Experimental field

The experiment was conducted during the period June 2017 to June 2018 in the experimental fruit culture area of the Center of Agricultural Sciences at the Federal University of Roraima, Cauamé Campus, located in the Boa Vista Municipality, Roraima State, Brazil (2° 52’ 17’’ NL and 60° 42’ 46’’ WL, at 90 masl). The climate, according to the Köppen classification, is type Aw. It is a rainy tropical climate, characterized by a rainy season from April to September, with higher pluviometric rates concentrated in May (291.2 mm), June (352.9 mm), and July (335.1 mm). The dry season is from September to March, with January (29.3 mm), February (24.7) and March (47.8 mm) showing the highest water deficits. The rainfall and average temperature are about 1,700 mm·year^-1 and 27 °C, respectively (^{Farias-Araújo, de Andrade, de Medeiros, & Sampaio, 2001}). The soil of the experimental area had acid pH (4.6), low nutrient content and a high aluminum index, which are typical characteristics of the uplands of Roraima.

Lysimeter installation

Six separate drainage lysimeters were installed 6 m apart. Each lysimeter had a surface area of 1.60 m² and a depth of 0.74 m. The lysimeters were filled with soil layers similar to those of the experimental area, in accordance with the recommendations of ^{Pereira et al. (1997)}, ^{Santos, Montenegro, Silva, and Rodrigues-Souza (2009)}, and ^{Miranda, Gonsaga-de Carvalho, Castro-Neto, and Balbino-dos Santos (2016)}.

Two (P2), 4 (P4) and 6 (P6)-year-old Myrciaria dubia plants were transplanted to each drainage lysimeter, two plants by age with similarities in terms of morphology, height, number of branches (basal and terminals) and diameter (of stem and canopy). The P2 were obtained as seedlings from the Brazilian Agricultural Research Corporation (Embrapa-RR) nursery. The other plants were brought from the Embrapa-RR experimental areas Água Boa and Serra da Prata. Prior to transplanting, liming and fertilization were carried out as recommended by ^{Viegas, Fração, and Silva (2004)}. In addition, 5 kg of chicken manure was incorporated as a source of organic matter and nutrients. After transplanting, a 15 cm mulch composed of grass (Trachypogon plumosus) residues, obtained from the area near the experiments, was applied around one plant of each age. Due to decomposition of the mulch during the experiment, this material was replaced every five months. The lysimeter arrangement and the development stages of the Myrciaria dubia plants are shown in Figure 1.

Figure 1 Development stage and arrangement of Myrciaria dubia plants of different ages in drainage lysimeters, with and without organic mulch (I and II, respectively): a) 2-year-old plants, b) 4-year-old plants, c) 6-year-old plants, and d) plant arrangement (C = with mulch; S = without mulch) and vertical section of lysimeter. 

Cultivation

The Myrciaria dubia plants were transplanted at the end of March 2017, and the evaluations of hydric balance began in June. Weed control was performed manually inside and outside the lysimeters. In addition, fertilization was performed every 3 months, as recommended by ^{Viegas et al. (2004)}, in order to replace the nutrients that were leached during lysimeter drainage. In the P2 lysimeters, irrigation levels of 7.5 and 10.3 mm were applied, in the presence and absence of mulch, respectively. For P4, 14.7 mm was applied on average, and in the P6, 21.7 and 29.2 mm was applied in the presence and absence of mulch, respectively. Irrigation was performed every four days in the afternoon.

Estimation of reference evapotranspiration

The ETo was estimated using the Penman-Monteith method, parameterized by the PM-FAO 56 scheme (^{Allen et al., 2006}). The climatic data considered were solar radiation (MJ·m^-2), average wind speed (m·s^-1), relative humidity (%), and minimum, maximum, and mean temperatures (°C). The data were provided by the Automatic Weather Observation Station operated by the Ministry of Agriculture, Livestock and Food Supply’s National Institute of Meteorology (INMET), from June 2017 to June 2018. An Excel spreadsheet was used to calculate the ETo following the method of ^{Conceição (2006)}.

