Potential of Malian Landraces in Hybrid Combination

The first possibility of economic hybrid seed production in sorghum has been discussed since the discovery of the genetic-cytoplasmic male-sterility. The greater advantage of hybrids to yields, compared to cultivars,


INTRODUCTION
Sorghum [Sorghum bicolor (L.) Moench] is one of the main cereals cultivated in Mali. It, along with pearl millet constitutes essential food sources of energy, protein, vitamins and minerals for the rural population in the country. It remains an essential culture for food security in rural areas with an area estimated of 1,204,652 hectares and an average grain yield of 1,055 kg/ha [1]. The crop is genetically adapted to the hot, dry, agro-ecologies where it would be difficult to cultivate other cereals. Sorghum is usually grown both for its grain for human food and its straw used as fodder.
Three major races of sorghum exist in the country: Guinea, Durra and Caudatum. Touré et al. [2] indicated that the race Guinea represents about 70% of the germplasm in the country, followed by the Durra (17%). It is produced under various conditions, from the arid regions of the north to high rainfall areas of the south.
The advantages of hybrid sorghum were first pointed out by Conner and Karper in 1927 [3]. The first possibilities of economic hybrid seed production in sorghum have been discussed since the discovery of the genetic male-sterility in sudangrass by Stephens in 1929 which was transferred to sorghum grain [4]. The greater advantage of hybrids to yields, compared to cultivars, has been demonstrated throughout the world [5,6]. Many studies showed that in stress conditions, the yields of both hybrids and varieties decline, but frequently the magnitude of difference, percent wise, is greater for hybrids in favourable conditions [7,8,9].
Seasonal precipitation is usually unpredictable and unreliable in most of the rain lands. Therefore, yield reductions and crop failures are predicted to occur. It is believed that superior hybrids identified under local conditions will have a rapid influence in increasing and stabilising yield levels in the rain lands. In general, F 1 sorghum hybrids, with their vigorous early growth with a fast rate, and ability to efficiently utilise limited moisture, produce higher yields under stress conditions than other varieties. The objective of this study was to monitor the potential combining ability of Malian landraces and their gene action with the introduced male sterile line. 13°35'). A completely randomised design was used. Each entry was planted in a 2-replicated trial with 2 planting dates at each location and 15 days between planting dates. Each plot consisted of two rows, which was 5 m long and 0.75 m apart. The equivalent of 100 kg of ammonia phosphate fertiliser and 50 kg of urea per hectare was applied. Hybrids were evaluated for photoperiod sensitivity, maturity, genetic traits (presence or absence of testa, cytoplasm A1 reaction, panicle shape etc.), yield, and agronomic desirability. Photoperiod sensibility was estimated by comparing days to 50% anthesis from the two (2) planting dates at each location. The software used for the data analysis is the GenStat with Duncan's procedure for mean separation. The estimation of heterosis or vigour hybrid was calculated for each hybrid following the formula:

MATERIALS AND METHODS
Mid heterosis = (F1-MP) x100 Where, F1 = Mean performance of the hybrid, MP = Average performance of the two (2) parents that produced the hybrid.
Subjective ratings were recorded by contrasting parent and hybrid from Cinzana, Baramandougou and Samé nurseries which received no appreciable rain at seedling stage and after flowering of the sorghums. Leaf stress symptoms, panicle blasting, miniature panicle size and endosperm texture softness were an indication for drought susceptibility.

