Morphological , Physiological and Nutritional Effects of Irrigation Frequency on Macaúba Palm Seedlings

The commercial nurseries of macaúba palm (Acrocomia aculeata) have experienced excessive seedling mortality, which is possibly due to the lack of information about the proper growing practices. The goal of this study was to investigate the response of macaúba seedlings to water stress using different irrigation frequencies during the early seedling stage. The experiment was carried out in a macaúba palm nursery located in João Pinheiro, Minas Gerais, Brazil. The treatments started 30 days after sowing. At 90 days after sowing, the morphological (number of leaves, stem diameter, shoot length, root weight and the fresh and dry matter content of leaves, stem and roots), physiological (allocation of photosynthates) and chemical (nutrient content in the shoot) characteristics were evaluated. The results indicated that shoot height and stem diameter became smaller as the frequency of irrigation was reduced. However, the accumulation of fresh biomass in leaves and stems, and the leaf and root dry biomass became higher under the same condition. When water stress was induced in younger seedlings, the allocation of photosynthates occured more proportionally across all organs of the plants. The highest accumulation of K, S and Fe in the shoot were observed in plants that were exposed to water stress. N accumulation was higher in the aerial part of the seedlings as the irrigation frequency increased. However, the opposite behavior was observed for P, which accumulation was lowest in the control treatments.


Introduction
The macaúba palm, Acrocomia aculeata, Jacq.Lodd.Ex Mart., is a perennial palm native to Amazonian regions.The species grow in the savannas and open forests of tropical America, which are distributed from Central America to southern South America.It is probably found naturally in almost all the Brazilian territory, especially across the Southeast and Central West regions (Pimentel et al., 2015;dos Reis, Pinto, da Assunção, & da Silva, 2017).This species has an annual flowering season in Brazil from September through February and with peak flowering from November through December (Berton, 2013), which coincides with the season of highest rainfall in the areas in which it grows (Colombo, Berton, Diaz, & Ferrari, 2017).Fruiting occurs throughout the year after the fourth year in the field and generally the fruits are ripe 12 to 13 months after fertilization (Giraldo Montoya, Motoike, Kuki, De Oliveira, & Gomes Honorio, 2015).
Due to the oleaginous properties of the fruits (Evaristo et al., 2016), macaúba palm is considered one of the most promising crop species as a source of oil for the biofuel industry (biodiesel, bio-kerosene and others), cosmetics and food.Currently, two types of oils are produced; one of them is extracted from the endosperm, which represents approximately 15% of the total oil of the plant.This oil is used in various products such as animal food cakes, food items for human consumption, cosmetics, etc., (Azevedo Filho, Colombo, & Berton, 2012;Santos et al., 2017).Additional oil is extracted from the mesocarp, which has good characteristics for industrial processing and can be used for the production of biofuels (Martins, 2011;Motoike et al., 2013).
The agricultural yield of macaúba palm varies between 4,000 and 6,000 liters of oil per hectare, which is short of only the oil palm (Elaeis guineensis, Jacq.) that yields up to 8,000 liters of oil per hectare.The macaúba tree can jas.ccsenet.be used in the fruits production correspond bunch pro human con phenols an The crop Brazil, ho Specificall rates.The hypothesiz physiologi the field a During th conditions of this stu seedlings g

Method
The exper Sementes altitude of According between th between a Figure 1.

Cultiva
The cultiv carried out placing the greenhous proprietary org n its entirety; fr (kernels) that n of fresh frui ding to approx oduces approxi nsumption wi nd β-carotene ( has been clas owever, in com ly, it was note e current study zed that a gra ical and morph after transplan is growth stag s are paramoun udy was to eva grown under v  The greenhouse structure had a total growing area of 1,200 m 2 , with a width of 20 m (and a post spacing of 10 m) and its orientation was of East-West.The length of the greenhouse was 60 m and gutter height was 4 m.The groud surface was covered with raffia and the greenhouse structure consisted of galvanized steel supports.The greenhouse structure was covered with low density polyethylene (LDPE) plastic film with a thickness of 152 µm.
The greenhouse was equipped with an internal movable shade curtain (with 50% light transmission).

Treatments and Sampling Procedures
During the first part of the experiment (from sowing to Day 30), the seedlings were grown under consitent conditions, including the irrigation (amount and frequency).When the seedlings reached a height of approximately 5 cm (after 30 days), the irrigation frequency for the plants in treatment 2 (T2) was changed from twice to once per day.Seedlings in treatment 1 (T1) were watered with a different irrigation frequency starting 45 days after sowing.Seedlings in the control treatment (T0) were irrigated twice per day throughout the experiment (Figure 2).For all treatments, the total amount of water applied during the experiment was 4 mm per day.A total of 160 seedlings, 10 plants per plot and 4 replicates per treatment were used.Note.T0 (Control Treatment): two irrigations per day, for 90 days, at 10:00 and 16:00 h; T1: one irrigation per day starting at 45 days after sowing, at 10:00 h; T2: one irrigation per day starting at 30 days after sowing, at 10:00 h (Figure 2).For all treatments, the total amount of water applied during the experiment was 4 mm per day.
At the end of the experiment (Day 90), a sample of 8 plants per treatment (2 plants per block) was used for destructive analysis and evaluation of the number of leaves, shoot height, stem diameter, root length, leaf area and dry matter, and fresh weights of the leaves, roots and stem.Each seedling was separated into leaves, stem and root and then weighed for the determination of fresh biomass.
The samples were then placed in paper bags and put in a drying oven with forced air circulation at a temperature of 65 °C until constant weight was obtained.At the end of this time, the samples were weighed to determine the dry weight of the leaves, stem and roots.The dry and ground samples of leaves, stem and roots were submitted to nitric-perchloric digestion to determine the macronutrient (P, K, Ca, Mg and S) and micronutrient (Mn, Cu, Fe and Zn) content.The P content was determined by the colorimetry method, K by flame photometry, Ca, Mg, Fe, Mn, Cu and Zn by atomic absorption spectrometry, and S by turbidimetry.Nitrogen was determined using the Kjeldahl method after sulfur digestion.The macro and micronutrient contents were expressed in units of g kg -1 and mg kg -1 , respectively.

