Physiological Response of Advanced Lines of Rice under Water Deficit in Campeche, Mexico

Yields of rice (Oryza sativa L.) respond to complex interactions between the genotype and the environment; rice has the particularity of being a semi-aquatic crop, and as a result of this, it presents lower adaptation to the limiting water content of soil and is extremely sensitive to stress from drought; therefore, it is the most important limiting factor in rice production. The objective of this study was to research the physiological response of rice genotypes to water stress. The grain yield and its components, leaf area and transpiration efficiency under irrigation (I) and drought (D) were evaluated in eight advanced lines from the nursery of the Latin American Fund for Irrigated Rice (Fondo Latinoamericano para Arroz de Riego, FLAR) and a control variety of rice. The experiment was established in Campeche, in the 2015 autumn-winter cycle. The grain yield and its components, as well as the leaf area were greater under irrigation conditions than under drought. The transpiration from the water stress was reduced and the plants under drought increased their transpiration efficiency. The genotypes P-V 2006 and P-V 2009 were identified, with genealogies FL05392-3P-12-2P-2P-M and FL08224-3P-2-1P-3P-M, respectively, as the most outstanding compared to the rest of the genotypes in the grain yield and its components, leaf area, total root biomass, and transpiration efficiency.


Introduction
Cultivation of rice (Oryza sativa L.) in Mexico occupies the fourth place after corn, bean and wheat in terms of surface, production and consumption, and is grown in an approximate area of 42,310 ha with a national average grain yield of 5.8 t·ha −1 [1]. Of this surface, 75% is cultivated under conditions of irrigation or permanent flooding during the cycle, with an average yield of 6.4 t·ha −1 and the remaining 25% is cultivated under rainfed conditions with average yields of 3.8 t·ha −1 [1]. However, since 2001, the national production has decreased as a result of the disarticulation of the rice chain, which is why there is use for importing it, and 85% of the national consumption of thin grain rice was imported in 2015 [2] [3] [4]. Therefore, the National Institute of Forest, Agricultural and Livestock Research (Instituto Nacional de Investigaciones Forestales Agrícolas y Pecuarias, INIFAP) in Mexico and the Latin American Fund for Irrigated Rice (Fondo Latinoamericano de Arroz de Riego, FLAR) in Colombia work together for the stockpiling, selection and biometric evaluation of materials of rice with thing long grain under the environmental conditions of the dry and humid tropics in Mexico [4].
Sowing rainfed rice is carried out in the humid tropics of the southeastern region of Mexico, which stretches from the Papaloapan Basin in part of the states of Veracruz and Oaxaca, the region of La Chontalpa (Tabasco) to the areas of Palizada and Yohaltun in the state of Campeche, zone considered as vulnerable to climate change due to the frequent losses of rice harvests caused by the "intraestival" summer drought and/or torrential rains in the autumn that cause flooding and lodging of rice crops [5]. Drought is considered as a deficit of sufficient water availability to cause a decrease in the yield of crops [6]. On the other hand, the magnitude of the decrease in the grain yield depends on the duration of the drought and severity of stress [7], in addition to the growth stage of the crop [8]. Thus, the sensitivity of rice (Oryza sativa L.) to drought increases, when the drought takes place in the flowering period [9]. As consequence, the water content is considerably reduced in the reproductive stage in plant cells, showing symptoms of withering and loss of turgidity, which has an effect on a decrease of the gas exchange and photosynthesis of the plant and, finally, on the grain yield [10].
The studies in advanced lines of rice from FLAR are directed toward parameters of stability of yields [11]. However, the low yields are related to climate change, and studies of the materials with different abiotic stresses are required for this, in this case, water stress to guarantee availability of materials with resistance or tolerance to drought [12]. Therefore, the objective of this study was to research the response of eight advanced lines and a control variety of rice (Oryza sativa L.), on grain yield and its components, leaf area, total transpiration and transpiration efficiency under irrigation (I) and drought (D) in a greenhouse, with the aim of selecting high yield materials with tolerance to drought and making recommendations to farmers of the region.

Soil and Climate in the Study Area
The experiment was established in the 2015 autumn-winter cycle in a greenhouse of Colegio de Postgraduados, Campus Campeche, five meters high with a white mesh roof, however, to avoid the incidence of rain, a polyethylene cover was placed at a height of three meters. To allow the plants to be exposed to room temperature, air was allowed into the greenhouse, lifting the mesh at the ends.

Genetic Material in Evaluation
Eight advanced lines (F6) of rice and a control variety (El Silverio) were included. The lines presented characteristics of high yield and resistance to the vector of the white leaf virus (Tagosodes oryzicolus) and rice "burn" (Magnaporthe oryzae, anamorphic Pyricularia oryzae), higher quality of grain, defined by its amylose content, appearance of the processed grain, high recovery of full grains and tolerance to the delay in harvest [14].

