Nitrogen and phosphorous are critical determinants of plant growth and productivity, and both plant growth and root morphology are important parameters for evaluating the effects of supplied nutrients. Previous work has shown that the growth of Acer mono seedlings is retarded under nursery conditions; we applied different levels of N (0, 5, 10, and 15 g plant-1) and P (0, 4, 6 and 8 g plant-1) fertilizer to investigate the effects of fertilization on the growth and root morphology of four-year-old seedlings in the field. Our results indicated that both N and P application significantly affected plant height, root collar diameter, chlorophyll content, and root morphology. Among the nutrient levels, 10 g N and 8 g P were found to yield maximum growth, and the maximum values of plant height, root collar diameter, chlorophyll content, and root morphology were obtained when 10 g N and 8 g P were used together. Therefore, the present study demonstrates that optimum levels of N and P can be used to improve seedling health and growth during the nursery period.
lass="Chemical">Nitrogen alass="Chemical">nd lass="Chemical">n class="Chemical">phosphorous are critical determinants of plant growth and productivity, and both plant growth and root morphology are important parameters for evaluating the effects of supplied nutrients. Previous work has shown that the growth of Acer mono seedlings is retarded under nursery conditions; we applied different levels of N (0, 5, 10, and 15 g plant-1) and P (0, 4, 6 and 8 g plant-1) fertilizer to investigate the effects of fertilization on the growth and root morphology of four-year-old seedlings in the field. Our results indicated that both N and P application significantly affected plant height, root collar diameter, chlorophyll content, and root morphology. Among the nutrient levels, 10 g N and 8 g P were found to yield maximum growth, and the maximum values of plant height, root collar diameter, chlorophyll content, and root morphology were obtained when 10 g N and 8 g P were used together. Therefore, the present study demonstrates that optimum levels of N and P can be used to improve seedling health and growth during the nursery period.
The aesthetic and economic values of lass="Species">Acer mono Maxim L. (Aceraceae) make it alass="Chemical">n importalass="Chemical">nt species for afforestatiolass="Chemical">n, gardelass="Chemical">nilass="Chemical">ng, alass="Chemical">nd road plalass="Chemical">ntilass="Chemical">ngs. The species is widely distributed ilass="Chemical">n the Yalass="Chemical">ngtze River basilass="Chemical">n of lass="Chemical">north alass="Chemical">nd lass="Chemical">northeast Chilass="Chemical">na [1]. Nursery practices, such as sowilass="Chemical">ng, seedbed delass="Chemical">nsity, prulass="Chemical">nilass="Chemical">ng, alass="Chemical">nd fertilizatiolass="Chemical">n, are usually stalass="Chemical">ndardized for ilass="Chemical">ndividual plalass="Chemical">nt species, ilass="Chemical">n order to produce high-quality seedlilass="Chemical">ngs [2]. Fertilizer applicatiolass="Chemical">n is widely used ilass="Chemical">n lass="Chemical">nurseries to improve plalass="Chemical">nt vigor alass="Chemical">nd productivity [3]; however, fertilizatiolass="Chemical">n calass="Chemical">n improve plalass="Chemical">nt growth by either ilass="Chemical">ncreasilass="Chemical">ng soil resources or by elass="Chemical">nhalass="Chemical">ncilass="Chemical">ng the ability of seedlilass="Chemical">ngs to garlass="Chemical">ner resources [4] by modifyilass="Chemical">ng soil pH [5]. As a result, plalass="Chemical">nts ilass="Chemical">ncrease their rate of photosylass="Chemical">nthesis, stem diameter, height, basal area, alass="Chemical">nd volume [6]. Deciduous alass="Chemical">nd evergreelass="Chemical">n hardwood species have differelass="Chemical">nt lass="Chemical">nutrielass="Chemical">nt requiremelass="Chemical">nts, alass="Chemical">nd deciduous species have beelass="Chemical">n reported to require 50% more N thalass="Chemical">n lass="Chemical">n class="Species">conifers, such as pines [7]. Therefore, the production of deciduous and pine seedlings have different nutrition and management requirements [8].
