Irrigation Depth and Carnauba (Copernicia prunifera) Straw Increase Water Use Efficiency in the Cherry Tomato in a Semi-Arid Region

This research addresses irrigation strategies for saving water, including the use of irrigation depths of 50, 75, 100, 125 and 150% of the crop evapotranspiration (ETc) and a ground cover of carnauba straw, in evaluating the response of the cherry tomato (Solanum lycopersicum var cerasiforme L) submitted to different levels of water availability in soil with and without ground cover, in the semi-arid region of northeastern Brazil. The physiological components showed higher values under ground cover and when submitted to 100% ETc. The greatest number of total and commercial fruit was obtained when the soil was covered, and irrigated with depths of 75 and 100%. Carnauba straw with irrigation depths of 100 and 125% resulted in the greatest fresh fruit weight. The highest values for fruit diameter and length were achieved in the soil under cover. Maximum productivity, 11,404.20 kg ha-1, was obtained at 107.5%, while for the soil with no ground cover, maximum productivity was 7,778.86 kg ha-1, obtained with an irrigation depth of 140%. Based on the results, soil productivity is possible without a cover of carnauba straw, at an irrigation depth of 50.5%, which can generate savings of 3400 m³ water-cycle-1 ha-1. In addition, water use efficiency in the cherry tomato was 3.06 kg m-3 and 1.95 kg m-3, with and without ground cover respectively. It can be concluded that the use of carnauba straw makes it possible to reduce water consumption in the 'red' cherry tomato under conditions of water scarcity.

Irrigation water is becoming a limited resource in arid and semi-arid regions. Therefore, controlling the water supply should be considered high priority (Mattar et al., 2020;Pé k et al., 2014). One of the alternatives for increasing water use efficiency in agriculture is through the adoption of irrigation systems that use little water in the soil; this also contributes to maintaining its aggregate structure, avoiding the risk of degradation and salinization (Wang et al., 2011a).
In addition to the use of more localized irrigation methods, such as micro-sprinklers and drips, one other alternative is the use of ground cover. This method, in addition to preserving and improving the quality of the soil in the long term, also serves as a barrier, preventing evaporation of the water, reducing the speed of surface runoff and, as a result, allowing greater infiltration in the soil, reducing infestation by invasive plants, and enabling greater temperature stability in the soil surface layers (Mulumba and Lal, 2008;Plaza-Bonilla et al., 2015).
Despite the issues described above, studies on the physiology of vegetables grown in different water regimes under the climate conditions of the semi-arid region are limited. Such a situation makes this study fundamental, as it can help, not only to improve water use efficiency in agriculture, but mainly because it allows the cultivated area to be increased without necessarily requiring greater water availability.
The aim of this work, therefore, was to evaluate the physiological and productive effects of the application of different irrigation depths, with and without ground cover, on cultivating cherry tomatoes in the semi-arid region of the Northeast.

Experimental Area
The experiment was carried out in the field from June 2016 to January 2017. The experimental area is located in the Vale do Curu, district of Pentecoste, in the state of Ceará . During the experimental period, the mean daily maximum and minimum temperatures in the area were 38.6 and 23.5°C respectively. The mean relative humidity as 58.5% and the mean daily reference evaporation, 6.21 mm. The mean annual rainfall is 797.0 mm, with the rainfall distribution considered to be irregular. According to Köppen, the climate is type BSw'h', hot semi-arid. 633 Eca -Evaporation of a Class A pan, in mm; Kp -Pan coefficient.
The irrigation required to replace the total volume of water lost through evapotranspiration (ETc) was applied in two instalments per day. As the irrigation system was localized, the ETc was corrected relative to the percentage wet area (PWA), resulting in the localized crop evapotranspiration (ETcloc) (Bernardo et al., 2009 Fd -Percentage irrigation depth defined by the treatment, in (%); Ea -Application efficiency, in (%); Se -Spacing between emitters, in m; Sr -Spacing between rows, in m; Qa -Mean dripper flow, in L. h -1 .
For the first 11 days after transplanting, each treatment received the irrigation depth required by the crop, to guarantee the uniform development of the plants until becoming established.
The different irrigation managements were started on the 12° (DAT -day after transplanting. A drip irrigation system was used, with Katif self-compensating button emitters installed in 16 mm polyethylene tubes at a spacing of 0.50 m, with a flow rate of 3.8 L h -1 and a working pressure of 20 mca. Irrigation was controlled by valves, with one in each plot.

