Special Series in the rootzone #5: Improving tomato quality

September 27, 2010
Written by Andrew Lee
In the fifth of six articles for Greenhouse Canada, Grodan® crop consultant Andrew Lee describes the processes and reasons why at certain times of the year tomato crops are more susceptible to a number of fruit quality disorders.

When is fruit most susceptible to quality issues?
■ Fruit is most susceptible to quality disorders of various kinds when the 24-hour temperature is high (>23℃), when the weather conditions fluctuate day to day, or in periods of dark, humid weather. During these times, stresses on the crop and fruit development from environmental influences, temperature, humidity, differences in water uptake and light levels are at their highest.

For these reasons, it is important to understand how the outside climate influences crop water use, a subject discussed earlier in this series (Greenhouse Canada, December 2009 issue, pg. 14). It is also why it is important to understand the basics of substrate functionality and why certain substrates are designed to provide growers the opportunity to stabilize, refresh or change EC and WC levels according to the prevailing weather conditions (Greenhouse Canada, April 2010 issue, pg. 14).

It is also the reason why it is important to have a plan that meets both marketing (size and quality) and production (kg/m2) goals. The plan, based on four key pillars, namely strategy, uniformity, strength and balance (Greenhouse Canada, June 2010 issue, pg. 18) should be robust enough to cope with extreme temperatures and facilitate strong and regular growth even in the darkest periods of the year.

Finally, it is about getting the most out of systems such as the climate computer and measuring tools to provide you with the necessary management information in order to steer to the plan on daily basis. In the last article, (Greenhouse Canada, September 2010 issue, pg. 12) I described how to optimize the start and stop times of irrigation, making reference to their impact on fruit quality in relation to changing weather conditions and how to stabilize and steer substrate EC.

I will now describe the benefits that this knowledge can provide and how it can be used to improve the financial returns to the company. Specifically, this article will address two of the most common fruit physiological disorders of tomato, namely blossom end rot (BER) and uneven colour (blotchy ripening). Furthermore, I will mention additional “tips” which, used in combination with substrate management, can help alleviate these problems.

KNOWLEDGE IS POWER
■ Travelling to different parts of the world, one of the most commonly spoken phrases I hear repeated by growers is, “it’s different here… we’re not the same as… so we can’t do that… it’s not like it is in….”
This is surprising to me. I usually reply with the following remark: “We all grow using the same source of light and using the same chemical composition of water and fertilizer, which not surprisingly means that wherever you are, the plants assimilate and dissimilate sugars in the same way. In most cases, we use the same varieties from the same seed companies whether we are in the Netherlands, Mexico, Australia, Canada, France or Poland and usually in greenhouses constructed by the same manufacturers. So how is it different here?”

What does differentiate growers is their knowledge of plant physiological processes (i.e., photosynthesis, respiration and transpiration) and how to translate this knowledge into making a plan and growing a crop in their specific climatic conditions (24-hour temperature, heat, vent and irrigation strategies). By taking this approach, growers have the potential to optimize production for their specific location. However, what is the point in producing 65-70 kg/m2 if a large proportion of this is waste? It is the yield of top grade fruit sold in the market that provides the financial returns to the business. Focus should be placed on volumes leaving the pack house, not on volumes leaving the greenhouse. Acceptable levels of waste from a cluster variety producing 65-70 kg/m2 would be in the region of two to five per cent. How do you compare?

Any improvements to fruit quality you make will also improve the productivity of the business elsewhere. A huge cost to the business is labour, which includes significant harvesting and packing costs. As a guide, it should be possible to pick and quality grade cluster varieties directly in the greenhouse into 5 kg marketing boxes at rates of 350-450 kg/hr. These labour rates can reduce by 30-40 per cent if fruit quality is poor (Table 1.0). Likewise, it should be possible to check weigh these boxes in the pack house, using automated machinery at rates of 1,800-2,000 kg/hr. Quality problems can reduce this by 50 per cent or more.

*Rates are from a Dutch greenhouse and are influenced by weekly production/m2 shown in Figure 1.0. These rates assume a weekly harvest of approximately 2.0 kg/m2.
 rootzone_fig_1

TIMES OF INCREASED RISK
To illustrate examples of how, when and why these fruit quality issues occur, I have used the Six Phase model, described in Greenhouse Canada, June 2010 issue, pg. 18.

