Abstract: Wheat production is often reduced by terminal heat stress therefore present study was aimed to characterize wheat genotypes for their heat tolerance for some important traits. Plant material was composed of sixteen wheat (Triticum aestivium L.) genotypes including two check cultivars. Wheat genotypes were sown in the field at two dates i.e., November 15 (normal sowing) and December 13 (delayed sowing) during winter seasons of 2003-2004 and 2004-2005. Combined analysis of variance (for two years) showed that mean values for years were highly significant at p=<0.01 for majority of the traits, differences were significant (p=<0.05%) for grain filling duration while non-significant for days to emergence. For seasons; differences were highly significant for all parameters, whereas for genotypes; the differences were highly significant for all parameters except grain filling duration where differences were non significant whereas, response for different interactions was variable. In present experiment, generally late sowing imposed negative effects on all traits. However, two years combined data showed that minimum difference between early and late sowing was 23 days for heading (CT-4), 7 cm for leaf area (CT-12, 13 and 14), 6.3 cm for plant height (CT-10) and 3 days for grain filling duration (CT-9) whereas, CT-10 matured 29 days earlier when sown late in comparison to the normal sowing time. From the experiment it was concluded that CT-4 and CT-10 had built in terminal heat stress escape mechanism which could be used for the development of new early maturing variety.
INTRODUCTION
Wheat is the world primer crop since it supplies 72% of the calories and protein in the average diet (Heyne, 1987) therefore; its grain and products are important components of our daily diet. The average per capita consumption of wheat in the country is 318 g daily contributing 83% of the total cereal intake (Byerlee et al., 1986). In addition, more than half of the daily requirements of iron and riboflavin are meet by amount of wheat available to an adult man.
In Pakistan area under wheat cultivation remained nearly constant in last two decades due to the cropping intensity and crop competition with little prospect of further increasing acreage in near future. Area under wheat is reported to be 8.2162 million hectares, out of which 7.1278 million hectares is irrigated and 1.0884 million hectares entirely depends on natural precipitation with an average yield of 2555 and 1182 kg ha-1 from irrigated and rainfed area, respectively (Anonymous, 2005). This indicates that wheat yield of the country is quite low both in irrigated and rain feed area is very low in comparison to its potential yield. The main reasons for lower yield of wheat per unit area in Pakistan are; delayed sowing due to late picking of cotton and late harvesting of sugarcane and rice crops, use of poor quality seed, imbalance use of fertilizers, shortage of irrigation water especially in rain-fed areas, infestation of weeds and diseases and unwanted rain showers at the harvest and many others. Among the environmental factors delayed sowing is one of the most crucial non-monetary factors for the growth and grain yield of wheat. It has been reported that 50% of wheat in Pakistan is planted late i.e., during the month of December. According to Khan (2003) a yield loss of 42 kg ha-1 day or 1% day occurs if planting is delayed from 10th November until 10th January. For obtaining good and healthy crop it must be provided with optimum climatic conditions during its various stages of growth and development.
Continual and terminal heat stress is a problem in 40% of the wheat growing areas of the world (Fisher and Byerlee, 1991) whereas, damage due to heat stress under late planting conditions has become an important factor limiting wheat yields in many parts of the Asian subcontinent, due to the rice-wheat cropping system (Aslam et al., 1989). Terminal or late heat stress especially during anthesis and grain filling stage of the late planted wheat is the major constraints of wheat yield (Macas et al., 2000; Khalifa et al., 1998). Growing food demand for the growing population and global warming is further pushing the wheat crop to heat stress environments (Chopra and Viswanathan, 1999).
Increased heat tolerance in late planted wheat is very essential to enhance and stabilize wheat productivity (Talbert et al., 2000, Dhanda and Sethi, 2002). Therefore breeding efforts at selecting genotypes with increased heat tolerance is therefore, one of the most vital objectives in wheat improvement programs (Macas et al., 2000). The sources of identifiable heat stress tolerance traits and availability of simple, fast and reliable screening test for heat tolerance have been major limitations of these programs.
Keeping in view the importance of wheat and its lower yield due to terminal heat stress imposed by late sowing in the country, an experiment was conducted to observe the influence of planting dates on final outcome in terms of various qualitative traits by comparing their performance with normal sowing. The executions of this research lead to the identification and selection of the improved wheat genotypes showing built-in genetic tolerance to late planting/terminal heat stress. This material could be utilized for development of early maturing high yielding, widely adapted and good quality varieties particularly suitable for cultivation under late planting conditions in the country.