Study Factors and Experimental Design
The experiment consisted in two treatments of soil moisture; irrigation (I) that    tubes were weighed every third day, to calculate the amount of water lost from evapotranspiration (ET) and add the water required, to take the soil from each tube to IWFC and maintain the level of moisture close to FC from sowing to physiological maturity of plants; the same procedure was done in the drought treatment (D) as in irrigation; water was added to the tubes, to take the soil to FC and the soil water content was maintained close to FC until 65 days after sowing (das); from this date the application of water was suspended and the weight was recorded only at 114 das, date when recovery irrigation was applied.
The amount of ET (g of water) in each tube was calculated as the difference be-

Study Variables and Statistical Analysis
The following variables were evaluated: grain yield (GY, g) which was obtained

Soil Moisture Content
The soil moisture content in the irrigation treatment was kept close to field capacity (FC), during the entire experiment; in the drought treatment the soil water content was kept close to FC until beginning the drought treatment at 65 days after sowing (das). In drought the soil water content reached values under the permanent wilting Point (PWP), as the cultivation cycle happened ( Figure   1).

Grain Yield and Its Components
In irrigation, the statistical analysis of the data detected highly significant differ-  The combined statistical analysis of irrigation and drought detected highly significant differences (p ≤ 0.01) between levels of humidity, between genotypes and interaction levels of humidity-genotypes in all the variables assessed (Table   5). Drought reduced grain yield, final aerial biomass, harvest index, number of panicles per plant, number of grains per panicle, weight of one thousand grains, and plant height ( Table 5). The reduction resulting from the water stress effect was higher in the final aerial biomass (52%), grain yield (45%), number of panicles per plant (36%), weight of one thousand grains (31%), plant height (28%), number of grains per panicle (27%), and harvest index (15%) ( Table 5).

Leaf Area
In irrigation, the statistical analysis detected highly significant differences (p ≤ 0.01) between genotypes for the leaf area and statistical differences (p ≤ 0.05) at 25 days after sowing (das) (Figure 2). Figure 2   In drought the statistical analysis detected that highly significant differences (p ≤ 0.01) were present between genotypes for all the dates of measurement except at 25 das, when no significant differences were found, and at 46 das when significant differences between genotypes were detected (p ≤ 0.05) (Figure 3). In the combined statistical analysis of irrigation and drought, highly significant differences were detected between moisture levels of the soil at 32 and 53 das, and from 67 to 109 das. Highly significant differences (p ≤ 0.01) were detected between genotypes in all the measurement dates, except at 25 das when significant differences (p ≤ 0.05) were found. The leaf area under irrigation was higher in irrigation than in drought, because starting at 65 das the water deficit began at the time of suspending water in the plants in drought treatment ( Figure   4).

Total Transpiration and Transpiration Efficiency of the Plant
In irrigation, the statistical analysis of the data detected highly significant differences (p ≤ 0.01) between genotypes for transpiration efficiency and total trans-  (Table 6). For total transpiration, it was higher in irrigation (8.1 kg) than in drought (3.8 kg); in irrigation genotypes 7 and 8 showed the highest values; no significant differences were found in drought (Table 6).
In the combined analysis of irrigation and drought, highly significant differences (p ≤ 0.01) were detected between environments of humidity and between genotypes for transpiration efficiency and total transpiration per plant; highly significant differences were found in the interaction of humidity environments-genotypes for total transpiration per plant and no significant differences were found for transpiration efficiency (Table 7).

Discussion
When comparing the result from nine rice genotypes under conditions of irriga-  [19]. The variable height is an aspect to consider in the selection of rice materials, in this experiment it was higher in irrigation than in drought and fluctuated between 67 to 87 cm, the height is associated with lodging problems which is why a taller height represents higher risk in the plant's fall, and what is sought in a material of long and thin grain is a compact height to avoid the loss of the yield from lodging [3] [12]. The least affected variables were days to anthesis (9%) and days to physiological maturity (12%), with a difference of eight and five days until flowering and days until physiological maturity, respectively, between irrigation and drought, with them being higher in drought; it was mentioned that drought before flowering delays the anthesis and this delay reduces the percentage of fertile spikelets and full grains [20]; as a result there is a lower photosynthetic rate, relative water content and stomata conductance, and these characteristics show a close relationship with the efficiency in water use, which is reflected in a reduction in grain yield [21]. The importance of leaf development for the crops depends on the interception of solar radiation, key element for the photosynthetic activity that is of vital importance in biomass production with a considerable contribution to the yield [22]. The leaf area in irrigation was higher than in drought, because starting at 65 days after sowing, the water deficit began when watering was suspended in the plants; this is because the plants generally limit the area and number of leaves in response to the water stress to reduce the consumption of water and avoid losses in the yield [23]. The transpiration efficiency was higher under drought (9.8 g·kg −1 ) compared to irrigation ( On the contrary, the total transpiration per plant was 8.1 kg of water, higher than the treatment under drought that showed 3.8 kg of water; when the soil moisture begins to decrease, the most effective response of the plant is to reduce transpiration. The first signs of water decrease in the soil are leaf rolling and closing of stomata, and they are the basic mechanism to reduce the impact of drought and are induced first in the vegetative phase and then in the reproductive phase [26] [27].

Conclusion
The advanced lines of long grain rice generally presented a higher capacity for adaptation to the producing zone of Campeche outperforming the control variety; the genotypes identified as one and two presented higher grain yield and its components, as well as plant height, leaf area, transpiration efficiency, both in irrigation and in drought, which makes them strong candidates to be used as progenitors in breeding programs and as commercial varieties in irrigation and drought production systems.