Effect of irrigation frequency on growth and production of a cucumber crop under soilless culture

doi: 10.9755/ejfa.2017.v29.i11.1496


INTRODUCTION
Despite the revolutionary expansion of hydroponics in commercial horticulture, there are still some constrains which restrict further expansion, especially in environmentally protected areas (i.e. Nitrate Vulnerable Zones) where the discharge of drainage is not compatible under the EU Nitrate Directive (Savvas, 2002;Ferreira et al., 2016). In particular, open hydroponic cultures have considerable amounts of nutrients (i.e. nitrates and phosphates) depleting into the environment, as leaching fractions of 25-35% of irrigation solution is often recommended (Schroder and Lieth, 2002;Savvas et al., 2007;Slamic and Jug, 2016).
Without doubt, the scarcity of water resources had led farmers to improve their irrigation strategies, for providing crops with their exact water requirements (Morille et al., 2013). