BULLETINS
& ARTICLES
Nutrition Helps
Improve Flavour of Tomatoes
Article reproduced with permission
of Horticulture News
IN THE SECOND OF HER TWO-PART ANALYSIS OF GREENHOSE
TOMATO FRUIT FLAVOUR MANAGEMENT, DR LUNETTE MORGAN OF SUNTEC INTERNATIONAL
HYDROPONIC CONSULTANTS LOOKS AT NUTRITION FOR FLAVOUR IMPROVEMNET AND SOIL-BASED
GREENHOUSE TOMATO SYSTEMS.
EC and nutrition for better flavour.
One of the simplest ways to improve tomato fruit
flavour lies with the root zone and how the grower manages inputs, such
as water and fertiliser.
In soil-less cultivation or hydroponics, fruit
yields and flavour can be manipulated more precisely than in soil-grown
crops.
Since the grower controls plant nutrition, via
a balanced nutrient solution, the composition and strength of this nutrient
can be changed to affect fruit flavour.
The simplest way of increasing the flavour constituents
of tomato fruit is in increase the concentration (EC) of the nutrient solution,
to produce fruits with more dry matter, sugar and acid, and, consequently,
better taste and firmness.
Such has been found to be the case with large-fruited
and cherry tomatoes. Some researchers have also reported improved
fruit flavour, with the use of increased solution conductivity levels.
It has been found that sugars and acidity levels
increased in the cherry cultivar, Gardeners’ Delight, with fruit grown
at an EC of 10 mScm-1 (CF of 100), as compared with 2.5 mScm-1.
Other studies have reported that the dry matter
content, sodium content and acidity of fruit grown at an EC of 8.0 mScm-1
was greater than fruit grown at 3 mScm-1.
An obstacle to improving the composition of tomato
fruit, via increased solution conductivity, is the well-documented relationship
between yield and the fruit dry matter content.
Using cultivation techniques, such as increasing
the nutrient conductivity to increase dry matter, also reduces the rate
of water accumulation and, thus, cell enlargement, so a loss in yield is
almost inevitable. Therefore, until recently, there has been little
incentive for a commercial grower, who is paid per kg of fruit, to increase
quality.
Fruit flavour is now more of an issue and techniques
are being investigated which allow improved fruit composition, without
the accompanying loss in yield which happened previously.
A Massey University study into the effect of hydroponic
nutrient solution concentration on greenhouse tomato flavour found increasing
the conductivity of the nutrient solution (EC) in NFT systems from only
2 to 4 mScm-1 resulted in an increase in fruit quality.
The study also found that even small increases
in brix or acidity could be detected by most consumers.
While these increases in tomato fruit flavour quality
are a positive aspect of applying higher conductivity solution to the crop,
there are other effects of increasing EC value.
The study’s trials found increasing conductivity
of the nutrient solution reduced fruit size and yield.
This was because the higher conductivity treatment
resulted in an increased concentration of phloem sap (carrying the sugars
for fruit growth) entering the fruit, thus less water accumulates and size
is reduced.
However, increased dry matter also results from
this reduced water accumulation and fruit quality is improved.
Yield loss resulting from high EC treatment can
be minimised by strengthening the root system and reducing plant stress,
because high rates of oxygenation in the root zone have recently proved
to be highly beneficial under increased EC treatments in tomatoes.
Adding mineral pyrophyllitic clay suspension to
the nutrient solution has also been shown to assist plant growth, yield
and fruit composition under conditions of plant stress (raised salinity
levels) in hydroponically-grown tomato crops (Lennard and Morgan, 2004).
New techniques such as these could well become
commonplace as growers concentrate on improving fruit flavour, while maintaining
high yields per square metre.
Soil-based greenhouse tomato systems
Numerous management techniques have been used to
increase yields, improve flavour and other qualities of soil-grown tomatoes.
Furthermore, for quite some time, plant breeders
have focused on developing cultivars with better yield and quality characteristics.
It is well known when salt-tolerant crops, such
as tomatoes, are grown under saline conditions, their flavour often improves.
This is only one of the two general methods available
for ensuring high-quality fruit production through a restricted in plant
growth.
The other methods involve restricting water availability
to the root system. This also acts to increase the ion concentration
around the roots just as raised salinity does.
Since more than 90% of the fruit is water, restricting
the content of the fruit tissue will then increase the concentration of
sugars.
Many growers using soil or organic media have
also developed irrigation management practices designed to impose a controlled
level of stress on their crops. In turn, this improves fruit quality
in terms of total soluble solids (TSS) and flavour.
Saline water has been used on field-grown tomato
crops to improve fruit quality and is a well-proven technique.
However, the improvement gained by water deficit
or saline water irrigation are usually accompanied by reduced yields because
of less water accumulation and cell enlargement in the fruit. Restricted
irrigation and salinity can be applied to adjust fruit number, size and
composition.
The effect of both methods is to reduce the quantity
of phloem sap entering the fruit. However, by increasing the concentration
of phloem sap, the quantity of sugars entering the fruit is maintained
at the same level.
Since the tomato plant is stable in its allocation
of assimilations to the fruit, this allows the process of increased dry
matter accumulations to occur, as a result of the increased salinity treatment.
Although less water accumulates in the fruit, dry matter still increases
to the same level as normal fruit.
The result is a high percentage of dry matter
in the fruit and a higher concentration of sugars in the fruit juice.
