Influence of Earliness per se Genes on Flowering Time in CIMMYT Wheats

The main environmental factors affecting phasic development in wheat are vernalization, photoperiod, and temperature (Pirasteh and Welsh, 1980). The adaptability of CIMMYT semidwarf wheats to diverse environments depends to a large extent on variation in these factors. Numerous studies have indicated that a factor (or factors) besides vernalization and photoperiod influences the rate of development of wheat. Syme (1968) found that the basic development period for wheat was influenced by mean daily temperature. Ford et. al. (1981) coined the term "earliness genes" and proposed that they were different from the genes controlling photoperiod sensitivity.

More recently, Miura and Worland (1994) found that genes on all three chromosomes of Group 3 could have striking effects on ear-emergence time. Slafer (1996) re-examined the assumptions that earliness genes are independent from photoperiod and vernalization and that differences in earliness genes apply only to the vegetative period up to floral initiation.

Methods

Seeds of 52 CIMMYT (or CIMMYT derived) lines and 12 testers were imbibed for 48 hours at room temperature and vernalized for eight weeks at 4^oC. After removal from vernalization, the seedlings were standardized for length (Fig. 1). Ten plantlets per entry were sown in pots and placed in a 24 h photoperiod regime at two day/night temperatures: 23/12^oC and 16/4^oC (Fig. 2). The number of days from transplanting to flowering was recorded for 8 plants per entry. Eps was calculated as the difference between days to flowering of the variety and days to flowering of the earliest variety in the set.


*Fig. 1. Eps* characterization at CIMMYT: seedlings were vernalized for 8 weeks at 4°C. Seedling length was standardized before transplanting to growth chambers.**		Fig. 2. Growth cambers at CIMMYT. Vernalized cultivars were grown at day/night temperatures of 23/12°C or 16/4°C under 24 h photoperiod.

Results and Discussion

From previous studies (Flood and Holloran, 1984) it is known that eight weeks of cold treatment and 24 hours of light will satisfy the vernalization and photoperiod requirements of wheat. Differences in days to flowering among the lines were considered to be due to the earliness per se genes. Figure 3 shows the mean days to flowering of CIMMYT wheats and testers after removing the response to photoperiod and vernalization. In general, all lines flowered later under the cold regime (top line). The range in the delay of flowering due to cold temperatures was 0 to 27 days. The number of days to flowering for CNO79/PRL was the same under both temperature regimes, indicating insensitivity to temperature. The varieties Nesser, Gen3*/PVN, and Chilero exhibited the strongest sensitivity to cold temperature by delaying flowering by 26-27 days.

To determine whether earliness genes act independently of temperature, the change in ranking between Eps at 23/12^oC and Eps at 16/4^oC was plotted in Fig. 4. A positive change in ranking indicates an acceleration in flowering under warm temperatures due to Eps genes. This would suggest that, relative to the other varieties in the set, the Eps genes in a given variety are sensitive to warmth and cause faster development. A negative change in Eps ranking indicates that Eps gene sensitivity is expressed as a delay in development under warm conditions, relative to the other varieties in the set. The magnitude of the sensitivity varied widely. In varieties to the right of the plot (Fig. 4), flowering was delayed by cold temperature, and the Eps ranking of these varieties was greatly affected by changes in temperature. This would suggest that the Eps genes in these varieties are, in all likelihood, temperature genes. The varieties whose Eps ranking changed due to temperature were: Pastor, Enkoy, PBW343, Chinese Spring, Rayon, Weaver, Irena, Turaco/Chil, PGO/Seri//BAU, Pitta, GOGATSUKOMUGI, Inquilab91, Kauz, VEE #5/Sara, CHUGOKU 114, TEMU 1032.94, Embrapa 16, Chum18//JUP/BJY, Stephens, ROQUE F 73, SONORA 64, Don Ernesto, Chilero, Gen*3/PVN, Nesser, CNO79/PRL, and TEMU1024.95.

In varieties to the left of the plot (Fig. 4), flowering was delayed by cold temperatures, but Eps ranking remained consistent regardless of temperature. For this group, the data suggest that Eps genes are not temperature genes but "static" genetic characters for early flowering. The varieties that ranked consistently for Eps effects were: Scan, Star, Pavon, Chil/PRL, Hubei, Ciano F67, Chilero/BUC, Nourin 61, Bacanora, HE/2*CNO79, Seri, Weaver/Roblin, and Temuco1024.95. For the variety CNO79/PRL flowering was not affected by temperature but the Eps ranking changed drastically. It is likely that the change in rank was not due to sensitivity to temperature but rather to the change in ranking of varieties within the set.

From these data it is clear that the rate of development in most CIMMYT genotypes is responsive to temperature. These results are in agreement with numerous studies (Angus 1981; Slafer and Rawson 1994) that found that not only were all genotypes responsive to temperature but there was genotypic variation in sensitivity to temperature. While all CIMMYT genotypes flowered earlier in warm temperatures, the degree of sensitivity to temperature also varied widely and resulted in interactions between temperature and genotype. Lastly, Eps appeared to be related to temperature genes in most CIMMYT varieties, but may also be associated with a "static" or constant character for early flowering. This is in sharp contrast with results reported by Slafer et. al. (1995) who found that in four genotypes the intrinsic earliness factor is a complex interaction between temperature and development. This study examined 52 CIMMYT (or CIMMYT derived) lines and 12 testers and found that Eps was related to temperature in 27 lines. However, there were 13 lines in which Eps appeared to be a factor related to "intrinsic" earliness in flowering and independent of temperature.

References

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