Of the necessary nutritional elements, N is required in the largest quantities, and its availability and internal concentration affect the partitioning of biomass between roots and shoots [9]. The amount and timing of N application can also alter plant morphology, nutrient availability, and net photosynthesis [10]. For example, Harper [11] reported that N supplementation is required to maximize seedling biomass during initialnursery stages of growth, even for some legume species, and Costa et al. [12] reported that root length and root surface area were increased under intermediate Nlevels and that root growth was reduced under both higher and lower fertilizationlevels. However, high N availability and its concomitant affect root and shoot biomass production [13]. n class="Chemical">Phosphorus is colass="Chemical">nsidered a primary lass="Chemical">nutrielass="Chemical">nt for plalass="Chemical">nt growth [14] alass="Chemical">nd is lass="Chemical">needed to sustailass="Chemical">n optimum plalass="Chemical">nt productiolass="Chemical">n alass="Chemical">nd quality [15]. The elemelass="Chemical">nt is esselass="Chemical">ntial for cell divisiolass="Chemical">n, reproductiolass="Chemical">n, alass="Chemical">nd plalass="Chemical">nt metabolism; moreover, its role is related to the acquisitiolass="Chemical">n, storage, alass="Chemical">nd use of elass="Chemical">nergy [16]. Ilass="Chemical">n additiolass="Chemical">n, P plays alass="Chemical">n importalass="Chemical">nt role ilass="Chemical">n lateral root morphology alass="Chemical">nd root bralass="Chemical">nchilass="Chemical">ng [17] alass="Chemical">nd ilass="Chemical">nfluelass="Chemical">nces lass="Chemical">not olass="Chemical">nly root developmelass="Chemical">nt, but also the availability of lass="Chemical">nutrielass="Chemical">nts [18]. Therefore, plalass="Chemical">nts have developed various strategies for obtailass="Chemical">nilass="Chemical">ng optimum P from soils, ilass="Chemical">ncludilass="Chemical">ng ilass="Chemical">ncreases ilass="Chemical">n root surface area, specific root lelass="Chemical">ngth (SRL), alass="Chemical">nd root-shoot ratio [19,20].
Both lass="Chemical">N and P are importalass="Chemical">nt lass="Chemical">nutrielass="Chemical">nts for ecosystem structure, processes, alass="Chemical">nd fulass="Chemical">nctiolass="Chemical">n, silass="Chemical">nce their availability limits the productiolass="Chemical">n of plalass="Chemical">nt biomass alass="Chemical">nd growth [21]. For example, the combilass="Chemical">ned applicatiolass="Chemical">n of lass="Chemical">n class="Chemical">N and P increases root surface area, root length, and root-shoot mass [22], and in Arabidopsis plant species, different nutrient levels have been shown to influence both root length and branching plasticity [23].
In the last few decades, the application of fertilizer in forest nurseries has attracted increasing attention throughout the world, as a result of increased demand for fiber, wood [24], and n class="Chemical">CO2 offsets [25]. These demalass="Chemical">nds calass="Chemical">n be satisfied through the productiolass="Chemical">n of healthy seedlilass="Chemical">ngs, which would ultimately ilass="Chemical">ncrease plalass="Chemical">nt biomass productiolass="Chemical">n; however, this requires the proper diaglass="Chemical">nosis of specific limitilass="Chemical">ng factors [26]. Ilass="Chemical">n fact, the amoulass="Chemical">nts of fertilizers used ilass="Chemical">n forest seedlilass="Chemical">ng productiolass="Chemical">n are relatively lower thalass="Chemical">n the amoulass="Chemical">nts used ilass="Chemical">n agriculture plalass="Chemical">nt lass="Chemical">nurseries [27]. Malass="Chemical">ny factors ilass="Chemical">nfluelass="Chemical">nce the effectivelass="Chemical">ness of lass="Chemical">nutrielass="Chemical">nt applicatiolass="Chemical">n olass="Chemical">n seedlilass="Chemical">ng growth. Ilass="Chemical">n particular, fertilizer type alass="Chemical">nd amoulass="Chemical">nt affect the growth of plalass="Chemical">nt seedlilass="Chemical">ngs [28].
Since growth of lass="Species">A. mono seedlilass="Chemical">ngs is reportedly lass="Chemical">n class="Disease">retarded under nursery conditions [29], species-specific combinations of N and P may be needed to ensure healthy seedling growth. Therefore, the present study was designed to determine the effects of N and P on A. mono seedling growth, with a specific focus on root morphology (i.e., root length, root diameter, SRL), and to establish a standard for N and P application in the production of A. mono seedlings.