Gas Exchange
Ninety days after sowing (DAS), the physiological aspects of the plants were evaluated with the LI6400XT® (LI-COR) infrared gas analyzer (IRGA). The third pair of fully developed leaves was used in the analysis.

Water Use Efficiency
The water use efficiency was determined from the ratio between the values for total fruit biomass and the amount of water applied (mm) in each treatment during cultivation, as per equation 6: W -Total irrigation depth applied during the crop cycle, in mm.

Statistical Analysis
Plant development data were submitted to analysis of variance by F-test, with the mean values compared by the Scott-Knott test (p>0.05) using the Sisvar statistical software (Ferreira, 2011). The physiological characteristics were analyzed graphically by regression analysis, and trend curves were prepared using the SigmaPlot 11.0 and TableCurve 2D v5.01 statistical software.

Results and Discussion
Daily evapotranspiration in the cherry tomato varied from 0.73 to 6.0 mm day -1 during the experimental period. A drop in the ETcloc was seen at the end of the crop cycle, probably influenced by the increase in cloud cover.
The values for localized evapotranspiration (ETcloc) were lower than those of the reference evapotranspiration (ETo), this difference being influenced by the wet-area correction (PWA), as well as by the crop coefficient (Kc), as shown in Figure 1. According to the researchers, the mean relative humidity at the experimental site was 58.5% and the maximum and minimum temperatures were 37.2 and 28.5º C respectively, different from the ideal temperatures reported by some researchers (Goto and Tivelli, 1998 ) for the tomato at different stages of crop development ( Figure 3). According to Goto and Tivelli (1998), the optimal temperature for the tomato varies depending on the stage of development: during the vegetative stage, from 20 to 30°C; during pollen formation, from 20 to 26°C; during growth of the pollen tube, from 22 to 27°C; during fruit setting, from 18 to 20°C, and during ripening, from 24 to 28°C. According to Taiz et al. (2017), the lethal temperature for the tomato is 45º C.
However, even at this temperature, with abundant access to water, most plants are able to keep the temperature of the leaves below this limit, using a defence mechanism known as evaporative cooling, including environments with high temperatures.
Evaluating the morphological characteristics of the 'red' cherry tomatoes, it was found that the different water levels applied to soil covered with carnauba straw and to soil with no cover resulted in variations in the production and quality characteristics of the harvested fruit ( Figure 4). The greatest number of fruit was obtained when the soil was covered with carnauba straw; the plants submitted to depths of 75 and 100% ETcloc produced the greatest number of total ( Figure 4a) and commercial fruit (Figure 4b), while the plants submitted to 150% ETcloc produced the greatest number of non-commercial fruit (Figure 4c).

Days after transplanting
The larger amount of non-commercial fruit resulting from the application of 150% ETcloc may be related to an excess of water in the soil, which was probably the main reason for the  (Taiz et al., 2017) as well as the low elasticity of the cell walls that comprise the fruit epidermis.
The results obtained in this study were superior to those obtained by Silva et al. (2011), who evaluated 11 strains of cherry tomatoes, harvesting an average of 148.92 fruits plant -1 .
For the total fresh ( Figure 5a) and commercial weight of the tomato fruit ( Figure 5B), the soil with ground cover, submitted to irrigation levels of between 50 and 125% ETcloc, afforded better conditions for obtaining greater fruit weight compared to the soil with no cover. The highest values obtained for these characteristics occurred in covered soil and with the application of 100% of the crop ETcloc ( Figure 5).  ISSN 2166-0379 2020 Fig. 5. Total, commercial and non-commercial fresh fruit weight in the 'red' cherry tomato. Uppercase letters compare water conditions for the same type of cover. Lowercase letters cover. Mean values followed by uppercase letters on a row and lowercase letters in a column do not differ by Scott-Knott test (p>0.05)