RESULTS AND DISCUSSION
Heterosis was observed in all stages of plant growth. Seedling vigour and seedling drought tolerance were emphatically better than the Malian parents. Similar results were obtained by Touré in 1980 [10]. Generally, plant growth was much more rapid and lush than that of parents. Patel et al. [11] and Blum et al. [12] obtained similar results showing that sorghum hybrids have a larger meristem than their parents and more rapid growth during the cell division growth process. Hybrids were tolerant to drought at all pre-floral stages of plant growth. Taye et al. [13] reported in their study that hybrids matured earlier than the adapted parents, and had higher grain yield, plant height, grain number and grain weight in all environments.
In Mali, leaf stress symptoms, panicle blasting and miniature panicle size, are the common manifestations of drought stress during panicle initiation. Selection against those traits was found to be effective. The multi-location evaluation was based heavily on those traits. Floral drought softens the endosperm texture of some varieties and hybrids. Under post floral drought conditions, the local Keninké sorghum grain became smaller than normal but maintained similar endosperm texture compared to the normal grain (Photo 2). Some varieties and hybrids maintained their grain size but became chaffy with little or no vitreous areas visible in the endosperm (Photo 1).
The extent of vitreousness depends on the amount of storage protein laid down in the endosperm. Since storage protein is one of the last components to be laid down in the grain, drought stress could block the physiological processes during the latter period of grain fill in some varieties. Most of the farmers in Mali preferred hard endosperm grain used to prepare the local dish called "To", a thick porridge. Soft endosperm grain is known to produce poor quality porridge. As reported by Touré [10] the dense panicle of Gadiaba (Durra) and Hegari (Caudatum) parents produced long and dense panicle hybrids combination with The ATx623 parent.
The female parent of hybrid had cytoplasmic male sterility, which was conditioned by both a sterile cytoplasm and the ms c ms c genotype at the Ms c locus in the nucleus. All of the male parents had fertile cytoplasm since they all produced normal pollen. The genotype of the Ms c locus of the male parents could be revealed by the male fertility of the plants from crosses to a cytoplasmic male sterile parent. If the hybrid plants were male fertile, the genotype of the male parent (Ms c Ms c ) was considered an R-line or "fertility restorer" line. If the plants were male sterile, the genotype of the male parent (ms c ms c ) was considered a B-line or "male sterility maintainer" line. Observations of the fertility reactions of these test crosses on A 1 cytoplasm showed that both maintainer and restorer genes were present in all taxonomic groups (Table 1). Those B-lines had the genotype ms c ms c . The frequency of landraces with fertility reactions was more frequent than those with maintainer reactions. No obvious geographic pattern was observed for the distribution of fertility reaction as reported by Touré and Scheuring [14]. However, the information on the distribution of fertility reaction in the local germplasm held in the development of female and male hybrid parents.
Heterosis could be exploited in Malian sorghums. That exploitation will be approached from a number of angles: the sterilisation and the dwarfing of Malian B-lines, the use of an array of different female parents in combination with the best Malian parents, pedigree recovery of Malian R-lines by introduced R-lines, recovery of R-lines with Malian grain characteristics from recurrent selection in breeding populations.
Significant and positive heterotic effects were recorded for grain yield per panicle (261-28%) and the panicle yield components namely, seed number (191-6%) and seed weight (45-2%) (  Table 3). The hybrids of photoperiod sensitive Malian parents were also strictly photoperiod sensitive. A photoperiod sensitive hybrid performance of insensitive x sensitive parental cross indicated the dominant gene action of photoperiod sensitivity. This model fitted well with the one described by Quinby and Karper [16]. The crossing of a photoperiod sensitive variety with a neutral variety gives a photoperiod sensitive product with a reduction of the threshold for floral induction [17]. The results obtained from this study concurred with previous studies that photoperiod sensitivity can be easily detected and can be used to breed for comparing flowering date from contrasting planting dates. The flowering of the ecotypes occurred in the 25 days which preceded the average date by the end of the rain season and floral initiation started and finished during the time to which the day is shorter than the night. This study showed a delay of 3 weeks in sowing, and the ecotypes lose 10 to 96% of their seeds with an average of 66% [18]. The sorghum varieties obtained by selection in recent years brought progress especially regarding productivity in favourable conditions. To meet the needs and constraints of farmers in Sudano-Sahelian zone, it is now necessary to incorporate agronomic characteristics of local varieties, including their sensitivity to the photoperiod sensibility which provides a large adaptability in the face of climate change.
There was an array of maturity genes available in Malian sorghums. The action of photoperiod sensitivity genes was to be distinguished from maturity genes before a clear understanding of their respective inheritances and interactions. Most of the photoperiod insensitive x sensitive the hybrids described above were 10 to 30 days earlier than the Malian parent, but, were still photoperiod sensitive. Heterosis was expressed at the early flowering stage by more tillers and reduced plant height. For example, in 1966, Kambal and Wegster [19] reported a 20% increase in average grain yield of hybrids, and 2.5 days hastening of flowering when compared to the parental mean. Similar results were also obtained by Kirby and Atkins [20], Sodani and Chaturvedi [21], Penga et al. [22] and Yanga et al. [6]. But there was one hybrid, ATx623 x CSM193 which had essentially the same maturity and photoperiod sensitivity as the CSM 193 parent. It was clear that CSM193 differed genetically from the other photoperiod sensitive parents in the study.

CONCLUSION
The seedling and plant vigour, the pre-floral drought tolerance, and the clear yield advantage of hybrids made with Malian male parents convinced that heterosis could be exploited in Malian sorghums. However, there was no scope for direct exploitation of hybrids involving the ATx623 and Malian variety parents. The typical Caudatum turtle-back seed shape was dominant in hybrid combinations with all Malian races. This seed shape rendered the grain more difficult to dehull than local cultivars. The thresh ability of the hybrids panicles was also very poor. The hybrid grains made with Guinea parents had thick brown sub coat with astringent tannins, which were undesirable for food uses. The development of acceptable hybrids using local germplasm could be addressed through various approaches. Hybrids based on introduced malesterile lines and landrace pollinators might be pursued through modification of hybrid parents to rectify traits determined by relatively few genes such as seed sub coat and pericarp thickness. However, development of hybrids with Guinea grain shape and grain quality would likely require the development of Guinea grain shape malesterile lines. The sterilisation and dwarfing of Malian B-lines, the use of an array of different female parents in combination with the best Malian parents, pedigree recovery of Malian Rlines by introduced R-lines, recovery of R-lines with Malian grain characteristics from recurrent selection in breeding populations could open up a new range of possibilities for developing hybrids for the country.