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Cultivos Tropicales

Print version ISSN 0258-5936On-line version ISSN 1819-4087

cultrop vol.40 no.2 La Habana Apr.-June 2019  Epub June 01, 2019

 

Original article

Management of AMF inoculated precedent crops to mycorrhize efficiently sweet potato Ipomoea batatas (L.) Lam in succession

Alberto Espinosa-Cuéllar1 

Ramón Rivera-Espinosa2  * 

Luís Ruiz-Martínez1 

Ernesto Espinosa-Cuéllar1 

Yasmani Lago-Gato1 

1Instituto de Investigaciones de Viandas Tropicales (INIVIT), Santo Domingo, Villa Clara, Cuba

2Instituto Nacional de Ciencias Agrícolas (INCA), carretera San José-Tapaste, km 3½, Gaveta Postal 1, San José de las Lajas, Mayabeque, Cuba. CP 32 700

ABSTRACT

The use of inoculated precedent crops as via to mycorrhize efficiently crops in succession have a practice high value and overall in crops like sweet potato which its direct inoculation needs a high amount of inoculants, but in the carried out studies only 30 days between both crops have been evaluated. With the purpose of precise the effectiveness in superior intervals and its dependence of the precedent crop and sowing season 12 experiments were executed in Brown soil, using maize and two species of bean as precedent seeded in rainy and little rainy seasons and in all cases under irrigation. The precedents presented a high response to inoculation with Rhizophagus irregulare/INCAM11 in both seasons with high percentages of mycorrhizal colonization and spores in maize and rainy season. The duration of the permanence effect in the sweet potato yield depended of the precedent and it was similar in both seasons, with leguminous it was extended until 45 days, although the maize at to the 45 days was achieved between 76 to 81 % of effect and at to the 60 days disappear for both precedents. The highest yields of sweet potato were achieved with the planting in the rainy season and in the bean-sweet potato succession. The potentiality of the precedent crops inoculated to provoke the permanence effect cannot be defined through the simple comparison of the indicators of mycorrhizal functioning that are presented in them.

Key words: beans; corn; crops in succession; permanence of mycorrhizal inoculant

INTRODUCTION

Arbuscular mycorrhizal symbiosis occurs in about 80 % of the plant species in which the bulk of the economic crops are found 1. This brings benefits to plants and the environment associated with the increase in the intake of plants are widely recognized nutrients 2 and water 3 as well as to the effect of bioprotection against the attack of some radical and foliar pests 4. Also to the tolerance to heavy metals 5, as well as other eco-services among which are improvements in the aggregates of soil, participation in the Carbon and Nitrogen cycles, promoting greater ecosystem resilience 6,7 and the conscious management of arbuscular mycorrhizal fungi (AMF) is a challenge to optimize these benefits in agrosystems 8,9.

The works developed in Cuba in this regard are based on the inoculation of efficient strains of AMF of a general nature with the plant species and recommended based on the edaphic environment 10. In this way, integral nutrient supply systems are designed from an effective mycorrhization of the different crops, obtaining high yields and satisfactory nutritional status, with lower doses of fertilizers 8,11-13.

One of the aspects found when working with mycorrhization-dependent cultures has been the presence of a positive effect of permanence of the inoculant applied to different crops on the first crop in succession, although it disappears for subsequent crops 8,14. This effect has been linked either with the use of inoculated green manures 15,16 or with the management of inoculants in crop sequences 14, with replacement in both cases of the inoculation of the first crop in succession; although it has also been evaluated in inoculated perennial crops 11,17 to define the moment of reinoculation. The disappearance of the permanence effect has been fundamentally associated with competition with the resident mycorrhizae 18, with the characteristics of the crop, its agronomic management and the edaphic conditions 11,19.

Ipomea batata (L.) Lam is a culture with a significant response to inoculation, but even using the pathway of the cuttings 14 requires very high amounts of inoculants. The recently developed works in this crop 20 established that, using inoculated precedents common crops in Cuban agriculture (a grass such as corn and legumes such as common beans or Vigna unguicolata, sown in the rainy and dry season). In addition, to the benefits of inoculation on these precedents, permanence effects were found equally effective with both precedents and in both periods. In this way, it is not necessary to inoculate the sweet potato, regardless of the higher values of mycorrhizal spores reproduced by corn and by crops planted in the rainy season.

