Do corm seeding rate and planting depth influence growth indicators of saffron (Crocus sativus L.)?

Highlights

• The maximum saffron crop growth rates were observed 180 days after the sowing date.

• The trend of saffron leaf area ratio was decreased from the early vegetative period.

• Saffron mother corms storage was reduced to zero during the growing season.

• Saffron daughter corms grow by using the storage of mother corms.

Abstract

Saffron, the primary in the market now, is a food spice by far the world’s most expensive. In order to study the effects of different corm seeding rates and sowing depth on physiological indices of saffron, an experiment was conducted as a factorial layout based on randomized complete block design with three replications at the research farm of the Ferdowsi University of Mashhad during 2015–2017 years. The experimental treatments were all combinations of different corms seeding rates in four levels (7, 8, 9, and 10 Mg ha⁻¹) and sowing depth in three levels (10, 15, and 20 cm). The studied traits were the dry weight of mother and daughter corms, cumulative dry matter, and some growth indices such as leaf area index (LAI), crop growth rates (CGR), relative growth rate (RGR), and Leaf Area Ratio (LAR). The results showed 8 Mg ha⁻¹ corm seeding rate and 10 cm sowing depth increased total crop dry weight, CGR, and RGR in saffron. Based on obtained results at the beginning of the vegetative growth period, CGR declined for a short time. So that by continuing the vegetative growth, the plant reached on growth compensation point, and then CGR increased. From 180 days after the sowing date and in the late stages of vegetative growth, the growth rates trend decreased again. The results also showed that the LAR trend decreased from the early vegetative growth period, which can be explained by the faster growth of the daughter corms and other underground parts of the plant than the leaves during the early vegetative growth period. According to the results and the natural trend of mother corm breakup during the growing season, increasing density due to increasing competition between mother corms caused intensification in a weight reduction of mother corms in the late stages of the growing season. Also, in the first three months after sowing, daughter corms did not grow anymore, and the beginning of their growth coincided with the end of the saffron flowering period.

Introduction

Saffron (Crocus sativus L.) belongs to the Liliaceae family. This plant is botanically an annual crop, but it is cultivated as a perennial crop. Saffron flowering occurs in autumn and has dormancy from late May to late summer (Gresta et al., 2016). Saffron is usually spread in low rainfall regions with cold winters and hot summers (Kothari et al., 2021). According to the statistics of Iran′s Ministry of Agriculture (2018–2019), the area under cultivation, total production, and the average yield of saffron in 2018 were 113938 ha, 404.5 Mg, and 3.62 kg ha⁻¹, respectively (Ministry of Agriculture-Jahad, 2019). Global statistics show that about 94 % of the total world production is allocated to Iran (Koocheki et al., 2017). Saffron is a triploid plant and, due to the inability of the sexual generation, is considered sterile. Therefore, corms have particular importance in saffron reproduction (Gresta et al., 2008; Kumar et al., 2008; Kothari et al., 2021).

Many factors are affecting the growth, quality, and quantity of saffron yield, including climatic factors, pests and diseases and weeds, irrigation, storage, sowing date, corm quantity as seed for planting in the first year, and sowing depth, which hereon, corm seeding rates and the optimal sowing depth in the first year are the most critical effective strategies for increasing saffron yield per unit area in the first and subsequent years (Bayat et al., 2016). The optimal sowing depth, while providing optimum conditions for flower emersion and plant emergence, is also effective in protecting corm from chilling and freezing in winter and also heat and drought in summer, but deep sowing prevents plant emergence; thus, it can reduce yield (Yildirim et al., 2016). On the other hand, considering that the cultivation and reproduction of saffron are performed through the corm, the optimal yield is considerably related to the corm quantity as seed for planting (Bekhradiyaninasab et al., 2020). Koocheki et al. (2014) also reported that planting density or corm seeding rates in the first year are one of the most critical factors affecting saffron yield. Selection of appropriate corm seeding rates in the first year in, while increasing the exploitation period of this crop, increases yield and reduces the period between sowing and product economization. Despite some performed studies, it seems that due to the heterogeneity of corms size at sowing, enough research has not been carried out on corm seeding rates in the first year and the optimal sowing depth of saffron; therefore, there is a necessity to review these principles in order to promote and improve the production of this crop. Besides considering that saffron's physiological changes occur below the soil surface, and unlike other plants, corms form below the soil surface. Hence, studying the plant's phenological stages, emphasizing the formation and development of underground organs, can provide a more accurate concept of plant growth changes during the growing season. In this regard, the present study was designed to investigate the effects of different corm quantities as seed for planting and sowing depth on physiological growth, flower and corm indices (aerial and underground organs) of saffron in an annual farm.