MATERIALS AND METHODS
To study the response of wheat genotypes to heat stress and to determine suitable planting time of wheat genotypes introduced from CYMMIT, an experiment was conducted in which sixteen wheat genotypes were tested at Nuclear Institute for Food and Agriculture (NIFA) Tarnab, Peshawar in two consecutive years 2003-04 and 2005. The list of genotypes along with their entry number and pedigree are given in Table 1.
Genotypes were evaluated for some important traits under normal (15th November) and delayed planting (13th December) conditions in RCBD design. Seeds of each genotype were planted in four rows of 2.5 m each. Row to row and plant to plant space were kept 50 and 30 cm, respectively. The experiment was replicated thrice. Normal agricultural practices e.g., hoeing, weeding, fertilizer application and irrigations were applied.
Ten plants from the two central rows of each genotype from each replication at random were selected for data recording. Days to 80% emergence were recorded from the date of sowing till 80% of the seedlings emerged from the soil and were clearly visible. Days to 50% heading (stage when ears emerged after the unfolding of the flag leaf) were recorded from the date of sowing to the date of 50% heading. Flag leaf area (cm2) was measured in centimeter using the formula suggested by Muller (1991) i.e., maximum widthxlengthx0.74. Plant height (cm) was recorded from the base of plant to the tip of the spike, excluding awns of the main tiller in centimeter. Grain filling duration was recorded as the period from anthesis/pollination to physiological maturity. Days to physiological maturity were recorded as the period from date of sowing to the date of maturity (when 90% of the crop turned yellow). Data recorded was then statistically analyzed with the help of computer software, MSTAT-C.
RESULTS AND DISCUSSION
According to combined analysis of variance (Table 2) mean squares for main factors viz., for years; differences were highly significant for days to heading, flag leaf area, plant height and days to maturity, differences were significant for grain filling duration while non-significant for days to emergence.
| Table 1: | List of wheat genotypes with their parentage and pedigree used to study the effects of terminal heat stress on some important traits |
| Table 2: | Mean square values for emergence, heading, flag leaf area, plant height, maturity and grain filling duration of wheat genotypes for terminal heat stress |
| DF = Degree of Freedom, DEM = Days to emergence, DHD = Days to heading, FLA = Flag Leaf Area, PHT = Plant Height, DMT = Days to maturity, GFD = Grain filling duration, ** = Significant at p≤0.01 probability level, * = Significant at p≤0.05 probability level NS = Non Significant at p≤0.05 | |
| Table 3: | Mean values for days to emergence of wheat genotypes grown in normal and late conditions |
For seasons; differences were highly significant for all parameters studied, whereas for genotypes; the differences were highly significant for all parameters except grain filling duration where differences were non significant. Variable responses were observed for interactions of the main factors, viz. YxS, YxG, SxG and YxSxG.
Days to emergence: During 2003-04, the data for day to emergence ranged form 5 (for CT-4 and CT-10) to 6 days (all other genotypes) for normal sowing and 14 days (CT-4) to 17 days (CT-8, CT-11 and CT-13) for late sowing. The difference between mean values of early and late sowing for emergence ranged from -9 to -11 days (Table 3). In 2004-05, data for emergence ranged from 5 days (CT-4, CT-10, CT-13 and Saleem 2000) to 6 days (all other genotypes) for normal sowing and 14 days (CT-4) to 17 days (CT-8, CT-11, CT-12 and CT-13) for late sowing. The difference between normal and late sowing ranged from -9 days (CT-1 to CT-7 and Saleem 2000) to -12 days (CT-12). The over all two years (2003-05) combined data for emergence showed a range of 5 days (CT-4, CT-10 and Saleem-2000) to 6 days (for rest of the genotypes) in normal sowing and 14 days (for CT-4) to 17 days (CT-8, CT-11 and CT-13) for late sowing whereas the differences ranged from -9 (CT-13) to -12 days (CT-1 to 7).
In our experiment generally early sowing produced earlier seedlings with few exceptions. Similarly, Micanovic et al. (1994) reported negative effect of delayed sowing on seedling emergence.