In the previous publications 14,16,20, the interval between the harvest of the inoculated precedents and the cultivation in succession was not more than 30 days. Therefore, it was necessary, in order to define the management of precedents inoculated at one time or another, to determine how intervals greater than 30 days influenced their effectiveness due to the practical importance of the reported results. Also the possible influence of the differences found in the reproduction of spores between the preceding and times in the duration of the effect and to the fact that both in the sequences of cultures 8,14 and in the inoculated perennial crops the positive response to inoculation gradually disappears in time 11,17.

MATERIALS AND METHODS

The research was carried out at the Tropical Food Research Institute (INIVIT) located at 22º 35' N, 80º 18' W and at 40 m a.s.l., in Santo Domingo municipality, Villa Clara province, Cuba, on Brown argenic soil moderately washed 21, also classified as Cambisol Eutric without carbonates 22, during 2015 and 2016.

The soil in the experimental area was homogeneous, without presenting significant differences (p<0.05) in the variables determined between the different experiments, years and planting seasons. Table 1 shows the average values corresponding to each planting season. The soil presented a neutral reaction, as well as low values of organic matter possibly associated with continuous cultivation. The phosphorus contents available were low and those of potassium were medium. Calcium and interchangeable magnesium showed high and typical values of these soils. The mycorrhizal spores were low, with approximate values to those reported in adjacent areas with similar soil type and management 12,14.

Table 1 Some initial properties of the soil in the experimental area and in both times of sowing of the previous ones. Sampling depth of 0-20 cm 

Sowing times of precedents pH Total nitrogen MO P 2 O 5 K 2 O AC Mg Na K Sp./ 50 g
H 2 O (g kg -1 ) mg kg -1 (cmol c kg -1 )
Rainy 7.12 1.60 20.02 20.20 21.44 30.20 4.29 0.37 0.61 41.5
Z 1-α * S ± 0.09 ± 0.07 ± 0.14 ± 0.14 ± 0.07 ± 0.38 ± 0.06 ± 0.02 ± 0.02 ± 0.64
little rainy 7.12 1.68 20.30 20.02 21.45 27.85 4.35 0.38 0.62 42.0
Z 1-α * S ± 0.10 ± 0.06 ± 0.16 ± 0.14 ± 0.06 ± 0.35 ± 0.06 ± 0.02 ± 0.02 ± 0.77

Z1-α * S = ± confidence interval (1-α = 0.05), where Z 1 = 1.96. Each value is averaged of 24 composite samples

Annual rainfall in 2015 and 2016 was 1211 and 1370 mm, slightly lower in 2015 to the historical average (1969-2016 period) of 1348 mm, although in the two years rainfall in the rainy season (May-October) reached 76 and 78 % of the annual total respectively, very similar to the historical percentage of 77 %. The average annual temperatures were 24.6 and 24.24 °C respectively, similar to the historical average of 24.3 °C, although in the rainy period they were 26.03 °C, higher at 3.8 °C those of the dry season.

The experiments carried out with the same previous cultures and times referred to in a previous publication 20. In the rainy season (Table 2) they were corn (Zea mays L./var.MC-4) and bean vigna (Vigna unguiculata (L.) Walp./var. Guariba); in the case of sowing of January the same maize variety were used and the common bean (Phaseolus vulgaris/var. BAT-304). Following the harvest of the previous crops and at three different times (30, 45 and 60 days), the sweet potato crop (Ipomea batatas (L) Lam/var. 'CEMSA 78-354') was planted.

As the previous cultures were different in the ages and also do not have the same extension of their biological cycle, the experiments did not include the cultures, nor the time as factors and were develop as independent experiments. For each crop and at each time, three experiments were carried out, each with the same sowing date of the previous one and varying the periods between the harvest of this and the plantation of the sweet potato to (Table 2) for a total of 12 experiments . The treatments (Table 3) and the design used in each experiment were similar: randomized blocks of four treatments, which included both the previous crop stage and the sweet potato stage (factor A) and repeated for two years (factor B), with 4 x 2 factorial arrangement and with four replicas.

The planting frames used were similar to those reported in a previous publication 20, 0.9 x 0.3 m for corn, 0.7 x 0.2 m for common beans and 0.9 x 0.1 m for the vigna. In the sweet potato cultivation, the plantation frame was 0.9 x 0.23 m for the successions started in the rainy season and 0.9 x 0.3 m for the others.