Solar Radiation
The accumulated solar radiation inside the greenhouse was estimated based on the hourly information obtained from a meteorological station located in the municipality of João Pinheiro, which belongs to INMET (Instituto Nacional de Meteorologia).The INMET uses an automatic meteorological station (EMA), brand Vaisala; model MAWS 301.The Sampling for solar radiation happens every 5 seconds.The "snapshot" value used in weather reports is the average of one minute (of 12 sample values).
The amount of light blocked by the plastic film that covered the greenhouse and the light reduction due to the shade cloth that was installed inside the greenhouse was subtracted from the global average monthly solar radiation.These quantities were estimated to be 20% and 50% of the global solar radiation, respectly.The results of the indoor global radiation can be calculated using Equation 1: hese values co ess (Figure 3).q is the tot

Morph
The

Analys
The irrigat differed w significant the treatm 46% for T 0.01), the respectivel variation in The accum during the the treatme to the aver (Figure 7B org    Pimentel (2012) were used, who described the nutrient content of 6-month-old macaúba seedlings (Table 1).
In a study investigating the impact of water stress on oil palm, Rivera-mendes, Cuenca, and Romero (2016) found that there is a significant reduction in the uptake of some macronutriments, in particular N, and that this is related to the gas exchange since water stress disrupts plant metabolism, causing stomatal closure and thus reduction in the transpiration and photosynthesis rates.This limits the transport of nutrients, particulary nitrogen and ultimately reduces the production of dry matter.It was observed that the difference in N content in treatments 1 and 2 was smaller between each of these two treatments compared to the difference between each of these two treatments and the control.
When comparing the nutrient content of each treatment with the reference values (Pimentel, 2012), it was observed that the seedlings in treatments T0, T1 and T2 experienced deficiencies for N, Mg and S.However, the treatment that showed the smallest difference with the reference data was T0.It was observed that the N content was higher in seedlings with high water availability (T0), indicating that the reduction in the number of irrigations affected the uptake and assimilation of N by the roots.
The values of K and Ca exceeded the optimal values for all treatments.This may be due to the high demand for these nutrients during the seedlings phase, since these nutrients impact stomatal control, which regulates the amount of water inside the stomatal cavities and cellular elongation (Pimentel, 2012).Higher amounts of K and Ca were found in the seedlings with the lowest irrigation frequency, possibly as a result of lower plant water uptake.
Considering that P in plants is used as an energy source, it seems plausible that the amount of P absorbed by the seedlings that received more water (T0) was used for the subsequent absorption and assimilation of N that later accumulated as dry biomass in the plants.In seedlings grown under water stress, P was kept in the organs due to a lack of water to transport it.This explains the high concentrations of P in the leaves that were submitted to water stress, since this decreases the nutrient absorption by the plants because the ions can only be transported from the roots to the stem and shoots by water (Novais, Barros, & Neves, 1990).
Table 1.Comparison of macro and micronutrients in leaves as reported by Pimentel (2012), and as measured at the end of the treatments reported here
Regarding the micronutrients, it was observed that the seedlings of all the treatments experienced deficiency of Mn and Fe as well as an excess of Cu and Zn.The Mn values were similar for all treatments; however, the Fe deficiency was highest in the T0 treatment.The absorption of Fe and Mn is linked to the presence of P in the plant, which is responsible for the movement of some macronutrients, such as the metals, as have been seen for T2: the amount of Fe and Mn was directly correlated to the amount of phosphorous present.
The alteration of irrigation in Acrocomia aculeata induces stress that caused changes in its physiology and consequently, its morphology, which makes it possible to obtain plants with desirable characteristics for high future productions.The study of the behavior of this species under conditions of water stress provides insight as to the places where it can be produced, because it is a rustic plant.Besides, as it was presented in this work, it allows to adapt well in conditions of water deficit because of the high survival rate.
In addition, these changes provide information on the proper management of the macaúba seelings in pré-nursery conditions and that have a direct impact on the economic values invested in the productions of this crop.
It is necessary to continue the study of the Acrocomia acualeata´s capacity for recovery when other environmental factors are altered.

Conclusions
 Macaúba seedlings grown under water stress showed specific changes in morphological characteristics: reduced aerial parts, stem thickness, and fresh and dry biomass of the leaves.


The photosynthate accumulation was more efficient in plants under water stress (T2: One irrigation per day, starting at 30 days after seeding), showing a photosynthate distribution more similar to the control treatment (T0: Two irrigations per day for the entire 90-day experimental period).


Increased content of K, Ca, S and Fe in the aerial parts was observed at the lowest irrigation frequency (T2).Reduced P content in the aerial plant parts was observed in the seedlings that were irrigated two times per day (T0).
var used was A t during the ex e tubes in grow e where they y information b

Figure 2 .
Figure 2. Representation of the three irrigation treatments used during the seedling production of Acrocomia aculeata, Jacq.Lodd.Ex Mart Figure 3.

Figure
Figure 6.C Figure 8