Days to 50% heading: In 2003-04, the data for 50% heading ranged form 118 days (CT-4) to 127 days (CT-14) for normal sowing and 88 days (CT-8) to 94 days (CT-5) for late sowing (Table 4). The difference ranged from 28 to 36 days for CT-4 and CT-13 and CT-14 respectively. During 2004-05 the data ranged form 119 days (CT-7) to 126 days (CT-14) for normal sowing and 102 days (CT-12 and CT-13) to 108 days (CT-5, CT-6 and Saleem-2000) for late sowing. The difference between sowing ranged from 15 (CT-5 and Saleem-2000) to 24 days (CT-12). The overall data for two years ranged from 120 (CT-2 and CT-11) to 127 days (CT-14) in normal sowing while data for late sowing ranged from 96 days (CT-8 and CT-13) to 101 days (CT-5).
| Table 4: | Mean values for days to 50% heading of wheat genotypes grown in normal and late conditions |
| Table 5: | Mean values for flag leaf area (cm2) of wheat genotypes grown in normal and late conditions |
CT-7 showed minimum difference (23 days) while CT-12, CT-13 and CT-14 showed the maximum difference of 29 days.
In our experiment earlier heading was recorded for all genotypes in late sowing in comparison to early sowing which could possibly be due to terminal heat stress. The present findings are in agreement to Waraich et al. (1982) who reported earlier 50% heading delayed due to sowing. Our results are also supported by Ihsanullah and Mohammad (2001) who reported genetic differences among wheat genotypes for days to heading. However, Khalifa et al. (1998) found variable response for days to heading depending upon the genotypes.
Flag leaf area: In 2003-04, data for flag leaf area ranged from 34.1 (CT-7) to 56.6 cm2 (CT-6) for normal sowing and 26.2 (Bakhtawar-92) to 44.8 cm2 (CT-9 and CT-11) for late sowing. The difference between normal and late sowing ranged from 21.7 (CT-6) to 0.0 (CT-12, CT-13 and CT-14) (Table 5). In 2004-05, data ranged from 49.5 (CT-9) to 30.5 cm2 (CT-3) for normal sowing and 44.8 (CT-9) to 26.2 cm2 (CT-1) for late sowing. The maximum difference of 17.4 was observed for CT-5, while the minimum difference (-1.5) was observed for CT-6 between normal and late sowing. Of the sixteen genotypes tested, only CT-6 produced greater flag leaf area in late sowing in comparison to early sowing (Table 5). Combined data for years (2003-05) ranged from 49.1 for CT-5 to 33.8 cm2 for CT-7 in normal sowing while 44.8 cm2 for CT-9 and CT-11 and 33.8 cm2 for CT-1 in late sowing. The over all difference of two years showed that CT-5 showed maximum difference of 18.0 cm2 while CT-12, CT-13 and CT-14 produced similar leaf area in both sowing (Table 5).
From the above data it can be concluded that generally normal sowing produced greater flag leaf area in comparison to the late sown plants. Whereas, CT-12, CT-13 and CT-14 produced similar flag leaf area in both sowing conditions. Since, biomass and canopy photosynthesis are highly related with flag leaf area and genotypic difference in flag leaf area duration, therefore, these genotypes needs to be retested and could be used in breeding programs. Similarly reduced leaf area in response to delayed sowing is reported by Ashraf and Bhatti (1998). Present results are in conformity to the earlier findings of Micanovic et al. (1994) who reported reduced leaf area in response to late sowing in wheat. However, according to Lonhard (1991) late sowing retarded leaf area index but had little effect on the size of flag leaf.
Plant height (cm): In 2003-04, data for plant height ranged from 99.9 cm (CT-5) to 123.9 cm (CT-2) for normal sowing and 82.5 cm (CT-13) to 106.7 cm (CT-2) for late sowing (Table 6). The difference for plant height between normal and late sowing ranged from 21.3 cm for Bakhtawar-92 to 13.3 cm for CT-1. In 2004-05 data ranged from 85.2 cm (CT-5) to 114.8 cm (CT-2) for normal sowing and 72.5 cm (CT-5) to 100.4 cm (CT-10) for late sowing. The maximum difference of 21.8 cm was observed for CT-4, while the minimum difference was -2.6 cm for CT-10. Combined data for two years (2003-05) ranged from 92.6 cm (CT-5) to 119.4 cm (CT-2) in normal sowing while 77.9 cm (CT-5) to 102.5 cm (CT-2) for late sowing. The over all performance of two years showed that CT-4 showed maximum difference of 20.9 cm while the minimum difference was observed by CT-10 (6.3 cm).
