چكيده لاتين :
Introduction: Successive developmental stages occur at different times over the growing season and are consequently exposed to different environmental conditions. Therefore, knowledge of the factors influencing variation in developmental stages duration is essential for adaptation and crop management (Hay and Porter, 2006). Under different environmental conditions, crop life cycle may be extended or curtailed, however, none of developmental stages are eliminated. Hence, the duration of different developmental stages is to vary as the crop life cycle is fit into the available growing season (Reynolds et al., 2001). Nearly %39 of wheat grain yield variation depends upon changes in environmental and climatic indices (Anderson and Ahmadi-Esfahani, 2010). Spike growth period (terminal spikelet to anthesis) is a crucial stage, which significantly affects grain yield formation. Spike growth duration decreases in stressful environments, thus negatively influences grain yield components (Refay, 2011). This study was carried out to investigate the effect of different environmental conditions arising from different sowing dates on the duration of various developmental stages, and canopy temperature depression in several wheat genotypes.
Materials and Methods: This study was conducted at Gonabad Agricultural Research Field Station, in the North East of Iran in 34º 23´ N, 58º 45´ E and 1060 m above sea level-in split plot arrangements using randomized complete block design with three replications during 2010-2011 growing season. The main plots were sowing dates with four levels (D1: Oct., 23, D2: Nov., 6, D3: Nov., 22, D4: Dec., 6). Ten bread wheat cultivars, (V1: (Bam), V2: (Nishabur), V3:(Falat) , V4: (Chamran) , V5: (Sivand) , V6:(Parsi), V7:(Pishtaze), V8:(Bahar), V9:(M-85-7), V10: (Sepahan)) were assigned to the subplots. To precisely determine the developmental stages at least five randomly selected plants from each plot were dissected according to Kirby, 1990. Double ridge (DR) stage and Terminal spikelet (TS) stage were determined by dissecting the shoot apex of sampled plants. Anthesis was recorded when anthers of middle florets in 50% of spikes were extruded (Kirby, 1990). Physiological maturity was determined when 50% of peduncles turned in to yellow. canopy temperature depression was calculated by the difference between air and canopy temperature (Kringwi et al., 2004).
Results and Discussion: The results showed that the highest grain yield obtained on 23 Oct. and 6 Nov. sowing dates (5.336 and 5.264 ton/ ha, respectively). Genotypes V6 :(Parsi), V1:(Bam), V8 :(Bihar) and V7:( Pishtas) had the highest grain yield on 23 Oct. sowing date (6.727, 6.099, 5.979 and 5.792 ton/ha, respectively), genotypes V6:(Parsi), V1:(Bam), V2: (Nishaboor), V5: (Sivand) and V8 :(Bahar) on 6 Nov. sowing date(6.333, 6.162, 5.915, 5.630, 5.615 and 5.435 ton/ha, respectively) (Table 1). The grain yield decreased significantly in all genotypes at the delayed (22 Nov. and 6 Dec.) sowing dates (3.200 and 2.216 ton/ha, respectively), due to reduced days to physiological maturity, anthesis, terminal spikelet, double ridge and grain filling period (Table 1). Delay in onset of developmental stages on 22 Nov. and 6 Dec. sowing dates caused young spike growth period (terminal spikelet to anthesis) and grain filling period to coincide with unfavorable environmental conditions. With sowing dates being delayed, the occurrence of different growth stages is postponed as compared to the earlier planting dates, resulting in the developmental stages coinciding with unfavorable temperatures. Canopy temperature depression (CTD) is used as a criterion in terms of the degree to which the plant canopy is cooler than its surrounding environment. Our results showed that there was no significant difference in CTD between 23 Oct. and 6 Nov. treatments, but there was high significant difference in CTD under 22 Nov. and 6 Dec. treatments during anthesis and milky grain stages. Transpiration causes canopy temperature to drop below the air temperature. Research indicates that decreased canopy temperature is better correlated with crop yield when vapor pressure is greatly reduced (under warm and sunny weather conditions during grain filling). Under these conditions, canopy temperature can be more than 10 C° lower than the environment temperature (Reynolds et al., 2001). This is in agreement with the findings of current work. Therefore, planting of suitable wheat cultivars from 23 Oct. till 6 Nov. caused pheonological stages to co-occur with more proper environmental conditions, leading to enhanced growth and grain yield.
