We use a kind of modified stronger rapid-aging test to study heat tolerance in seeds. We use 48 C for 96, 104, and 120 hours within an incubator with water on the bottom. The entries are put within paper bags. After this preconditioning the seeds are put within paper dolls in a germinator in the usual way, and counted for dead and alive seeds.
A set of 18 translocations with wx and two inversions, Inv9a with wx and Inv2a with gl2, listed in MNL 40:184-185, 1966, not yet in uniform background, were used. They were crossed with IAC Maya latente, an open pollinated variety with a source of heat and frost tolerance from Michoacan 21 Comp 1-104, and a flint single cross from Brazilian Cateto. The F2 progenies were compared in split-plot design with paired waxy and non waxy seed and the F3 of glossy-2 and non glossy-2 by the treatment described above. The method of analyzing statistically the results was that described by L. T. de Miranda (MNL 55:18-19, 1981). The results showed linkages of tolerance to heat from latente, for short, lte1, with wx T2-9b (2S.18; 9L.22) and Inv2a (2S.7; 2L.8), both in chromosome 2. With the Cateto there was very strong linkage with wx T9-10b,(9S.13; 10S.40), for chromosome 10. We suggest calling it Lte2 by analogy to the known latente.
To check this last result the original MNL marker stock, g R sr2, was crossed to the component lines of the flint Cateto single cross C1 and C2. Only selfs were obtained. C1 has no plant color, and crossed to supersweet (a1 sh2) gave color. C2 has anthocyanin plant color. With the marker stock both segregated as lacking two pairs of factors: C1 gave a nearly perfect fit, 761 colorless to 976 colored, or 0.4381 colorless (expected 0.4375). C2 gave a poorer fit, 575 to 652, or 0.468 colorless, with more quantitative effects. The data are shown in Table 1, and the analysis in Table 2. We didn't measure the effect of g or sr2 in seed heat tolerance. The results show linkage of a factor for heat tolerance from Cateto with r, for which is proposed the symbol Lte2.
The IAC Maya latente was obtained as follows. We crossed the line Michoacan 21 Comp 1-104 with adapted material and selfed two times to get pure and segregating families. The families were tested in the field against frost. Remnant seed of the plants more tolerant to frost were planted, and recombined and compared with adapted material for tolerance to heat in seeds by the test described above. We backcrossed to IAC Maya three more times, selfing two times between each backcross and testing by the heat test in seeds. Most plantings were made out of normal season and occasional frosts confirmed that heat and frost tolerance came together.
R. M. Castleberry and R. J. Lerette (Proc. 34th Annu. Corn and Soybean Res. Conf., 1979) obtained adapted latente lines from the same source as ours, using basically synchrony of pollen shedding and silking following early season drought stress as the criterion for selection. Our observations and preliminary physiological measures made in our institute agree with their conclusions.
The available information suggests that the genetics of environmental resistance up to now follows the genetics of control of distribution of anthocyanin color. Latente-1 (lte1) from Michoacan follows the R2 B super-gene and must have together at least two more loci. A factor controlling the SH-SS (Levitt's hypothesis) tolerance type is the lte1 proper, that we tested for heat and frost, a factor for abscisic acid control of stomata. Latente-2 (Lte2) goes with the locus of the R1 super-gene and adds tolerance to heat. It is probably an hydrophobic protein type of tolerance, definitely different from that of Michoacan.
L. T. de Miranda, L. E. C. de Miranda, E. Sawazaki and N. C. Schmidt
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