Fruit firmness is an important aspect in the production of quality tomatoes. Tomatoes that have firm fruit can be allowed to ripen more fully on the vine thus imparting greater flavor and higher consumer demand. Indeed, fruit firmness is used by consumers as an indicator of high quality and often is the last test before selecting a tomato for purchase. Conversely, soft fruit are prone to injury during harvesting, grading and marketing. Additionally, since tomatoes continue to ripen (and soften) after harvest, tomato fruit that are inherently soft have a relatively short shelf life for both the retailer and consumer.
Ultimately, anything that influences cell wall structure and strength affects fruit firmness of tomato. Genetic (hereditary) factors have been identified that cause certain varieties to bear firmer fruit than others. For example, "meatier", beefsteak types, because of their internal structure, are likely to have firmer fruit than conventional types. Additionally, environmental factors such as temperature, relative humidity, fruit shading, soil fertility and soil salinity all have been shown to affect fruit firmness. Of the preceding, temperature seems to be the most frequent cause for soft fruit.
Tomato is a warm-season vegetable that is susceptible to chilling injury. The latter term is used to describe physiological changes (damage) that occurs in some plant species in response to cool (non-freezing) temperatures. Chilling injury in tomato fruit has been associated with cell wall degradation and is usually classified in one of three levels according to its severity:
Level 1: Loss of fruit firmness and non-uniform color development.
Level 2: Translucent, water-soaked patches and/or yellow, mealy spots on (otherwise) red fruit.
Level 3: Large green areas on (otherwise) red fruit along with uneven fruit surface due to tissue collapse.
Chilling injury and the associated loss of fruit firmness are most often encountered late in the growing season or in the winter greenhouse. While no "benchmark" temperature has been established for chilling injury to occur in tomato, suffice to say the cooler night temperatures are the more likely associated symptoms will develop.
Any discussion of tomato fruit firmness would not be complete without the mention of calcium as a plant nutrient. Calcium is used by the plant for the manufacture of new cell walls and is especially important in maintaining proper cell wall structure. Calcium deficiency in tomato has been associated with fruit cracking, blossom-end rot and soft fruit. Since calcium is immobile in the plant it is important to supply adequate amounts as a mineral nutrient during all stages of plant growth. It is important to remember that excessive magnesium in the soil is known to antagonize (block) the uptake of calcium by plants. Therefore, the proper calcium:magnesium ratio is needed for proper calcium nutrition. Generally, plants need two calcium ions for every one magnesium ion they uptake.
Finally, the way tomatoes are harvested and handled can also affect fruit firmness. Tomato fruits allowed to vine ripen should be harvested in flats or lugs and stacked no more than two to three fruits in deep in the container. Deeper or taller harvesting containers (e.g. a five-gallon bucket) tend to put excessive pressure on the lower fruit, causing soft spots or splitting.
Editor's note: tomatoes developing soft spots in the wall was a problem of a number of growers in August at the North Missouri Produce Auction.
REVISED: November 30, 2015