BLOSSOM END ROT
■ BER is probably the most common of all fruit quality disorders. The external symptoms are characterized by blackening at the end of the fruit (Picture 1.0). BER can occur suddenly and extensively, and often with disastrous financial consequences. The symptoms are caused by local Ca2+ deficiency in the fruit tissue, which leads to a breakdown in the structure of the plant cell wall. Fruit is at greatest risk during the phase of fruit enlargement, 10 to 15 days after flowering (Picture 2.0). Despite large advances in greenhouse crop production, BER still remains a tiresome quality problem for tomato growers in all corners of the world.

rootzone1  
Picture 1.0: Characteristic symptoms of blossom end rot.  
 rootzone2  
Picture 2.0: Fruit are susceptible shortly after flowering when maximum cell division takes place.  
The supply of Ca2+ in the drip solution in 99.9 per cent of situations where BER is evident in the crop is more than adequate for plant and fruit growth, provided it has been prepared, mixed and is dosing correctly. Lack of Ca2+ in the feed is therefore unlikely to be the primary cause of BER. However, poor control of the aerial environment resulting in low water uptake, poor aeration or adverse temperature in the rootzone resulting in root disease and poor substrate management leading to high EC can all reduce the uptake of Ca2+. This is because Ca2+ is transported primarily to the leaves within the water conducting xylem vessels by the process of transpiration (Greenhouse Canada, December 2009, issue, pg. 14). Fruit possess no stomata and only a small number of xylem vessels and therefore have limited means of attracting and transporting Ca2+. To minimize the incidence of BER, it is important to control the greenhouse environments, aerial and rootzone, creating a balanced plant and balanced rate of fruit growth to ensure the demand from fruit for Ca2+ is matched by supply from the rootzone. Growers must understand the risks associated with certain planting strategies and growth phases.

Tip 1: Maintain balance in the crop. It is important to maintain the right balance in the crop. Fruit affected with BER will ripen at an earlier stage, reducing the fruit load and creating a vegetative crop. This further increases the risk of BER on subsequent clusters. It may look unsightly, but to help create the right generative balance leave the fruit in place until it has turned red.

PHASE ONE: PLANTING AND ROOTING IN
■ Obviously, there is no fruit load in this phase, so there will be no visible signs of BER. However, the foundations for BER can be laid if rooting into the substrate is delayed, especially in hot conditions. When young plants are planted into the greenhouse under high light and temperature conditions, temperatures in the rootzone can become too high. High rootzone temperatures (>26℃) increase the risk of root disease such as Pythium, which if present will reduce root function and therefore Ca2+ uptake.

To minimize the risk in this situation, the slabs should be initially saturated the night before the young plants are delivered to the greenhouse. This will prevent the substrate temperature from rising too high. The initial saturation of the substrate should be fast and uniform throughout the greenhouse, allowing the crop to be planted the following morning. It should then root quickly into the substrate, preferably within 24 hours. Fast rooting-in facilitates easy growth and rapid development of leaf area that ultimately helps shade the slab. Once rooted-in, the plants are no longer reliant on the propagation block for water and nutrients and the irrigation strategy can be adjusted to avoid irrigating during the peak solar hours when slab temperatures are highest. Irrigation should be applied only to refresh the substrate solution in the morning and again if required in the evening.

PHASE TWO: ROOTING THROUGH AND PLANT DEVELOPMENT
■ Under these same climatic situations, as the plants are rooting through the substrate and setting the initial clusters, plant and fruit growth will be very fast. This will increase the substrate pH (>6.2), resulting in low P-PO42- availability; ideally, P-PO42- levels in the substrate should be between 40 ppm and 45 ppm. With low P-PO42- availability, Ca2+ is not partitioned effectively to the ends of the fruit and under these circumstances it is possible to induce BER on the initial (1-3) clusters. It is therefore important to have chosen a substrate as part of the overall strategy where the nutrient solution is freely available and not buffered by organic material and is easy to balance and refresh with the minimal volumes of irrigation (Table 2.0). In this phase, the addition of small quantities of NH4+ (3-5 ppm measured in the slab) will also help by lowering the pH and increasing P- PO42- availability.

Tip 2: Maintain the correct nutritional balance in the rootzone for optimum fruit quality. Look to the analysis in the stonewool slab (Table 2.0); it is identical to that originally supplied. Then compare this to the analysis in the coco slab. Look in particular at the high K+ levels and then the K+/Ca2+; ideally this should be 1:1, as it is in stonewool. This balance of elements is important and if not corrected can lead to BER. The reason is simple: K+ is a monovalent ion, whereas Ca2+ is a divalent ion. In essence, this means that K+ is more readily available to the plant. High levels can therefore compete and limit Ca2+ uptake. In this respect, also take note of the high levels of Na+, another antagonist to Ca2+ uptake. Of course, over time this imbalance can be corrected by recalculation of the drip solution; in the short term it increases the risk of physiological disorders and makes reuse of the initial drain solution difficult. In this case, it would not be advisable to reuse this solution at an EC >1.0mS, whereas the stonewool runoff can simply be reapplied.