Materials and methods
Study site and soil collection
This experiment was conducted at Maoershan Forest Research Station (127°′‒127°′E, 45°23′‒45°26′ N, 390 m above sea level) of Northeast Forestry University, in Heilongjiang, China. The site has a cold, continental monsoon climate, with an average annual air temperature of 2.8°C. Average temperatures in January and July are -19.6°C and 20.9°C, respectively, and the annual average humidity, annual precipitation, and annual evaporation are 70%, 723.8 mm, and 1094 mm, respectively. The frost-free period usually lasts from 120 to 140 d, and the soil is mostly Hap-Boric n class="Chemical">Luvisol [30], which was described previously by Walass="Chemical">ng et al. [31].
Before starting experiment ten soil samples were collected from the experimental field at 0–20 cm soil depth and were thoroughly mixed to make a representative composite soil sample that had a totalN, P, and K content of 3.98 g kg-1, 820.8 mg kg-1, and 14 g kg-1, respectively, and an available N, P, and K content of 4, 7.23, and 176 mg kg-1. The totalN was determined by an elemental analyzer (vario MACRO cube; Elementar, Hanau, Germany). The available n class="Chemical">nitrogen was measured by alkali solutiolass="Chemical">n diffusiolass="Chemical">n method alass="Chemical">nd total P alass="Chemical">nd available P by flow ilass="Chemical">njectiolass="Chemical">n alass="Chemical">nalyzer method (Seal Autoalass="Chemical">nalyzer 3, Seal Alass="Chemical">nalytical, Norderstedt, Germalass="Chemical">ny). Total K alass="Chemical">nd available K were determilass="Chemical">ned by the flame photometer (FP640, Shalass="Chemical">nghai, Chilass="Chemical">na). The soil test was performed ilass="Chemical">n State Forestry Admilass="Chemical">nistratiolass="Chemical">n Key Laboratory of forest resources developmelass="Chemical">nt ilass="Chemical">n Northeast Chilass="Chemical">na.
Plant materials and fertilization treatments
The seeds of n class="Species">Acer mono Maxim L. were collected from Chalass="Chemical">ngbai Moulass="Chemical">ntailass="Chemical">n Jililass="Chemical">n provilass="Chemical">nce, Chilass="Chemical">na. Seeds were exposed to cold stratificatiolass="Chemical">n for six molass="Chemical">nths from November to April. Ilass="Chemical">n early May, stratified seeds were sowlass="Chemical">n olass="Chemical">nce alass="Chemical">n opelass="Chemical">n field lass="Chemical">nursery. After germilass="Chemical">natiolass="Chemical">n, seedlilass="Chemical">ngs were lass="Chemical">not treated with alass="Chemical">ny kilass="Chemical">nd of fertilizatiolass="Chemical">ns ulass="Chemical">ntil startilass="Chemical">ng the experimelass="Chemical">nt. After four-year ulass="Chemical">niform size, seedlilass="Chemical">ngs were selected ilass="Chemical">n the same lass="Chemical">nursery, with plalass="Chemical">nt-to-plalass="Chemical">nt alass="Chemical">nd row-to-row distalass="Chemical">nces kept at 20 alass="Chemical">nd 30 cm, respectively. Each seedlilass="Chemical">ng was treated with olass="Chemical">ne of four levels of N (0, 5, 10, or 15 g) alass="Chemical">nd olass="Chemical">ne of four levels of P (0, 4, 6, or 8 g). Telass="Chemical">n plalass="Chemical">nts were ilass="Chemical">ncluded ilass="Chemical">n each treatmelass="Chemical">nt, alass="Chemical">nd each treatmelass="Chemical">nt was replicated three times (total lass="Chemical">n = 480). We applied N ilass="Chemical">n two split doses durilass="Chemical">ng May alass="Chemical">nd July, alass="Chemical">nd P was applied olass="Chemical">nce as a basal dose ilass="Chemical">n May. Stalass="Chemical">ndard cultural practices were performed durilass="Chemical">ng the experimelass="Chemical">nt (i.e., weedilass="Chemical">ng, hoeilass="Chemical">ng, irrigatiolass="Chemical">n, etc.), ilass="Chemical">n order to produce healthy seedlilass="Chemical">ngs.
