Zinc deficiency in tomato plants

Zinc is involved in a range of enzyme reactions similar to manganese and magnesium. It is important for the development and function of growth regulators (e.g. auxin) that influence internode elongation. It is also involved in chloroplast development.

In the early stages of zinc deficiency the younger leaves become yellow and pitting develops in the interveinal upper surfaces of the mature leaves. As the deficiency progresses these symptoms develop into an intense interveinal necrosis but the main veins remain green, as in the symptoms of recovering iron deficiency.

Zinc can be applied throughout the season. Applications should ensure sufficient zinc is available prior to flowering. Foliar applications are the most appropriate means of treating a deficiency. Deficiencies occur only rarely in greenhouse crops.

The internodes are thinner and shortened (by one third to half compared to normal plants) giving the plant a rosetted appearance. Leaves are small and thicker giving a ‘leathery’ appearance. Because the leaf hairs are dense, the smaller leaves take on a silvery gray green sheen. Irregular yellowish green chlorotic blotches appear on the leaves. These may develop into brown necrotic lesions over the whole leaf. Leaves die and fall off; flowers die back. Any fruit developing will remain small and ripen prematurely. High phosphorus levels or anaerobic conditions can induce deficiencies. Where excess zinc is available, plants exhibit symptoms close to that for iron deficiency – i.e. chlorosis of young leaves.

Deconstructing Flavor in Tomatoes

I’ll start with this caveat: growing conditions can have a huge influence on taste/flavor in tomatoes.  Fruit harvested early or late in the season is usually inferior in flavor compared with those harvested mid-season; high flavor is often favored with plants in light drought stress; and growing area, soil type/fertility and disease incidence/plant health can significantly effect fruit taste.  Perceived “likeability” is also often confounded with non-flavor fruit quality attributes, such as color, texture and skin thickness.  For example, most folks struggle to see anything that is not red and round as a proper tomato.  We have used blind taste tests (literally – I’m talking blindfolds) to get a non-biased flavor assessment on something like GWR bicolors.  Taking off the blindfolds is fun, and informative.  However, there is a genetic component to flavor per se that is important, and subject to modification through plant breeding.

Knife at the ready

In our tomato breeding program, selection for flavor has always been primary.  We taste several thousand tomato plants in our breeding nurseries each year, and it is generally not possible for us to do anything other than apply a simple subjective selection criterion – select what you think tastes good.  We can manage that in our moderately sized breeding program – large companies cannot.  In an effort to perhaps apply more objective criteria and to design more efficient breeding strategies, we’ve been reading more on what is known about the biochemical and genetic basis for tomato taste and flavor

We have conducted tomato tastings at Frogsleap Farm for the last 5-6 years, and are always surprised with the person-to-person variation in ranking tomatoes for flavor.  As with most things that we eat and drink – taste is personal.  That said, there are trends in what people like in tomato taste/flavor – as illustrated in recent scientific studies by Harry Klee and colleagues at the University of Florida.

For decades it has been known that tomato flavor is primarily influenced by three categories of compounds: sugars, acids and volatile aromatics - and that it is a balance between these compounds that is important, not the just level of an individual component per se.  In the next several paragraphs I’ll try and outline the current understanding of the individual flavor components, how they interact, and the potential implications for tomato breeding.

Yilmaz, 2000 (reference) shows the following typical chemical composition of a tomato fruit:

Constituent	% DM
Fructose	25
Glucose	22
Saccharose	1
Citric acid	9
Malic acid	4
Protein	8
Dicarboxylic amino acid	2
Pectic substances	7
Cellulose	6
Hemicellulose	4
Minerals	8
Lipids	2
Ascorbic acid	0.5
Pigments	0.4
Other amino acids, vitamins, and polyphenols	1
Volatiles	0.1

To further set up this discussion; flavor = taste + aroma.  The tongue recognizes five basic tastes; sweet, sour, bitter, salty and savory - all of which come from non-aromatic/volatile constituents.  There are various aromas in tomatoes, favorable and unfavorable, and these are due to volatile compounds, to which we have receptors in the nose.

Sugars

Glucose and fructose are the major sugars in tomato, with glucose predominating early and fructose increasing with advancing fruit maturity.  In a mature ripe tomato the glucose:fructose ratio is generally about 1:1.  The tomato wild relative S. chmielewskii accumulates sucrose, rather than glucose – the result of a recessive loss-of-function allele of the gene controlling production of the enzyme acid invertase.  This allele and its high sucrose phenotype have been introgressed into commercial tomatoes.  Total sweetness index (TSI) is used to indicate sweetness. The contribution each sugar makes to this parameter is described relative to sucrose, and calculated as: [(1.00 × sucrose) + (0.76 × glucose) + (1.50 × fructose)].  When tomato samples are spiked with any of the three sugars, tasters perceive less overall aroma and less ripe tomato flavor.  Flavor intensity was highest with samples spiked with sucrose or fructose (Baldwin andThompson, 2000).