Journal of Agricultural Studies
The soil with ground cover, submitted to irrigation levels between 50 to 125% ETcloc, resulted in the greatest fresh weight production of non-commercial fruit (Figure 5c). For the plants grown in covered soil and submitted to 150% ETcloc, the lower value for non-commercial fresh weight is due to most of the fruit being cracked, causing a loss of moisture to the environment, and resulting in a lower weight for this treatment.
Despite the number of total and commercial fruit at the irrigation depth of 50% being higher for the soil with no cover, they were not greater for total or commercial fresh weight. A similar result was found by Abdel-Razzak et al. (2016) and Abdel-Razzak et al. (2013), where the weight, size and total yield of the cherry tomato fruit decreased as the amount of water applied was reduced under the treatment with water stress (50% ETc).
This lower production may be related to the greater number of inflorescences seen in the plants under this treatment compared to those grown under other treatments. The large number of flowers produced can influence the production of photoassimilates (Picanç o et al., 1998), which may be insufficient, thereby not meeting the demand of the plants during the flowering and fruiting stages, causing the flowers to abort.
Controlling the number of flowers, fruit and stems reduces competition for photoassimilates by the drainage organs, especially the fruit. This practice tends to lead to larger fruit (Beckles, 2012), mainly due to the reduction in competition by the drains (flowers and fruits) for the photoassimilates produced by the sources (leaves).
For the characteristics of diameter ( Figure 6a) and length (Figure 6b) in the cherry tomato fruit, a difference was also seen between plants grown under the different irrigation depths, both with and without ground cover. The mean values for diameter varied from 11.59 mm to 21.65 mm for soil with and without cover at water depths of between 50 and 150% ETcloc.
In general, the greatest values for diameter and length were obtained with the plants grown in soil with ground cover, especially at the irrigation depth of 150% ETc, since the largest fruit diameters were seen for this combination. ISSN 2166-0379 2020 Fig. 6. Fruit length (a) and Diameter (b) in the 'red' cherry tomato. Uppercase letters compare water conditions for the same type of cover. Lowercase letters compare water levels between the two types of cover. Mean values followed by uppercase letters on a row and lowercase letters in a column do not differ by Scott-Knott test (p>0.05).

Journal of Agricultural Studies
The length and diameter of the fruit are affected by turgor pressure (Silva et al., 2013), i.e. the pressure exerted by water on the membranes and cell walls of the fruit. Water stress caused by the lack of water reduces the potential for cell wall expansion, with the fruit remaining small compared to fruit produced with a greater supply of water. The above statements were confirmed in this study, since when water availability was from 50 to 150% ETcloc, an increase was seen of 57.22 and 49.10% in the diameter and length of the fruit respectively. These results agree with those obtained by Abdel-Razzak et al. (2016), according to whom, as the level of water supplied to the plants is increased, the weight and mean size of the fruit increases.
For the physiological characteristics evaluated 90 days after sowing, the plants grown with ground cover and those grown with no cover showed different responses. In general, the highest values were usually found in plants grown in soil covered with carnauba straw, with  ISSN 2166-0379 2020. Gas exchange in the 'red' cherry tomato grown in soil with ground cover (Carnauba straw) and with no ground cover 90 days after sowing, with both submitted to different levels of irrigation.

