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Introduction
Agroclimatic suitability for fruit growing depends on the dormancy characteristics in deciduous trees; if the accumulation of chilling in dormancy is deficient, physiological disorders occur in the production of biomass and fruits (Gariglio, Dovis, Leva, García, & Bouzo, 2006). In this sense, temperature is one of the main controls over plant distribution and productivity, with effects on physiological activity at all temporal and spatial scales (Sage & Kubien, 2007; Yepes & Silveira, 2011). Plants are adapted to temperatures between 5 °C and 40 °C, in which dry mass production and growth occur (Yepes & Silveira, 2011). High temperatures cause stress (heat stroke) that produces adverse effects on plants, but it can be prevented with good nutritional status. Heat stroke is a series of metabolic dysfunctions and physical restrictions that can not be adjusted, avoided or corrected, and that causes excessive transpiration that can alter a tree’s food reserves. Trees find optimum growing conditions across a range of temperatures from 21 °C to 29 °C, so high temperatures can damage and cause death at a threshold of 46 °C (Coder, 2016); damage varies depending on the duration of high temperatures at the site. Irradiation is also important for the physical and biological processes of living beings: plants use it to produce carbohydrates through photosynthesis (Taiz & Zeiger, 2002). Also, irradiation, when interacting with temperature and precipitation, influences plant growth and yield (Rivetti, 2006).
The pecan tree (Carya illinoinensis K. Koch) is a deciduous fruit tree of the family Juglandaceae (Briceño et al., 2018; Muncharaz, 2012; United States Department of Agriculture - Natural Resources Conservation Service USDA-NRCS, 2016), native to Mexico and the United States, leaders in global production (Gardea, Martínez, & Yahia, 2011; Orona, Sangerman, Fortis, Vázquez, & Gallegos, 2013). According to the Instituto Nacional de Innovación Agraria (INIA, 2004), pecan does not require chilling in winter. The buds respond to temperatures above 10 °C, regardless of the accumulation of chilling. Climate influences pecan tree development since it directly generates physiological damage (Medina & Cano, 2002; Grageda, Ruiz, Jiménez, & Fu, 2014). Pecans develop at an average of 26.7 °C; in winter they require from 7.2 to 12.3 °C. High temperatures in the Comarca Lagunera occur from May to August (25.3 to 26.7 °C), and the coldest occur from December to February (7.2 to 13.0 °C), which are optimal for the development and production of biomass and pecan fruits (Medina & Cano, 2002). The temperature in the Comarca Lagunera between 2013 and 2016 was 27.06 °C from May to August and 14.82 °C from December to February (Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias INIFAP, 2017). On the other hand, pecan requires an adequate amount of light from 700 to 800 (μmol·m-2·s-1) (Lombardini, Restrepo, & Volder, 2009) to achieve an optimal photosynthetic rate. This light intensity requirement exceeds that of other fruit trees, so pecan is a fruit tree that is relatively intolerant to shade. Pecan leaves require high light intensities due to their high photosynthetic capacity (Gómez, Arreola, Trejo, & Flores, 2006).
Currently, studies concerning vegetative reserves in trees are still incomplete or have been carried out on a limited number of species. The starch concentration in perennial organs is an indicator of the ability of trees to cope with stress conditions such as defoliation, drought, and pollution (Kozlowski, Kramer, & Pallardy 1991). In the case of pecan, at varietal level, there are no approaches to study the physiological processes in starch in relation to environmental factors. Therefore, the aim of the present study was to determine the influence of temperature and irradiation on the starch concentration in the root and trunk of C. illinoinensis varieties Wichita and Western.
Materials and methods
Study area
The study was carried out at the Laguna Unit Experimental Field of the Universidad Autónoma Agraria Antonio Narro (25° 33' 22.63'' N and 103° 22' 07.77'' W) in Torreón, Coahuila de Zaragoza, Mexico. The region has a desert climate with average annual rainfall of 230 mm (Instituto Mexicano de Tecnología del Agua ?αμπ;#91;IMTA?αμπ;#93;, 2005) and an elevation of 1 120 m (Instituto Nacional de Estadística y Geografía ?αμπ;#91;INEGI?αμπ;#93;, 2012). The C. illinoinensis orchard is under agronomic management with flood irrigation using well water and a planting density of 100 trees·ha-1 established in a real frame. Scheduling includes eight irrigations with intervals of 12 to 47 days, depending on the phenological stage, with a total irrigation depth of 748 mm·year-1. The water has pH 8.2 and electrical conductivity of 1 480 µΩ·cm-1.
Tree sampling
A systematized sampling method consisting of randomly selecting trees was used (Valenzuela et al., 2011). Four adult trees of the Western (WN) and Wichita (WA) varieties were selected with an average age of 30 years. The plots of each variety were 1 ha in size; in each plot, sampling was carried out in an area of 0.3 ha in monthly periods during an annual production cycle. Two root and two trunk samples (Valenzuela et al., 2011) were taken from each tree. The trunk samples were obtained in the form of chips with a Pressler® drill; for the root samples, a conventional pick was used to make a small trench to locate the main root and extract the sample. The samples were carefully cleaned by removing soil debris and placed in perforated and labeled aluminum bags in a refrigerator at 5 °C.