Total soluble solids, measured as Brix, is the sum of sugars (65%), organic acids (13%) and other minor components, and is intrinsically correlated to sugar content in tomatoes.  Although Brix is also highly correlated to sweetness in various tomato tasting experiments, Tieman, et al. found that specific volatile aromatic compounds enhanced perceived sweetness in their tasting experiments (more on this later).

Sweet (beef x cherry) segregate

Smaller fruited types generally have higher sugar content/Brix.  Tomato breeders at the University of Florida crossed a high sugar cherry tomato with a low sugar large fruited type and evaluated segregating progeny (Georgelis, et al. 2004).  They found a negative correlation between sugar content and fruit size or earliness; higher sugars in indeterminate vs determinate types; and no relationship between sugar content and fruit yield.  Others have reported a negative correlation between sugar content and fruit yield, which better matches our personal experience.  We’ve made a lot of crosses between cherry and beefsteak types.  Can you recover cherry level sweetness in a beefsteak segregate? – no.  We have found some pretty tasty larger fruited lines from such crosses, but they are not sugar bombs.

Photosynthesis is the conversion of CO₂ to sugar.  It is the ultimate source of energy in all the food we eat and in the fossil fuels we burn.  Chlorophyll is a green pigment, stored in plastids (chloroplasts), that is critical to photosynthesis.  In most plants chlorophyll is concentrated in leaves.  Tomato plants have chlorophyll/chloroplasts in leaves, stems and in unripe fruit.  In fact we now know that the chloroplasts in unripe fruit are an important source of sugars for the fruit, and that more chloroplasts generally result in improved fruit quality (Nguven, et al 2014).  Dark green unripe fruit is good for fruit quality.  GLK2 is a transcription factor that controls the formation of chloroplasts in tomato fruit.  The GLK2 wild type gene “U” has a phenotype with darker green unripe fruit and lingering green shoulders in ripe fruit.  The loss-of-function mutant “u” has lighter green unripe fruit and no green shoulders – so called uniform ripening.  The uniform ripening trait has been bred into most commercial tomatoes to facilitate commercial harvest and to present a more uniform red fruit (in grocery stores).   There are a lot of reasons why supermarket tomatoes generally taste bad – and this is one of them (Powell, 2012).   Since all heirloom types contain the wild type allele at this locus, any breeding program for improved taste should insure preservation of the wild type.  This is particularly true for progeny from crosses between heirloom and commercial types.

Green shoulders on non-uniform ripening wild type

Numerous studies have shown that generally, perceived sweetness is the most powerful determinate of “likeability” for taste in tomatoes.  That is certainly supported by our less rigorously designed tasting exercises here, although it is not uncommon for someone to say, “that one is too sweet”.  Sweet and sour - the ying and the yang.

Organic acids

Organic acids are what provide tomatoes the sour/tangy counterbalance to sweet.  The two primary organic acids in tomato are citric acid and malic acid.  Malic acid content is high in developing fruit, but decreases as the fruit matures.  Citric acid level increases somewhat during fruit development and generally represents 65-70% of the organic acids in ripe tomato fruit.  Tieman et al. report that tomato fruit citric acid concentration, when corrected for fructose content, was correlated to flavor intensity in their tasting trials.  Tomato fruit also contain low levels of free amino acids.   Glutamic acid, g-aminobutyric acid, glutamine, and aspartic acid comprise about 80% of the total free amino acids in
tomatoes.  Glutamic acid (glutamate) is concentrated in the gel around the seed and although it is a component of the umami (i.e savory) taste in foods, it seems to be negatively correlated with flavor in fresh tomatoes (Buchell et al., 1999).  

There is significant genetic variation for organic acid content and pH in tomatoes, with pH ranging from 4.1 to 4.8.  Low acid tomatoes are generally bland or one-dimensional, and a combination of high acid and high sugar is generally favorable.  In a classic study at UCD (Stevens et al., 1979, see graph below) tasters found that overall flavor intensity was correlated with both acidity (sourness) and sugars (sweetness), but that acidity (ph or titratable acidity) was most closely related to flavor intensity.  Tomato-like flavor, something we all love, was not strongly correlated with either sugar or acids – and is driven instead by volatile aromatics. 

Stevens, et al., 1979

Volatile aromatic compounds
Over 400 volatile aromatic compounds (AKA volatile organic compounds = VOCs), are found in tomato, and are either derived from carotenoid pigments, fatty acids, phenylalanine or free amino acids (Klee, 2010).  VOCs generally accumulate during fruit ripening.  Volatile compounds can be perceived by humans in two ways – sniffed through the nostrils (smell) or forced up behind the palate after chewing/swallowing (flavor).  It is generally agreed that taste is a complex interaction between smell, flavor, sugar and acids. 

