Moisture stress areas generally exist when there is insufficient water available to a crop or a condition in which available soil moisture is depleted to a point that the plant growth is adversely affected. The critical period of plant growth usually starts at the time when reproductive organs are formed, and pollination and fertilisation take place.
Maize being the king of the crop plants, is known for its numerous industrial uses like corn bread, corn chips, paper, insulator, card board, pipes, chemicals, plastics, methanol, tar, green corn, baby corn, starch, glucose, oil, besides human and animal consumption.
It is estimated that, of the total agricultural land available on earth (14 billion hectare), 6.0 billion hectare are arid or semi arid. About 26% of the world's total cultivable land falls in arid and semi arid areas where water is the major limiting factor to crop production.
In moisture stress areas, maize grain losses may reach 24 million tons per year, equivalent to 17% of well-watered production in the world. On global level, water for humans and their crops is rapidly becoming a critically short commodity.
Thus, this figure is expected to increase in future. Therefore, each unit of water should be used effectively, equitably and soundly. Hence, the development of maize with high and stable yield under low moisture is an important priority for today's needs, as access to drought -adapted cultivars may be the only affordable alternative to many small-scale farmers.
A study was conducted to evaluate the performance of some germplasm (inbred lines) of maize under normal and water stress conditions. For this purpose, inbred lines of maize with diverse origin were collected from various national and international sources and maintained in the field for two consecutive seasons to ensure their identity and purity.
Eighteen lines were selected and tested under greenhouse conditions for various seedling traits to screen out drought tolerant and susceptible lines by growing them both under normal and moisture stress conditions. Eight inbred lines viz. F-133, F-l41 and F-128 (drought tolerant), F-131, F-135 and SR-402 (moderately drought tolerant), and F-149 and SEL-8 (drought susceptible) lines were crossed in an 8 x 8 diallel fashion during April-May 2002.
The parents and their 56 F1s were planted the following season in a triplicated randomised complete block design under normal as well as water stress conditions. All other agronomic and cultural practices were kept same except irrigation. By controlling irrigation, water stress was applied at the time of antithesis, silking, pollination and grain filling stages of crop growth in the water stress experiment only.
The results revealed highly significant differences among inbred lines for all the characters studied under both normal and water stress conditions. On overall grand mean basis, a considerable reduction in almost all parameters was recorded under water stress condition as compared to normal planting except for osmotic potential, soluble sugar content, protein content and stomatal frequency which showed 27.1, 32.9, 13.76 and 7.0% increase under water stress condition.
Grain yield per plant, as a consequence of reduction in all its components, showed a maximum reduction of 36.1% under water stress condition.
Performance of hybrids was quite variable under both planting conditions. Hybrids showing high mean values for some traits displayed lower mean values for the other traits.
However, under normal planting condition, some of the crosses were prominent displaying higher mean values for grain yield and most of its components, while under water stress condition; some hybrids were good performers for most of the traits.
Diallel analysis conducted for all the characters revealed that under normal planting, both additive and dominant genetic effects were significant for all the traits viz, plant height, leaf area per plant, number of kernels per ear, 1000-grain weight, biological yield, grain yield per plant, harvest index, osmotic potential, soluble sugar content, protein content, stomatal frequency and stomata size.
Under water stress condition, both additive and dominant genetic effects were significant in case of plant height, leaf area per plant, number of kernels per ear, 1000-grain weight, biological yield, grain yield per plant, harvest index, protein content stomatal frequency and stomata size, while only additive effects were significant for osmotic adjustment and soluble sugar content.
Under normal condition, mean squares due to general combining ability (GCA) were highly significant and greater than specific combining ability (SCA) mean squares for plant height, biological yield, grain yield per plant, osmotic potential, soluble sugar content, stomatal frequency and stomata size indicating the preponderance of additive genetic effects whereas for leaf area per plant, number of kernels per ear, 1000-grain weight, harvest index and protein content. Mean squares due to SCA were highly significant displaying the importance of non-additive genetic effects.
Under water stress condition mean squares due to GCA were highly significant and greater than SCA mean squares for plant height, leaf area per plant, number of kernels per ear, biological yield, grain yield per plant, osmotic adjustment, soluble sugar content and protein content indicating the importance of additive genetic effects.
Reciprocal effects were found highly significant for all the traits, except the number of kernels per ear, harvest index and stomatal frequency under normal condition while soluble sugar content and stomatal frequency under water stress condition.
The exposure of the breeding material to water stress environment, significantly affected the growth of the maize plant as expressed in terms of altered morphological, physiological and biochemical traits studied.
On the basis of the overall performance of the genotypes, parental inbred lines F-133 and F-135 were the best on the basis of mean performance under both normal and water stress conditions. Additive gene action for plant height and soluble sugar content predominated under both conditions.
Over dominance type of gene action for leaf area per plant, number of kernels per ear, 1000-grain weight, biological yield, grain yield, protein content, and stomata size under normal condition remained unchanged under water stress conditions.
Additive gene action for harvest index and stomatal frequency under normal planting changed to over dominance under water stress conditions. Inbred lines F-133, F-135 and F-149 were the best general combiners under both planting conditions. Thus, these inbred lines are suggested to be used in future breeding strategies for the production of drought tolerant maize cultivars.
The best combinations on the basis of mean performance and specific combining ability under normal conditions were F-133 x F-149, F-141 x F-131, F-133 x SEL-8 and F-l31 x F-141, whereas under water stress condition, the best performing crosses were:--133 x F-141, F-133 x SEL-8, F-141 x SEL-8, F-131 x F-l35 and SR-402 x F-149.
These crosses may be exploited for the selection of transgressive segregants in subsequent generations for improved yield potential.
(The writer belongs to the Plant Breeding and Genetics Department, University of Agriculture, Faisalabad.)