Plant growth measurement
Plant height and root collar diameter were recorded for all selected seedlings before fertilizer application (May) and again at the end of the experiment in (November), in order to quantify plant growth during the experimental period. A digital venire caliper was used for the root collar diameter measurements (Shanghai measuring and cutting tools work. Co. Ltd).
Estimation of chlorophyll contents and carotene
The lass="Chemical">chlorophylls a and b alass="Chemical">nd lass="Chemical">n class="Chemical">carotene were measured, as described by Arnon [32]. Briefly, fresh leaves (0.2 g) were ground in a mortar with a small amount of quartz sand and calcium carbonate powder and 2–3 mL 95% ethanol. Absorbance of the supernatant was measured at 665, 649, and 470 nm using a spectrophotometer (Hitachi-U2001; Hitachi, Tokyo, Japan), and concentrations of chlorophylls a (Ca) and b (Cb) and carotene (Cx-c) were calculated using the following formulae:
where A indicates absorption at wavelength i, V is the volume of the extract, and W is the weight of fresh leaf tissue (g).
Root morphology measurement
Root samples were carefully taken from single plant in each treatment group during harvesting, using the procedure described by Guo et al. [30]. The root samples were kept in an icebox and transported to the laboratory within 4 h of collection. The individual samples were cleaned with de-lass="Disease">ionizedlass="Chemical">n class="Chemical">water to remove residual soil particles and stored in a refrigerator. The root samples of each treatment were then divided into different branch orders, as described by Pregitzer et al. [33], i.e., the distal branch order as the first order. The separated samples were then scanned with an Expression 10000XL 1.0 scanner (dpi = 400; Epson Telford, Ltd., Telford, UK), and the images were analyzed using the WinRHIZO (Pro2004b) software (Instruments Regent Co., Ville de Québec, QC, Canada) to determine the average root diameter (ARD) and total root length (TRL). Finally, the roots were oven dried to constant mass at 65°C, in order to determine dry mass, and SRL was calculated as the TRL from each root order divided by the corresponding dry mass [34].
Statistical analysis
The experiment was conducted with a randomized complete block design, with split plot arrangements, in order to test the effects of lass="Chemical">N and P fertilizer alass="Chemical">nd their ilass="Chemical">nteractiolass="Chemical">n olass="Chemical">n seedlilass="Chemical">ng growth, lass="Chemical">n class="Chemical">chlorophyll and carotene content, and root morphology. The data were analyzed using Statistix 8.1 software (Analytical Software, Tallahassee, FL, USA), and the experimental treatments were randomized and repeated three times, in order to reduce any variation caused by soil heterogeneity [35]. Furthermore, N treatment was used as the main block, whereas P treatment was used as the sub-block.
Results
Effect of N and P on plant growth
Seedlings treated with N fertilizer exhibited significantly greater plant height and root diameter than untreated (i.e., 0 g N) controls (P<0.05; S1 Table, Fig 1), and values for both parameters were highest in seedlings treated with 10 g N, followed by those for seedlings treated with 15 and 5 g N, respectively. Similarly, seedlings treated with P fertilizer exhibited significantly greater plant height and root collar diameter than untreated (i.e., 0 g P) controls, and values for both parameters were highest in seedlings treated with 8 g P, followed by those treated with 6 and 4 g, respectively. We also observed a significant interaction effect of n class="Chemical">N and P for both plalass="Chemical">nt height alass="Chemical">nd root collar diameter alass="Chemical">nd foulass="Chemical">nd that values for both plalass="Chemical">nt height alass="Chemical">nd root diameter were highest ilass="Chemical">n seedlilass="Chemical">ngs treated with 10 g N alass="Chemical">nd 8 g P (Fig 1).
Fig 1
Combined and single effect of NP on plant height and collar diameter of Acer mono seedling.
Different letters show the level of significance. Abbreviation N, Nitrogen; P, Phosphorous; NxP; Interaction.
Combined and single effect of NP on plant height and collar diameter of Acer mono seedling.
Different letters show the level of significance. Abbreviation N, n class="Chemical">Nitrogen; P, lass="Chemical">n class="Chemical">Phosphorous; NxP; Interaction.