The plots consisted of five 6-meter long furrows, except for beans, which had six furrows. In each experiment, the previous cultures and the sweet potato of each treatment were always located in the same plots.

Table 2 Dates of sowing and harvesting of crops in different successions in both years 

Season1 Culture preceding Sowing Harvest Sweet potato plantation Harvest
30 days 45 days 60 days
Rainy Corn Year 1 5/11/2015 9/12/2015 10/15/15 10/30/15 11/16/15 4 months
Vigna year 1 6/10/2015 9/12/2015 10/15/15 10/30/15 11/16/15 4 months
Corn Year 2 4/11/2016 8/12/2016 9/15/16 9/30/16 10/15/16 4 months
Vigna year 2 5/10/2016 8/12/2016 9/15/16 9/30/16 10/15/16 4 months
Little rainy Corn Year 1 1/15/2015 5/16/2015 6/18/15 3/7/15 7/18/15 4 months
Bean year 1 2/9/2015 5/16/2015 6/18/15 3/7/15 7/18/15 4 months
Corn Year 2 1/16/2016 5/18/2016 6/18/16 4/7/16 7/19/16 4 months
Bean year 2 02/10/2016 5/18/2016 6/18/16 4/7/16 7/19/16 4 months

1The epoch is defined by the beginning of the succession

Table 3 Treatments studied in each succession and for each of the periods between harvest of the precedent and plantation of the sweet potato 

Treatments Previous crop stage Sweet potato cultivation stage
1 50 % NPK + AMF 50% NP 25% K + AMF
2 50 % NPK + AMF 50% NP 25% K
3 50 % NPK 50% NP 25% K
4 100 % NPK 100% NPK

Precedents in the rainy season: corn (var. MC-4) and vigna (var. Guariba). Precedents in the dry season: corn (var. MC-4) and common beans (var. BAT-304). Sweet potato variety 'CEMSA 78-354'. HMA: R. irregular / INCAM 11.

Inoculating and mycorrhizal inoculation

The inoculant was prepared at the National Institute of Agricultural Sciences (INCA), Mayabeque, Cuba, based on Rhizophagus irregular23/INCAM 11 and using Urochloa decumbens (Hochst. Ex A. Rich) As a host plant. This strain is recommended as an efficient strain for this edaphic condition 10. The inoculant had 30 g-1 spores. Inoculation carried out using to seed coatings. In the case of grains was done with an equivalent amount of inoculant to 8 % by weight of the same 24, at doses of 2 and 4 kg ha-1 for maize. Respectively bean and sweet potato was coated the lower third of cuttings with a mixture of 1 kg of inoculant in 5 liters of water and applying 35 kg ha-1 (14.

Fertilization and cultural care

In the case of fertilization for high yields (100 % NPK) they correspond to those of the respective Instructions 25-28, and they were 90, 130, 170 kg ha-1 of N, P2O5 and K2O for corn. 80, 60, 90 kg ha-1 for beans and vigna, as well as 120, 100, 300 kg ha-1 for N, P2O5 and K2O, for sweet potato. The doses applied to obtain optimal mycorrhizal functioning in the different inoculated precedents were 50 % NPK 24 and in the case of sweet potato 50 % NP 25 % K 29-31 were applied. Work on the previous crops and sweet potatoes, including irrigation, was carried out according to the Technical Instructions for these crops 25-28. It should be noted that when the previous crops were harvested, the soil was prepared in a conventional manner, with a sequence of rotational work (plowing), two harrows, and finally furrowed, adjusting the times between workings at the three intervals studied.

Evaluations

Soil analysis

Composite samples of soils were taken in the depth of 0-20 cm in each of the replicas of the experiments and at the beginning of these. The determinations made were established in the soil and plant tissue laboratory of the National Institute of Agricultural Sciences 32 and it was consisted of pH in KCl and H2O, with a soil-solution ratio of 1:2.5; total nitrogen (Nt) by the micro-Kjeldahl method. Also, organic matter according to Walkley-Black; P2O5 and K2O with extraction with solution of (NH4)2CO3 with pH 9 and the exchangeable cations (Ca, Mg, Na and K) by extraction with AcNH4 1N at pH 7.0.

