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INTRODUCTION
Rice (Oryza sativa L) is a crop with high demand for world food and its production reaches more than 700 million tons (Méndez del Villar, 2019). Cuba is one of the highly consuming nations of this cereal, which amounts to 80.38 kg / person / year.
Part of the national production comes from the Agroindustrial Grains Enterprise "Sur del Jíbaro" which is located in La Sierpe Municipality (Sancti Spíritus Province) and this entity is the second largest producer in the country, although the negative effects of climate change have caused the productivity loss of rice crop (Zedeño et al., 2016).
Different researchers have examined the relationship between the decrease in rice crop yields and climatic variables such as air temperature and solar radiation at certain fixed periods of rice growth, which lead to the definition of regression models for productivity estimation of irrigated rice (Pedro Júnior et al., 1995; Steinmetz et al., 2013 and Macedo, 2014).
On the other hand, in the evaluation of agrometeorological conditions for rice production in Cuba, Delgado (2012) considers that the combined action of climatic variables such as temperature, relative humidity, wind strength, solar brightness and rainfall have a determining effect on rice cultivation under production conditions. In this regard, Rivero-Vega et al. (2018) report that the decrease in temperature and the decrease in solar radiation lead to an increase in the potential yields of rice.
Considering the aspects discussed above, this research work was carried out with the objective of estimating crop yields of irrigated rice varieties depending on different climatic variables.
MATERIALS AND METHODS
The research was conducted during the period July / 2011-July / 2012 in the experimental areas of the GrainsTerritorial Research Station Sur del Jíbaro, located at 21 ° 41'3 "north latitude and 79 ° 11'6" west longitude at an elevation of 30 meters above sea level, sowing the varieties monthly under flooding conditions in a Gleysol Vertico soil (Hernández et al., 2015).
Soil preparation was done with direct puddling technology and the method of direct broadcast sowing of pregerminated seed was used with a standard sowing of 120 kg ha-1. Pregermination seed and cultural attentions were carried out according to the technical recommendations of rice crop in Cuba defined by the Instituto de Investigaciones del Arroz (2008). Experimental plots size was 100 m2 with a calculation area of 75 m2, arranged in random blocks for each treatment (months) with five replications.
For the precision of a development period of rice plant, the grain ripening phenophase was considered according to Yoshida & Parao (1976), which was determined under field conditions according to the current methodology for this crop (CIAT, 1980).
Daily recording of climatic variables carried out compiling weather data: air temperature (maximum, average and minimum) in ° C and relative humidity (%), in the duration of the phenophase analyzed, taking the Meteorological Station as a reference. No. 78 341, located at the geographic coordinates 79.191581 North Latitude and 21.691545 West Longitude and elevation of 30 meters above sea level in El Jibaro Town of La Sierpe Municipality, Sancti Spíritus Province.
Crop yield data in t ha-1 at 14 % humidity and climatic variables were tabulated in Microsoft Excel 2003 program. Statistical processing was performed in SPSS version 21 program on Windows IBM Corporation (2011) by multiple linear regression (stepwise method). In this way, correlation coefficients were obtained with regression equations that express relationship between climate and crop yield of the rice crop at that site, as well as yield estimation by interpolation method.
RESULTS AND DISCUSSION
The results of the correlation - regression analysis in sowing months between the main elements of the climate in Sur del Jíbaro (Sancti Spíritus) are reflected in Table 1.
TABLE 1 Main results of multiple linear regression (stepwise method) between the climatic variables and crop yield per variety
| Variety | r | Signification | Equations |
|---|---|---|---|
| IACuba 31 | 0,818 | 0,021 | Rend = 21,674 - 0,391*Tmáx - 0,060*Hr |
| IACuba 40 | 0,520 | 0,123 | Rend = 7,079 - 0,164*Tmin |
| Jucarito 104 | 0,865 | 0,031 | Rend = 2,914 - 0,466*Tmin + 0,468*Tmáx - 0,050Hr |
| Procequisa-4 | 0,839 | 0,050 | Rend = - 2,808 - 1,276*Tmin - 0,205*Tmáx + 1,517*Tmed |
Rend: Crop yield at 14 % humidity
Tmax: Air temperature (maximum)
Tmin: Air temperature (minimum)
Tmed: Air temperature (mean)
Hr: relative humidity
Generally, the correlation is acceptable between crop yields with air temperature (maximum, average and minimum) in Procequisa 4 variety. This result was also obtained in IACuba 31 and Jucarito 104 varieties, where relative humidity constitutes one of the factors associated with the expression of crop yield. In the case of the IACuba 40 variety, the correlation between variables was regular.
Another important aspect is that negative coefficients were obtained in linear equations which expresses that, the increased value of the independent variables, reduces crop yield (dependent variable). That is shown in the estimation by means of linear equations in Figures 1, 2, 3 and 4.
FIGURE 1 Crop yield estimation in IACuba 31 variety depending on Tmax variation and mean value Hr = 87.74 %.
FIGURE 3 Crop yield estimation in Jucarito 104 variety depending on Tmin variation, considering the mean values of Hr = 90.11 % and Tmax = 31.44 °C.
FIGURE 4 Crop yield estimation in Procequisa 4 variety depending on Tmin variation and the mean value Tmax = 31.44 ° C.
The nature of the statistical model that describes the relationship between the variables coincides with De Datta (1982) investigations where it is stated that low relative humidity influences the glume opening. Also, these results contrast with Franco & Ramírez (2000) studies on the sowing schedule in rice crop at this site, where relative humidity values were negatively correlated with crop yield.
Estimating crop yields showed that this parameter decreases approximately 5% for the variety IACuba 40, 10% for the variety IACuba 31, 13% for Procequisa 4 and Jucarito 104 varieties in increments of 1 °C produced from maximum and minimum temperatures in cultivars analyzed for phenophase maturation. These results are below that reported in the literature where Mohandass et al.(1995) and Peng et al. (2004) assert that the decrease in rice productivity is above 14.5% and 15%, respectively, in these climatic conditions.
CONCLUSIONS
Biometrically, it was shown that the variables air temperature and relative humidity have a significant effect on the productivity of the rice crop.
The decrease percentage of crop yields ranged between 5% and 11% for each increase of 1 °C of the maximum and minimum temperatures, with greatest effect involvement in Procequisa 4 and Jucarito 104 varieties.