rootzonetable2

VIEW LARGER TABLE

PHASE FOUR: PRODUCTION AND BALANCE

■ The risk of BER in the early phases of growth is not that great for winter planted crops due to the low development speed, low transpiration and low root activity of the crop. However, they can be at risk shortly after the first harvests are taken, expressing symptoms 14-21 days later. This is because, following the first harvests, the crop shows significant re-growth as the fruit load is released. The set speed of the flowering clusters also increases, resulting in many more, smaller fruit of the same age and size and therefore a higher demand for Ca2+. This usually coincides with increase in light levels, changing outside temperatures and the requirement to ventilate the greenhouse faster and to a greater extent, all of which lead to a sudden increase in the transpiration rate. For this reason, the crop must be balanced during Phase Three and re-growth controlled in Phase Four (Greenhouse Canada, June 2010 issue, pg. 18). Too many leaves at this stage or a strong vegetative crop can result in BER.

Tip 3: Alleviate the symptoms of BER on successive clusters. High rates of transpiration will concentrate Ca2+ moving in the transpiration stream to the leaves and not the fruit. It is possible to manipulate the distribution of Ca2+ to the fruit by removing two or three additional leaves from the bottom of the plant during the weekly de-leafing process if successive clusters are affected. A pre-night temperature set point can also be used (if outside temperatures dictate) to create root pressure and “pump” Ca2+ to the fruit.

PHASE FIVE: MAXIMUM PRODUCTION
■ A high substrate EC in the summer is the most common cause of BER. This is because at high EC levels, the xylem vessels in the fruit become more constricted, limiting Ca2+ deposition. In the last article (Greenhouse Canada, September 2010 issue, pg. 12), I reviewed the basic thought processes on how to manage substrate EC during this growth phase and how to keep EC stable from day to day (Figure 2.0). The ideal substrate EC is of course influenced in some respects by the variety grown; for example, cherry types are usually grown at higher EC to attain certain minimum Brix values. For most cluster types, the slab EC in this phase should be stabilized in the region of 3.5-4.5mS, depending on the prevailing weather with a fluctuation in the slab during 24 hours of 0.5-0.8mS.

It is, however, important to have the EC stable and at its lowest during the hours of peak solar radiation. This is because the crop needs to transpire at its maximum to keep it and the greenhouse environment as cool as possible.

If the EC is deemed too high, it should be lowered as soon as possible. Do not react by irrigating only fresh water; this will result in more BER, as no Ca2+ will be supplied. However, do decrease the dripping EC based on outside light levels (W/m2), i.e., 3.0 mS with a -0.5mS reduction in the range 500-900 W/m2 and try to keep the EC slab – EC drip between 1.0 and 1.5mS. In the morning, check the time of first drain. This should occur at approximately 400 J/cm2 or 600 W/m2. At midday, check the irrigation volume in relation to light; as a rule-of-thumb, aim for 3.0 ml/J (2.0 ml for uptake and 1.0 ml for drain). Also, ensure that the minimum rest time setting is not restricting the maximum volume of water that can be applied, so check the start status on the climate computer. In the afternoon, check the drain per cent – if it is too high, use smaller, more frequent irrigation sessions (ratio 3.0 ml/J) as this will make more water available to the plant if it is too low; look to the irrigation volume (ratio 3.0 ml/J).
rootzonefirugre2
 
Figure 2.0: Optimized start and stop times and “irrigation volume” maintain a stable rootzone EC despite large fluctuations in weather.  

Tip 4: Maintain water uptake in extreme weather conditions. Ideally, the crop should be strong with a strong healthy root system in this phase of growth, a result of good planning and crop management through Phases One to Four. Water uptake (applied – drain) as a minimum should be in the region 2.0-2.2 ml/J. Tools such as shading and fogging, if required for the climatic conditions, should not reduce this. They should be used to allow the plant to keep pace with higher levels of transpiration and therefore prevent it from closing its stomata under pressure of water stress and inducing BER.

UNEVEN OR ‘BLOTCHY’ RIPENING
■ The symptoms are characterized by orange blotches on the surface of the ripening fruit (Picture 3.0). Fruit is at greatest risk in hot or changeable, bright to dark, weather conditions.
The supply of K+ in the drip solution in 99.9 per cent of situations where uneven ripening is evident is more than adequate to allow the fruit to colour naturally. Indeed, evidence from trials conducted during the 1990s suggests that levels in the substrate must fall below 160 ppm to induce symptoms in this way. Lack of K+ in the feed or rootzone is therefore unlikely to be the primary cause of uneven ripening. In this respect, remember that the addition of extra K+ to the feed solution as a knee jerk reaction to alleviate uneven ripening on the lower clusters can actually induce BER in the developing clusters due to the K+/Ca2+ (Tip 2).

