and fish alike. To do this, however, we must evaluate another water component parameter: alkalinity.
Alkalinity is commonly referred to as the buffering capacity or the ability of a solution to neutralize an acid. High alkalinity infers the solution can have a relatively large amount of acid or base added without sudden swings in pH. Fish and plants do not respond favorably to dramatic changes in pH. You can buffer the water naturally by allowing an accumulation of phosphates from fish food and nitrates from fish waste. Still, depending on your water source (reverse osmosis, deionized), it might be appropriate to add a buffering agent—a compound like potassium phosphate could have a dual purpose in that it also provides two macronutrients: potassium and phosphorus. Personally, I do not like dabbling with excessive chemical supplementation. I find a certain wisdom in personal restraint because the lethal dose for a particular compound might or might not be published for a given fish species. Even if the compound(s) are not at a lethal dose, chronic exposure can depress health, which reduces growth.
Try to effectively deliver all essential elements through fish feed. Also, to avoid nutrient deficiencies, I recommend targeting those micronutrients that can be locked up at a high pH or are limiting within a given fish food. If you try for only one these, make sure you target iron. While the requirement for iron is low, the importance in plant growth and function is vast (look for interveinal chlorosis or yellowing of the immature leaf with green veins as indications of iron deficiency). If nutrient deficiencies still exist, however, foliar supplementation is one way to go (refer to the University of Florida’s extension publication HS1163 for more information). In fact, iron is arguably one micronutrient that an aquaponics grower needs to supplement. Supplementing your water with a chelated form of this element is effective as chelated iron (synthetic or organic) is readily absorbed directly by the plant. Potassium and calcium deficiencies can also arise, but are easily corrected by supplementing with calcium chloride (CaCl2) or KH2POH4. Supplementation can also be combined with pH adjustment when using calcium hydroxide (Ca(OH)2) and/or potassium hydroxide (KOH).
In conclusion, aquaponics is truly a balancing act in which growers are always searching for that ideal middle ground between hydroponics and aquaculture. Sometimes “trial and error” provides the best way of determining what is right for a particular system with its specific plant and fish species. While I have described aquaponics as an “art,” every grower should be diligent in his/her scientific methodology and always adhere to general principles and err on the side of caution when adding chemicals that could potentially harm the fish or the end consumer (that is, people). As such, always measure certain water-quality parameters (temperature, pH, alkalinity, ammonia, nitrite, nitrate, etc.). Also, always try to deliver nutrients through the fish feed. Just feed your fish to apparent satiation and then weight out and add additional food for your plants. If deficiencies still occur, check water quality and increase the amount of excess food. If that does not correct the deficiencies (especially iron), only then consider individual supplementation.
References
Treadwell, D., Taber, S., Tyson, R., & Simonne, E. (2010). Foliar-Applied Micronutrients in Aquaponics: A Guide to Use and Sourcing. University of Florida IFAS Extension Publication HS1163. Retrieved from http://edis.ifas.ufl.edu/hs408
Francis-Floyd, R., Watson, C., Petty, D., & Pouder, D. B. (1990). Ammonia in Aquatic Systems. University of Florida IFAS Extension Publication FA16. Retrieved from http://edis.ifas.ufl.edu/fa031
Tyson, R. V. (2007). Reconciling pH for Ammonia Biofiltration in a Cucumber/Tilapia Aquaponics System Using Perlite Medium. (Doctorial dissertation). Retrieved from http://etd.fcla.edu/UF/UFE0019861/tyson_r.pdf
