The type of rootstock can affect the physiological, phenological and reproductive performance of the cultivar grafted onto it (Salazar-García, 2002a). The greatest variability occurs when seedlings rootstocks are used (Salazar-García, Velasco-Cárdenas, Medina-Torres, & Gómez-Aguilar, 2004a). This variation is not only between species, but also among trees of the same race and even within seed-grown plants, which due to cross pollination can be highly heterozygous (Salazar-García, Borys, & Enríquez-Reyes, 1984a). In fact, only rootstocks obtained by asexual methods are genetically identical to the mother plant (Salazar-García, 2002a).
The rootstock used can have an important effect on the leaf nutrient concentrations of the grafted cultivar. Embleton, Matsumura, Storey, and Garber (1962) found that rootstocks of the Guatemalan race led to a lower Cl- concentration than those of the Mexican race, and that both the race of the rootstocks and their botanical variety affected the leaf concentrations of N, P, K, Ca and Mg of the grafted cultivar. For their part, Labanauskas, Stolzy, and Zentmyer (1978) report that ‘Duke’ seed rootstock favored higher leaf concentrations of N, P and Cu in ‘Hass’ scions, compared to ‘Topa’ rootstock. In Australia, ‘Duke 7’ (clonal, Mexican race) benefited the leaf concentration of Zn in ‘Hass’ trees in full production, while ‘Velvick’ (clonal or from seed, West Indian) did not modify leaf nutrient levels (Marques, Hofman, & Wearing, 2003). In California, leaf nutrient concentrations in ‘Hass’ trees grown on 10 clonal rootstocks were measured, and although there were variations among them, the concentrations of N, P, K, Ca, Mg, S, Zn, Cl, Mn, B, Fe and Cu were within, or near, the optimal standards recommended for California (Mickelbart, Bender, Witney, Adams, & Arpaia, 2007).
The search for avocado rootstocks tolerant to water stress has been limited; one possible reason is that in most producing countries it is grown under irrigation. However, in Mexico, more than 111,000 hectares with avocado lack irrigation. In Nayarit just 5 % of 5,300 ha with avocado have irrigation (SAGARPA-SIAP, 2014). The shortage of water for agriculture, which is exacerbated by the variation in interannual rainfall and its annual distribution, reduces avocado yields and quality, making it necessary to obtain rootstocks with higher productivity than those currently used.
During the 1980s in Mexico, rootstocks tolerant to salinity or drought were selected (Salazar-García et al., 1984a; Salazar-García, Borys, & Enríquez-Reyes, 1984b; Salazar-García, Borys, & Enríquez-Reyes, 1984c; Salazar-García et al., 2004a). Once the technique for rooting avocado stems was refined (Salazar-García, 2002b), the most promising selections were cloned, and in 2000 their evaluation as rootstocks for ‘Hass’ began in Nayarit (Salazar-García et al., 2004a; Salazar-García, Velasco-Cárdenas, Medina-Torres, & Gómez-Aguilar, 2004b).
It is unknown whether the aforementioned rootstocks affect the leaf nutrient concentrations of ‘Hass’, which have to be considered for the commercial management of their nutrition. Therefore, the objectives of this research were: a) detect differences in leaf nutrient concentrations of various clonal avocado rootstocks prior to being grafted, b) determine the effect of the rootstock on leaf nutrient concentrations in young and adult ‘Hass’ trees grown without irrigation.
Materials and methods
The orchard under study is located in Platanitos, municipality of Tepic, Nayarit, Mexico, at 1,060 masl; the area’s mean annual minimum temperature is 13.1 °C, the mean annual temperature is 20.3 °C and the mean annual maximum temperature is 27.5 °C. The orchard did not receive irrigation and mean annual rainfall was 1,240 mm, distributed from June to September.
