| Results Analysis of variance indicated sufficient variation among genotypes for all the characters under irrigated as well as rainfed condtions. Comparative evaluation of the three genotypes for root growth and distribution envisaged that root growth (inferred from root length and counts) was, in general, better in irrigated than rainfed conditions (Table 1). However, root growth in variety C306 was at per in both the environment. Maximum root density was found in 10 - 20 cm of the soil profile wall in all the varieties and in both the environments. An over all basis, root counts in WH147M were approximately two times more than WH147 in both the environment and C306 in irrigated conditions. Further, WH147 in both exhibited considerably better root growth than C306 in rainfed conditions too. Also, mutant developed a very stable root system below 10 cm profile downwards. The mutant recorded significantly higher number of spikes per m2, greater spike length and higher grain yield per m2 as compared to WH147 and C306 under irrigated as well as rainfed conditions (Table 2). The mutant (WH147M) showed parity for number of tillers per plant with that of C306 in rainfed and WH147 in irrigated conditions. On an average, the mutant recorded 47 per cent and 10 per cent higher grain yield over WH147 in rainfed and irrigated conditions respectively. Compared to WH147, the mutant recorded approximately two times higher number of leaves per plant (46.3) and dry weight of these leaves (45.4g) and three times more leaf area per plant (1702 cm2). Discussion With the advent of newer high yielding wheat varieties which incorporate dwarfing genes, concern has been expressed that root system of these varieties must also be shortened, with undesirable consequences when they are grown in water stressed environments. As such, genotypic variation in the root system of wheat has been the subject of numerous studies (O'Toole and Bland 1987). Differences in root characters of cultivars have been demonstrated and various parameters are correlated with top growth. Klepper et al (1984) observed that tiller development in cereals may be synchronized with root axis production and the number of roots on a tiller may be predicted from the number of leaves on the same tiller. In present study, higher number of leaves per plant coupled with better root growth in case of WH147M, substantiating this fact. Further, water is most rapidly taken up by younger roots localized in 10 - 15 cm profile (Fischer and Turner 1978). However, in semiarid rainfed environments, invariably and the moisture in this soil profile is used for vegetative growth and plant encounters stress at the time of grain filling. In most cases the problems can be tackled by developing cultivars with improved root system, capable of extracting moisture from lower soil stratum to ensure adequate supply of water to the growing plant which can maintain favourable water potential and avoid dehydration during grain filling period which hampers the starch deposition in grains (Singh and Mehta 1986). Ability for rapid lateral root distribution with higher rate of root elongation would thus be advantageous. Therefore, breeding semi-dwarf high yielding wheat genotypes with developmental plasticity for root system merits prior consideration, particularly, for rainfed areas. This kind of specifilc variability can be generated through mutagenesis. Marx (1983) has reviewed such developmental mutants in annual seed crops. WH147M, a high thermo tolerant genotype (Sheoran et al 1983) with considerably better yield compared to its locally adapted mother cultivar and standard check C306, seems to be one such developmental mutant with some compensatory mechanism for adaptation in rainfed conditions. This was corroborated by our earlier studies as WH147M was found to maintain better water potential at grain filling stage (Behl et al 1986). Therefore, the mutant WH147M can be used as a source material to incorporate vigorous root system and dense crop canopy in other desirable back grounds to ensure maximum water use rainfed environment by way of extracting more moisture from lower soil profiles and avoiding evapotranspirational losses. |
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