Effect of N and P on chlorophyll and carotene contents
Seedlings treated with N fertilizer exhibited significantly greater levels of lass="Chemical">chlorophylls a and b alass="Chemical">nd lass="Chemical">n class="Chemical">carotene (P<0.05; S1 Table, Fig 2A and 2B), and values for all three parameters were greatest in seedlings treated with 10 g N. Similarly, seedlings treated with 8 g P fertilizer exhibited significantly greater levels of chlorophyll and carotene contents, whereas the values of seedlings treated with 6 and 4 g P were not significantly different than the values of plants treated with 0 g P. We also observed a significant interaction effect of N and P for levels of chlorophylls a and b and carotene and found that levels of all three were highest in seedlings treated with 10 g N and 8 g P (Fig 2A and 2B).
Fig 2
a-b. Combined and single effect of NP on chlorophylls a, b and carotene of Acer mono seedling. Different letters show the level of significance. Abbreviation N, Nitrogen; P, Phosphorous; NxP, Interaction.
a-b. Combined and single effect of lass="Chemical">NP olass="Chemical">n lass="Chemical">n class="Chemical">chlorophylls a, b and carotene of Acer mono seedling. Different letters show the level of significance. Abbreviation N, Nitrogen; P, Phosphorous; NxP, Interaction.
Effect of N and P on root morphology
The first, second, and third root orders of seedlings treated with N fertilizer exhibited significantly greater TRL, ARD, and SRL values than those of untreated (i.e., 0 g N) seedlings (P<0.05; S1 Table, Fig 3A–3C), and for all root orders, values for the three parameters were highest in seedlings treated with 10 g N. The TRL, ARD, and SRL values varied among each of the root orders, depending onNlevel. Furthermore, both TRL and SRL decreased with increasing in root order but increased with increasing Nlevel; whereas ARD increased with increasing root order and increased further with increasing Nlevel.
Fig 3
a-c. Combined and single effect of NP on root length, root diameter, and specific root length (SRL) of first, second and third order root of Acer mono seedling. Interaction values of specific root length (SRL) of second and third order root were non significant. Different letters show the level of significance. Abbreviation N, Nitrogen; P, Phosphorous; NxP, Interaction; RL, root length.
a-c. Combined and single effect of lass="Chemical">NP olass="Chemical">n root lelass="Chemical">ngth, root diameter, alass="Chemical">nd specific root lelass="Chemical">ngth (SRL) of first, secolass="Chemical">nd alass="Chemical">nd third order root of lass="Chemical">n class="Species">Acer mono seedling. Interaction values of specific root length (SRL) of second and third order root were non significant. Different letters show the level of significance. Abbreviation N, Nitrogen; P, Phosphorous; NxP, Interaction; RL, root length.
Similarly, the first, second, and third root orders of seedlings treated with P fertilizer exhibited significantly greater TRL, ARD, and SRL values than those of untreated (i.e., 0 g P) seedlings (P<0.05; S1 Table, Fig 3A–3C). The root morphology values (i.e., TRL, ARD, and SRL) increases with increasing Plevel, and for all root orders, values for the three parameters were highest in seedlings treated with 8 g P. Furthermore, both TRL and SRL decreased with increasing in root order but increased with increasing Plevel; whereas ARD increased with increasing root order and increased further with increasing Plevel.We also observed a significant interaction effect of n class="Chemical">N and P for TRL alass="Chemical">nd ARD but foulass="Chemical">nd that ilass="Chemical">nteractiolass="Chemical">n effect was olass="Chemical">nly lass="Chemical">nolass="Chemical">n siglass="Chemical">nificalass="Chemical">nt for the SRL values of secolass="Chemical">nd alass="Chemical">nd third order roots (P < 0.005; S1 Table). The TRL alass="Chemical">nd ARD values of all three orders, as well as the SRL values of first order roots, were highest ilass="Chemical">n seedlilass="Chemical">ngs treated with 10 g N alass="Chemical">nd 8 g P (Fig 3A–3C).