Determination of crop evapotranspiration

The ETc determination for Myrciaria dubia plants started 60 days post-transplanting, from the water balance (Equation 1), which is based on the law of mass conservation (^{Reichardt and Timm, 2012}):

where R is rainfall (mm), I is irrigation (mm), D is drainage depth (mm), ETc is evapotranspiration of the crop (mm), and Δh is the variation of soil water storage within the lysimeters (mm). The value of Δh was calculated using Equation 2:

where θ ₂ is the average soil moisture in the volume at one day after rainfall or irrigation (m³·m^-3), θ ₁ is the average soil moisture in the volume on the day before irrigation (m³·m^-3), and Z is the depth adopted hydric balance (mm).

Soil moisture was determined with an FDR 10HS (Decagon) sensor calibrated prior to the experiment. The depth setting was 150 mm, because the sensor is 10 cm long and the soil influence volume is 1 L (^{Cobos and Chambers, 2010}).

Crop coefficient determination

The Kc was determined using the ratio of ETc to ETo values (Equation 3) presented by ^{Doorenbos and Pruitt (1977)}:

For a better understanding of the Myrciaria dubia crop coefficients, plants were characterized by phenological stages according to the chronological ages studied; P2, P4 and P6 were classified in stages EI, EII, and EIII, respectively, which have a canopy coverage of up to 10, 80 and 95 % over the soil area, respectively.

Climatic conditions and reference evapotranspiration

The climatic characteristics and the reference evapotranspiration (ETo) during the experiment are presented in Table 1. The mean temperature was between 25.54 and 29.64 °C. The maximum and minimum temperatures varied from 26.06 to 30.3 °C, and from 25.02 to 28.99 °C, respectively. It should be noted that the smallest variation was recorded for minimum temperature values, at only 3.97 °C between the highest and lowest minimum temperatures. ^{Farias-Araújo, Fonseca-Conceição, and Bittencourt-Venâncio (2012)} reported that stability of the minimum temperature is characteristic of regions at low latitudes and altitudes. In addition, according to ^{Ramos, Santos, and Fortes (2009)}, the mean, maximum and minimum temperatures recorded in this study are consistent with historical average temperature ranges, indicating that the collected data correspond to a normal year in the region.

The lowest relative humidity percentages were recorded in November and December 2017, and in January, February and March 2018. However, in these months a higher wind speed, higher solar radiation and lower rainfall were also recorded (Table 1).

The ETo varied due to local climatic factors during the evaluation period (June 2017 to June 2018). On average, an ETo of 3.3 mm·day^-1 was seen during June and July, and of 5.2 mm·day^-1 from August to December. From January to March 2018, ETo values close to 6.2 mm·day^-1 were recorded, attributed to the increasing temperature, wind speed, and solar radiation, and a decrease in relative humidity. From April to June, ETo declined to 3.6 mm·day^-1 as a consequence of lower air temperature and solar radiation in the early rainy season. ^{Murga-Orrillo et al. (2016)} reported similar ETo values, ranging from 1.7 to 6.6 mm·day^-1, in the same locality and using the same method of analysis (Penman-Monteith). ^{Farias-Araújo, Antunes-Costa, and dos Santos-Araújo (2007)}, who estimated ETo by different methods, reported averages ranging from 3.3 to 5.0 mm·day^-1 in the Boa Vista municipality, with the minimum value reported in June and the maximum in March.

At the monthly level, in June and July 2017, the average ETo was approximately 102 mm. In the following months, there was a considerable increase, and during January, February, and March 2018 the ETo monthly accumulation values were 179.2, 184.9, and 196.5 mm, respectively. Over the final three months the monthly ETo value decreased to 106.54 mm. The total ETo accumulated during the entire evaluation period (12-month) was 1,915.4 mm. A lower value of 1,566.6 mm per year was estimated by ^{Megna-Francisco, Mainar-de Medeiros, Moreira-de Matos, Santos, and Falle-Saboya (2017)} for the State of Paraiba, indicating that in the Boa Vista-RR region there is greater evaporative demand by the atmosphere, due to climatic conditions.

