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Genetics of Gigantism in Tobacco

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greenmonster714

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deluxestogie

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This is an interesting read. Apparently, most "giant" tobacco is the result of a simple Mendelian recessive trait. Although this article is undated, its latest reference is 1918, so we can assume that to be near the publication date. Serious study of Mendelian genetics was in its infancy. But they got it right. The "gigantism" that they observed after a hybridization between a normal-height parent and a giant parent, appeared only in the F2 generation, and in a ratio of 25% giants to 75% normal height. And these "giant" F2 plants, when selfed, produced 100% "giants." Textbook for a simple recessive trait, caused by a change in a single gene.

What I find even more interesting is that it is not actually "gigantism", but rather a failure to go to blossom in a timely way. Tobacco plants stop getting taller (and cease the initiation of new leaf nodes) when they begin to produce their reproductive buds. If the budding process is delayed, for whatever reason, then the stalk continues to produce more leaves, and to gain further height.

A few decades later than this article, researchers discovered that this form of "gigantism" was the result of a change from "day neutral" blossoming to "short day" blossoming. The plants keep on growing until the day length is very short. Actually, it turns out that it's not the day length, but the night length. When the uninterrupted period of darkness is sufficiently long, the plants will blossom. If (experimentally) the period of darkness is interrupted with brief light, then these plants never go to blossom.

Here's a brief discussion of Photoperiodism: http://www.hammiverse.com/lectures/39/1.html

In 2014, I grew Colombia Garcia that never produced buds, after 142 days. The plants were in a less than ideal growing bed, but yet reached 78" in height, producing 42 leaves. [Leaf size 18.5" x 8"] This particular planting bed (I'm now realizing) was exposed directly to the neighbor's mercury vapor insecurity light--all night, every night.

In 2015, having produced no seed the previous year, I germinated more of the same seed batch of Colombia Garcia (so...the same "generation" as the 2014 plants). These were planted in a highly productive bed alongside my house. These 2015 plants grew to 116" (over 9-1/2 feet) to the crowfoot, and similarly produced 42 leaves. [Leaf size 21" x 13"] Here's the punchline. This bed is mostly shielded from the neighbor's light, but two windows in my house overlook that bed. This 2015 season, the plants finally produced buds and seed (maturation time ~120 days), but only in the few plants that were hidden from my bedroom window, and not subjected to light shining from indoors at night! The other window is my study, which I use at night only in the winter.

Garden20150831_2058_ColumbianGarcia_justBudding_200.jpg


My conclusion, as of today, is that Colombia Garcia is a long-night giant, and that in order to obtain seed, it needs to be grown in a location that is not exposed to lights at night.

Thank you, greenmonster, for posting that article link.

Bob
 

greenmonster714

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Your welcome Bob. As usual you took it to new heights and I learned even more. Thank you.

Id always thought the seeding was brought on by time and not light cycles. Obviously it does have an effect.
 

Tutu

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I'm curious to learn if it matters what type of light might distort this night cycle. My planting area has two lamps close to it, and they are permanently on at night. Never seemed to have had problems with plants forming buds. Never had the giants either I suppose. But it'd be interesting to learn how light might distort the cycle of normal plants growing and blossoming too.
 

deluxestogie

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I recommend the Coursera online course "What a Plant Knows": https://www.coursera.org/learn/plantknows (It's possible to enroll, but not pay for a certificate. This allows you to access the course videos and readings.)

Also, the instructor of that course, Daniel Chamovitz, published a wonderful small book by the same title: https://www.amazon.com/What-Plant-Knows-Field-Senses/dp/0374533881

Both the course and the book expect only a basic knowledge of biology.

These go into the fine details of light frequencies that affect plants in various ways and at various timings.

My understanding is that most tobacco varieties are day-neutral, which means that their growth is determinate, and they will grow to a specific size or number of leaves, then bud, regardless of exposure to light. The giants are apparently indeterminate, and depend on dark period to trigger budding. Certainly other factors, such as root restriction and sudden alteration of light periods have some impact, as evidenced by some plants budding very early, but I don't really have a good grasp of what exactly causes that.

Bob
 
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