Abstract: A field investigation was carried out to study the influence of irrigation on phenology and accumulated heat units of four cultivars of bread wheat (Triticum aestivum L.) at different phenological stages over two growing seasons. There were three treatments: rainfed, 10 and 20 mm irrigation. The rainfed plants had significantly shorter phenological stages and lower growing day degree than the irrigated plants in all the four cultivars. Cultivars Akbar was found to be early and C 306 was found to be late maturing. Akbar had the lowest and C 306 had the highest growing degree days during all the phenological stages under each irrigation regime. Heliothermal units also showed more or less the same trends. The phenothermal indices and heat use efficiency were found to be higher in the irrigated plants than the rainfed plants.
INTRODUCTION
Temperature is one of the most important climatic events affecting the growth, phenology, development and yield of wheat[1]. Despite the improved ability to predict onset and modify the impact, it remains as an important factor affecting third world food security. Temperature plays a vital role in almost all biological processes of crop plants. Growth and development of crop plants markedly depend on temperature. Integrated effect of temperature on the physiological processes significantly determines the nature of crop growth and development. The usefulness of different temperature indices like growing degree days (GDD) of heat units has been reported by many workers for predicting crop growth and development[2-4]. Most proposed temperature indices show significantly greater correlation with plant growth and development than does accumulated time. Significant effect of soil moisture on heat units was noticed by several workers[5,6]. Influence of soil moisture stress on the heat unit requirement of crop plants should be assessed in the field as the moisture of farms in a zone varies considerably. The effect of different soil moisture regimes on phenological development and heat use efficiency (HUE) of four bread wheat cultivars grown under field condition was studied in the present investigation through the accumulated heat unit system.
MATERIALS AND METHODS
The experiment was conducted in the research field of Rajshahi University Botany Department with four bread wheat (Triticum aestivum L.) genotypes, raised from four regular wheat varieties grown in Bangladesh, viz., Akbar, Opata, Protiva and C 306. The experimental field was prepared after repeated ploughing and harrowing. A basal dose of urea (80 kg ha–1), TSP (60 kg ha–1) and MP (30 kg ha–1) were added to the soil before sowing. The soil of the experimental field was silty loam having a pH 7.5, low in organic carbon (0.44%) and total N (82 ppm). The average field capacity of the soil within a depth of 0.5 m was 34% for the first and 39% for the second year, respectively. The experiment was arranged in a split plot design having three replications with water regimes as main plots and cultivars as sub plots. Three different water regimes were adopted for each growing season, namely, rainfed (I0), 10 mm irrigation (I1) and 20 mm irrigation (I2). The first irrigation was applied at 18 days after sowing and the laters were at 15 days interval. The measured amount of water was uniformly added manually over the irrigated plots with sprinklers. The experiment was conducted in 1998-1999 and 1999-2000 growing seasons. In both the growing seasons, the sowing date was 25 November. The total amount of water received as irrigation was 50 mm for the I1 plots and 100 mm for the I2 plots in both growing seasons excluding the natural rainfall. The natural rainfall recorded was 0 mm for the first year and 36 mm for the second year.
The most important stages like crown root initiation, booting, anthesis and grain filling were recorded in days when 50% of a cultivar of each replication reached a definite stage as the representative of that stage. The daily meteorological data from the Regional Meteorological Office, located at about 1 km away from the experimental field, were used. The growing degree days (GDD), heliothermal unit (HTU), phenothermal index (PTI) and heat use efficiency (HUE) were calculated according to the formulae of Rajput[7]:
| I. | Growing degree days (GDD) = Σ (Tmax + Tmin)/2 - Tb |
| ii. | Heliothermal unit (HTU) = GDD X duration of sunshine hour |
| iii. | Phenothermal index (PTI) = GDD � Growth days |
| iv. | Heat use efficiency (HUE) = Grain yield ha–1 � GDD |
| Tmax | = | Maximum temperature |
| Tmin | = | Minimum temperature |
| Tb | = | Base temperature = 10°C |
RESULTS AND DISCUSSION
Only the data of 1998-1999 growing season have been reported in the present report because both the data of 1998-1999 and 1999-2000 showed similar trends in the above-mentioned parameters.