All genotypes used in the present experiment produced shorter heights in response to late sowing in comparison to their early sowing. The decrease in plant height may have been occurred due to shortening of growth and photosynthetic period due to terminal heat stress associated with late sowing. The present results are in agreement with those of Mishra et al. (2000) who observed significantly negative effects of delayed planting on plant height. Present results are also similar to the earlier findings of Knapp and Knapp (1978). In addition, Ibrahim et al. (1986) found a reduction of 20% in plant height due to late sowing.
Grain filling duration: The data for grain filling duration (Table 7) in 2003-04 ranged form 43 days (CT-4) to 39 days (CT-1 and CT-13) for normal sowing and 38 (CT-9) to 30 days (CT-14) for late sowing. The difference between normal and late sowing ranged from 4 days (CT-9) to 10 days (CT-14). During 2004-05 the data ranged form 47 days (CT-7) to 41 days, (CT-9) for normal sowing and 39 days (CT-2, CT-3, CT-7, CT-8, CT-11 and CT-12) to 33 days (CT-5) for late sowing. The difference ranged from 3 days (CT-12) to 13 days (CT-5). The combined data for two years ranged from 40 days (CT-14) to 45 days (CT-7) for normal seasons while 33 days (CT-5) to 38 days (CT-2, CT-7, CT-8, CT-9 and CT-10) in late sowing. CT-9 showed minimum difference (3 days) while CT-5 showed maximum differences of 10 days for grain filling duration.
In our experiment generally, genotypes required more days for grain filling when grown earlier in comparison to late sowing which could have positive effects on grain yield. Negative effects of shorter growing period on all phases of plant growth such as tillering, flowering and grain filling is reported by Khan (2003). Talbert et al. (2000) have reported that terminal heat stress especially during grain filling stage of the late planted wheat is considered one of the major environmental factors drastically reducing wheat production. Randhawa et al. (1981) found lower grain filling duration in response to delayed sowing. Gebeyehou et al. (1982) reported that longer grain filling duration had a positive effect on kernels/spike and kernel weight.
Days to maturity: During 2003-04, the data for days to maturity ranged form 161 days (CT-4) to 167 days (CT-14, Bakhtawar-92 and Saleem-2000) for normal sowing and 124 days (CT- 8 and CT-14) to 128 days (CT-6) for late sowing.
| Table 6: | Mean values for plant height (cm) of wheat genotypes grown in normal and late conditions |
| Table 7: | Mean values for grain filling duration of wheat genotypes grown in normal and late conditions |
| Table 8: | Mean values for days to maturity of wheat genotypes grown in normal and late conditions |
The difference between mean values ranged from 35 days (CT-4) to 42 days (CT-14, Bakhtawar-92 and Saleem- 2000) (Table 8). In 2004-05, data ranged from 166 days (CT-4, CT-7 and CT-14) to 169 days (CT-5) for normal sowing and 140 days (CT-and CT-14) to 144 days (CT-10) for late sowing. The difference between normal and late sowing ranged from 23 days (CT-5) to 28 days (CT-1, CT-2 and CT-10). The two years (2003-05) combined data showed that maturity ranged from 165 days (CT-10) to 168 days (CT-5, Bakhtawar-92 and Saleem-2000) in normal seasons and 132 days (CT-14) to 136 days (CT-10) in late sowing. The over all difference ranged from of 29 days (CT-14) to 35 days (CT-10).
In present experiment, for late sowing, genotypes were planted 30 days later than normal sowing but maturity was recorded almost at the same time in both normal and late sown populations. Hence on over all bases, early maturity was recorded in late sowing which could be beneficial for avoiding terminal heat stress. Present results coincide with those of Begum and Saifuzzaman (1987) who reported that sowing dates have direct relationship with physiological maturity and the advancement in sowing dates attains early maturity. These results are also supported by the finding of Ashraf (1968), who concluded that wheat sown early required more days to mature as compared with crop sown late in the season.