|Osborn, G - UNIV. OF ARKANSAS|
Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: December 1, 2003
Publication Date: June 1, 2004
Citation: Pinson, S.R., Osborn, G.S. 2004. Improved methods for identifying fissure resistant rice. In: Rice Technical Working Group Meeting Proceedings, February 29-March 4, 2004, New Orleans, LA. 2004 CDROM. Technical Abstract: Kernel fissuring is one of the leading causes of reduced milling yield. Any reduction in fissuring results in direct increases in yield and profit for both the producer and the miller. Breeder interest in selecting varieties with improved resistance to pre-harvest fissuring rose rapidly in the 1990's after the agronomic value and genetic variability within U.S. rice germplasm for this trait were actualized with Cypress. Breeders soon adopted several methods for comparing relative fissure resistance among rice lines. One method involves exposing dried rice kernels to controlled humid air conditions followed by visual observation of kernel fissures. This method is laborious as it involves careful handling and observation of each treated kernel. Also, grain samples must be evaluated for fissures prior to laboratory treatment so that pre-existent stress and kernel fissuring do not introduce error into the evaluation. A second method breeders use to evaluate fissure resistance is to sequentially harvest seed samples from single plots during maturation and evaluate them for loss of milling quality over sequentially lower grain moisture contents while exposed to actual field conditions. Varieties that exhibit little sequential loss of milling quality have 'milling stability' or be 'fissure resistance'. Much seed is required to plant plots large enough to accommodate numerous sequential harvests, and much labor is required to cut, thresh, and evaluate harvest-moisture and milling quality of some 15 samples/plot. These fissure resistance evaluation methods require such large amounts of seed and/or labor that they cannot be applied until late in the breeding process, after six or more generation advancements, when much breeding effort has already been invested in each advanced line. Present evaluation methods allow a reactive final selection to prevent release of a fissure susceptible line. What is needed is an early-generation proactive selection tool. Our plan was to first identify environmental, genetic, and kernel component (physical and chemical) factors that determine fissure response in rice, then learn to 'control' them, resulting in an improved selection technique. Before the impact of various environmental and grain components on fissuring could be measured, a 'measuring stick' - comprised of varieties with established order of resistance to field fissuring - needed to be established. At the initiation of this project, it was known that Cypress was more fissure resistant than Lemont, and it was suspected but not proven that LaGrue and Teqing were less fissure resistant than Lemont. Our replicated multi-year milling stability data documents that the order of fissure resistance among the following varieties is Saber, Cypress, Jodon, Cocodrie, Lemont, LaGrue, Teqing, Jefferson. Saber was consistently the most fissure resistant line in all replications. Presently, breeders use the same large plots for studying yield, optimum milling quality and milling stability. To optimize the plots for yield evaluation, they are planted early (i.e., the first half of April in Beaumont, TX). Present data show, however, that seed harvested from plots planted later (i.e., May) reveal more distinct differences between fissure resistance and susceptibility because of a larger loss of milling quality among fissure susceptible lines. We documented that a wider range in kernel maturity, as indicated by moisture content, was associated with increased field fissuring, and we found that kernel maturity was less uniform in plots planted after May 3. As expected, once average grain moistures reached 16 - 18 percent, the drier uppermost seed of fissure susceptible rices exhibited more field fissuring and lower head rice yields than the less-over-mature seed collected from lower portions of the plot canopy. In contrast, Saber and Cypress exhibited a narrower range of kernel moisture, and the uppermost seed had higher or similar milling quality as lower-placed seed even at low (14 percent) harvest moistures. This suggests the possibility of reducing the number of seed samples per plot from 15 sequential harvests to a single harvest of paired top- versus bottom-or bulk-harvested seed. If proven valid, the paired-sample method requires such small plots that fissure resistance selection could begin in the F3 or F4 generations. The present data do validate a more efficient manner for documenting loss of milling quality within sequentially harvested seed. Percentage whole kernels after dehulling was found as accurate as duplicate milled samples for predicting head rice yield, yet required less processing time and as little as 25 grams paddy rice. By reducing the labor and seed required for milling stability evaluation, breeders can now evaluate fissure resistance in additional plots per year. The current project also verified that the "fissure index" computer model used to evaluate post-harvest fissuring from chemical and physical properties of grain accurately predicts pre-harvest fissuring as well. The model was further improved with inclusion of the third grain dimension, thickness. The contribution of the hull, bran and endosperm (white rice) toward the overall fissure resistance of various varieties was measured and modeled. The high fissure resistance of Cypress was primarily attributed to hull composition/tightness, and bran composition. Saber's high resistance was predominantly from endosperm composition, suggesting that even higher levels of fissure resistance can be attained by combining key attributes of these two varieties through breeding selection.