Planting date affects biochemical characteristics, assimilate remobilization, and yield of safflower under water stress

Adjusting the planting date is an inexpensive and effective method to mitigate the possible effects of water stress. A 2-year field study was conducted to investigate the effects of planting dates on the biochemical and physiological characteristics and yield of safflower (Carthamus tinctorius L.) under water stress in the 2018 and 2019 growing seasons. The treatments consisted of three planting dates (December 6 and 26 and January 15), three irrigation regimes (normal irrigation, irrigation cut-off at flowering and irrigation cut-off at seed filling) and two safflower cultivars (Goldasht and Isfahan). During the 2-year period, pigment content, enzyme activity, canopy temperature, assimilate remobilization, water use efficiency (WUE) and yield were measured. Planting date and irrigation regime had a significant effect on all parameters, with the most significant differences observed when irrigation was cut-off during flowering and seed filling, resulting in yield reductions of up to 31% in Goldasht. Irrespective of planting date, irrigation cut-off reduced total chlorophyll by 7.5-28.4% and increased canopy temperature. When irrigation was cut off, the relative water content (RWC) of Goldasht was 10.8-11.7% higher than that of Isfahan on December 26. Although irrigation cut-off at flowering stage increased canopy temperature, Goldasht planted on December 26 had the lowest value. Chlorophyll a, carotenoid content, and catalase and peroxidase activities were generally highest on December 26, especially in Goldasht. In addition, dry matter remobilization and remobilization efficiency were higher in the first year due to higher average temperatures and evaporation. In Goldasht, the highest WUE was observed in a planting at the end of December under normal irrigation. Oil content was also highest in Goldasht under normal irrigation, especially when planted in late December. A positive relationship was observed between seed yield and total chlorophyll, RWC, number of seeds per capitulum, remobilization efficiency and WUE. Overall, December 26 was the optimal planting date under water stress, especially for Goldasht, which resulted the highest yield in both years.