In general, it was observed that the plants continued with their vegetative and productive development during the 12 months of evaluations. ETc and Kc values were calculated for 2-, 4- and 6-year-old plants, and there was only fructification in 6-year-old plants. The harvest occurred in April and May 2018, with an average production of 6 kg per plant.

Myrciaria dubia crop evapotranspiration

The ETc results of Myrciaria dubia for P2, P4 and P6, in the presence and absence of mulch, are shown in Figure 2, where it can be seen that ETc varied directly with ETo and with plant vegetative development. In addition, mulching influenced soil moisture conservation in the lysimeter area, avoiding water loss through evaporation. ^{Allen et al. (2006}) mention that values different from the standard ETc can be obtained when the soil surface is covered, when intercropping (in presence of organic mulching), and as a result of other specific cultivation practices.

Figure 2 Monthly mean evapotranspiration in Myrciaria dubia plants of different ages: a) 2-years, b) 4-years, and c) 6-years, in the presence (pm) and absence of mulch (am). 

As illustrated by Figure 2a, the P2, in the presence and absence of mulch, had an estimated cumulative water consumption of 805.3 and 1,063.9 mm·year^-1, respectively. The mean, minimum and maximum water consumptions by the plants with mulch were 2.0, 0.8, and 3.2 mm·day^-1, respectively, and without mulch the mean water consumption was 2.7 mm·day^-1, with a minimum of 1.0 mm·day^-1 and a maximum of 4.7 mm·day^-1. Consequently, at the beginning of the rainy season (from April to May), ETc decreased to 1.5 and 1.6 mm·day^-1 in the presence and absence of mulching, respectively. The cumulative ETc value in uncovered soil showed an estimated 24.3 % higher water consumption than that of plants growing in covered soil.

P4 plants, in the presence and absence of organic mulching, showed ETc values of 1,264.6 and 1,539.6 mm·day^-1, respectively. Thus, plants with mulch consumed on average 3.2 mm·day^-1, ranging from 2.0 to 4.7 mm·day^-1. However, when mulch was not used, the values ranged from 2.7 to 6.2 mm·day^-1, with a mean value of 3.9 mm·day^-1 (Figure 2b). Water consumption in the soil without mulch was thus 17.9 % higher than that of the soil managed with mulching. A comparable difference of 16.7 % was reported by ^{Murga-Orrillo et al. (2016)} in corn (Zea mays).

P6 plants had an ETc of 2,395.2 and 2,816.4 mm·day^-1, in the presence and absence of organic mulching, respectively. The plant with mulch had on average an ETc of 6.1 mm·day^-1, ranging from 2.3 to 10.3 mm·day^-1. Without mulch, values ranged from 2.7 to 12.2 mm·day^-1, with a mean of 7.2 mm·day^-1 (Figure 2c). As a result, the water consumption for plants in uncovered soil was 15 % higher than that in covered soil.

Considering the above results, it can be stated that mulch composed of native grass (Trachypogon plumosus) residues reduced ETc. About it, ^{Scholz-Berça, Grandizoli-Mendonça, and Fonseca-Souza (2019)}, and ^{Cortez, Nagahama, Olszevski, Patrocinio-Filho, and de Souza (2015)} mention that the soil coverage with plant residues is presented as an agricultural practice that conserves the natural processes that maintain a stable temperature and reduces natural water losses by the evaporation process on the soil surface, providing an increase in water-use saving.

Crop coefficient of Myrciaria dubia plants

In general, Kc values for stages EI, EII and EIII changed in relation to the plants’ vegetative and productive development status. In addition, mulching influenced the Kc value in all phenological stages (Figure 3). ^{Freire, Cavalcante, Rebequi, Dias, and Viera (2012)} state that soil cover, in semi-arid conditions, is used to mitigate the effects of high soil surface temperatures and to avoid water losses by evaporation.