Total colonization of roots with AMF (%)

45 days after planting corn it was carried out, vigna and common beans and 90 days after planting sweet potatoes. For total colonization, samples of fine roots were from eight plants per plot, located in the central furrows. The roots were stained 33 and evaluated by the method of the intercepts 34.

Mycorrhizal spore count

The number of mycorrhizal spores was determined at the beginning and at the time of harvest of each crop and expressed as a total number of spores in 50 g of soil. A sample composed of 10 subsamples (0-20 cm) per plot was taken and the spore extraction was performed using the wet decanting method 35 and the stereomicroscope count.

Yield

For its estimation in t ha-1, the plants of each plot harvested, excluding those found in the two edge furrows. In corn, vigna and bean crops, it performed at 120, 90 and 100 days respectively and expressed based on 14 % humidity. The sweet potato made at 120 days and only commercial roots (>100 g) quantified.

The effect of permanence (EP) in each interval was estimated from the s response s in the yield of the sweet potato of the inoculated-sweet potato (R1) and inoculated-sweet potato (R2) sweet potato treatments in relation to the sweet potato yield in the homologous sequence not inoculated and calculated as follows EP (%)=R1R2x 100

Statistical analysis

The ANOVA performed according to the design used. For the previous cultures, by having information on three experiments for each preceding combination x seasons, it was proceeded first to assess whether there was interaction treatments x repetition of experiments x years and the results are presented from that analysis. For sweet potato cultivation, the results expressed according to whether the interaction treatments x years in each experiment was significant or not. In addition, regression analysis between colonization percentage vs. yield and vs spore quantities in 50 g for each succession and season established in the sweet potato culture, using the data corresponding to the three planting intervals. The comparison between experiments was made with the confidence intervals (p<0.05), calculated from the Esx obtained in the ANOVA performed for each variable in the different experiments.

RESULTS

In none of experiments, the interaction terms were significant, whereby only the effects of the treatments on each of the variables in the different experiments will present factor.

Precedent crops in rainy season and principal culture in little rainful season

Both maize and the cowpea showed significant responses (p<0.05) at doses of fertilization, as inoculation with AMF efficient strain, so that no differences between yields inoculated treatments receiving average doses of fertilization and those who received the highest doses of fertilization in each crop (Table 4A and B). The mycorrhizal inoculation also caused significant increases in the mycorrhizal colonization percentage and the amounts of spores in relation to the non-inoculated treatments, showing the non-inoculated treatments that received the highest doses of fertilization, the lowest values; however, the colonization and spore values obtained in the previous inoculated corn were higher (p<0.05) than when vigna was used as such.

Table 4 Effects of inoculation with AMF on the precedents and effectiveness of the effect of permanence on the successions initiated in the rainy season 

Corn rainy season Sweet potato-rainy season
30 days 45 days 60 days
Treatments t ha-1 Col # sp Treatments t ha-1 Col # esp t ha-1 Col # esp t ha-1 Col # sp
AMF 50% NPK 3.59a 72.8a 735 a AMF 50% NPK 25.75a 72.3a 715a 25,1a 71.8a 713a 25.9a 73.5a 715a
AMF 50% NPK 3.58a 71.4a 736 a 50% NPK 25.49a 70.8a 712a 24.1b 70.1a 712a 20.4b 37.1b 310b
50% NPK 3.36b 10.9b 71b 50% NPK 21.33b 10.1b 69b 20.9c 9.9b 69b 20.6b 8.5c 70c
100% NPK 3.58a 9.5b 54c 100% NPK 25.84a 9.8b 61c 25.5a 8.9b 60c 25.8a 8.5c 60d
Esx 0.01 0.53 0.99 Esx 0.235 0.38 1.1 0.20 0.49 1.1 0.21 0.39 1.3
p 0.000 0.000 0.000   0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Vigna -rainy season Sweet potato-rainy season
AMF 50% NPK 2.47a 58.8a 521a AMF 50% NPK 25.8a 54.3a 517a 26,1a 53.9a 516a 26,1a 55.3a 516a
AMF 50% NPK 2.48a 57.8a 520a 50% NPK 25.9a 54.5a 516a 26,1a 52.9a 513a 21,1b 41.8b 226b
50% NPK 2.26b 9.3b 56b 50% NPK 20.9b 10.4b 56b 21.2b 9.8 56b 21.2b 10 c 56 c
100% NPK 2.49a 8.3b 51c 100% NPK 26,1a 8.6c 41c 26.3a 8.4 37c 26.3a 8.3d 41d
Esx 0.03 0.27 0.44 Esx 0.14 0.31 1.0 0.18 0.33 1.5 0.21 0.43 1.5
p 0.000 0.000 0.000   0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

col (% mycorrhizal colonization); #sp (spores 50 g -1 ). AMF: R. irregulare/INCAM 11. Interactions with repetitions and years were not significant. For each previous crop or sweet potato planting interval, different letters in each column lead to significant differences at p <0.05 according to Duncan's Test. p: p- value of the treatment factor in the different ANOVA performed.

