Determination of Basal Temperature and Its Relationship With Jatropha Crop in Irrigated and Non-irrigated System

Full plant growth and development require, among others, air temperatures and water availability at levels appropriate to each crop. The effect of temperature on plant development can be represented by the thermal sum, which requires the lower basal temperature for each plant species. However, plant responses may be different when associated with different soil water contents. This work determined the lower and upper basal temperature of Jatropha curcas L. and verified the relationship between thermal sum and crop development under different water regimes, in the climatic conditions of Goiânia, GO, Brazil. We evaluated twenty-four plants cultivated at the planting density of 2,222.2 plants ha. Of these, twelve plants were irrigated from October 2010 to October 2012, whereas the other twelve remained unirrigated. Basal temperatures were estimated by four different methods described in the literature, in two phases of observation, maturity and total cycle. From the results, regression analysis was performed. The lower basal temperature was 4.9 and 7.2 °C, and upper basal temperature was 38.8 and 36.8 °C, respectively, for the maturity and total cycle stages. The accumulated thermal sum for the complete plant development of jatropha was 10,314.55 DD (±1574.73) for the non-irrigated treatment, and 9,260.67 DD (±735.06) for the irrigated treatment. The results of plant development showed good coefficient of determination in relation to the accumulated thermal sum.


Introduction
Jatropha curcas L. is a perennial monoic species belonging to the family Euphorbiaceae, which also includes species of castor bean (Ricinus sp.), manioc (Manhiot sp.) and rubber tree (Hevea spp.).Regarding production, its main characteristic is to supply raw material for Biodiesel generation.It is considered a rustic, sunlight-demanding crop with high resistance to drought.Jatropha presents great adaptability to diverse edaphoclimatic and low soil fertility conditions (Arruda, Beltrão, De Souza, Jamil Marur, & Severino, 2004;Freitas, Missio, Matos, Resende, & Dias, 2011).Pena, Evangelista, Casaroli, and Alves Júnior (2016) determined the temperature range suitable for the development of jatropha: between 15 and 28 °C.Moreover, the authors observed that there are no regions unfit for crop development in the state of Goiás.The water demand of jatropha, for its full development, is 1,200 mm annual water.Tolerant to drought, jatropha also tolerates rainfall between 360 and 2,400 mm.However, so as not to affect production, ideal cultivation would be in regions where the deficit does not exceed 720 mm (Yamada & Sentelhas, 2014).
Temperature is one of the main environmental parameters delimiting environmental conditions for the development of a plant species.In plants, metabolic processes are highly dependent on the temperature of the site where they grow.Environmental temperature exerts a great impact on plant photosynthesis and respiration, since it influences several biochemical reactions related to these two physiological processes (Taiz & Zeiger, 1998;Beltrão & De Oliveira, 2008).Extreme air temperatures do not favor the development of certain enzymatic reactions in plants.For this reason, each plant species develops best within a certain air temperature threshold (Bonhomme, 2000).
One simple way to describe plant growth and its development as a function of air temperature is through degree-days (DD) (ºC day), which is the difference between the average daily temperature and the minimum temperature required by the species, called lower basal temperature (Müller et al., 2009;Pilau, Battisti, Somavilla, & Schwerz et al., 2011).Degree-days are used to include the effects of air temperature on biological processes as a function of time, on different developmental stages, allowing a rough estimate of the time at which a particular phenological stage will occur (Salazar-Gutierrez, Johnson, Chaves-Cordoba, & Hoogenboom, 2013).
Most studies on the subject neglect upper basal temperature in the assessment of degree-days since it shows a high value, rarely reached in the field during crop development (Müller et al., 2009).Notwithstanding, some models use this measure for determining degree-days, highlighting the importance of knowledge of both basal temperatures to optimize monitoring strategies, to improve yield and quality, to support selection and breeding programs, and to choose species that are more adapted to the climatic conditions of the growing site (Müller et al., 2009;Martins, Reis, & Pinheiro, 2012).With temperature values below or above the basal ones, development does not occur.
Several authors have proposed different classes of agroclimatic aptitude for jatropha.Due to the lack of bibliographic records evidencing the estimated value of basal temperatures for the development of jatropha, there is no consensus on the ideal temperature range.Jatropha is a crop valued due to its great importance in the biodiesel production system, which grows largely in the country.In view of this, and since temperature is an intrinsic development factor, this work determined the lower and upper basal temperature of Jatropha curcas L. Furthermore, we verified the relationship between thermal sum and plant development under different water regimes, in the climatic conditions of Goiânia, GO, Brazil.

Experimental Site
The study was conducted in an experimental area of the Agronomy School of the Federal University of Goiás (EA-UFG), Goiânia, GO (16º35′44.22″S, 49º16′44.40″W; 722 m).According to the climatic classification of Köppen, the climate of the region is type Aw (tropical wet, with dry winter), with annual average temperature, relative humidity and rainfall of 23 °C, 70% and 1,498 mm, respectively (Silva, Heinemann, Paz, & Amorim 2014).
In the mentioned area, 768 Jatropha curcas L. plants, spaced 3.0 m apart and 1.5 m between rows, were cultivated.From this total, 24 plants were evaluated, twelve of which were irrigated from October 2010 to October 2012, and twelve unirrigated ones.

