Impact of crop stand, Rhizobium inoculation, and foliar fertilization on pea root parameters

Roots are a fundamental part of the plant. They are needed to absorb water and nutrients for growth (Smucker, 1993). Optimum root growth is especially crucial in arid areas or in regions with temporary drought, because in such conditions crops may have limited access to water and nutrients (Lynch, 2013). Songsri et al. (2009) stated that root parameters are important traits for water efficiency under moderate and severe drought and that genotypes with larger root systems could maintain a relatively higher water efficiency under water stress conditions compared to those with a smaller root system.

Morphological root traits, such as root length, root diameter, and root dry matter, are key determinants of plant resistance to adverse habitat conditions (Laio et al., 2006; Bodner et al., 2013; Klimek-Kopyra et al., 2017). Ingram et al. (1994) noted that root thickness determines drought resilience of plants and that root length density (RLD) enables water uptake. Root biomass is dependent on root diameter (Eissenstat and Yanai, 2002), and biomass production increases with a larger root diameter because of better developed transport of water and nutrients (Qaderii et al., 2019). Klimek-Kopyra et al. (2015a) indicated that variation of morphological root traits differs between crop stands (pure stands versus intercrops) and soil types; the average root diameter of pea grown in a Luvic Phaeozem was higher for pure grown pea compared to pea inter-cropped with linseed, whereas it was the other way around in a Eutric Cambisol.

Drought resistance can be improved by changing the plant to increase the efficiency of water uptake by a root system better adapted to drought. This can be done by breeding new cultivars with altered root plasticity, capable of effectively acquiring both deep and shallow water and nutrients’ resources (Ho et al., 2005; Tron et al., 2015; Lynch 2018). Successful results of biological progress regarding changes in root morphology are noticeable mainly in legumes, such as chickpea, cowpea, soybean, field pea, and white clover, which are most sensitive to water deficiency during growth and development (Matsui and Singh, 2003; Annicchiarico and Piano, 2004; Benjamin and Nielsen, 2006). Under dry conditions, the right cultivar selection in combination with proper agrotechnical treatments can mitigate yield losses (Klimek-Kopyra et al., 2018a; 2019). The distribution of RLD in pea was indicated by Klimek-Kopyra et al. (2018a) to be affected by the interaction of pea cultivars × years, with a fodder cultivar of pea obtaining a higher RLD in a semi-drought year compared to a food cultivar in a very humid year. A significant interaction between seed inoculations and years was also proven with a gel seed inoculant, which significantly increased the RLD compared to a powder inoculant in a dry year, whereas the gel inoculant was least effective in a humid year.

Another approach was presented by Fernández and Eichert (2009), who indicated that foliar fertilization can effectively reduce the negative effects of unfavorable weather conditions in crop growth by reducing the negative impact of soil effects that limit the uptake of hard-to-reach nutrients by plant roots. Foliar fertilization has many advantages if used during the plant ripening period, when the root system is inefficient, by effectively complementing shortages of micro- and macronutrients (Wójcik, 2004). However, the effectiveness of foliar fertilization may depend on the physiological state of the plant, for example, osmotic potential of the roots, hydraulic conductance, or nutrient status (Weinbaum, 1988). Wójcik (2004) indicated that foliar fertilization is most effective when soil nutrient availability is low in the dry topsoil and root activity during the reproductive stage is low. Similar to that, Klimek-Kopyra et al. (2018a) have shown that under periodic drought conditions, foliar fertilization significantly increased the weight of pea roots, whereas a combination of Rhizobium inoculation and foliar fertilization significantly increased the RLD. Another study by Klimek-Kopyra et al. (2018b) showed that Rhizobium inoculation together with foliar fertilization effectively promoted the growth of biomass and pea yield components, suggesting that foliar fertilization can also effectively affect pea underground biomass, minimizing its reduction in intercrops of pea with linseed. Current literature does not show the influence of Rhizobium inoculation and foliar fertilization on root system development in the condition of pea intercrops with linseed or wheat. The aim of the research was therefore to show the effect of crops stands (pure stands versus intercrops) and the single treatments of Rhizobium inoculation and foliar fertilization as well as a combination of both on pea roots parameters.