In relation to sweet potato, this crop always responded significantly to inoculation and increased fertilization so that both treatments achieved similar yields in the different experiments with one and the other preceding (Table 4 A and B). In the treatments in which the precedent inoculated and the sweet potato not inoculated, the permanence effect was maintained in the first 30 days for both precedents with similar yields with the treatments, in which the sweet potato was also inoculated, but already at 45 days, it was only completely effective in the inoculated vigna-sweet potato succession. Although with the precedent of inoculated corn the yield achieved was superior to the homologous treatment without inoculation, the effect was 76 % compared to the increase in yield obtained by the treatment in which both crops in the succession inoculated. At 60 days, the effect disappeared completely with both precedents.

Both colonization and spore percentages responded significantly to sweet potato inoculation, with higher values (p<0.05) when corn was precedent. For each precedent and in the treatments with inoculated precedents and uninoculated sweet potato, similar values for both mycorrhizal indicators were presented during the intervals of 30 and 45 days in relation to the treatments. In this two cultures in succession were inoculated and both indicators decreased sharply (p<0.05) in the 60-day interval, although with values higher than non-inoculated homologous treatments. The treatments not inoculated and that received the highest fertilization doses, always exhibited the lowest values of the mycorrhizal performance indicators evaluated.

In both sequences, sweet potato yields were strongly associated with mycorrhizal colonization percentages (Figure 1). In the corn-sweet potato sequence, the highest yields of sweet potatoes were achieved with colonization rates in the range of 70 to 75 %; while using vigna as a previous crop, similar yields were achieved with colonization percentages lower than those obtained in the previous sequence (p <0.05) and of the order of 52 to 56 %.

Figure 1 Relationships in sweet potato cultivation between yield and colonization percentages for corn-sweet potato (A) and vigna-sweet potato (B) sequences 

The data corresponding to the three intervals, the two years and four replications of the three treatments that received the same dose of fertilizers with and without inoculation were used (n=72). AMFp60d (precedent inoculated-sweet potato without inoculation, interval 60 days).

It should be noted that in the graphs of performance vs colonization and for both sequences (Figure 1A and 1B ), it can be observed that values of the order of 40 % of colonization achieved in the 60-day interval presented the same performance as the treatments not inoculated with about 10 % colonization. Also in both sequences, the reproduction of spores by sweet potatoes was significantly associated with the colonization percentages achieved by this crop (R2 = 0.99), with always increasing values for both variables, although the spore production when the vigna was the preceding was much lower than with corn (Table 4).

Previous crops in a little rainy season and main culture in a rainy season

With both precedents a significant response was found in the yield to inoculation and fertilization (Table 5A and B), so that the treatments inoculated with the efficient strain AMF and average fertilization dose reached yields similar to those obtained with the highest doses of fertilization. Likewise, both inoculated corn and beans had much higher values (p<0.05) of colonization percentages and spore quantities than non-inoculated, although both were significantly higher in corn than in beans (p<0.05). In turn, the highest doses of fertilization had the lowest values of these two indicators of mycorrhizal functioning.

In addition, in the rainy season the corn not reached only higher yields than when it was sown in the rainy season, but also higher percentages of mycorrhizal colonization and spore production. Likewise, the vigna reached higher values (p<0.05) of the mycorrhizal performance indicators evaluated than the beans.

The sweet potato also presented a positive response to inoculation and mineral fertilization (Table 5 A and B) with similar yields between them in the different experiments, although higher (p<0.05) when the previous crop was beans. Similar to the previous period, the effect of permanence achieved with both precedents in the 30-day interval; at 45, it only kept completely with the precedent of inoculated beans, although with corn it was of the order of 81 % and disappeared completely in both successions at 60 days.

