Influence of water stress imposed at different stages on growth and yield attributes in bread wheat genotypes (Triticum aestivum L.)
N. F. Veesar, A. N. Channa, M. J. Rind and A. S. Larik
Department of Plant Breeding & Genetics, Sindh Agriculture University, Tandojam, Sindh, Pakistan
Corresponding author: M. J. Rind
The studies were carried-out so as to assess the influence of water-stress treatments on growth and yield attributes of wheat genotypes. The water stresses were imposed at tillering, booting and at grain formation stages of crop growth and development. The wheat genotypes evaluated were: Sindh-81, Mutant of Sindh-81, Indus-66 and Mutant of Indus-66. The experiment was conducted in a Randomized Complete Block Design with three replications in a plot size of 1.0 x 3.0 m. The means squares from analysis of variance revealed significant differences among genotypes, water stress treatments and their interactions except interaction was non-significant for seed index only. These results further suggested that genotypes not only performed differently but also responded variably to water stress conditions, hence some being more tolerant than the others at various growth stages. Results further revealed that, on an average over the genotypes, taller plant heights, higher number of tillers per plant, maximum grain yield per plant and higher seed index were recorded in Indus-66 whereas, more number of spikelets per spike was formed in genotype Sindh-81. Since plant height, numbers of tillers per plant, grain yield per plant and seed index are very important parameters in wheat and also serve best indicators of water stress tolerance, therefore genotype Indus-66 by and large proved to be more stress tolerant as compared to other genotypes. Averages over water-stress treatments indicated that stress imposed at tillering stage caused significant declines in plant height, number of tillers per plant and spikelets per spike whereas maximum reductions in grain yield and seed index occurred when stress was given at grain formation. The significance of genotype x water stress interactions for almost all the attributes suggested differential response of genotypes to water-deficit conditions which further implied that genotypes with relatively little declines in important characters at crucial stages may be preferred either for general cultivation or to be used in hybridization programmes to develop new water stress tolerant wheat genotypes.
The drought, salinity and high temperature stresses are the main environmental constraints and are major threats to wheat crop not only in Pakistan but throughout the world, specially in arid and semiarid regions. Abiotic stresses, notably extremes in temperature, photon irradiance, and supplies of water and inorganic solutes, frequently limit growth and productivity of major crop species such as wheat (Triticum aestivum L.). Furthermore, one abiotic stress can decrease a plant's ability to resist a second stress (Mark and Antony 2005). However, if a single abiotic stress is to be identified as the most common in limiting the growth of crops worldwide, it most probably is low water supply (Araus et al. 2002). The main consequence of moisture stress is decreased plant growth and development caused by reduced photosynthetic activity.
All phases of plant growth are not equally vulnerable to water shortage. However some phases can cope-up with water shortage very well, while others are more vulnerable and water shortages at such stages may result in serious yield losses. Moisture stress is known to reduce biomass, tillering ability, grains per spike and grain size at any stage when it occurs. So, the over all effect of the moisture stress depends on its intensity and length of stress (Bukhat 2005). Substantial losses in wheat grain yield have been reported due to water deficiency depending on the developmental stages at which crop plant experiences stress. Water stress at various stages specially before anthesis can reduce number of ear heads and number of kernels per ear (Dencic et al. 2000; Mary et al. 2001). While water stress imposed during later stages might additionally cause a reduction in number of kernels/ears and kernel weight (Gupta et al. 2001).
In water scarcity conditions, wheat fields are usually not irrigated on the basis of crop demand or the quantity of water required, but the crop fields are irrigated when the farmer has its turn. Under such situations, wheat crop suffers from different degrees of stresses and leads to great reduction in yield. One of the best solutions to resolve the issue of shortage of irrigation water is to evolve drought or water stress tolerant varieties those could withstand very well with water stress or require less numbers of irrigations but still produce optimum yields.
