Under water-deficit stress, the uptake of many nutrients declines due to reduced nutrient mass flow and diffusion (Zhao et al. 2020). Our results and other studies showed that osmotic stress caused by PEG impairs the uptake of micro and macronutrients (Mouradi et al. 2016). Although AM improved nutrient levels and reduced the stress damage by expanding root depth and more soil access through their hyphae, the variation trends of the uptake, accumulation, and transfer of nutrients vary in different species of plants and AM under water-deficit stress condition. In many studies, AM has reported to increases the uptake of nutrients such as N (Hashem et al. 2019), P (Zardak et al. 2018), K (Zhao et al. 2015), Cu, Zn, Fe (Abbaspour et al. 2012), and Mn (Wu and Zou 2009) under the water-deficit condition in different plants. Moreover, AM can affect the uptake of nutrients by producing different compounds. For instance, AM increased the amount of P absorbed by plants via increasing the activity and production of enzymes such as phosphatase (Hu et al. 2013). It has been shown that AM not only improved the P uptake but also increased the uptake of N and nutrition in the plant through enhancing the hydraulic conductivity of the root under water-deficit stress (Gholamhoseini et al. 2013; Kong et al. 2014).
Another compound produced by AM is chelating agents such as siderophores, which can ameliorate the uptake of micro-nutrients such as Zn and Fe in the plants (Dehghanian et al. 2018). Although some studies have shown that increasing the concentration of P by mycorrhiza has a positive effect on the Zn content, but another reason for the increase in the amount of Zn in the roots and shoots of mycorrhizae-inoculated plants is the rise in the diffusion-limited process of Zn (Lehmann et al. 2014). By increasing the osmotic potential, the pores will be closed which will reduce transpiration and imbalance the active transport, thereby reducing the transfer of nutrients from the root to the shoot (Silva et al. 2009) while increasing the K content of the shoot by mycorrhizae stimulated the stomata and improved the transport of nutrients from the roots to the shoots (Ruiz-Lozano and Azcón 1995).
The increased activity of POD, CAT (Uzilday et al. 2012) and PPO (Thipyapong et al. 2004) enzymes under stress condition indicated their crucial role in enduring water-deficit stress. CAT has been considered as the most indispensable enzyme for counteracting the hydrogen peroxide produced under stress conditions (Khanna-chopra and Selote 2007). POD is among the major H2O2-binding enzymes in cytosol and chloroplasts whose level also rapidly increases rose under water-deficit stress. Under water-deficit stress, an increment was observed in the CAT and POD activities in diverse members of the Asteraceae family, such as Silybum marianum (Nouraei et al. 2018), Carthamus tinctorius L (Chavoushi et al. 2019), Helianthus annuus L (Ghobadi et al., 2013). In line with our results, other studies have also reported that AM has increased the levels of POD and PPO (Meddich et al. 2015; Tyagi et al. 2017) in various plants under water-deficit stress. One of the reasons for the increase in POD enzyme by AM could be the expression of its encoding genes in inoculation with AM (Mustafa et al. 2017). Although CAT is a metalloenzyme and thus its activity depends on the availability of metal nutrients (Armada et al. 2016), in the present study, AM improved the uptake of metal nutrients; however, the effect of mycorrhizal inoculation on CAT enzyme levels under stress conditions was very different and depended on the plant species and even the species of mycorrhizal fungi (Wu and Zu 2009). The increase in CAT activity by F. mosseae (Amiri et al. 2015) and other species has been reported in many plants under water-deficit stress (Aalipour et al. 2020; Al-Al-Arjani et al. 2020). Osmolytes such as total soluble sugars and proline increased under water-deficit stress; playing a significant role in regulating the osmotic potential of the plant (Khan et al. 2015). Proline is an amino acid and can be stored in the cytoplasm, which in addition to osmotic regulation of the cell, detoxifies ROS and protects membrane integrity and stabilizes proteins/enzymes, and serve as one of the plant's solutions to reduce stress damage (Ashraf et al. 2007). In the current study, the increased leaf proline level was observed by Funneliformis mosseae under water-deficit stress.
The increase in proline content can be assigned to the effect of AM on increasing the N content of the plants under water-deficit stress (Augé 2001). High N levels in the plant under water stress can significantly influence the genes involved in proline biosynthesis which finally increase proline (Monreal et al. 2007; Wang et al. 2011). In another study, an increase was reported in total soluble sugar under drought stress conditions, which is consistent with our results (Al-Arjani et al. 2020). AM increased the level of total soluble sugar in plants as it increased the activity of sucrose-metabolized enzymes which had a positive and significant relationship with glucose, fructose, and sucrose contents (total soluble sugars) (Wu et al. 2017). As observed, under water-deficit stress, plant growth decreased due to reduced osmotic regulation ability, disruption of the solute uptake system, disturbance of osmotic balance, and excessive energy requirements to produce osmolytes (Munns et al. 1993). Based on the findings of this study, a loss was observed in the dry weight of shoots, roots, and flowers of chamomile under water-deficit stress (Baghalian et al. 2011). One of the causes of reduced chamomile growth under stress may be the osmoregulation imbalance and the disruption in the salt absorption system or the high level of energy required for counteracting the stress (Salehi et al. 2018). An increment was also detected in the dry weight of shoots, roots, and flowers of chamomile (Bączek et al. 2019) due to the improved absorption, distribution of nutrients, the increment of proline, total soluble sugars, and antioxidant enzymes by AM, which improved the growth performance, lowered the stress damage, enhanced the plant growth and elevated the dry weight (Al-Arjani et al. 2020).
According to the results, the dry weight of chamomile shoots, roots, and flowers reduced under drought stress (Baghalian et al. 2011). Under drought stress, plant growth was reduced due to the reduction of osmotic regulation, osmotic imbalance, and the requirement of excessive energy needs to cope with stress (i.e. the production of osmolites and disruption of the nutrient uptake) (Munns et al. 1993). All nutrients play a vital role in plant growth; the nutrients (macro and micro) were positively correlated with the plant growth was positive (Daur et al. 2011). The effects of each nutrient on the uptake of other nutrients are very complex. According to the correlation shown in Fig. 4, the synergistic effect between many nutrients in chamomile reflects the diverse roles of these nutrients in the growth, yield, and uptake of other nutrients by chamomile. For example, sufficient Mg causes a proportional distribution of carbohydrates in the root and shoot; promoting the chamomile root growth (He et al., 2020). On the other hand, Mg affects biomass production and plant growth by proper distribution of carbohydrates and the appropriate allocation of hydrocarbons to different parts of plants (Verbruggen and Hermans 2013) or improving the plant access to N (Haberman et al. 2019) and iron (due to its vital role in photosynthesis) (Dong et al. 2019) with an effective role in vegetative growth and ultimately the accumulation of plant dry weight. This is consistent with a positive and high correlation of the dry weight of chamomile with the mentioned elements. According to the results, one of the most important causes of reduced growth of chamomile under stress conditions is the disturbed nutrients uptake (Salehi et al. 2018). The level of nutrients in the plant and the ability to uptake these nutrients are important factors in selecting the best cultivar under stress. In this regard, the Sor cultivar was almost superior to Bod in terms of both factors. On the other hand, improved absorption and distribution of elements as well as increased proline, total soluble sugar and antioxidant enzymes by mycorrhiza inoculation enhanced the growth while reducing stress-induced damages (Al-Arjani et al. 2020). In line with previous reports (Bączek et al. 2019), mycorrhiza increased the dry weight of shoots, roots, and flowers of chamomile.