The rainfed plants of all the cultivars took shorter time than the irrigated plants in all the four phenological stages. Cultivar C 306 took the longest time (97.8 days) to attain grain filling stage and it was followed by Opata (92.9 days). Akbar took the shortest time to attain grain filling (Table 1). Water stress caused reduction in required days for a particular phenological stage in the present investigation and the number of days taken for attainment of different phenological stages were in the order I2>I1>I0. Longer duration of the phenological stages might be due to more tillering, maximum leaf emergence, higher plant height and greater yield components. These occurred due to favorable environmental conditions, especially optimum temperature and adequate irrigation throughout the whole growth period. In contrast, the rainfed plants suffered from water stress throughout the growing period and this water insufficiency reduced the number of days for all the phenological characters by supplying the lower amount of assimilates which reflected to different growth stages resulting in quick maturity of plants. Day and Intalap[8] reported that the maturity of wheat plants hastened due to water stress. Gales and Wilson[9] noticed that the plants suffering from water shortage matured earlier than the well-watered controls.
| Table 1: | Time (days) taken for different phenological stages of four wheat cultivars as influenced by different irrigation regimes |
| Table 2: | Growing degree days (GDD) of four wheat cultivars as influenced by different irrigation regimes at different phenological stages |
*Difference within variety between treatments, In a column, means followed by a common letter are not significantly different | |
| Table 3: | Helio-thermal units (HTU) of four wheat cultivars as influenced by different irrigation regimes at different phenological stages |
| Table 4: | Phenothermal index (PTI) of four wheat cultivars as influenced by different irrigation regimes at different phenological stages |
| Table 5: | Heat use efficiency (HUE) of four wheat cultivars as influenced by different irrigation regimes |
*Difference within variety between treatments In a column, means followed by a common letter are not significantly different | |
As cultivar C 306 is late maturing, all the phenological characters showed higher values of required days in the present study (Table 2). Water deficiency decreased the duration of phenology as compared to irrigated condition due to the drought and thereby the requirement of heat units (GDD) decreased for different phenological stages with rainfed condition (Table 2). On average, the highest and the lowest GDD were found in cultivars C 306 and Akbar, respectively for all the four phenological stages. It is likely that maturity days will be more in that cultivar which had the highest GDD and the reported results corroborate the findings of Singh et al.[6], Rajput and Sastry[10].
To obtain the HTU of a definite phenological stage, the product of the degree days with the base temperature (10°C) and actual duration of bright sunshine hours was computed (Table 3). Generally the I2 plants had the highest HTU for all the phenological stages on average and were followed by I1 and I0 plants. Rahman et al.[11] also reported higher HTUs in the irrigated plants of wheat. Among the cultivars, C 306 had the highest and Akbar had the lowest HTU for almost all the phenological stages. The irrigated plants passed their growth period with bright sunlight and sufficient water that influenced the photosynthesis effectively. In contrast, the rainfed plants passed their growth period with bright sunlight and insufficient water that hampered the different metabolic activities. Water stress compelled the plants to complete its life cycle with a short period of time resulting in decreased HTU that happened in cultivar Akbar in the present investigation.
On average, the PTI during later phenological stages were in the order I2>I1>I0 as well in most cases (Table 4). This result was in agreement with Rahman et al.[11]. Among the cultivars, C 306 had the highest and Akbar had the lowest PTI during the grain filling stage although the PTI of the cultivars did not differ significantly during the sowing to crown root initiation stage. The lowest PTI was noticed during booting stage for all the cultivars. The variation in the PTIs for different phenological stages indicated that the accumulated temperature could be utilized for studying the pattern of biomass accumulation at different phenological stages which ultimately influence the crop productivity.
The irrigated plants had higher HUE than the rainfed plants for all the four cultivars (Table 5). Irrigation resulted in higher grain yield via optimum metabolic activities and thereby the irrigated plants had higher HUE. On the other hand, deficiency of water hampered the normal metabolic process and resulted in lower grain yield as well as lower HUE. Higher HUE in the irrigated plants than the rainfed plants was also noticed by Rahman et al.[11]. Due to its lowest GDD, Akbar had the highest HUE although the highest grain yield was not obtained by Akbar. On the other hand, Opata had the poorest grain yield as well as higher GDD which resulted in the lowest HUE. Although C 306 had the greatest grain yield, it did not show the highest HUE like the other heat units due to its highest GDD and late maturing nature.