Discussion
Fertilization and plant growth
Previous studies have shown that N supplementation can significantly affect the shoot morphology and nutritional status of nursery seedlings [36,37]. The growth-promoting effect of N (up to the optimum level) increases cytokinin production, which subsequently affects cell wall elasticity [38], number of meristematic cells, and cell growth [39]. In addition, N fertilization also increases seedling height and root collar diameter [40,41]. The present study demonstrated that N supplementation can increase growth parameters to a certain extent but has a negative effect at higher levels. Researchers have reported both positive and negative effects of fertilizer application on subsequent seedling growth and survival [42,43].Meanwhile, P fertilizer application is necessary to ensure optimum plant production and quality [15], as well as for the acquisition, storage, and use of energy [16]. The present study demonstrated the positive relationship between Plevel and plant growth, which is supported by previous findings that P application increases plant height and root collar diameter [44], as well as basal stem diameter [45], and that P application has a positive effect on the growth of various species, including lass="Species">Tectona grandis [46], Casuarilass="Chemical">na spp. [47], lass="Chemical">n class="Species">Dalbergia sissoo [48], and Tecoma grandis [49]. The growth-promoting role of P application has been reported previously [50,51].
Furthermore, previous studies have also shown that root and shoot morphology are affected by the level and form of the fertilizer applied [52] and that the application of lass="Chemical">N and P fertilizer, either alolass="Chemical">ne or ilass="Chemical">n combilass="Chemical">natiolass="Chemical">n, siglass="Chemical">nificalass="Chemical">ntly ilass="Chemical">ncreases the stem growth of hybrid poplar seedlilass="Chemical">ngs [53], alass="Chemical">nd combilass="Chemical">ned P alass="Chemical">nd N fertilizatiolass="Chemical">n has beelass="Chemical">n showlass="Chemical">n to promote the growth of youlass="Chemical">ng lass="Chemical">n class="Species">Eucalyptus grandis and birch (Betula pubescens) seedlings [54]. Therefore, the increased growth observed in A. mono seedlings treated with the optimal P level might have resulted from a favorable balance of nutrients (i.e., N and P).
Fertilization and chlorophyll and carotene contents
The addition of N promotes the formation of active photosynthetic pigments by increasing the amounts of stromal and thylakoid proteins in leaves [55,56], as well as by increasing the formation of chloroplasts during leaf growth [57]. Although N is the most important elemental factor in lass="Chemical">chlorophyll biosylass="Chemical">nthesis, N applicatiolass="Chemical">n is also capable of producilass="Chemical">ng lass="Chemical">negative effects [58]. For example, excess N has beelass="Chemical">n showlass="Chemical">n to shortelass="Chemical">n the life spalass="Chemical">n of leaves, ilass="Chemical">ncrease their susceptibility to disease [59,60]. lass="Chemical">n class="Chemical">Chlorophyll and carotenoid synthesis are dependent upon mineral nutrition [61]. It might be due to the optimum availability of N, which plays a vital role in cell division and the formation of active photosynthetic pigments, including chlorophyll. Green pigments in leaves depend on P concentration, since it facilitates the plant for stability in unfavorable condition [62]. However, the facilitation of biochemical characteristics [51] and biosynthesis of pigment molecules depends on the uptake of optimal P levels [63]. In apricot seedlings, optimal P conditions have been shown to increase total chlorophyll content and plant growth [64]. Previous studies have also reported that P application increases the biomass and carotenoid production of a blue-green alga (Spirulina platensis) [65], whereas higher P concentrations were reported to reduce the chlorophyll content of the blue-green alga (Azolla pinnata) [66]. However, P deficiency decreases protein and chlorophyll content [67].
Fertilization and root morphology
In the present study, optimum N fertilization had a favorable impact on root growth, which is supported by previous findings that N availability has significant effects on root biomass, production, and mortality [68]; root elongation [17]; and higher root-order development and branching [69]. The application of N fertilizer can also affect lass="Chemical">water use efficielass="Chemical">ncy by ilass="Chemical">nfluelass="Chemical">ncilass="Chemical">ng root growth alass="Chemical">nd distributiolass="Chemical">n ilass="Chemical">n the soil [70], alass="Chemical">nd optimal N fertilizatiolass="Chemical">n has beelass="Chemical">n showlass="Chemical">n to elass="Chemical">nhalass="Chemical">nce root lelass="Chemical">ngth alass="Chemical">nd diameter [12], whereas higher alass="Chemical">nd lower N levels have beelass="Chemical">n showlass="Chemical">n to reduce root growth alass="Chemical">nd biomass [71,72,73]. Ilass="Chemical">n additiolass="Chemical">n, N fertilizatiolass="Chemical">n siglass="Chemical">nificalass="Chemical">ntly ilass="Chemical">ncreases the diameter of lass="Chemical">n class="Species">Larix gmelinii root tips [74] and the growth, root length, and root diameter of Pongamia pinnata seedlings [75].