Both colonization and spore percentages responded significantly to sweet potato inoculation, with higher values (p<0.05) when corn was precedent. For each precedent and in the treatments with inoculated precedents and sweet potato without inoculation and with the intervals of 30 and 45 days, similar values of both mycorrhizal indicators presented in relation to the treatments in which the two cultures in succession were inoculated. However, both indicators decreased sharply (p<0.05) in the 60-day interval, although with values higher than non-inoculated homologous treatments. In non-inoculated treatments, higher fertilization doses reduced (p<0.05) the values of mycorrhizal performance indicators evaluated.

In addition, in the two sequences, sweet potato yield was strongly associated with the percentages of mycorrhizal colonization obtained in this culture (Figure 2). When the precedent was corn, the highest yields of sweet potatoes were achieved with colonization rates in the range of 59 to 64 %. While in the bean-sweet potato sequence, the colonization percentages that were associated with the highest yields were lower than those achieved with corn were (p<0.05) and they were in the range of 51 to 57 %; however, sweet potato yields were higher in this last succession. In both sequences, the spore reproductions made by the sweet potato were strongly associated with the colonization percentages achieved by this crop (R2 = 0.98), with always increasing values for both variables, although the sporulation with the preceding bean was much or less than with corn (Table 5).

Table 5 Effect of inoculation with AMF on the precedents and effectiveness of the effect of permanence on the successions initiated in the dry season 

Corn- rainy season Sweet potato -rainy season
30 days 45 days 60 days
Treatments t ha-1 col # sp Treatments t ha-1 col # sp t ha-1 col # sp t ha-1 col # sp
AMF 50% NPK 3.14a 60.6a 562a AMF 50% NPK 29.7a 62.5a 553a 29.4a 61.75a 552a 29.8a 61.5a 552a
AMF 50% NPK 3.13a 59.25a 562a 50% NPK 29.3a 61.3a 551a 28.4a 60.6a 549a 25.5b 36.5b 229b
50% NPK 2.61b 10.5b 62b 50% NPK 24.9b 10b 61b 24.1b 9.5b 60.8b 24.2b 10c 61c
100% NPK 3.15a 9c 58c 100% NPK 30.2a 8.6b 57c 29.3a 8.5b 55.6c 29.3a 9.8c 55d
Esx 0.03 0.36 0.5 Esx 0.14 0.41 0.7 0.14 0.36 0.9 0.21 0.4 0.8
p 0.000 0.000 0.000   0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Bean- little rainy season Sweet potato-rainy season
AMF 50% NPK 2.42a 51.5a 449a AMF 50% NPK 31.5a 54.0a 438a 31,1a 53.8a 437a 31.03a 55.5a 437a
AMF % NPK 2.41a 50.8a 448a 50% NPK 31.2a 53.5a 437a 30.7a 53,1a 436a 25.08b 35.5b 139b
50% NPK 2.06b 9.1b 49b 50% NPK 25.4 b 11,1b 47 b 25.4b 10.1b 47 b 25.11b 9.1c 46c
100% NPK 2.42a 8.4b 44c 100% NPK 31.4a 9.5c 40c 31.2a 8.6 39c 31.38a 8.6c 38d
Esx 0.02 0.38 0.70 Esx 0.235 0.40 0.81 0.271 0.42 1.0 0.21 0.36 1.0
p 0.000 0.000 0.000   0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

col (% mycorrhizal colonization); #sp (spores 50 g-1 ). AMF: R. irregulare/INCAM 11 . Interactions with repetitions and years were not significant. For each previous crop or sweet potato planting interval, different letters in each column lead to significant differences at p <0.05 according to Duncan's Test. p: p value of the treatment factor in the different ANOVA performed.

Similar to the other era, values of the order of 40 % colonization achieved with both precedents in the 60-day interval resulted in the same performance as non-inoculated treatments with colonization percentages close to 10% (Figure 2A and 2C). There were significant differences between periods (p<0.05) for the colonization percentages and the amount of spores in the sweet potato, with the highest values when the precedents were sown in the rainy season; however, the yield of sweet potato was superior to planting it in the rainy season, that is, with the precedents sown in the rainy season.

Figure 2 Relationships between yield and percentage of mycorrhizal colonization in sweet potato cultivation, for corn-sweet potato (A) sequences and for vigna-sweet potato succession (B) 

The data corresponding to the three intervals, the two years and four replicates of the three treatments that received the same dose of fertilizers were used (n=72 in each case). AMF p60d (precedent inoculated- sweet potato without inoculation in 60 days interval).