Determination of starch concentration
The starch concentration was determined using the technique established by Ebell (1969) and Haissig and Dickson (1982). For the determination, 10 mg of pulverized root and trunk dry matter was weighed in microtubes on an analytical balance, and 1 mL of distilled water was added and mixed with a vortex mixer for 1 min. Subsequently, the root and trunk samples were boiled on an electric plate for 10 min at 100 °C to gelatinize the starch.
The samples were centrifuged at 2 500 rpm for 2 min, and 300 μL of the extract was taken and placed in new microtubes. Then 900 μL of absolute ethyl alcohol was added and centrifuged at 10 000 rpm for 5 min to precipitate the starch. The microtube alcohol was carefully emptied to leave only the precipitated starch, and 1 mL of distilled water was added. The microtubes were placed in the vortex mixer for 3 min, and 50 μL of iodine solution (0.01 N) was added to each of the microtubes. Finally, the absorbance was recorded in a UV-Visible spectrophotometer (Genesys 20, Thermo Scientific®, USA) at 595 nm using 1 mL of distilled water and 50 μL of iodine as controls.
Environmental factors
Monthly data on average temperature (°C) and irradiation (MJ·m-2) were obtained from the INIFAP weather station at the La Laguna Experimental Field (INIFAP, 2017). The units and conversion factors of solar irradiation for plants were obtained from the International System of Units and INRA (Institut National de la Recherche Agronomique) models (Bonhomme, 1993).
Statistical analysis
Pearson correlation and multiple regression analyses were performed. In the latter, the relationship that offered a better fit (linear or quadratic) was considered to determine the relationship between the starch concentration in each compartment (root and trunk) and environmental factors (temperature and irradiation); residual analysis was developed for each test. The tests were considered significant with P < 0.05, and the analyses were carried out in the SPSS (2009) version 18.0 statistical program.
Results
Correlation and regression analyses of the Western variety root
The value of the multiple correlation between starch concentrations and the temperature and irradiation in the root of the WN variety was significant (R = 0.954, P < 0.001). The multiple linear regression analysis indicated a significant relationship between root starch concentrations and environmental factors (F = 44.394, df = 2,9, P < 0.001, R2 = 0.908). The regression equation obtained was:
The R2 of 0.908 indicates that 90.8 % of the variability in starch concentration can be explained by the temperature and irradiation present in the crop. The remaining 9.2 % is explained by other factors not analyzed in this research. There was no violation of the normality assumptions of the residuals. Figure 1 shows the relationship of temperature and irradiation in relation to the starch concentration in the Western variety.
Correlation and regression analyses of the Western variety trunk
The value of the multiple correlation between starch concentrations and environmental factors in the trunk of the WN variety was not significant (R = 0.725, P = 0.200). The best fit in the regression analysis was quadratic; however, it was not significant (F = 1.946, df = 4,7; P = 0.207, R2 = 0.526). There was no violation of the normality assumptions of the residuals. The regression equation obtained was:
Correlation and regression analyses of the Wichita variety root
The value of the multiple correlation between starch concentrations and the temperature and irradiation in the root of the WA variety was significant (R = 0.922, P < 0.001). The multiple linear regression analysis indicated a significant relationship between root starch concentrations and environmental factors (F = 25.608, df = 2,9; P < 0.001, R2 = 0.851). The regression equation obtained was:
The value R2 = 0.851 means that 85.1 % of the variability in concentration can be explained by the temperature and irradiation present in the crop, while the remaining 14.9 % is explained by other factors not analyzed in this research. There was no violation of the normality assumptions of residuals. Figure 2 shows the relationship of temperature and irradiation with respect to the starch concentration in the Wichita variety.
