Study on bread wheat cultivar developmental stages and canopy temperature depression changing at environmental conditions due to different sowing dates

Introduction
Successive developmental stages occur in different time over the growing season and are exposed to different environmental conditions, therefore, knowledge of factors influencing variation in developmental stages duration is essential in adaptation and crop management (Hay and Porter, 2006). Under different environmental conditions crop life cycle may extend or curtailed, however, none of developmental stages are eliminated. Hence, the duration of different developmental stages are to vary as the crop life cycle is fit in the available growing season (Reynolds et al., 2001). nearly %39 of wheat grain yield variation is depended to environmental indecses.(Anderson and Ahmadi-Esfahani, 2010). Spike growth period (Terminal spikelet to Anthesis) is a crucial stage and significantly affects grain yield determination. Spike growth duration decreases under environmental stress, thus negatively influences grain yield components (Refay, 2011). This study was carried out to investigate the effect of different environmental conditions due to different sowing dates on the duration of different developmental stages, and canopy temperature depression with in 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 see level-in split plot arrangements using randomized complete block design with three replications, during 2010-2011 growing season. 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: (Nidhaboor), V3:(Falat) , V4: (Chamran) , V5: (Sivand) , V6:(Parsi), V7:(Pishtaze), V8:(Bahar), V9:(M-85-7), V10: (Sepahan)) were assigned to subplots. To precisely determine the developmental stages at least five randomly selected plants from each plot were dissected following 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 diffrence between air and canopy temperature (Kringwi et al., 2004).
Results
The results showed that the highest grain yield obtained in 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 in 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) in 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, in delayed (22 Nov. and 6 Dec.) sowing dates (3.200 and 2.216 ton/ha respectively), due to reduction days to physiological maturity, anthesis, terminal spikelet, double ridge and grain filling period (Table 1). Delay in appearance of developmental stages in 22 Nov. and 6 Dec. sowing dates caused young spike growth period (terminal spikelet to anthesis) and grain filling period happened in unsuitable conditions. There was no significant difference in Canopy temperature depression (CTD) in 23 Oct. and 6 Nov. treatments, but there was high significant difference in CTD in 22 Nov. and 6 Dec. treatments, in anthesis and milky grain stages. So planting of suitable wheat cultivars in the period of 23 Oct. to 6 Nov. caused pheonological adjustment with more suitable environmental conditions and imporoved grain yield in recommended variety in the same environmental conditions.