It seems that by reducing the number of plants per square meter, the positive effect of organic fertilizers on the growth characteristics of the plant was revealed obviously, probably due to more availability and synchrony of the nutrients in organic fertilizers to the plant needs. It seems that with decreasing plant density, plant access to growth resources such as light, water and nutrients is increased and resulted to improved growth characteristics of the plant including its height. Some researchers studied the effects of organic and chemical fertilizers on yield and essential oil percentage of basil (Ocimum basilicum L.) and reported that vermicompost-treated plants had higher plant height, leaf yield, shoot yield, fresh and dry yield than other treatments (Liang et al., 2005; Tahami Zarandi et al., 2010). The results of a study on sesame (Sesamum indicum L.) showed that the application of chemical fertilizers improved leaf nitrogen content and significantly increased plant photosynthesis and consequently the biological yield of plant through increased availability of nutrients in particular of nitrogen (Kumar et al., 1996).
The positive effects of organic fertilizers on the qualitative and quantitative characteristics of different plants have been emphasized in many studies (Lakhdar et al., 2011; D'Hose et al., 2014; Ebrahimi et al., 2019). It was reported that organic fertilizers increased activity of microorganisms in soil (Arancon et al., 2004) and improved soil physiochemical and biological properties, increased nutrient storage capacity (Arancon et al., 2005) and slow release of nutrients. Nutrients improved the growth characteristics of the plant, including its height. Physiologically, when water and nutrients are sufficiently supplied to the plant, the accumulated water in the cells increases and is transmitted to the adjacent cells by turgidity and eventually increases the plant height. The effect of organic fertilizers on the qualitative and quantitative characteristics of medicinal plants of Plantago ovate Forsk., Alyssum homolocarpum L., Lepidium perfoilatum L. and Lalementia iberica L. were investigated and it was reported that cattle manure treatment produced the highest plant height compared with the other treatments, while the height of all plants was higher than the control due to application of vermicompost, coffee compost and mushroom compost (Koocheki et al., 2013). The positive effects of vermicompost application on plant nutrition and growth was also reported for sorghum (Kamaei et al., 2019).
It seems that the average plant density played the most role in increasing the flower number per plant. In low plant density, flower number per plant decreased compared with the average plant density probably due to excessive access to food and growth resources by the plant. In high plant density, this decrease was related to plant competition over water and nutrients and lack of efficient use of resources.
Organic fertilizers appear to be likely to increase the flower number per plant by supplying the plant with the micro nutrients (Motta and Maggiore, 2013). Organic fertilizers might enhance number of flowers through improving soil microbial activities (Padmavathiamma et al., 2008; Ebrahimi et al., 2019), increasing water holding capacity (Shamsodin et al., 2007) and supplying more essential nutrients (Motta and Maggiore, 2013), increased photosynthesis and plant dry matter (Atiyeh et al., 2002; Ebrahimi et al., 2019), which eventually led to increased flowering. The effects of different organic and biological fertilizers on the safflower were studied and it was reported that vermicompost solely or combined with Nitroxin® and Nitrajin® biofertilizers improved the quality and quantity of the plant (Rezvani Moghaddam et al., 2013). In a same study, it was reported that application of 10 t ha− 1 vermicompost increased flower number, plant height, 1000-seed weight, biological yield and essential oil content of Foeniculum vulgare Mill. (Darzi et al., 2006).
In high plant densities, it seems that intra-specific competition was increased and growth resources, particularly radiation, would not been adequately provided to the plant (Ndabamenye et al., 2013), thus resulted in a decrease in dry flower yield. In a study, the effect of distance between planting rows (60, 70 and 80 cm) and within rows (25, 35 and 45 cm) on yield and yield components of Satureja khuzistanica Jamzad, was investigated. The results showed that the highest flower yield and canopy diameter were observed in 45 cm within row distance and density of 7 plant m− 2 had the highest dry matter yield (Hekmati et al., 2012).
The vermicompost probably played an important role in supplying the water needed for the plant (Shamsodin et al., 2007) because of its high moisture holding capacity, thereby producing more flower dry yield. Cattle manure at low levels of plant density significantly increased dry flower yield probably through increased nitrogen release in soil (Motta and Maggiore, 2013). Some studies have shown that the application of organic fertilizers reduce the salinity effects and increase the uptake of phosphorus and nitrogen thus improve the qualitative and quantitative characteristics of plants (Sabahi et al., 2010; Ebrahimi et al., 2019). In a study, the effects of different levels of vermicompost (0, 5, 10, 15 and 20 t ha− 1) on the qualitative and quantitative characteristics of German chamomile (Matricaria chemmomilla) were investigated. The highest dry and fresh flower yield, and the maximum plant height were obtained from vermicompost application of 20 t ha− 1 (Haj Seyyed Hadi et al., 2013).