DISCUSSION

The response to inoculation with an AMF effective strain of previous crops in both periods, expressed to that with only 50 % of fertilization ensure high yields. Even higher than homolog uninoculated (p<0.05) and the like to those obtained with high doses of fertilizers (100 % NPK) corroborates the results obtained previously in different crops and edaphic conditions 8,24 and makes clear one of the benefits of inoculation.

Likewise, the previous inoculated cultures reach high colonization rates of an effective mycorrhizal functioning 8. Thus, in corn, the records are higher than those of legumes are and, in turn, in the rainy season of planting the percentages are higher than those obtained in the dry season are and corroborate what previously obtained 20. The latter may be associated with higher growth s and yield obtained at the season rainy, which can lead to an increase in the needs of fungal structures that meet the requirements of plants, such as with different species of forage inoculated when they grow at 1 season and another 36.

The higher spore counts obtained in the sowings of the precedents in the rainy season can be explained by the higher colonization percentages and higher growth existing in both crops inoculated at this time. Similarly, in corn its most profuse radical system, together with the highest colonization achieved, should explain the greater number of associated spores. In addition, that these results were similar to those obtained previously with these cultures 20. Several authors have reported in different cultures inoculated with efficient strains of AMF, higher amounts of spores associated with the highest percentages of mycorrhizal colonization and with the highest yields of biomass or yield 29,30,36,37, as well as for the same crop, higher in the rainy season than in the dry season 36. In correspondence with the above other authors 38 argue that an efficient mycorrhizal operation can leads to earnings C obtained by the fungus invest in the production of spores. A mechanism that not only is favored in species of generalist that they are characterized by a high production of spores 39, but it is presented for a broad group of Glomeromicota genera and species 40.

The information corroborated the effect of permanence found in the first 30 days for the different precedents in both periods 20, but it only becomes fully extensive until 45 days with the precedents of legumes, although in that case there are less spores that when the precedent was corn. However, with the inoculated corn, a yield equivalent to about 80 % of the effect achieved at 45 days, which has a remarkable practical importance. It is noteworthy that the effect achieved even when the common work of soil preparation prior to the sweet potato plantation was executed.

The soil work is recognized to affect the stability of the resident mycorrhization 41. In the various works carried out in Cuba on the management of inoculation in crop sequences 8,14,20 this effect has been reported in the presence of soil work and even in a work in which minimum and conventional tillage were compared. Similar permanence effects were obtained between the two 42 and possibly related to the fact that generalist strains such as R. irregulari are characterized by the high production of spores 39, propagules more resistant to tillage than extraradical mycelium.

In different works carried out with micotrophic cultures and inoculating an efficient strain 11,14,17 the effect disappears at some point, depending on its extension of the interaction with resident mycorrhizal fungal populations and the competitiveness of the inoculated strain itself 18, although they also influence the edaphic condition, the type of crop and its management 11,19. The disappearance of the effect seems to be important since it reduces possible ecological risks associated with the inoculation of non-resident strains 18. In the specific case of R. irregulare it has been reported very low endemism 43 and these authors consider that the risk The introduction of an “exotic” strain of this species seems smaller than with other species.

An interesting aspect is that the magnitudes of the colonization and quantities of spores reproduced by the inoculated precedents seem to maintain a direct influence on the colonization percentages and quantity of spores obtained in the sweet potato in succession. Probably the most existing mycorrhizal spores to plant the potato and derived from the inoculation of the preceding crop origin to greater colonizations thereof, as has been found in previous experiments with these same cultures 20 or other author with varieties of forage inoculated in the sowing and subjected to periodic cuts 44. It is bases with the amount of spores present when making a cut, the permanence or not of an effective mycorrhization for the new growth stage.

Likewise, the greater colonization of sweet potato should cause high amounts of spores, which is a similar result to that found in the inoculated precedents and has been found in this same culture 29-31. Also in various experiments comparing strains of AMF in that inoculation of the strain not only causes efficient colonization percentages and higher performance but also greater is amount is of spores 45-47; however, it does not explain why similar amounts of spores are maintained throughout each succession in the inoculated treatments.