The Kc value for plants in the EI stage, regardless of mulch status, increased during the evaluation period; in the first months, water consumption was lower due to the slow development of the plants, which had, on average, four terminal branches, a height of 50 cm, and a canopy diameter of 15 cm. At the end of the experiment, plants had 27 branches, a height of 125 cm, and a canopy diameter of 65 cm.

The average Kc values, for EI stage plants with and without mulch, were 0.4 and 0.6, respectively, ranging from 0.3 to 0.5 in the first four months, and from 0.4 to 0.7 in the last four months (Figure 3). Similar results were found by ^{Doorenbos and Pruitt (1977)} in tropical and Mediterranean climates for banana (Musa spp.) (0.5), mango (Mangifera indica L.) (0.4), passion fruit (Passiflora edulis) (0.4), and citrus (0.5). Similarly, ^{Allen et al. (2006)} mention that the guava (Psidium guajava) Kc is around 0.5 during initial vegetative growth.

Figure 3 Crop coefficient (Kc) in camu-camu plants in the presence (pm) and absence of mulch (am) by phenological stage. 

^{Flumignan and Teixeira-de Faria (2009}) obtained results different from those of the present study when evaluating coffee plants in the first and second year of cultivation. They reported a Kc of 0.9, which was attributed to a large leaf area and shrub characteristics with a high number of branches.

The Kc value obtained in plants at EII also showed an increase over time due to canopy development. At the beginning of the experiment these had two main stems, 25 terminal branches, a height of 130 cm, and a canopy diameter of 70 cm. Twelve months after transplanting, they had 175 terminal shoots, a plant height of 187 cm, and a canopy diameter of 194 cm.

The average Kc values, for EII stage plants with and without mulch, were 0.6 and 0.8, respectively, ranging from 0.5 to 0.7 in the first four months, and from 0.7 to 0.9 in the last four months (Figure 3). ^{Doorenbos and Pruitt (1977)} reported similar results to those obtained in this study in tropical and Mediterranean climates for banana (Musa spp.) (0.7 to 0.85), mango (Mangifera indica L.) (0.5 to 0.75), passion fruit (Passiflora edulis) (0.75 to 0.8), and citrus (0.75 to 0.8) during stage II (vegetative development), defined as up to 80 % of soil covered by the plant’s canopy.

Water consumption in the P6 at the EIII stage also showed a significant increase during vegetative and productive development. At the beginning of the experiment, P6 had three main stems, 60 terminal branches, a height of 180 cm and a canopy diameter of 130 cm. After 346 days, the plants had, on average, 365 terminal shoots, a height of 245 cm, and a canopy diameter of 265 cm. The average Kc values, for plants with and without mulch, were 1.1 and 1.3, respectively, ranging from 0.8 to 1.3 in the first four months and from 0.9 to 1.6 in the last four months (Figure 3).

The highest water consumption occurred between 180 and 240 days, which corresponds to the canopy vegetative development, flowering and fruiting phases, for which average Kc values were 1.3 and 1.5 for plants with and without mulch, respectively. After fruit maturation, Kc values decreased, possibly due to leaf senescence, which reduces transpiratory capacity by causing turgidity loss and increased stomatal resistance. Similar results were obtained by ^{Silva et al. (2000)} in Mangifera indica in the São Francisco Valley, Petrolina, Pernambuco, Brazil. In contrast, ^{Calgaro and Braga (2012)} report lower Kc values, ranging from 0.70 to 1.45, with a mean of 0.98, in the Acerola/Surinam cherry crop in an edaphoclimatic study in Fortaleza, CE, Brazil. ^{Allen et al. (2006)} found similar Kc values in banana (Musa spp.), cocoa (Theobroma cacao), and coffee (Coffea arabica L.), which averaged 1.1, 1.05, and 0.95, respectively. Doorenbos and Pruitt (1979) mention that Kc assume low values in the emergency phase, maximum values during vegetative/fruit development and decline in the maturation/senescence phase.