Correlation and regression analyses of the WA variety trunk
The value of the multiple correlation between starch concentrations and environmental factors in the trunk of the WA variety was not significant (R = 0.732, P = 0.154). The best fit in the regression analysis was quadratic; however, there was no significant relationship between starch concentrations and environmental factors (F = 2.030, df = 4,7, P = 0.194, R2 = 0.537). There was no violation of the normality assumptions of the residuals. The regression equation obtained was:
Discussion
In Mexico, pecan trees develop well where the average temperature in summer is 25 °C to 30 °C without wide variation between day and night, with an average of 26.7 °C. In the coldest months, an average between 7.2 °C and 12.3 °C is required for pecan trees to have good probabilities for proper development and production (Medina & Cano, 2002). Potisek, González, Chávez, and González (2009) state that pecan in its natural environment develops with average annual temperatures ranging from 10 °C to -1 °C during the winter and maximums of 41 °C to 46 °C during the summer; INIA (2004) indicates that the species does not require chilling in winter. The floral and vegetative buds respond to temperatures above 10 °C that occur at the beginning of spring, regardless of the accumulation of chilling during the winter period. Yepes and Silveira (2011) note that terrestrial plants are adapted to temperatures between 5 °C and 40 °C, within which dry mass production and growth takes place. Finally, Restrepo-Díaz, Melgar, and Lombardini (2010) note optimal temperatures in other fruit trees: Malus domestica Borkh, 20 °C (Higgins et al., 1992); Mangifera indica L., 30 °C (Yamada, Fukumachi, & Hidaka, 1996); Annona cherimola Mill., 20 °C (Higuchi, Sakuratani, & Utsunomiya, 1999); Vitis vinífera L., 27 °C (Higgins et al., 1992); Prunus armeniaca L., 25 °C (Wang, Wang, & Wang, 2007) and Ficus carica L., 28 °C (Can & Aksoy, 2007). In the present study with pecan, the winter stage temperature was 15 °C to 17 °C and in summer it was 25 °C to 30 °C, an interval that covers the temperatures indicated by the aforementioned authors, which gives guidelines for subsequent studies to focus on evaluating the growth and production of pecan in the region. The inversely proportional behavior of the starch concentration in the pecan root with respect to temperature coincides with that reported by Vasconcelos-Ribeiro, Caruso-Machado, Espinoza-Núñez, Augusto-Ramos, and São Pedro-Machado (2012), who indicate that high temperatures promote vegetative growth and thereby decrease starch in the Citrus root.
Chávez, González, Valenzuela, Potisek, and González (2009) reported that PAR (Photosynthetically Active Irradiation) levels range from 1 011 to 1 181 μmol·m-2·s-1 in the final period of the maximum vegetative growth stage in pecan seedlings. Yepes and Silveira (2011) explain that, in Hymenaea courbaril L. plants, the photosynthetic response to light is 150 to 800 μmol·m-2·s-1. Finally, Restrepo et al. (2010) report the following sunlight saturation values of other fruit trees: 1 000 to 1 100 μmol·m-2·s-1 in Olea europea L.; 1 800 to 1 900 μmol·m-2·s-1 in M. domestica; 1 800 to 1 900 μmol·m-2·s-1 in V. vinífera; 1 100 μmol·m-2·s-1 in F. carica; 1 300 μmol·m-2·s-1 in Prunus persica L. (Higgins et al., 1992); 1 130 to 1 330 μmol·m-2·s-1 in Prunus dulcis (Mill.) D. A. Webb (De Herralde, Biel, & Savé, 2003); 750 to 1 000 μmol·m-2·s-1 in Citrus sinensis L. (Caruso, Tambelli, Lázaro, & Vasconcelos, 2005) and 700 to 800 μmol·m-2·s-1 in C. illinoinensis (Lombardini et al., 2009). During the present study, the pecan trees received irradiation of 13 to 19 MJ·m-2 or its equivalent of approximately 700 to 900 μmol·m-2·s-1, so the solar irradiation received in the study area is suitable for development by avoiding stress, since light interception is an important factor affecting photosynthesis in fruit trees. Taiz and Zeiger (2002) and Restrepo-Díaz et al. (2010) reported that the effect of environmental factors (drought, temperature and irradiation) on plant growth and development results in response mechanisms such as acclimatization and adaptation.
This study showed that temperature influenced starch concentrations, mainly in the root, obtaining significant regression for both pecan varieties, slightly higher in WA (WN, R2 = 0.854, P < 0.001; WA, R2 = 0.897, P < 0.001), which indicates that this variety is better adapted to the region’s climate. The results show that temperature has an effect inversely proportional to the starch concentration in the root; that is, if the temperature increases, the starch concentration in the root decreases. This is due to the demand for carbohydrates in the aerial parts of the plant, which occurs when the environmental temperatures increase during the whole growing period, mainly for the emission of foliage (Restrepo-Díaz et al., 2010; Taiz & Zeiger, 2002, 2006). In addition, it was observed that, for both the Western and Wichita varieties, the maximum starch concentration in the root is recorded when temperatures are in the order of 15 to 20 °C. On the other hand, irradiation had a similar impact, since it was observed that the starch concentration in the root decreases at a higher irradiation rate; in both varieties, the highest concentrations were obtained between 10 and 15 MJ·m-2. According to the results, the Wichita variety is the best adapted to the temperature conditions and high irradiation rates in the region.
Conclusions
Of the two varieties with the largest cultivated area, the Wichita variety was found to have better conditions for the accumulation of starch reserves in the root and trunk, since its requirements for development are lower than in the Western variety. The influence of environmental factors on a given variety is of great importance, as having the proper conditions does not affect the growth and development of the plant; that is, selecting a good variety depends on knowing the effect that an environmental factor can have on the physiology of the crop. The study was carried out on adult trees, so it would be advisable to make a comparison of carbohydrate reserves in young trees, as well as relate environmental factors to the starch concentration at different ages, to know at what point in the life of the tree its starch reserves are most affected.
