Summary and discussion– Dorais, Papadopoulos, & Gosselin 2001

“Influence of electric conductivity management

on greenhouse tomato yield and fruit quality”

M. Dorais, A.P. Papadopoulos, A. Gosselin

Published 2001, Agronomie 21, pp 367-383.

[Hypertext to this explanation of EC] A short background on Electrical Conductivities (ECs)- they are a simple way to estimate how much salt (fertilizer) is in the irrigation water. EC is actually measuring how well the solution conducts electricity, and the EC probe cannot distinguish between a solution with high calcium from one with high sodium. EC must be considered as an estimate of the total fertilizer content of the irrigation water. Just to confuse the issue, ECs can be reported in several different units of measure. But fortunately, all of the units are easily converted from one to another. The scientific literature tends to favor µS/cm (microSiemens per centimeter) as the unit of measure, but those values are identical to an older unit called micromhos per centimeter (µmhos/cm) (1µS/cm = 1 µmho/cm). The change from one system to another started in the 1970’s. Sometimes, milliSiemens per centimeter (mS/cm) is used, which is just µS/cm x 1000 (1 mS/cm = 1000 uS/cm). Other units include “total dissolved solids” (TDS) or “parts per million” (PPM), which are both simply calculations assuming certain salts are present in “normal” relative proportions. In general, to convert from µS/cm to PPM, simply multiply the µS/cm value by 0.64. To convert from mS/cm to PPM, multiply mS/cm by 640 (1 mS/cm = 640 PPM).

This paper comes out of the highly respected Greenhouse and Processing Crops Research Center under the umbrella of the Agriculture and Agri-Food Canada in Harrow, Ontario. Dr. Papadopoulos has been publishing very applied and reliable papers on hydroponic crops for many years, and this paper is yet another excellent source of information. The issue of Electrical Conductivity (EC) management and its effect on yield and quality is a hot topic. This lengthy literature review paper explores how high ECs cause fruit size to decrease while increasing the dry matter content. This means that the average fruit is smaller, but may have a higher carbohydrate content (and therefore, more sugars) than fruits from plants grown under lower fertilizer EC regimes.

EC levels affect yield and fruit quality differently depending on how the fertilizer solution interacts with other factors. These other factors include the specific tomato variety being grown, various environmental parameters such as humidity and temperature, the composition of the nutrient solution and specific crop management strategies. Some studies showed that ECs above 2.3 to 5.1 mS/cm reduced yield to an unacceptable level, while at the same time, ECs of 3.5 to 9.0 mS/cm improved tomato fruit quality. Manipulating the environmental conditions such as humidity, temperature and ambient CO₂ levels might offset some negative effects on yield and quality issues, such as blossom end rot (BER).

Flavor and Nutritive Properties

In general, tomato flavor is determined by the levels of soluble sugars and organic acids in the fruit at the time of harvest. Most commercial markets in the US and Canada favor low-acid, high-sugar tomatoes. Some growers are using sodium chloride (NaCl, the same compound as table salt) in their fertilizer solution to produce a fruit with reduced acidity and increased sweetness and improve the overall flavor intensity. However, some consumers, particularly home gardeners in the Southeast US, prefer a high-acid, less-sweet tomato flavor, so care must be taken to produce fruit that is acceptable to the consumer base. It is well documented that the levels of soluble solids (carbohydrates and sugars), acids, volatile compounds (which produce the characteristic aroma of tomatoes), minerals, carotene (red color) and vitamin C all increase with increasing EC. However, very high ECs can lead to fruits with “off-flavor” characteristics that are described as “moldy,” “bitter,” and even “burning.”

Fruit Size and Flower Set

Most North American consumers prefer a larger size tomato (200g, or 0.4 pounds, minimum). High salt levels in the fertilizer solution reduce the water potential in the plant, which depresses the water flow from the roots into the fruit and therefore reduces the rate of fruit expansion. This osmotic effect varies with light intensities (higher light levels will reduce fruit size even more than low light conditions under high EC stress), but in general, high EC in the irrigation water means that less water is moving through the plant and the fruits will be smaller. Flower production is not affected by high ECs, and fruits do not mature any faster or slower under high EC conditions. However, under very high ECs (15 mS/cm), one researcher did observe that fruit set on the upper trusses is reduced, but the mechanism for this behavior is not yet understood. Fortunately, growers do not intentionally try to grow tomatoes under such severely high ECs.