The RLD did not differ between years and crop stands but was significantly higher after foliar fertilizer application by Photrel having higher values than Nitragina and Nitragina + Photrel, showing intermediate values (Table 2). The RLD was ranked as follows in the individual root fractions (root diameter in mm): 0.1–0.2 > 0.2–0.5 > 0.5–1 > 0.05–0.1 > 1–2 > in fractions of > 2 and < 0.05 (mean overall years, crops stands, and treatments; Table 3). Although total RLD did not differ between crop stands, there was a significant interaction of crop stand × root fractions and also a significant interaction of treatment × root fractions. Pure pea had a lower RLD in the root fraction of 0.1–0.2 mm than in intercrops with linseed and wheat; in the root fractions 0.2–0.5 and 0.5–1 mm, the RLD of pea/linseed was highest, with no significant differences between crop stands in other root fractions (p < 0.001, Figure 1 A). Among treatments, the RLD was highest with Photrel and lowest with Nitragina with Nitragina + Photrel showing intermediate values in the three root fractions from 0.1 to 1 mm, with no significant differences between treatments in other root fractions (p < 0.001, Figure 1 B).

There was a significant year × crop stand effect on the MRD: the MRD of pea for main effects was ranked as follows: Y – 2009 > 2010 > 2011; S – pea/linseed, pure pea > pea/wheat. In 2009, the MRD did not differ among crop stand, whereas in 2010 and 2011, the MRD of pea/wheat tended to be lower than that of pure pea and pea/linseed. Treatment did not affect the MRD (Tables 2 and 4).

The RDM of pea for main effects was ranked as follows: Y – 2011 > 2009, 2010; Nitragina + Photrel ≥ Nitragina ≥ Photrel, with no main effects of crop stand. However, there were significant interactions of year × crop stand, year × treatment, and crop stand × treatment (Table 5). The year × crop stand interaction was given as follows: RDM tended to be lower in pea/wheat than in pure pea and pea/linseed in 2010 and 2011, whereas, in 2009, it tended to be lowest in pure pea compared to the other treatments. The difference of RDM between 2009 and 2011 was lower for pea/wheat than for other crop stands. The year × treatment interaction was given as follows: RDM of pea was significantly higher with Nitragina + Photrel than with the other treatments only in 2011, whereas no differences between treatments occurred in 2009 and 2010. The crop stand × treatment interaction was given as follows: RDM of pea tended to be higher with Nitragina + Photrel than the two other treatments in pure pea and pea/linseed and with Nitragina and Nitragina + Photrel than with Photrel in pea/wheat.

The ICE_ROOT indicator shows that the RDM of pea in the pea/linseed intercrop was higher in all 3 years than in the pure stand and the RDM of pea in the pea/wheat intercrop was higher in 2009 but was lower in 2010 and 2011 than in the pea pure stand (Table 6).

Number of root nodules per plant among years was ranked as follows: 2011 > 2009 > 2010. Crop stand and treatment did not affect the number of root nodules per plant (Table 2).

Current research indicates that there are significant interactions between legumes and non-legume crops in intercrop-ping, affecting above-ground biomass production, formation of yield components, plant height, and stem and leaf characteristics (Zając et al., 2013; Neugschwandtner and Kaul, 2014; Klimek-Kopyra et al., 2020) and the uptake and use efficiency of macro- and micronutrients (Neugschwandtner and Kaul, 2015, 2016a, 2016b; Zając et al., 2018). Under Central European conditions, grain and bio-mass yield advantages have been shown for intercrops of naked oat/faba bean (Klimek-Kopyra et al., 2015b) and wheat/rapeseed (Ebrahimi et al., 2017). Klimek-Kopyra et al. (2018b) also indicated a negative effect of intercropping on selected physiological parameters with a significant reduction in the green area index (GAI) and the normalized difference vegetation index (NDVI) values in pea/linseed intercrops. Zając et al. (2013) noticed that mutual aggressiveness between species is higher in less favorable weather conditions when the development of one species is overly favored.