Biometric Measurements
The growth characteristics of the evaluated plants were: i) plant height-measured using graduated ruler; (ii) crown area-determined from measuring the plant crown lengths in the direction of the planting row and perpendicular to it; (iii) stem diameter-measured at a height of 10 cm from the soil surface; iv) number of branches in the main stem-by direct counting of the branches observed in the plant.Moreover, seed yield was evaluated in two harvests, one in the 2010/2011 crop season and another in the 2011/2012 crop season, both between February and March.

Weather Reports
The variables minimum air temperature (Tm, °C), maximum air temperature (TM, °C), average air temperature (Ta, °C), relative air humidity (RH%), rainfall (R, mm) and wind speed (W, m s -1 ) were obtained at the automatic meteorological station of the Agronomy School of the Federal University of Goiás, located 287 m from the experimental area.
In the SD DD model, the plant's Tb is considered to be the resultant of the lowest standard deviation, thus Tb will be the one in which the sum of degree-days (DD) generates less dispersion (Equation 2): Where, DD i is degree-days accumulated in the ith period using a Tb series, DD m is the mean of the degree-days accumulated for all itch periods, and n is the number of periods.
The SDday model is based on the basal temperatures previously selected in the preceding method, resulting from the lowest standard deviation between the different planting times (Equation 3): Where, SDday is the standard deviation (day), SD DD is the standard deviation (DD) and Tag is the global average air temperature from all observations (ºC).
CV DD was determined by the relationship between SD DD and the accumulated degree-days of all plantings (DDm) (Equation 4).CVday, in turn, was obtained according to SDday and the number of days to reach a certain development stage (Xd) (Equation 5): Upper basal temperature (TB) was determined following the same methodology applied for Tb.However, the TB value was found when the standard deviation became constant.For this, temperatures between 20 °C and 40 °C were used, varying in 0.5 °C.
After the determination of Tb and TB, we performed the calculations of degree-days (DD, °C day), estimated by the methodology proposed by Ometto (1981), from the following set of equations: Case 1, TB > TM > Tm > Tb: Case 2, TB > TM > Tb > Tm: Case 3, TB > Tb > TM > Tm: Case 4, TM > TB > Tm > Tb: Degree-day values were added in each of the evaluated stages, vegetative growth and maturity, determining the thermal sum necessary to complete each one of them.

Statiscal Analysis
The values of plant growth and development were correlated to the thermal sum by adjusting logistic and sigmoid equations.

Results and Discussions
The estimated values of lower basal temperature in the maturity and total cycle were equal to 4.9 and 7.2 °C, respectively (Figure 1), regardless of the estimation method.between 15 and 28 °C.By concept, the crop presents the highest growth rates with such values.Nonetheless, jatropha plants do not show zero growth when air temperatures below 15 °C occur, initiating only a reduction of these rates, tending to zero.This makes the value of 7.2 °C for Tb (total cycle), found in this study, to be a coherent value.Jatropha has a wide range of thermal adaptability; however, it has little tolerance to low air temperatures, and its lethal range is between -3 and 4 ºC (Andrade, Caramori, De Souza, Jamil Marur, & De Arruda Ribeiro, 2008).Jatropha seeds remain vigorous (90% germination) when submitted to temperatures of 10 °C (Oliveira, Hilst, Da Silva, Sekita, & dos Santos Dias, 2015).
The values found for upper basal temperature (TB) were 36.8 and 38.8 ºC, corresponding to maturity and total cycle, respectively, again with little difference in the methodologies used (Figure 3).2014), who found high thermal adaptability for the same crop when performing its agroclimatic zoning.Thus, these studies corroborate the TB values determined in the present study (Figure 3).
The thermal sum accumulated in the maturity stage of jatropha (Tb = 4.9 °C; TB = 38.8°C), in the two production cycles evaluated, was 3,271.00 and 2,054.05DD for non-irrigated system, taking on average 182 days to complete the development stage and conclude fruit ripening, and 2,726.60 and 1,593.35DD for irrigated system, concluding the cycle with a mean of 151 days.The number of branches was considered as a reference to development (Figure 4).for s the Table 1.Logistic and sigmoid equations, with three parameters, adjusted to the jatropha growth data, described by crown area, plant height, stem diameter and number of branches, for irrigated and non-irrigated system, as a function of the accumulated thermal sum (ST, °C day)

Equations
Figure 1

Figure 4 .
Figure 4. Growth rates: crown area, height, stem and branches (white columns), as a function of time (months-year) and accumulated thermal sum in periods of increasing or constant growth rates (black dashed lines and circles), as well as the monthly thermal sum (gray squares) and the moving average of the growth rates (gray dashed line), for non-irrigated (a) and irrigated (b) treatments