The rootstocks (RSs) were transplanted in July 2000 (start of rainy season), at a spacing of 6 x 8 m, and grafted with cv. Hass in May 2001. A total of 14 different clonal RSs previously selected for their tolerance to salinity or drought were included (Salazar-García et al., 2004b) (Table 1). As control, creole seedling RSs with phenotypic characteristics of the West Indian race were included.
The trees were fertilized according to the procedure described by Salazar-García, Cossio-Vargas, and González-Durán (2009). The fertilizers applied were: ammonium sulfate, urea, diammonium phosphate, potassium sulfate, Sul-Po-Mag, calcium carbonate, zinc sulfate and boronat. In plants from zero to three years old, the annual nutrient units (g) applied per tree were: N (38.4), P2O5 (103.7), K2O (95), CaO (300), MgO (131.4), ZnO (33) and B (1.2), and from the fourth year they were: N (330), P2O5 (300), K2O (1125), CaO (1000), MgO (135), ZnO (270) and B (20).
Two soil samplings were made. The first one was carried out once the RSs were established (September 2000) in five randomly-selected sites. At each site, four subsamples were obtained from 0-30 and 31-60 cm deep. Of the 20 subsamples from each depth, a composite sample was obtained from which its physical and chemical characteristics were determined in the Fertilab laboratory, which is accredited by the Soil Science Society of America’s NAPT program (http://www.naptprogram.org/pap/labs). The second sampling was in September 2007, obtaining a composite sample for the aforementioned depth, although in this case two trees were sampled for each of the one-third sections into which the orchard was divided. Texture, pH (1:2 water) (Hendershot, Lalande, & Duquette, 2008), organic matter (Nelson & Sommers, 1982), inorganic-N (Keeney & Nelson, 1982), P-Bray (Bray & Kurtz, 1945), K, Ca, Mg (Rhoades, 1982), Na, Fe, Zn, Cu, Mn (Lindsay & Norvell, 1978) and B (Bingham, 1982) were determined.
Leaf sampling prior to grafting
Sampling was conducted in April 2001 to determine the nutritional status of some rootstocks prior to being grafted. Eight clonal rootstocks were included: Plat-2, Plat-3, Plat-4, Plat-5, Plat-6, Plat-7, Plat-8 and Plat-11. For analysis, six-month-old leaves from the first vegetative growth flush after transplant were collected from the middle part of the shoot. Ten mature, healthy and whole leaves were collected from each tree, after which they were washed with tap and distilled water, and then dried in a forced-air oven (Lab Line Mod. Imperial V) at 60 °C for 48 hours. The dried leaves were pulverized in a stainless steel mill (Thomas Scientific, Whiley Mini Mill 3338-L10, Swedesboro, NJ, USA) and sieved in No. 40 mesh.
Chemical analyzes were conducted in the aforementioned lab. Total-N was determined by semi-microKjeldahl digestion modified to include NO3 (Bremer, 1965). P, Ca, Mg, S, Fe, Cu, Mn and Zn were removed by wet digestion with a mixture of HNO3 and HClO4 (Jones & Case, 1990) and K was separated in water (rapid extraction method). With the exception of P, they were determined by atomic absorption using an iCE 3000 Series spectrometer (Thermo Scientific, Madison, Wisconsin, USA) (Association of Official Analytical Chemists [AOAC], 1990). P was quantified by the ascorbic acid method and B was determined by calcination using the azomethine-H spectrophotometric method (Enríquez-Reyes, 1989), both in a Genesis 20 spectrophotometer (Thermo Scientific, Madison, Wisconsin, USA).
Leaf samplings in ‘Hass’ scions
For the nutritional analysis leaves were sampled in 14 clonal rootstocks and the control. The first sampling was in September 2004 (three years after grafting) and the second in August 2007 (six years after grafting). The number of trees included in each sampling was variable. In each tree 30 healthy, whole leaves, from six to seven months of age from the winter vegetative flush (emerged in February), were collected from the middle part of non-fruit-bearing shoots. The leaves were prepared and analyzed as described in the previous section.
