Seed Yield in Three Species of Brassica (Brassica napus L., B. rapa L., B. junceae L.): Effect of Rainfall and Photothermal Quotient in Rainfed Conditions of Gonbad

Increasing terminal drought and heat stresses in Gonbad area necessiate evaluation and selection of superior genotypes from different species of Brassica. A field experiment was conducted at Agricultural Research Station of Gonbad during 2003–2006 growing seasons. Twenty genotypes from three species of Brassica (five genotypes of spring type, B. napus (STN), five genotypes of winter type, B. napus (WTN), five genotypes of B. rapa (turnip), and five genotypes of B. junceae (mustard)] were sown at optimum sowing date. Combined analysis of variance showed that the mean seed yield (SY) of STN genotypes was considerably greater than other genotypes. The mean SY of WTN, STN, turnip, and mustard genotypes were 1381, 2191, 1780, and 1915 kg ha-1, respectively. A great proportion of SY variation was related to environmental factors such as rainfall, and interaction of temperature × radiation (photothermal quotient, PTQ). In STN and turnip genotypes, there was a positive linear relationship between SY and rainfall during growing season. For each 0.1 unit increase in PTQ during flowering period, SY of WTN, mustard, STN, and turnip genotypes increased by 582, 574, 336, and 303 kg ha-1, respectively. For each 0.1 unit increase in PTQ from termination of flowering to physiological maturity, SY of WTN, mustard, STN, and turnip genotypes also increased by 749, 732, 658, and 240 kg ha-1, respectively. The number of days from emergence to physiological maturity in WTN and mustard genotypes was longer than STN and turnip genotypes. Therfore, the reproductive stages of WTN and mustard genotypes coincided with higher temperatures, led to a greater response of SY to increase in PTQ. The SY superiority of STN genotypes showed that STN genotypes are better adapted to rainfed environmental conditions of Gonbad.