View Single Post
Old January 4, 2013   #74
Fusion_power
Tomatovillian™
 
Fusion_power's Avatar
 
Join Date: Feb 2006
Location: Alabama
Posts: 2,250
Default

I am working on defining the basic concepts and issues with developing a cold tolerant tomato. These are some more thoughts along lines I've already looked at and adding some new items. The first and most important statement is that there are a bit over 500 genes involved in the stress response biopaths for S. Lycopersicum tomato. At least 1/3 of them are not activated by cold temperatures. By comparison, S. Habrochaites activates nearly 200 unique genes when exposed to cold. The number of genes involved indicates that multiple external traits are invilved.

One item I needed to define is exactly what temps a tomato can stand.

120°F = Severe heat, but if plenty of water is available, the plants are fine. This temp is way above levels at which pollination can take place. Plants with heavy fruit set may show stress.

92°F = This is the temp at which pollen starts clumping and blossoms begin to drop.

70°F to 92°F = This is the goldilocks zone. Tomatoes grow prolifically, flowers set readily, plants need maximum fertility in the soil. The high end of this range is optimum for spread of several foliage diseases.

65°F to 72°F = the best temperature to grow seedlings.

50°F to 65°F = this is the beginning of cold stress. Tomato plants in this range grow slowly, often produce anthcyanins (turn purple), and become pale green from loss of chlorophyll function.

32°F to 50°F = This is the range where normal tomato plants show severe cold stress. Leaves shrivel, turn yellow, wilt, stems lose turgor, roots stop absorbing water.

28°F to 32°F = This is the maximum range most tomatoes can withstand without freezing. Note that if frost forms on the leaves, then the leaves will freeze and die. The plant may live and can form new leaves, but the stunting effects take quite a bit of time to overcome.

22°F to 28°F = This is the range that a few select varieties can withstand for brief periods of time but stipulating that frost on the leaves will still kill them.

15°F to 22°F = This is the range that a few Russian cultivars are reported to survive, again only if frost does not form. The reports I have read indicate that this tolerance is only for a limited time period, in other words, repeated low temps for 3 days or more will still kill the plants.

0°F to 15°F = A few Russian cultivars are able to handle temps this low for brief periods of time. This is the low end of the range that wild tomato species S. Habrochaites, S. Chilense, and S. Lycopersicoides can withstand.

As the temperature goes below 60°F, tomato plants enter a state where normal photosynthesis ceases. Sugar accumulates in the leaves, rubisco - a crucial chemical in the plant- begins to be deactivated by free radicles. This process causes the leaves to become dysfunctional in such a way that they can not recover. One very special trick that greenhouse growers MUST know is that if plants are exposed to overnight lows below 45°F then the greenhouse must be let rise to a high temp near 100°F the next day. If this is done, then the plants totally reverse all effects of being too cold the night before.

Here are a few of the breeding objectives.

Able to tolerate extremely low temps for short periods of time
There are several components to this high order trait. The leaves must be able to close stomata to avoid dessication. The leaves must be thicker than normal again to avoid desiccation. The stem must be able to maintain elasticity and conduit ability for photosynthate from the leaves and nutrients from the roots. The roots must be able to maintain absorption of nutrients.

Fruit must be able to tolerate freezing temps
The fruit must not freeze damage as a result of cold where typical damage is gray soft areas inside the fruit skin. The fruit must maintain expansion even when cold temps limit biological processes. The plant must be capable of translocating nutrients from the leaves and roots to the fruits.

Flavor must be maintained regardless of temperature, especially at fruit maturity
This requires that the leaves must be able to produce the volatiles that compose aroma and the terpenes associated with flavor. There is an implied problem with calcium channel signalling at low temperatures. Selection must be maintained for fruit that are healthy and well developed.

Pollination must be able to occur at temps from 32 to 50 degrees F
There are several issues with pollination since most tomato plants do not set fruit at temps below 50F. The first is precocious flowering which would seem to be a required trait. Another is for pollen production which is necessary for fruit enlargement to occur. I have LA2006 with the ft gene for flowering at low temps, but that does not necessarily include production of pollen. S. Habrochaites produces pollen in the desired temp range so it may be the best source of these genes. I do not want to use the parthenocarpic genes because they infer several physiological processes are dysfunctional.

Able to grow at low temps where most tomatoes shrivel up and shiver (range 32 to 50 degrees F)
This is currently a non-existent high order trait in S. Lycopersicum. I am not certain if any of the cold tolerant cultivated varieties I have accumulated so far will help any in this regard. I have researched enough to know that this ability is in S. Habrochaites and in S. Lycopersicoides. I plan on relying heavily on these species to bring in the required traits.

Able to grow and produce in a cool climate, i.e. with less growing degree days for the season
This is a significant new direction for the breeding work. Areas such as large parts of Oregon and Washington have normal summer temps that are almost too cool for tomatoes to grow. The physiological adaptations required for growing at 32 to 50 degrees F should also help with growth in areas that have low average temps. The objective then should be to reduce the number of growing degree days for tomatoes to produce a crop.

Here are varieties I have accumulated so far and reasons for including them in a cold tolerance breeding program.

Bellstar - Jointless and was developed in Canada with a degree of cold tolerance
Wheatley's Frost Resistant - has cold tolerance, but seems oriented toward late season rather than early spring
Glacier - Small early and productive with some cold tolerance
Stupice - Small early and productive with some cold tolerance
Siberian - Small early and productive with some cold tolerance
Tastiheart - my selection that survived 22 degrees F April 7th 2007
LA2006 - ft gene for setting fruit at low temps.

Crosses I plan on making to see what can be done:
Kimberly X Eva Purple Ball - to move the precocious flowering gene into a larger fruited variety
(BBxEPB) X LA4026 to combine jointless, high lycopene, F1, F2, F3
LA4454 X Druzba to combine the sucr gene with a good flavored line
Tastiheart X LA2006 to combine cold tolerance with fruit set at 40°F
Perth Pride X (disease tolerant breeding line from R. Gardner) to get ph3 into a dwarf
Doublerich X LA0722 to move the ascorbic acid gene(s) into a high vitamin C line. Hopefully will be able to increase vitamin C significantly.
Nepal X ? - To bring in good flavor genes
KBX X Bellstar - to get the jointless gene into a good flavored large fruited orange.
Tastiheart X PI 126256
Atkinson X LA3969 - to move the chromosome 12 segment into an adapted southern variety

I'll add more crosses as I think of them.

DarJones
Fusion_power is offline   Reply With Quote