Tieman, et al., 2012 (reference) summarizes the results from tasting panels and the relationship between chemical composition and “likeabiliy” in a broad collection of 152 heirloom varieties compared with samples of store-bought tomatoes.  Levels of reducing sugars (fructose and glucose), organic acids (citrate and malate) and 28 VOCs were measured, and multivariate analysis was used establish relationships between concentration of these compounds and various flavor attributes determined by the tasting panel. Flavor intensity was positively correlated with twelve different VOCs, seven of which were significant after accounting for fructose content.  Sweetness was also positively correlated with twelve VOCs, eight of which overlapped with those enhancing flavor, and three of which were independent predictors of sweetness after accounting for fructose content.  Although other studies have reported volatile induced enhancement of sweet, fruity, sour, bitter, smoky, and salty tastes, this study found the most significant enhancement was to sweetness. 

Tomato plants engineered to lack fatty acid-derived VOCs (the predominant family of VOCs in tomato) scored similarly for “likeability” to those with normal levels of these compounds.  In contrast, plants without carotenoid-derived VOC’s were perceived as significantly less sweet, and with a lower “likeability” score.  Other research finds that the concentration and composition of the various carotenoid pigments in tomatoes is directly related to the concentration of specific carotenoid-derived VOCs.  Carotenoid cleavage dioxgenase (CDD) genes code for various CDD enzymes, which in the chromoplast enable the conversion of pro-lycopene (tangerine), lycopene (red), and β-carotene (orange) to specific VOCs, most of which impart a fruity flavor.  The VOC geranial, derived from lycopene, is one of which Tieman et al. reports enhances sweetness.  In yellow and GWR fruited types the cartenoid pathway is truncated  early (see Genetic control of fruit color intomatoes) and virtually no carotenoid-derived VOCs are produced.   Although I have never tasted a true yellow-fleshed tomato that I found exceptional, there are a handful of GWR fleshed heirlooms and several of our GWR breeding lines that have excellent flavor – which in light of these data, is perplexing.

Green flesh - outstanding flavor

Klee and Giovannoni, 2011 (reference) make an interesting point.  Virtually all of the VOCs that contribute to tomato flavor are derived from a chemical compound essential for the human diet – essential fatty acids, essential amino acids and carotenoid pigments.  For example β-carotene is a precursor for Vitamin A, and it’s VOC derivative β-ionone is a key flavor volatile often associated with an intense fruity flavor.  They suggest an effective co-evolution between plants and animals – plants produce essential nutrients that have a characteristic flavor profile and animals evolve the ability to seek out such products, eat them and thus distribute the seed. 

Tieman et al. used standard genomic tools to segregate the tomato lines in their study into genetically related subgroups.  Their conclusion: “Based on these data, we found no obvious genetic subgroups that could explain liking, sweetness, or tomato flavor intensity. There was no obvious genetic clustering of good versus bad taste when varieties were sorted by chemical composition. These latter data also indicate the chemical complexity of liking, as there is no simple pattern of chemical content that separates high from low consumer liking scores.”  They also found extraordinary genetic variation for concentration of critical VOCs among the tomato lines studied, much more so than for sugar or acid concentrations.  This suggests a rich opportunity for breeding once specific VOC linked molecular markers are identified and deployed to enable efficient selection in large populations.

Conclusions

Wet lab characterization of the various flavor components is expensive and labor intensive.  Deployment of molecular markers associated with various genes that together contribute to perceived tastiness will allow more efficient selection and should help deal with the complexity of this trait.  In the mean time I think we (Frogsleap Farm) are stuck with the olfactory tools Mother Nature has provided, which although subject to individual preferences, can certainly send us in the right direction.  We will continue to combine tasting with objective measurements of soluble solids (Brix).  This literature review provided some new guidance – dark green unripe fruit are favorable (avoid uniform ripening types), high cartenoid pigment content is good, and some pigments probably better than others (e.g. β-carotene).  To the extent that the clear skin mutant allele y generally depresses the carotene pathway (see Genetic control of fruit color in tomatoes), I’m steering toward the yellow skinned wild type.  The green flesh phenotype (gf/gf) provides for an incomplete breakdown and conversion of chloroplasts to chromoplasts, which would seem to lead to a decrease in the cartenoid pigment content vs the wild type Gf (e.g red vs purple).  However, we routinely find our best tasting tomatoes carry the gf phenotype – on this point I am perplexed. 