Journal of Agricultural Studies
The maximum value seen for stomatal conductance (gs) was 0.46 mol m -2 s -1 , while in the soil with no ground cover, the maximum conductance was 0.22 mol m -2 s -1 . It can be seen in Figure 7a that the use of cover resulted in the cherry tomato always showing higher values than when grown in soil with no cover.
The results for stomatal conductance reflect the positive effect of using a ground cover of carnauba straw compared to the soil with no cover. This is because the use of cover results in soil moisture being maintained over time, with the water in the soil available to the plants for a longer period.
Water being available to the plants for a longer period favours the maintenance of stomatal opening, with a direct influence on conductance, which increases (Taiz et al., 2017). This corroborates Lima et al. (2009), who cite the use of mulch as retarding water deficiency.
Similar behaviour to that of stomatal conductance was seen for transpiration rate ( Figure 7B) and photosynthesis ( Figure 7C), confirming the assumption that soil covered with straw is more efficient at keeping moisture, and becomes more efficient over time, allowing greater opening of the stomata, through which water loss (transpiration) and the entry of carbon dioxide for photosynthesis takes place.
According to Manzoni et al. (2013), transpiration depends on the amount of water in the soil, as well as the density and depth of the root area.
The physiological factors are all interconnected and reflect decisively on each other. The reduction in the hydraulic conductivity of the soil probably limited the amount of water available to the plants, which reduced stomatal conductance, thereby affecting transpiration and photosynthesis (Manzoni et al., 2013).
Monitoring gas exchange through stomatal closure is considered a difficult process, as plants  ISSN 2166-0379 2020 have to 'choose' between reducing CO2 assimilation by closing the stomata, or losing water by opening them, both of which are detrimental to plant development; the plant, however, generally tends to favour the photosynthetic rate (Pereira et al., 2012).

Journal of Agricultural Studies
Water use efficiency, evaluated by the relationship between the net photosynthesis of the plant and its rate of transpiration, related to the amount of water lost by the plant and the amount of carbon that the plant is able to fix (Taiz et al., 2017), was superior in plants grown with ground cover (Figure 7d), since these were more efficient in instantaneous water use compared to those grown in soil with no cover. However, a reduction was seen when irrigation depths greater than 104.7% ETcloc were applied to tomato plants grown with ground cover. This may be related to a possible excess of water in the root system that could be reducing the availability of O2 necessary for energy production and cell multiplication, among other recurrent physiological processes in the root system. This reduction in oxygen availability induces stomatal closure (Mesquita et al., 2013), apparently without detectable changes in the water potential of the leaf.
A decrease in oxygen availability to the roots has a similar result in the plant as a water deficit, since both can stimulate the production of abscisic acid (ABA) and its being directed to the leaves (Taiz et al., 2017).
Unlike in this study, Marouelli et al. (2003) and Sá et al. (2005) found a linear increase in water use efficiency when the tomato was submitted to increasing levels of soil water tension. In the soil with no ground cover, the plants grown under a water availability of 113.6%, achieved greater maximum efficiency in instantaneous water use, with a WUEi value of 2.38, corresponding to 58.04% less than the soil with ground cover.
Regarding the internal CO2 concentration in the substomatal chamber, and its ratio to the atmospheric CO2 concentration and instantaneous carboxylation efficiency, the cherry tomato plants grown with ground cover (Figure 7e, Figure 7g and Figure 7h respectively), showed similar behaviour, compatible with the other physiological characteristics already studied, this being directly related to stomatal conductance and photosynthesis. The greater the stomatal opening, the greater the tendency for CO2 to enter the sub-stomatal chamber.
This larger volume of CO2 enabled higher rates of photosynthesis, at least up to a certain limit, which in the case of this study appeared to be when there was an excess of irrigation, with ETcloc greater than 100%. The possible 'flooding' effect may have caused the stomata to close and reduced photosynthesis (Taiz et al., 2017), and may have maintained a high concentration of the gas in the sub-stomatal chamber, since its assimilation was compromised by the reduction in the photosynthetic rate.
No difference was found in leaf temperature between plants grown in soil covered or not with carnauba straw (Figure 7f), regardless of the ETc depth used.
A difference in yield was seen between the treatments under evaluation, with the irrigation depths combined with the ground cover of carnauba straw being noteworthy, as these always 645 showed greater productivity compared to the treatments with no ground cover but with the same level of water replacement (Figure 8). Fig. 8. Productivity in the 'red' cherry tomato grown in soil with ground cover (Carnauba straw) and with no ground cover, with both submitted to different levels of irrigation Other researchers working with the cherry tomato obtained different yields. Preczenhak et al. (2014), working with the 'Sweet Grape' cultivar, obtained a yield of 2.08 kg plant-1, a higher result than that seen in this research. Menezes et al. (2012) achieved a production of 544.8 g plant-1 from the CLN1561A accession, and 452.35 g plant -1 from the Carolina cultivar, i.e. lower than the greatest yields per plant seen in this study.
Through derivation of the equation that modelled the results of this variable, the ETcloc replacement rate that provided maximum productivity (11,398.36 kg ha -1 ) was 107.5%, i.e. 226.85 L plant -1 cycle -1 . When the soil had no ground cover, the maximum productivity was 7,784.6 kg ha -1 , acquired at a water content of 296 L plant-1 cycle -1 , and achieved at the maximum water depth of 140% ETcloc.
From the data, it can be seen that it is possible to obtain the same productivity as the soil with no ground cover (7,778.86 kg ha -1 ), using carnauba straw with a water depth of 59.5% ETcloc, generating a saving of 3,400 m 3 water cycle -1 ha -1 ; it is also possible to increase the area cultivated with tomatoes without necessarily requiring greater water availability.
A different response was found by Harmanto et al. (2004) for a water replacement of 75% ETc, estimated by the Penman-Monteith equation, which gave the best result for the tomato grown in a greenhouse.
According to Silva et al. (2013), the tomato is sensitive to water stress, and when irrigated with depths of less than 100% Etc, low productivity may result due to the lack of water, affecting the photosynthetic rate and, consequently, the productivity of the species.
Relating the physiological characteristics studied so far to the productivity achieved, a direct  ISSN 2166-0379 2020 646 relationship between them can clearly be seen, with the greatest productivity obtained in those treatments that presented the best results for stomatal conductance, photosynthetic rate and instantaneous carboxylation efficiency.

