The process by which tetraploid Louisiana irises came into existence is fascinating. The chapter devoted to this process in The Louisiana Iris, while thorough, can’t begin to describe the work required to induce tetraploidy by chemical treatments. Nor can words express the frustrations hybridizers currently face in getting seed-set on tetraploids. These are pretty large obstacles to overcome, but they are solvable problems .
My goals for working to improve the tetraploids focuses on increasing their gene pool to include all of the varied forms known in diploid Louisianas. This wealth of various flower-forms really makes these irises unique, and must be preserved. To this end, crosses have been made utilizing I.brevicaulis, I.giganticaerulea, I.hexagona, I.fulva and I.nelsonii X the modern hybrids. Keep in mind that these crosses are not intended to produce immediate introductions, but rather to lay a foundation for future hybridizing.
In 1989 two hundred and fifty I.brevicaulis rhizomes were collected in northeast Texas while in a completely dormant state and placed in wet vermiculite. Once growth had resumed, the rhizomes were treated in a colchicine solution. The first group was subjected to a series of 12-hour treatments, while the second group received 24-hour treatments. The initial results were encouraging. The root tips all showed a pronounced swelling, and new foliage seemed to be thicker. Mortality was roughly 25%. The surviving plants bloomed in 1991 and were a total let-down. None were marked for further study and have been discarded.
After my initial attempt to induce tetraploidy failed so miserably, I decided to try a more proven method – the treatment of newly germinated seedlings. During the 1990 bloom-season, 86 diploid crosses were made. These crosses were made with light-colored irises, primarily whites, pinks and yellows. I.brevicaulis, and predominantly I.brevicaulis– based hybrids were crossed with hybrids of I.giganticaerulea and I.nelsonii backgrounds – and in some cases, directly back to these species.
Seeds were soaked in water for several weeks then peeled and planted in a peat-based potting medium. (Removing the outer corky seed covering allows for more even germination and makes the seedlings much easier to handle.) The pots were covered with saran wrap and placed in the greenhouse under constant illumination. The pots were kept covered until germination commenced.
When the seedlings reached a height of about an inch, they were carefully dug and rinsed, then placed in a .05% colchicine solution. Two pint jars were used for each cross, one for the colchicine solution and one for the water. After 24 hours in the colchicine solution, seedlings were thoroughly rinsed with running water before being placed in jars of water for another 24-hour soak. Three complete cycles were used. At the end of the first colchicine treatment a pronounced swelling was noted at the root tips. A total of 887 seedlings resulting from 86 crosses received this treatment.
The treated seedlings were planted in individual 1.5-inch pots in a sterile peat-based potting soil with vermiculite added to ensure that the mix did not stay too wet. Instead of placing the potted seedlings back in the greenhouse where temperatures can fluctuate, they were kept indoors. Constant illumination was provided until the seedlings finally began to shows signs of renewed growth, then reduced to 18 hours.
In early spring 1991, the surviving 539 seedlings were re-potted into 2-gallon containers and held in wading pools. Seeds which germinated after the treatment process was finished were retained under identical conditions for comparison. The untreated seedlings responded immediately making substantial growth. By summer’s end, the treated seedlings had also responded very well, but there were marked differences. The treated seedlings had much nicer foliage with very little leaf-tip burning. The appearance of the diploid seedlings is best described as bedraggled.
At this time, it is not known how successful the treatments have been. Many of the treated seedlings have grown to the point that they should bloom in 1992.
During 1991 over 100 diploid crosses were made. I.brevicaulis variants of white and pink were heavily used, as was I.hexagona. These crosses are now germinating and will be treated with the herbicide method described by Dr. Kevin Vaughn.
While all of the five species have been used, the emphasis of my hybridizing program is on I.brevicaulis and I.hexagona. The goal is to have a genetic melange.
In observing I.brevicaulis growing in my garden and others as well as the wild, significant differences have been noted amongst the variants. Many of the modern hybrids derived from I.brevicaulis have a tendency to go dormant in summer. This is not true of all L brevicaulis. Variants which made it through the summer without the need of extra watering were obtained over the past 3 years. They range in color from blues, white, rosy-pink to smoky-lavender. Other good traits of L brevicaulis are the bud count and branching. All positions were double-budded on the white, pink and one blue with an especially showy signal and spray-pattern. The stalks were wonderful! Five bud positions, sometimes with a branch, or 4 positions with 2 branches. It is important to note that these stalks remained upright during bloom. Some variants lay their stalks on the ground. Such traits are not desired. This species definitely deserves a thorough trial.
Not very much is really known about the species I.hexagona. It is safe to comment that its contribution to the development of the modern hybrids is even more obscure. T.A Washington, a hybridizer of the 1930s, extensively used irises he called I.hexagona hybrids. None of his work remains, and were not utilized in other hybridizers’ programs. Frank Chowning believed the iris known as MISS PRISCILLA (’35), may have been the primary or exclusive form of I.hexagona in the Washington registrations. This iris is no longer available, and was not an important factor in the Chowning line. Until recently, its validity as a species was still much in question. The plants I obtained of I.hexagona have left me with no doubts as to their authenticity.
Flowers of 1. hexagona have wonderful substance, better by far than I have observed on any of the other species. Blossoms last from 1 to 2 days longer than normal. Stalk height varied from 24″ to 34″. Four bud positions were the norm, but there were a good number of stalks carrying 5 positions. Typically, only the terminal position is double-budded. The last stalk of the season was the best. It had 4 double-budded positions with three perfectly placed branches, which were also double budded. A truly awesome sight with 3 or 4 blooms opened at once. Foliage coloring is yellow- green, but held up extremely well in our Texas heat. This iris would be a boon to any breeding program, diploid or tetraploid.
Now that the fall re-planting is finished and all of the seedlings bedded, thoughts turn to spring. Will it have all been worth it?
Only time will tell, but I think the future of Louisiana tetraploids is bright.