The horticultural influence of rootstock on leaf nutrient concentrations in six-year-old scions was obtained with the “System for leaf nutrient diagnosis of ‘Hass’ avocado in Nayarit” (Salazar-García, Álvarez-Bravo, & González-Durán, 2015). This system uses leaf nutrient standards for this region and classifies the concentration of nutrients as: deficient, below normal, normal, above normal and excessive.
The harvests from each tree in September-October 2004 and 2007, years in which the leaf samplings were made, were recorded.
Soil moisture and temperature
These parameters were recorded monthly during two water stress periods at 30 cm deep in the north and south directions of the tree’s canopy drip area. The first period was from August 2000 to July 2001, taking measurements on five trees per sampling date. The second period was from April to June 2006 and was made in three trees per treatment. Soil suction was quantified with a portable 2900 “Quick Draw” Series tensiometer (Soilmoisture Equipment Corp., Santa Barbara, CA. USA), graduated from 0-100 centibars (cbar = kPa). Soil temperature was obtained at 14:00 h with an Aquaterr Temp-100 unit (Aquaterr Instruments, Fremont, Calif., U.S.A.).
A completely randomized experimental design with 14 treatments (RSs), plus the control, was used. The experimental unit was a tree and a different number of replications was used. Analysis of variance was performed using the GLM procedure of the Statistical Analysis System (SAS, 2009) statistical package and comparison of means with the Waller-Duncan test (P ≤ 0.05).
Soil fertility at baseline (2000) and seven years later (2007) showed some variations (Table 2). At first, the soil pH was 5.8 and seven years later it dropped to 5.1. The organic matter content and the concentrations of Ca and P were similar in both years. However, the concentrations of N, K, Fe and Zn increased, while the opposite occurred for Mg and Mn.
Leaf nutrient concentrations of rootstocks prior to being grafted
The different RSs showed no differences in the leaf concentration of any of the nutrients analyzed (Table 3).
Leaf nutrient concentrations in young and adult ‘Hass’ trees
Young trees (three years old) showed no differences in leaf nutrient concentrations due to the rootstock (Table 4). In the case of adult trees (six years old), there was only a difference (P ≤ 0.033) in the P concentration (Table 5). ‘Hass’ on Plat-4 showed higher values of P (0.11 g∙100 g-1) than Plat-17 (0.09 g∙100 g-1), Plat-9 (0.08 g∙100 g-1) and the native rootstock (0.08 g∙100 g-1). However, Plat-14 did not statistically differ from 11 of the rootstocks evaluated.
Six years after the graft with ‘Hass’, the diagnosis for the nutrients of greatest importance in avocado showed the following: N, eight RSs with normal levels (1.7 to 1.8 g∙100 g-1) and seven below normal (≤1.6 g∙100 g-1); P, normal in nine RSs (0.10 to 0.11 g∙100 g-1) and below normal for the rest; K, Mg and Fe, all RSs with normal levels; Ca, all RSs were normal or above normal; Zn, normal in four RSs and below normal in the rest; B, one RS with normal level (Plat-7), eight in deficiency, including the creole one, and the rest below normal (Table 5).
By comparing the dataset of ‘Hass’ on all the clonal RSs (the control was excluded), it was found that the age of the scion affected leaf nutrient concentrations (Table 6). The values of N, P, Ca, Mn and B were higher in young trees. Adult trees only had a higher concentration of Fe and Zn than young trees.
Influence of the nutrient concentration of rootstocks prior to grafting
Linear correlation analysis was performed for nine clonal RSs in order to look for any type of relationship between the leaf nutrient concentrations of the rootstock before being grafted and that of ‘Hass’ at three and six years after grafting. Of the 198 correlations obtained, 59 were significant. Of the 11 nutrients analyzed in ‘Hass’, B was the most affected by the type of RS, followed by P, Ca and S. In addition to Cu, which was not affected, the least affected were N, Mg and Zn (Table 7).