Journal of Agricultural Studies
According to Bandeira et al. (2011), for vegetables in general, development is influenced by soil moisture, water deficit being the most limiting factor to high productivity and good quality fruit; excess water in the soil can also cause serious damage, as seen in this study at water depths of 125% ETcloc or more. Therefore, replacing water in the soil through irrigation, in the right amount and at the right time, is a determining factor for success in the production of the cherry tomato.
Water use efficiency (WUE) is a parameter related to the total harvest yield per unit of water used (Silva et al., 2013). Based on the irrigation management adopted in this experiment, a WUE of the order of 3.06 kg m -3 was seen when using carnauba straw and an irrigation depth of 205 mm; whereas in the soil with no ground cover, the greatest acquired efficiency, of 1.95 kg m -3 , was seen at an irrigation depth of 211 mm, showing a large difference between the plants grown with or without a ground cover of carnauba straw (Figure 9). Fig. 9. Water use efficiency in the 'red' cherry tomato grown in soil with ground cover (Carnauba straw) and with no ground cover, with both submitted to different levels of irrigation Considering that the water depth applied in irrigation constitutes an important parameter in calculating the efficiency indices of both types of soil, the benefit of covering the soil with carnauba straw in relation to the soil with no cover is confirmed for the soil and climate conditions of the region.
When comparing the irrigation depth, a water use efficiency of 3.19, 2.67, 2.87, 2.11 and 1.31 kg m -3 was found in the covered soil for an ETcloc replacement rate of 50, 75, 100, 125 and 150% respectively; while for the soil with no cover the water use efficiency was 1.95, 1.40, 1.76, 1.55 and 1.16 kg m -3 .  ISSN 2166-0379 2020 Other studies have also demonstrated water use efficiency in the tomato (Yang et al., 2017), and show the importance of the careful use of irrigation management, as well as the need to determine the correct level of water for the plants that would allow better use of the water resources and give greater economic return (Oliveira Neto et al., 2011;Carvalho et al., 2011).

Conclusion
The results show that the use of carnauba straw together with an irrigation depth of 100% Etcloc results in greater water use efficiency in cultivating the 'red' cherry tomato under the conditions of recurrent water scarcity in the semi-arid region.