In wheat breeding programmes, seeking increased yields have usually attempted to improve drought tolerance of plants. However before successful genetic manipulation could be made, it is important to characterize the physiological parameters of known drought tolerant or sensitive cultivars. Analyzing physiological determinants of yield responses to water stress may also be helpful in breeding for higher yields and stability of genotypes under drought conditions. The traits to select (either for stress escape, avoidance or tolerance) and the framework where breeding for drought stress is addressed will depend on the level and timing of stress in the targeted areas. However, selecting for yield itself under stress-alleviated conditions appears to produce superior cultivars, not only for optimum environments, but also for those characterized by frequent mild and moderate stress conditions. This implies that broad avoidance/tolerance to mild/moderate stresses is given by constitutive traits also expressed under stress-free conditions (Araus et al. 2002). Keeping in view the importance of identifying water-stress tolerant wheat genotypes, the water-stress conditions were imposed to wheat plants at various stages of crop growth and development. The stresses were given at tillering, at booting and at grain formation stages. Thus, the objectives of present research were to determine the effect of water stress on yield and yield contributing traits of bread wheat genotypes.
Material and Methods
A field experiment was undertaken to assess bread wheat genotypes (Triticum aestivum L.) for water stress tolerance during 2005-2006 at the experimental area of Plant Breeding and Genetics, Sindh Agriculture University, Tandojam. The water stress treatments were created by withholding the irrigation at various stages and for specified period of time as under:
T1 = Six irrigations, each was applied at 15 days intervals (control without water-stress at any stage)
T2 = Five irrigations, 1st applied at 20 days after sowing and subsequent four irrigations at 15 days intervals (stress at tillering stage).
T3 = Five irrigations 1st, 2nd, 3rd irrigations were applied at 15 days intervals and 4th irrigation after 25 days (stress at booting stage), fifth irrigation after 15 days.
T4 = First four irrigations were applied at 15 days intervals and 5th irrigation after 30 days (stress at grain formation stage).
The fifteen days in Pakistani conditions are considered as normal irrigation interval without any stress. Four bread wheat genotypes i.e. Sindh-81, Mutant of Sindh-81, Indus-66 and Mutant of Indus-66 were grown in a Randomized Complete Block Design (RCBD) with 3 replications arranged in a plot size of 1.0 x 3 m, thus there were four rows, each 3 m long. The spacings between row to row and plant to plant were kept at 6.0 and 4.0 inches respectively. The sowing was done on normal planting date of November 22, 2005. The seed was sown by single coulter hand drill with the seed rate of 125 kg ha-1. The bunds and channels were made according to experimental design so that controlled irrigations could be applied to the experimental plots. Thus the plots were separated by leaving 1.5m space as buffer zone to avoid water seepage from water courses. All the required cultural operations were adopted uniformly in all the plots throughout the growing period on as and when required basis. Fertilizer DAP was applied at the rate of 150 kg ha-1 at the time of sowing and two bags of nitrogen ha-1 as urea was applied after sowing in three split doses i.e. with first and third irrigations and the last dose at anthesis stage. The crop was harvested on April 8, 2006 after 135 days of sowing. The observations for yield components such as plant height, number of tillers per plant, number of spikelets per spike, grain yield per plant (g) and seed index (100-seed wt. g) were recorded on 10 plants per replication, thus a total of 30 plants were studied for each treatment and genotype. The data collected were subjected to analysis of variance according to Gomez and Gomez (1984) so as to discriminate the treatment mean differences among genotypes and water-stress conditions. Mean comparisons were also made by using L.S.D. (5%). For these statistical calculations, MSTAT-C software package was used.
Results and Discussion
All the growth and developmental stages of wheat plant are not equally vulnerable to water stresses, but some stages are more critical than the others. Some wheat genotypes may with-stand very well to water deficiencies whereas others could sustain severe yield losses in water deficit conditions.