Leaf nutrient levels prior to grafting showed a higher number of positive correlations for the leaf concentrations of Ca and Mn, and negative correlations for B in three-year-old ‘Hass’ scions. In the case of six-year-old ‘Hass’ scions, the main correlations were negative for P and B, and positive for S and Fe (Table 7).
In general, the significance of the correlations varied with the rootstock and nutrient in question. When the correlation was significant, it was positive for N, Ca, Mg, S, Fe, Mn and in one case for Zn and B. The negative correlations occurred for P, K and B, and in one case for Zn. At the level of each rootstock, it was found that Plat-11 was the one which in most cases increased leaf nutrient concentrations, both in three-year-old scions (N, Ca, Mg, S and Mn) and six-year-old ones (Ca, Mg, S, Fe and Mn). However, other rootstocks had a higher frequency of negative correlations, highlighted by Plat-5 for P, K, Zn and B (three-year-old scions), as well as P, K and B (six-year-old scions), and Plat-8, which caused reductions in the concentrations of P, K and B in young and adult ‘Hass’ scions (Table 7).
Fruit production of the two harvests evaluated showed no significant differences among the rootstocks evaluated. At three (2004) and six (2007) years after grafting, crop production ranged from 0.5 to 18.8 kg∙tree-1 and from 12 to 80 kg∙tree-1, respectively (Tables 4 and 5).
Analysis of overall correlation of the fruit productions (2004 and 2007) and leaf nutrient concentrations of the ‘Hass’ scions in those years showed positive association (P ≤ 0.0001) of N (r = 0.486), P (r = 0.486), K (r = 0.497), Ca (r = 0.483), Mg (r = 0.504), S (r = 509) and Zn (r = 0.289). On the other hand, the association of those variables was negative (P ≤ 0.0001) for Cu (r = -0.443), Fe (r = -0.412) and Mn (r = -0.857). There was no correlation between leaf B concentration and the production obtained (data not shown).
Soil moisture and temperature
Soil temperatures remained within the range of 21 °C (October) to 33 °C (June). The highest temperature occurred before the rainy season (June) and the lowest in October when the rains ended (Figure 1).
During the rainy season and shortly thereafter (August to November), soil moisture was high (5 to 10 cbar). From December moisture gradually decreased to 34 cbar (in 2000) and 37 cbar (in 2006) at the time of maximum soil drought.
The decrease in soil pH (5.8 to 5.1) was noticeable seven years after the orchard’s establishment. However, Salazar-García (2002a) mentioned that in Michoacán and Nayarit, Mexico, it is common for avocado to be grown successfully in soils with pH between 4.8 and 6.5. The contents of some nutrients did not change between soil samplings, while others increased and others decreased. However, the trees showed no symptoms of nutritional deficiencies in the foliage and fruit.
Leaf nutrient concentrations did not vary among the different rootstocks prior to grafting, or three years later. No clonal rootstock evaluated differed from the others or with the creole one. Something similar happened six years after grafting, since the only difference among the RSs used was for P, highlighted by Plat-4 (Mexican race) with the highest concentration and the native rootstock (West Indian race) with the lowest (Table 5). However, no relationship between rootstock race and leaf nutrient concentrations was detected.
The leaf nutritional diagnosis identified ‘Hass’/rootstock nutrient interactions of interest for nutrition management. In this sense, the RSs that favored normal or above normal concentrations of the macronutrients N, P, K, Ca and Mg were Plat-6, Plat-8, Plat-11, Plat-14 and Plat-7, with the last being the only one that favored a normal B level. Interestingly, the native creole rootstock, locally referred to as “adapted to the region,” led to below normal levels of N, P, S, Mn, Zn and deficiency of B. This may explain the chronic deficiency of Zn and B in ‘Hass’ orchards in Nayarit (Salazar-García, Ibarra-Estrada, Gutiérrez-Martínez, & Medina-Torres, 2014).