Yield

Several researchers have studied the relationship between EC and yield, and found that ECs of 4.6 to 8 mS/cm decreased yield only because of a reduction in fruit size (not a reduction in the number of fruit). However, a very high EC of 12 mS/cm reduced both the number and size of fruit. This effect varies with cultivar, and not all tomato cultivars are affected to the same extent. Some varieties are more resistant to EC-related problems such as BER and cracking than other varieties. Varieties that normally produce smaller fruits, such as some of the cluster tomatoes varieties, would not be as affected by high ECs than cultivars producing larger (beefsteak) fruits.

Problems and Disorders

One researcher found that increasing ECs from the normal range of 2.0 to 3.5 mS/cm to a higher range of 2.6 to 4.6 mS/cm reduced fruit cracking in a spring crop of greenhouse tomatoes. This may be due to a thicker cuticle (fruit skin) and lower fruit turgor pressure (softer fruit) found at slightly higher ECs.

Generally, blossom end rot (BER) increases with increased ECs, especially when the fruit is at a very young stage of development (7-21 days after blooms open). Internal BER can also manifest as blackened seeds inside the fruit, causing the fruit to stop expanding and mature at a very small size. Some varieties are less susceptible to BER than others. One researcher reports that the ideal EC of the nutrient solution for preventing BER in greenhouse tomatoes is 2.0 to 2.5 mS/cm, with a calcium concentration of 7mM (need to convert mM to ppm?) However, the same researcher admitted that these levels may not be optimal for other qualities, such as fruit flavor, firmness and cracking.

Post-harvest shelf life

Fruit shelf life is dependent on several pre-harvest factors, but increasing EC may increase shelf-life of some varieties. This is thought to be due to the thicker skin that can resist cracking and splitting and the reduced water content of the fruits’ cells. However, these effects where not conclusive under moderate ECs (ranging from 3.0 to 5.6 mS/cm) so growers should not count on increasing EC as a strategy for improving post-harvest shelf life.

Diurnal Adjustment of EC and Humidity

Growers with sophisticated (computer controlled) irrigation and fertilizer injector systems are able to adjust the EC of the fertilizers for different day/night regimes and under changing humidity conditions. Adjusting EC to a higher level (8.0 to 9.0 mS/cm) during the evenings and nights while maintaining a lower EC (1.0 to 2.0 mS/cm) during the days when the plants are actively transpiring and need more water can reduce the incidence of BER. Diurnal changes of EC are one strategy for maintaining higher fruit size while increasing sweetness of the tomato. Increasing humidity during the day can also reduce BER, but growers should be aware of other negative effects of high humidity, such as increased disease risk and reduced movement of nutrients from the roots to the plants’ canopy. Researchers did observe that increasing humidity under moderate ECs did not increase fruit size, nor did it enhance the flavor of the fruits. Growers should be warned against drastic changes of EC (such as running out of fertilizer and flushing with clear water) that will invariably lead to fruit quality problems.

Growing Media and EC

Different growing media, such as rockwool, coir, peat, perlite, and sawdust show different responses to nutrient solution EC levels. It is imperative that the leachate of the irrigation solution from the media be sufficient so that the EC of the root zone does not spike so high as to negatively affect plant growth. Some media, such as peat, can hold onto fertilizer ions to the point of increasing the EC of the leachate to as high as four times that of the incoming dripper solution. This can lead to a severe decrease in plant growth and fruit yield. Therefore, the EC of the dripper solution, the frequency of irrigation, and the quantity of the nutrient solution provided to the plants changes with different media. Under low light conditions, leachate rates should generally range from 10-15%, while under high light conditions, 30-50% leachate may be required to avoid toxic salt accumulation in the root zone.