In addition, even the inoculated cultures are not colonized only with the strain applied and the different cultures have different capacities both to colonize with one or another strain 6,38,40. So although the colonization percentages are an indicator of the functioning and they are generally positively related to yield 48,49, mycorrhizal functioning is a very complex phenomenon and better performances cannot be inferred when comparing inoculated cultures, only by the values of these percentages.

Similarly in an experiment with 17 species of crotalaria 50 to evaluate the response to mycorrhizal inoculation and the effect of permanence on an onion crop (Allium cepa L.) in succession, the highest percentages of colonization in the onion are not associated with the species of crotalaria that presented the highest percentages of colonization.

On the other hand, although some authors have positively related the existence of the permanence effect with the amount of spores associated with the inoculated culture. For example in the succession Canavalia ensiformis inoculated-banana 12 and in forage areas of several species of inoculated brachiae 44. The different characteristics in the reproduction of spores by the different cultures 51, the differences in the resident mycorrhizal community, the influence of the seasons and the very fact that the spores are not the only infective propagule 6,38. They may allow evaluations for each culture inoculated in a particular agrosystem; they do not allow comparing the potential of the different previous cultures inoculated to obtain the permanence effect, from the total spores reproduced by each of them.

The information obtained confirms the existence of an effect of permanence of the inoculant applied with a high importance for its use in crop technologies. Some authors have established using molecular techniques a positive confirmation of the strains applied in the inoculated cultures until a period of two years after inoculation 18. However, others suggest that the positive effect of inoculation not necessarily linked to a direct effect of the inoculated strain 52 and therefore it is essential to develop the protocols for the follow-up of the INCAM11 strain using specific molecular markers, to establish the mechanism present in the permanence effect found.

The use of inoculated beans as a precedent seems to present another additional benefit, associated with the higher yields found in sweet potatoes compared to those obtained in the corn-sweet potato succession. It may be related among others to the positive influence of the residues of legumes easier to mineralize the corn stover with higher C/N and also, the importance of nitrogen for yams 29.

It was interesting that the intermediate colonization values reached in the sweet potato in the 60-day interval by the different successions with inoculated precedents and uninoculated sweet potato did not cause yields. Even if they were slightly higher than those obtained in the homologous treatments without inoculating, indicating that this amount of fungal structures is insufficient to guarantee a positive response in performance 53. The percentage of colonization as assessed is nonspecific and in this situation of disappearance of the permanence effect, it is logical to consider that the proportion in which the efficient inoculated strain is present in the sweet potato colonization has also decreased, which masks possible assessments of efficiency of the inoculated strain. However, similar results have been reported in an inoculation management experiment with this same strain in sequence of crops and in the same type of soil 14, in which sweet potato was the third crop in succession. In addition, the percentages of colonization reached of 38 % did not originate yield superior to the inoculated controls, which presented much lower values of colonization.

However, in inoculated perennials crops a gradual disappearance of operation mycorrhizal with proportional decreases between colonization and associated yields has been reported 17,44. The apparent contradiction may be related among others with different characteristics of mycorrhizal functioning in crops 38. As with the fact that it may not be the same to establish an effective colonization, which should require a minimum number of efficient strain propagules 53, which will decrease the effectiveness of an existing operation in a perennial crop in which there is undoubtedly an accumulated effect on growth. It can mask the relationship between the percentage of current colonization and the response of the plant 54.

In the different crops the higher doses of fertilizers presented the lowest values of the performance indicators, corroborating the more general aspects already reported that the amounts of fertilizers designed for high yields decrease or inhibit mycorrhizal functioning in both inoculated crops 8 as from the resident mycorrhizal communities 6,38.

CONCLUSIONS

  • The inoculation with AMF efficient strains from the previous cultures is an adequate way to achieve an effective mycorrhizal function in the sweet potato in succession in the two planting seasons of this crop.

  • The use of cowpea and common bean crops as above, can exploit this effect permanency inoculation 45 days after harvest of these crops and s uperior to the results obtained with corn.

  • The potential of the inoculated precedents to develop the permanence effect not established through the simple comparison of the performance indicators obtained in each of these.

ACKNOWLEDGEMENT

To the Fund for Science and Innovation (FONCI) from Cuba for financial support for the execution of this research within the framework of the FONCI 56-2016 project.

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Received: October 18, 2018; Accepted: March 27, 2019

*Author for correspondence. rrivera@inca.edu.cu

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