Furthermore, thin-root species have been shown to exhibit greater plasticity in response to fertilization, both in root growth rate and reducing mycorrhizal colonization [76]. Greater plasticity was also observed in thin-root species than in thick-root species in a subtropical forest [77]. High Nlevels have been shown to increase fine-root length and surface area in lass="Species">loblolly (lass="Chemical">n class="Species">Pinus taeda L.) and ponderosa pine Pinus ponderosa L. seedlings [78], which may potentially enhance nutrient and water acquisition from the soil. Plants exposed to high nutrient deficiencies exhibited a progressive reduction in TRL [23]. Therefore, N deficiency can result in a remarkable decrease in length of both first- and second-order lateral roots [79], a relationship that has been widely confirmed in both annual [71] and perennial plants [72].
In contrast, N availability has no significant effect on fine root biomass, SRL, ARD, or TRL [33,80]. For example, N fertilization increased the ARD of larch (n class="Species">Larix gmelinii) but had lass="Chemical">no effect olass="Chemical">n TRL [81] alass="Chemical">nd was showlass="Chemical">n to elass="Chemical">nhalass="Chemical">nce all three parameters (i.e., SRL, ARD, alass="Chemical">nd TRL) ilass="Chemical">n other tree species [82]. Furthermore, N is likely to have complemelass="Chemical">ntary effects olass="Chemical">n root morphology developmelass="Chemical">nt because it stimulates root elolass="Chemical">ngatiolass="Chemical">n [83]. The results of the preselass="Chemical">nt study are colass="Chemical">nfirmed by previous filass="Chemical">ndilass="Chemical">ngs that N fertilizatiolass="Chemical">n ilass="Chemical">ncreases SRL, up to a certailass="Chemical">n level [74,81], alass="Chemical">nd SRL has also beelass="Chemical">n associated with rapid root proliferatiolass="Chemical">n [84].
Previous studies have also shown that P can have a profound effect on root system morphology and that P generally stimulates root growth [85]. For example, P application significantly increases ARD and TRL in the perennialleguminous shrub lass="Species">Lespedeza davurica L. alass="Chemical">nd the perelass="Chemical">nlass="Chemical">nial herbaceous grass lass="Chemical">n class="Species">Bothriochloa ischaemum L. [20]. The present study also confirmed the results of Jin et al. [18], who reported that P application increases TRL and ARD. In most species, P-deficiency results in decreased ARD [86]; however, some species, such as Arabidopsis thaliana, develop larger roots in P-deficient conditions [87]. The SRL of fine roots varies, depending on nutrient availability [88]. Longer fine roots are more efficient in nutrient acquisition than shorter fine roots and allow plants to form larger root systems at a lower carbon cost than those of plants with shorter roots; whereas thicker roots may provide benefits in infertile or competitive environments [89]. Therefore, higher SRL values (i.e., thinner roots) indicate higher exploitation efficiency under intensive competition conditions, since plants with thinner roots are generally more competitive [90].
Conclusions
The present study demonstrates that the growth and root morphology of lass="Species">A. mono seedlilass="Chemical">ngs are siglass="Chemical">nificalass="Chemical">ntly affected by lass="Chemical">n class="Chemical">N and P fertilization. Untreated seedlings exhibited lower measures of plant height, root collar diameter, chlorophyll and carotene content, and several parameters of root morphology; whereas maximum values were achieved when seedlings were supplemented with optimal levels of N (10 g plant-1) and P (8 g plant-1). Therefore, the results of the present study suggest that optimal levels of N and P can be used to ensure the production of vigorous and healthy A. mono seedlings.
Summary of ANOVAs (F&P values) for the effect of fertilization on plant height, root collar diameter, chlorophyll and carotene content and root morphology of A. mono seedling.
Authors: Abdul Latif Khan; Sajjad Asaf; Raeid M M Abed; Yen Ning Chai; Ahmed N Al-Rawahi; Tapan Kumar Mohanta; Ahmed Al-Rawahi; Daniel P Schachtman; Ahmed Al-Harrasi Journal: Microorganisms Date: 2020-02-05
Authors: Henry O Awika; Amit K Mishra; Haramrit Gill; James DiPiazza; Carlos A Avila; Vijay Joshi Journal: Sci Rep Date: 2021-05-05 Impact factor: 4.379
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