Uneven ripening can be caused by a number of other factors, such as high fruit temperatures (>30℃). This is because lycopene, the red pigment in fruit, is actively synthesized between 15-32℃ and beta carotene, the orange pigment, is actively synthesized at temperatures >30℃. Fruit has no means of cooling itself, and therefore need protection from direct radiation. This is normally achieved naturally by maintenance of an adequate leaf area index, and also by placing a shade screen over the central roadway and white-washing the sidewalls in summer.

rootzonepic3
 
Picture 3.0: Uneven ripening of cluster tomatoes.  
By far the most common error is supplying too much water to the crop on dark days when transpiration is minimal due to poor adjustment of the start and stop times, or growers trying to “chase EC” in order to stabilize it by supplying high volumes of irrigation water. This creates too much root pressure (remember: transpiration, then irrigation), which damages cell structure in the skin and the fruit does not colour properly during the ripening process. You may also see “growth mottle spots” on the leaves and the crop exhibiting strong vegetative pale growth (Picture 4.0) as another reaction to mismanagement in the rootzone. To minimize the incidence of uneven ripening and radial skin cracking that can also be induced in the same way, it is important to understand how to manage the rootzone environment in response to changing weather conditions (Greenhouse Canada, September 2010 issue, pg. 12) and the risks associated in certain growth phases.

Tip 5: Avoid situations which can lead to increased root pressure. Optimize start and stop times of daily irrigation. Avoid low substrate EC especially at the end of the day, maintain a constant fruit load and avoid high rootzone temperatures.

PHASE THREE: GROWTH AND BALANCE
■ The level of transpiration in this growth phase in spring for winter planted crops will vary widely. It is therefore important to manage the rootzone accordingly. Please refer to Greenhouse Canada, September 2010 issue, pg. 12, on the thought processes that are involved to optimize the start and stop times of irrigation. You will know if you are giving too much water on the dark mild days – the drain per cent will increase and the substrate EC will decrease in comparison to a bright day.

rootzonepic4
 
Picture 4.0: Growth mottle spots are a result of too much water on dark humid days.  
Remember the design feature, “nutrient refreshment efficiency,” (Greenhouse Canada, April 2010 Issue, pg. 14) and how easy it is to refresh and replace nutrient solution in stonewool. This allows you to steer EC in the rootzone on the dark days, and when used in combination with late start and early stop times, you can protect fruit quality with minimal irrigation volume (Figure 3.0). In fact, on these days it would not harm the crop if substrate EC increased a little in the range 0.2-0.3mS.

Tip 6: Optimize the number of irrigations per hour. As a general rule-of-thumb the number of irrigations you give per hour can be linked to global radiation, i.e., 200 W/m2 = one irrigation; 600 W/m2 = four irrigations; 800 W/m2 = six irrigations; and 1,000 W/m2 = seven to eight irrigations per hour. This guide can be used to adjust the maximum rest time setting, which of course will govern the irrigation volume on dark days.
   
PHASE SIX: FINAL PRODUCTION
■ Finally, when the heads are removed towards the end of the crop, the demand for water is dramatically reduced. Uneven ripening will occur in this phase if the irrigation strategy is not adjusted accordingly.

 rootzone_figure3  
Figure 3.0: Stabilized EC with minimal irrigation volume on dark days will help maintain fruit quality. The combination of a late start and early stop time with large irrigation volumes has refreshed and stabilized the substrate EC on these two dark days with a total radiation sum 538 J/cm2 and 348 J/cm2 respectively, preserving fruit quality.  

FUTURE ARTICLES FROM GRODAN
■ The rootzone environment can be described as the engine room of the crop. A good quality root system will allow the crop to transpire and supply Ca2+ to the fruit, avoiding BER. However, the time that transpiration starts and the rate of transpiration during the day are governed by interaction with the aerial environment. The rootzone climate needs to be managed accordingly in order to maintain optimum plant balance, production and fruit quality. Furthermore, the intrinsic properties that the substrate has in respect to nutrient management and nutrient availability can go a long way to minimizing risk of fruit physiological disorders in extreme conditions.

It is also important to remember that maintaining regular production of high quality fruit will have significant impact on costs to the business elsewhere, not least harvesting and grading costs. In the next issue of Greenhouse Canada, I will define the additional benefits in terms of water and fertilizer savings that can be gained by correct rootzone management.


Andrew Lee works for Grodan BV as Business Support Manager for North America and Export Markets. He is a PhD graduate from the University of London, England.

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