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close this bookAmaranth to Zai Holes, Ideas for Growing Food under Difficult Conditions (ECHO; 1996; 397 pages)
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View the documentAbout this book
View the documentAcknowledgements
Open this folder and view contents1: Basics of agricultural development
Open this folder and view contents2: Vegetables and small fruits in the tropics
Open this folder and view contents3: Staple crops
Open this folder and view contents4: Multipurpose trees
Open this folder and view contents5: Farming systems and gardening techniques
Open this folder and view contents6: Soil health and plant nutrition
Open this folder and view contents7: Water resources
Open this folder and view contents8: Plant protection and pest control
Open this folder and view contents9: Domestic animals
Open this folder and view contents10: Food science
Open this folder and view contents11: Human health care
Open this folder and view contents12: Seeds and germplasm
Open this folder and view contents13: Energy and technologies
Open this folder and view contents14: From farm to market
Open this folder and view contents15: Training and missionary resources
Open this folder and view contents16: Oils
Open this folder and view contents17: Above-ground (urban) gardens
View the document18: What is ECHO?
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Open this folder and view contentsECHO development notes - issue 52
close this folderECHO development notes: issue 53
View the documentFifty-one issues of edn in one book!
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View the documentThe nutritive value of chaya, one of the most productive green vegetables
View the documentSolar water disinfection
View the document"Why don't my tomatoes set fruit?"
View the documentInsights from a biogas project.
View the documentMalnutrition and child mortality
View the documentList of distance learning courses is available from ECHO.
View the documentFrom ECHO's seedbank
View the documentEchoes from our network
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Open this folder and view contents28 additional technical notes about tropical agriculture
Open this folder and view contentsPrinciples of agroforestry
Open this folder and view contentsGood nutrition on the small farm
 

"Why don't my tomatoes set fruit?"

is a common question asked by gardeners from temperate climates who move to the tropics. A related question is, "Local markets only have cherry or plum type tomatoes. Can you send seed of a larger tomato?"

If you have a tomato plant that is healthy and flowering but not setting fruit, the reason is likely related to temperature. Both daytime highs and nighttime lows have a variety of effects on the ability of a tomato to set fruit. Cherry, plum, and other small tomatoes seem to be less adversely affected by these extremes, which is why those types are the ones in local markets. We had hoped to find clear-cut guidelines, but could not, so we will venture our own: If daytime temperatures are not less than 33øC (92øF) and nighttime temperatures less than 22øC (72øF), you may experience difficulty. If daytime temperatures are over 40øC (104øF) or nighttime temperatures over 26øC (79øF), you will almost surely have poor fruit set and possibly damaged fruit.

These are fine rules-of-thumb, but the reasons are too complex to be precise. Understanding some of the factors may help you find a solution.

Nighttime temperatures. These can be too low or too high. Night temperatures that do not drop to at least 26øC (78øF) are clearly damaging to fruit set. Cultivars developed for early production in temperate regions are able to also set fruit earlier-when temperatures are low, some as low as 4.4øC (40øF). On the other hand, those developed for warm climates typically will not set fruit if temperatures fall below 10øC (50øF).

Pollen grains must germinate before the ovule can be fertilized. At 25øC (77øF) germination takes about an hour; at 10øC (50øF), 5 hours; at 5øC (41øF), 21 hours. Once it germinates, the pollen tube must grow until it reaches the ovule. Growth rate increases with temperature from 10-35øC (50-95øF), but is reduced outside that range. The ovule may deteriorate before it is fertilized.

High daytime temperatures. The anther must dehisce (burst open) before pollen grains can be released. This process is inhibited by temperatures that are too high. Over 35øC (95øF), the surfaces of both the pollen grain and the stigma may dry out, causing poor fruit set. The pollen germination rate increases with temperatures up to a point, but is inhibited over 37øC (99øF).

A high of 40øC (104øF) seems to be a critical point. Exposure to temperatures greater than this can damage both ovules and pollen production. E.g., if the ovule has been exposed to very high temperatures nine days before flowering, it can deteriorate. Once fertilized, the endosperm of the developing seed can deteriorate over 40øC (104øF) for between 1-8 days after fertilization.

The difference between daytime highs and nighttime lows (diurnal variation). In regions and seasons where days are long, tomatoes are not productive unless the difference between day and night temperatures is at least 5.5 Cø (10 Fø). We have been told that a very high diurnal variation, as might occur in a desert or high in the mountains, can apparently overcome some of the above effects of high temperatures.

Fruits that do set at high temperatures are often so badly damaged or misshaped that they are not marketable. Red varieties may become more orange at higher temperatures. This is because synthesis of the red pigment, lycopene, is slowed at high temperature but the orange pigment, þ- carotene, continues to accumulate normally.

Heat-tolerant varieties have been developed which can extend the range a bit. Recent examples are 'Solar Set' and 'Heatwave' which are supposed to give improved fruit set at temperatures around 32-35øC (low 90s F). Presumably tomatoes grown under shadecloth would be a little less damaged by heat.

[References: Vegetables: Characteristics, Production and Marketing by Lincoln Peirce, Wiley & Sons, 1987; The Tomato Crop, Atherton and Rudich, Chapman & Hall, 1988; personal conversation with Dr. Don Maynard, Florida Gulf Coast Research and Education Center.] Return to Index.

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