Winner in 2014 "blind taste test"

We’ll be cautious with determinate and dwarf types which just may not have the leaf area to support maximum sugar accumulation, though I know there is a practical place for both types for some growers.  Nailing flavor in cherry and grape types is relatively simple, it gets harder with larger fruited types – which is where we are concentrating now.  Another challenge is to get great flavor with acceptable fruit yield and high fruit quality (non-splitting, firm flesh and good shelf life).  We are making progress on all fronts.  We will also anxiously follow progress in Harry Klee’s lab at the University of Florida, where they continue to provide a better understanding of flavor in tomatoes, and also create positive energy for challenging the bland tasting stereotype we are routinely presented in grocery stores and restaurants.

Gardening in the California Drought

California is in the middle of a multiyear, perhaps decades long drought.  Governor Jerry Brown is imposing the first mandatory water conservation restrictions ever for the state.  The Sierra Nevada snowpack, from which a third of the state gets its water, is at 5% of normal after a nearly snowless winter.

From USGS - Trinity Lake Reservoir, completely dry, usually holds up to 2.5 million acre feet (or 797,684,293,296 gallons) 

This is a shame for the county's most populous and biggest agricultural state.  More water in California is used for irrigation than for any other purpose.  However, in the large urban areas in southern and central California, water is primarily used directly by people.

From U.S. Drought Monitor - Darker red areas are in more severe drought

Over 37 million Californians (over ten percent of the country) have been affected by the drought.  Those who like to garden or landscape have likely been more affected by this drought - with no end in sight.

So, what is the plant lover to do without sufficient water?  The answer is to use water more intelligently, and to do this by using the Groasis Waterboxx.

The Groasis Waterboxx

Most water poured into the soil is not directly used by the plant.  It either flows away through the soil or evaporates into the air (especially in hotter climates).  There are multiple techniques to prevent these losses (planting in containers, using mulch) but these have serious drawbacks (cost, easy drying out of roots, poor evaporation blocking ability of mulch).  The Groasis Waterboxx deals with all of these issues, as explained below.

The Groasis Waterboxx was initially designed to plant and grow trees in the desert, with water only added at planting.  In this it has been remarkably successful.  In a Saharan Desert planting trial, single trees planted with the Waterboxx had 88% one year survival (vs. 11% survival for trees watered once weekly but without the Waterboxx).  The survival rate for at least one tree increased to 99% when two trees were planted in the Waterboxx as intended.  When planting a tree with the Waterboxx, 10 gallons is poured into the soil before planting, the bare root tree is planted, and then the Waterboxx is filled with 4 gallons of water.  No water ever again needs to be added as the Waterboxx collects dew and rain water and slowly releases it to the growing plant through a wick in the base of the Waterboxx.  The Waterboxx is left in place until the tree outgrows it and can be reused for up to ten years.  The Waterboxx planted tree is then resistant to drought because of its deep, capillary water fed roots.

What about gardening?  Well the Waterboxx can be used to grow many fruits and vegetables as well.  Generally, since vegetables require more water than trees, we recommend inserting a second wick into the base of the Waterboxx for water loving garden plants.  If this is done, water generally needs to be added to the Waterboxx every 3-4 weeks - again 4 gallons.  However, as the Waterboxx prevents evaporation of soil moisture immediately beneath it (by acting as a type of impermeable evaporation barrier, a far better alternative to mulch), the roots of the plant won't dry out. Below you can see results of a customer growing tomatoes with the Waterboxx (with extra wicks inserted) in Hemet, California in late 2014.

Three weeks growth of tomatoes with the Groasis Waterboxx by a gardener in Hemet, California

Can the Waterboxx also be used to grow greens rather than traditional vegetables?  Yes.  The same avid gardener in California ingeniously found a way to extend the reach of the Waterboxx by adding 3 extra wicks, adding water permeable material to each of the wicks and then spreading these out into the soil raised bed.  The gardener then planted 27 greens on either side of the Waterboxx, and placed a evaporation cover over them as they began to grow.  The customer found that the greens grow great with only 2.5 to 3 gallons of Water per week (all poured into the Waterboxx), and no need for overhead watering.  He is able to enjoy fresh greens every day now.

If you want to start gardening with the Waterboxx (or growing trees), please visit our main website, www.dewharvest.com.  There you will be able to see many other garden plants growing with the Waterboxx, including zucchini, cucumbers, watermelon, pumpkin and eggplant.

We would love to hear your comments below - to leave one, please click on "Comments".

Image Sources:
Trinity Lake Image:  
Credit: U.S. Geological Survey,
Department of the Interior/USGS
Photo by: Tim Reed, USGS California Water Science Center Supervisory Hydrologist; taken February 4, 2014.

California Drought Map:

US Drought Monitor

  The U.S. Drought Monitor is jointly produced by the National Drought Mitigation Center at the University of Nebraska-Lincoln, the United States Department of Agriculture, and the National Oceanic and Atmospheric Administration. Map courtesy of NDMC-UNL.