Effects of Gibberellic Acid and Plant Density on Antioxidant Activity and Secondary Metabolites of Saffron (Crocus sativus L.)

Saffron (Crocus sativus L.) belongs to the Iridaceae family, whose dry stigmas are the most expensive spices in the world and are widely used in food and pharmaceutical industries. Correct management of planting density is one of the most important factors in the formation of saffron yield. Choosing the appropriate planting density in saffron, while increasing the exploitation of this agriculture, increases the yield and reduces the period between planting and economic yield. On the other hand, gibberellins are classified as a diverse group of plant growth regulators that enhance some physiological and biochemical pathways in plants. Gibberellins are involved in many plant development processes and the improvement of some desirable traits such as increasing stem length, uniform flowering, reducing time to flowering, and increasing flower weight. Also, considering that the study of planting density factors and the simultaneous application of gibberellic acid hormone on the qualitative traits of the saffron medicinal plant has not been done, the purpose of this experiment is to investigate the effect of dense cultivation and gibberellic acid hormone on the antioxidant activity and effective substances of saffron.

To investigate the effects of gibberellic acid and plant density on the antioxidant activity and active ingredients of saffron, an experiment was conducted with a factorial arrangement based on a randomized complete block design at the Agricultural Research Station, University of Birjand during 2017-2018 growing season. Treatments consist of gibberellic acid (0 and 20 ppm) and plant density (100, 200, and 300 corms.m-2) with three replications. After the land preparation operation including initial plowing, disk, and land leveling, plots with dimensions of 1x2 meters were created. The distance between the plots was 50 cm and the distance between the blocks was 2 meters (including the irrigation ditches). Since it is important to choose a quality root for planting in order to create a high yield, healthy roots without wounds and scratches and free from any kind of disease were prepared and planted after weighing (the average weight of each root is 8 grams). Anthocyanin, phenol, antioxidant activity, safranal, crocin, picrocin, dry stigma yield, and photosynthetic pigments were measured. Data were analyzed using SAS 9.1 software and means were compared based on the Duncan test.  

The results showed that gibberellic acid had a significant effect on different traits (P<0.05). The highest amount of phenol (629.77 mg. 100 g dry weight-1) and antioxidant (39.53%) was obtained from the gibberellic acid treatment. Therefore, the effect of gibberellic acid at the level of one percent on stigma anthocyanin was also significant and the rate of this trait was 24% higher in the treatment of gibberellic acid application than the non-application treatment. Active ingredients of saffron are influenced by gibberellic acid treatments. The highest safranal (32.5 %), crocin (140.4 %), and picrocin (58.1%) obtained from the use of gibberellic acid, and the lowest safranal (28/3%), crocin (125.1 %), and picrocin (56.6 %) obtained from control. A planting density of 300 corms.m-2 had the highest dry stigma yield. Chlorophyll a and total were increased by application of 20 ppm of gibberellic acid by 14% and 12%, respectively, compared to the control. Plant density treatments were significantly affected only on some traits of active ingredients (safranal and picrocin) at the level of one percent and the highest these were obtained from plant density with 100 corms.m-2, but it had no significant effect on other traits. 

The current study's findings revealed that the use of 20 ppm gibberellic acid had positive effects on qualitative traits of saffron and low planting density (100 corms.m-2) affected only some traits of saffron active ingredients (safranal and picrocin) and high planting density (300 corms.m-2) affected the dry stigma yield.