The effect of growth and development periods on grain yield of spring bread wheat under terminal heat stress in Ahwaz

Introduction Temperature is the most important determining factor in growth and development duration length of crops. Temperature increase result in faster growth and development and shorter life duration of plant. This is one of the main factors of yield loses in hot environments (Aggravel, 1991). Studies showed that one centigrade increase in mean temperature of growth duration causes eight days (6 percent) decrease in vegetative growth phase, two days (five percent) decrease in filling period duration and 21 days (eight percent) decrease in life cycle duration of wheat (Koocheki and Hosseini, 2006). Modhej et al. (2007), planted six wheat cultivars (three bread wheat cultivars and three durum wheat cultivars) in two sowing dates (November 21 as optimum sowing date and January 20 as late sowing date) and reported that grain filling period of late sowing date confronted to terminal heat stress and resulted in significantly decrease in grain weight and grain yield. In this study, the most important aim is to study the effect of terminal heat stress on growth periods length, grain yield and its components and grain yield formation rate and its components in 20 spring bread wheat cultivars in Ahwaz condition. Materials and methods In order to study the effect of terminal heat stress on growth periods length, grain yield and its components and grain yield formation rate and its components in 20 spring bread wheat cultivars, a field experiment was conducted in a stripe block arrangement using a complete randomized block design (CRBD) with three replications in research field of Ramin Agriculture and Natural Resources University of Khouzestan (in 31º N, 48º E, 35 Km north-east of Ahwaz, and 20 m above the sea level) during two cropping seasons (2007-2008 and 2008-2009). Experimental factors were four sowing dates (06 Nov. (early), 06 Dec. (timely), 05 Jan. (late) and 04 Feb. (very late)) in vertical plots and 20 spring bread wheat cultivars (Atrak, Arvand1, S-80-18, Star, Inia66, Bolani, Bayat, Pishtaz, Chamran, Chenab70, Darab, Dez, Roshan, Shoeleh, Kavir, Maroon, Hamoon, Hirman and Vee/Nac) in horizontal plots. Results and discussion Results showed that year, sowing date, genotypes and sowing date× genotype had significant effect on measured traits. Mean comparisons revealed that with delay in sowing date and increase in mean temperatures of growing season, decreased the day from sowing to anthesis, day from anthesis to maturity, day from sowing to maturity, grain number in m2, thousand grain weight and grain yield that resulted to decrease in grain formation rate, grain filling rate and grain yield formation rate. Generally, Maroon and Shoeleh cultivars (early- lately maturity cultivars) with short period of sowing to anthesis and with long period of anthesis to maturity, had longer grain filling period.  Conclusions Generally and according to these results could say that delay in sowing and increase in temperature of growth cycle duration resulted in confrontation of terminal phases of growth and development to high temperatures and occurrence the terminal heat stress. These stress decreased the day from sowing to anthesis, day from anthesis to maturity and day from sowing to maturity. These effects resulted in lower grain number in m2, thousand grain weight and grain yield. Finally, above decreases lead to lower grain formation rate, grain filling rate and grain yield formation rate.