In California, Mickelbart et al. (2007) found differences in leaf nutrient concentrations in ‘Hass’ on 10 different clonal rootstocks, although most nutrients were close to or within the optimal standards recommended for California. Additionally, they found differences in their ability to supply nutrients to the ‘Hass’ scions. This is consistent, since it is known that the roots of rootstocks often vary in their ability to absorb and transport nutrients to the aerial part of the tree.
In the present study, the higher leaf nutrient concentrations in young trees (three years old) than in adults (six years old) can be attributed to lower crop nutrient demand in the young trees or to an interannual variation. In adult ‘Fuerte’ avocado trees on Mexican race rootstock seedlings, higher leaf concentrations of N, K and Ca were observed in the first year of sampling, and in the second they were of P and Mg (Herrera-Basurto et al., 2008).
The results of the present research correspond to trees that were not irrigated. When considering the increase in nutrient demand, due to the age of the trees (biomass and fruiting), it is theorized that the rootstocks must express their abilities to take water and nutrients from the soil to meet demand. The fact that the adult scions (six years old) had a higher leaf concentration of Fe and Zn differs from the findings reported by Havlin, Beaton, Tisdale, and Nelson (1999), in the sense that the low soil moisture can induce deficiencies in these nutrients. In the present study the soil moisture was slightly higher in young trees (up to 34 cbar) than in adults (up to 37 cbar). However, as already mentioned, all the rootstocks included in this evaluation, except the creole one, were selected for their tolerance to salinity or drought, which may explain the reduced presence (except B) of deficient leaf concentrations. Rootstocks may differ in the anatomy of the root, the characteristics of the secondary xylem vessels and the rate of water movement (Fassio, Heath, Arpaia, & Castro, 2009; Reyes-Santamaría, Terrazas, Barrientos-Priego, & Trejo, 2002), which can vary among avocado races.
Generally, the main correlations between the nutrient levels of the RSs, prior to grafting, and those of the six-year-old ‘Hass’ scions were negative for B and P and positive for S and Fe. However, the RSs showed different behavior, such as Plat-11, which consistently increased the concentrations of Ca, Mg, S and Mn in three- and six-year-old ‘Hass’ scions. Other RSs, such as Plat-5 and Plat-8, were characterized by lowering the concentrations of P, K and B in three- and six-year-old ‘Hass’ scions. In any case, this quality of the aforementioned RSs can help manage their fertilization. Mickelbart et al., 2007 mentioned that RSs should allow the productive potential of the scion to be expressed, although this factor is usually relegated to second place after the material has been selected primarily based on its tolerance to a specific stress.
The ‘Hass’ productions obtained in 2004 and 2007 showed no differences among the rootstocks evaluated. In California, Arpaia, Bender, and Witney (1992) evaluated different RSs grafted with the cv. Hass where ‘Duke 7’ and ‘Borchard’ stood out with 103.5 and 93.1 kg∙tree-1. The latter production resembles that recorded in our study for 2007 in Plat-8 (80 kg∙tree-1 amounting to 16.7 t∙ha-1).
Under the conditions in which this research was conducted, positive correlations between ‘Hass’ production and leaf concentrations of N, P, K, Ca, Mg, S and Zn, as well as negative correlations with Cu, Fe and Mn, all important nutrients for avocado production, were established. This indicates that several rootstocks were able to absorb and transport nutrients from the soil, especially in low moisture and nutrient (except K) availability conditions, as in the site where the study was conducted.
The constant biotic and abiotic stresses limiting ‘Hass’ avocado production in Mexico will encourage the use of clonal rootstocks to maintain the productivity of the orchards. In preparation for this, this research determined that: a) young ‘Hass’ trees, grown without irrigation, on various clonal rootstocks had a higher concentration of nutrients than adult trees, b) clonal rootstocks did not change leaf nutrient concentrations in young ‘Hass’ trees and in adult trees only that of phosphorus, c) the type of clonal rootstock used did not affect per-tree fruit production. This will make it possible to manage, at commercial level, the nutrition of the studied rootstocks.