Competition among crops not only affects yield structure but may also affect root parameters such as root length, root distribution, and root density (Craine and Dybzinski, 2013). Intra-specific belowground competition of roots between species was observed in our studies. The MRD and RDM of pea tended to be higher in pure pea and pea/linseed intercrops compared to pea/wheat intercrops in 2 of the 3 years. Furthermore, soil type is affecting the root parameters of pea grown in pure or in intercrops (Klimek-Kopyra et al., 2015a). Jensen et al. (2007) revealed that root systems demonstrate a high level of plasticity in response to the local heterogeneity of soil profile and plant density. These findings were supported by Klimek-Kopyra et al. (2015a) who showed that environment strongly determines the level of interspecies competition: legumes grown on a rich soil competed better with linseed than under pure soil conditions, where species grown in intercrops had generally lower values of RLD, MRD, and RDM. Also, our findings confirm that in unfavorable weather conditions, the cultivation of pea/wheat intercrop was less valuable compared to pure pea stand.

Zając et al. (2012) analyzed morphological–developmental reactions of pea in pure stand to the application of seed inoculation of Nitragina and application of Nitragina in combination with foliar micronutrient fertilization of Photrel. They found both to be effective for mitigating unfavorable weather conditions, proving that the pre-inoculation of pea with Nitragina resulted in an increase in selected plant parameters and biomass yield. Pea seed inoculation in combination with foliar fertilization also increased the number of nodes per plant, which was particularly evident in a year with climatic conditions less favorable for plant growth. As a result, Zając et al. (2012) concluded that the highest seed yield per single plant of pea was achieved as an effect of the combined treatments application. Our results showed an increase in RDM in selected years and crop stands with a combined application of treatments, but no differences were observed compared to single treatments of Nitragina or Photrel for RLD, MRD, and the number of root nodules per plant. Photrel showed a higher RLD of the root fraction 0.1–0.5 mm compared to Nitragina. Klimek-Kopyra et al. (2018b) found that the use of Nitragina and Photrel improved the yield and yield structure of species in intercrops of pea/linseed and pea/wheat under unfavorable weather conditions. Nitragina significantly increased the number of seeds per pod and seeds per stem of pea, whereas Nitragina + Photrel decreased straw weight and spike length for wheat intercropped with pea.

The aim of our study was to assess the impact of the Rhizobium inoculant Nitragina and the micronutrient fertilizer Photrel on root parameters depending on crop stands. Our findings revealed that Photrel and the combination of Nitragina and Photrel had an impact on the root system of pea. Klimek-Kopyra et al. (2015a) observed that, for pea grown in intercrops, there were as much as 70% of pea roots under drought conditions in the root fractions below 0.5 mm. In this study, and also in general, RLD did not differ among years, whereas foliar fertilization with Photrel increased the RLD of pea in the root fractions of 0.1–0.5 mm. In addition, we showed that the RLD differed between crop stands, with pea having a lower RLD in the root fractions of 0.1–1 mm when grown as pure crop stand compared to an intercrop with linseed.

The number of root nodules per plant differed among years but was not affected by crop stand and treatment. Farzaneh et al. (2009) also reported a year effect on root nodule number. The missing increase in root nodules per plant with Rhizobium inoculation indicated that rhizobia strains suitable for pea infection were present in the soil (cf. Farzaneh et al., 2009). Also the missing effect of the foliar fertilizer Photrel, which contains several micronutrients, might be explained by the high content of available nutrients in the experimental soil. Fertilization with sulfur can increase both the number and dry matter weight of root nodules of pea (Zhao et al., 1999; Scherer et al., 2006). Crop stand did not affect the number of root nodules per plant, whereas Zhao et al. (2020) reported that the number of root nodules per plant of pea increased with increasing density of intercropped maize.

The application of Rhizobium inoculation (Nitragina) or foliar fertilization (Photrel) and intercropping can affect root parameters of pea. Intercropping of pea with linseed resulted in a higher RDL of pea in the root fractions of 0.1–1 mm compared to pure pea and pea/wheat. Also, MRD was higher and RDM tended to be higher in pea/linseed than in pea/wheat. Foliar fertilization with Photrel resulted in a higher RLD in the root fractions of 0.1–0.5 mm than with Rhizobium inoculation using Nitragina. MRD was not affected by the treatments, but a combination of Nitragina + Photrel increased RDM in 1 year. The number of root nodules per plant was not affected by crop stands or treatments but varied among years. The same observation has been made for MRD and RDM but not for RLD. Intercropping of pea with linseed and the application of a foliar fertilizer might be a strategy to improve pea root characteristics.