Flower yield is a complex feature that is influenced by many physiological processes and its measurable performance would be revealed in phenological, morphological, and physiological traits (Hobbs and Mahon, 1982). Weak correlations between some traits appeared to be related to differences in the time of traits measured, as traits such as flower number per plant and flower weight per plant were measured during flowering, while the traits such as plant height, number of branches per plant and canopy diameter were evaluated at the end of the flowering period. Therefore, causality analysis was performed to accurately determine the contribution of each of the traits to improvement of dry flower yield.
Although the yield of most crops, particularly medicinal plants, has increased over the past decades, but the morphological and physiological processes underlying this increase of yield are not well identified (Tollenaar, 1991). Researches revealed the positively correlation between the physio morphological traits and yield of medicinal plants including Mentha pulegium, Peppermint (Mentha piperita) and Thymus vulgaris (Mirzaee Nadooshan et al., 2001; Kukreja et al., 1992). If the origins of increased yield of medicinal plants are identified, paths to improve their actual potential by better crop management practice and effective nutrition supply may be identified (Fraser and Eaton, 1983). In this study physio morphological traits affecting yield Echium amoenum were identified using multiple regression and causality analysis.
The coefficients of Eq. 1 show the relative impact of changes in each of the variables in the model on flower yield. For example, the change in flower yield was 0.32 units per unit of change in shoot yield per plant, while this change would be 0.27 per unit increase in plant height. In other words, the relative share of shoot yield per plant was about 16% higher than that of plant height, implying some important growth characteristics of borage such as producing numerous branches and flower formation at the end of them and finally the effect of these traits on flower yield. However, to better interpret these results, the unit of measurement for each variable should also be considered, which is why multiple regression was performed on standardized traits data. Thus, due to the effects of different treatments (different types of fertilizer and plant densities) in the above model, it is possible to quantitatively evaluate the response of borage based on the rate of increase or decrease of the variables affected by the treatments.
According to Table 6, direct effect of shoot yield per plant on dry flower yield was more than direct effect of plant height via flower number per plant. Plant height had more direct effect on dry flower yield than flower number per plant.
Shoot weight per plant affected dry flower yield indirectly in three ways:
(A) Indirect effect of shoot weight per plant through plant height (P31 + P42)
(B) Indirect effect of shoot weight per plant through flower number per plant (P31 + P43)
(C) Indirect effect of shoot weight per plant by plant height and number by flowers per plant (P21 + P32 + P43)
As it shown in Table 6, shoot weight per plant had the most indirect effect on dry flower yield (0.0594), which resulted in increased dry flower yield mediated through plant height. It was reported that the yield of flowering branches of Camphorosma monspeliaca L. was positively correlated and affected by shoot yield. Also, the number of tillers that had the most direct effect on the yield of flowering branches was also indirectly affected by plant height (Abbaszadeh et al., 2011).
The results (Table 6) showed that the direct effect of plant height on dry flower yield (P42 = 0.27) was more than the indirect effect through flower number per plant (P43×P32 = 0.0012). In a study, investigation of morphological characteristics affecting yield of medicinal plant revealed that improvement of plant height and branch number per plant increased flowering branches yield (Abbaszade et al., 2011). The results of causality analysis indicated that flower number per plant had only a positive direct effect on dry flower yield. From a physiological point of view, the flower number in plants such as borage is the last component of yield and cannot transmit yield fluctuations (in this case, flowers) to another component, thus the causality analysis performed is fully consistent with the physiological bases. Comparison of morphological traits affecting dry flower yield showed that shoot weight per plant affected dry flower yield more than the other traits, as the total direct and indirect effects of shoot yield per plant was more than the other traits. Considering the paths coefficient affecting dry flower yield (Fig. 2), it seems that management practices and treatments that would increase shoot yield per plant would lead to improved yield of Echium amoenum. Results of a study on sunflower (Helianthus annus L.) showed that there were positive correlations between biological yield, shoot yield and grain yield (Amirian et al., 2013). In another study, shoot yield, plant height and number of grains per plant were identified as the most influential traits on the yield of Trigonella foenum-graecum L. (Singh et al., 2012). Some researchers reported that shoot yield, particularly umbrellas dry weight and 1000-seed weight of Coriandrum sativum L. were the most important traits affecting the yield of this medicinal plant (Dyulgerov and Dyulgerova, 2013). Guler et al., (2001) determined negative and significant relationship between 100-seed weight and seed yield of chickpea using path coefficient analysis.