The statistical analysis was carried out so as to determine the differences among four wheat genotypes in response to four water-stress conditions. The mean squares from analysis of variance (Table 1) revealed that main effects (genotypes and water stress treatments) and their interactions were significant for all the characters studied except seed index where interactions were non-significant (Table 1). The significance of genotypes and water stress treatments indicated that varieties performed differently and water stress treatments also significantly affected the plant traits. The interactions were significant further implied that varietal response to stress environment was also variable some being less affected than the others, it means choice of the tolerant genotypes could be made for important attributes. Results with respect to plant height revealed that maximum culm lengths of 98.4 and 100.5 cm were attained by genotype Mutant of Induds-66 when stresses were subjected at both tillering and booting respectively indicated its being more water stress tolerant whereas minimum plant heights of 74.4 and 78.4 given by genotype Sindh-81 implied its being more vulnerable to water stresses. On an average, declines of 8.49% at tillering and 3.08% at booting were caused by the water stress conditions against no stress control (Table 2). Gupta et al. (2001) and Muzammil (2003) also observed substantial decline in plant height when irrigation was withheld at booting stage, however tolerant genotypes attained more plant height. The number of tillers per plant has got direct contribution towards grain yield. It means, as the number of productive tillers increase, there will be simultaneous increase in yield. On an average over genotypes, Indus-66 produced more numbers of tillers. However, stress at tillering caused significant declines in number of tillers, yet maximum reduction occurred in genotype Mutant-81 (15.8 tillers) and minimum in Indus-66 (20.8 tillers) suggesting mutant-81 being highly susceptible and Indus-66 highly tolerable to water-deficit environments. Over the stress treatments, stress imposed at tillering caused greatest declines of 19.1% in tillers as against no stress control. These results thus suggested that if grain yield in wheat is to be increased via numbers of tillers, then water stress at tillering stage may be avoided. Similar to present findings, Rana et al. (1999) and Kimutro et al. (2003) found that water stress at tillering or at booting significantly affected the formation of tillers in wheat.
Generally, spikes with more number of spikelets are supposed to produce more grains per spike, consequently higher yields per plant. Under water stress conditions, on an average, number of spikelets per spike declined, nevertheless prominent reduction occurred (10.98%) when stress was given at tillering stage. As regards to varietal performance, particularly at tillering stage where maximum reduction in number of spikelets was expected, genotype Sindh-81 by producing maximum numbers of spikelets per spike (22.6) ranked as the most water stress tolerant genotype (Table 2). These results thus suggested that stress may be avoided at tillering stage in order to increase the numbers of spikelets per spike. The grain yield is the total out-put of all the yield components. The average yield of all the genotypes dropped considerably under all the water-deficit conditions, yet the declines of 20.74, 46.85 and 101.23% were recorded when stresses were subjected at tillering, booting and at grain formation respectively (Table 2). Averaged over all the stress environments, Indus-66 produced maximum grain yield per plant, thus less affected, thus being more water stress tolerant genotype. Surprisingly, Indus-66 which formed less number of tillers per plant still gave more grain yield per plant could be explained probably having more number of productive tillers rather than having non-productive as in case of genotype Sindh-81. A large reduction in yield with stress at grain formation suggested that water stress must be avoided at grain filling so as to save the yield losses. Solomon et al. (2003) and Ozturk and Aydin (2004) also found yield reductions of 79.7 and 65.5% when water stress was imposed either at earlier stages or at grain formation. Seed index is also considered as one of the most important indicators of stress tolerance via kernel weight. In all the stress environments, seed index declined subsequently as 26.90% at tillering, 51.89% at booting and 81.45% at grain formation (Table 2). On an average over genotypes, the maximum seed index was noted in Indus-66, hence being highly tolerant to water stress conditions. Seed index results therefore suggested that stress may be avoided at grain formation and genotype Induss-66 may be preferred in water deficit environments.
For growth parameters like plant height, tillers per plant and spikelets per spike, considerable declines were observed when stress was imposed at tillering stage, nonetheless, maximum reductions in grain yield per plant and seed index (100-seed wt. g.) occurred when stresses were subjected at grain filling stage. Generally, genotype Indus-66 performed well in water stress conditions as it attained a reasonable plant height, produced more tillers, gave higher grain yields and seed index as compared to other genotypes.
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