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Led by light in west africa
Led by light in west africa
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Led by light in west africa

  1. 14 Living organisms use day-length to synchronize their daily and seasonal activities. Photoperiod-sensitivity is a spectacular expression of this phenomenon affecting reproduction, daily leaf movements or seasonal stem growth, dormancy and leaf fall. In the West-African savannahs the duration of the rainy season is linked with the latitude, and the movements of the Inter-Tropical Convergence Zone (ITCZ) – being gradually shorter from the south to the north of the area. Sorghum and pearl millet have been cultivated in West Africa since millennia and have consequently co-evolved with their natural pests. The pest pressures suffered by both crops grown in the wet and warm climate of the rainy season are very high. Therefore, the crops must be sown within the days at the time of the last large rainfall of the season. last two months in the far northern areas, and seven months in the far southern zones covering all the intermediate durations of the intermediate regions. The photoperiod-sensitivity allows the species to cover this whole range, since there are no late varieties that would be photoperiod-insensitive. The cycle duration within the species is lengthened Alteration of the days and nights synchronize a modify the duration of the vegetative phase, from emergence of the plantlet until the reproduction. Thus sorghum and pearl millet are classed as short- is triggered from July to August, when days are the longest. The triggering was strictly repressed from March to June and allowed from the beginning of July, at a date that is genetically controlled. The Led by Light in West Africa
  2. 15 only environmental signal whose annual evolution can explain this pattern is the daily change of the sunset hour. Flowering would be inhibited by sunset delay (more than 24 hour-cycle), during spring, and allowed 24 hour-cycle), during summer. The tropical landraces would therefore respond more to the daily variation of the sunrise and sunset hours than to the duration between the two events. Altered development and growth During the long vegetative phase of the West-African sorghum and pearl millet landraces, the rate of the leaf initiation by the apex and the rate of the leaf appearance from the whorl after they grew inside the sheaths, are reduced by half at the onset of the internode elongation, around the appearance of about the 15th leaf. The pattern is observed for sowings in the rainy season, before 1 July. These rates also depend on the sowing date, being minimal for sowing at the beginning of June and then increasing from July to September. Due to the internal co-ordinations, the rate of the stem elongation is thus reduced for plants sown early, while the growth of roots remain unchanged irrespective of early or late sowing. Since the stem is the heaviest organ in the tall tropical sorghums, instant time for roots to explore a larger volume of soil. This strategy explains the rusticity of sorghum (known for growth of the plants during the stem elongation. Breaking the low yield stability? The nodes of the panicle are produced at the same rate as the leaf, thus showing dependence on the sowing date. Since the growth rate is parallely changed, the time for date. Consequently, the potential size of the panicle remains stable for plants sown within the rainy season, Thus the West-African landraces possess a multitude of adaptable characters needed in their environment, including the quality and taste of their grain. But their strong defect is their inability to respond to the increased soil fertility, linked with their yield stability. To improve the situation, the potential amount of grain per area has to be increased, possibly through the increase of the plant density, while keeping the other characters unchanged. Knowledge newly acquired on different aspects of the photoperiod-sensitivity will step will be to understand the genetics of the threshold that gives each landrace a particular calendar date for panicle initiation in order to derive varieties adapted to a range of latitudes from the best varieties and hybrids bred from the African diversity. 0 30 60 90 30 20 10 0 Numberofleaves Days after emergence + Leaves initiated by the apex + Leaves appeared from the whorl
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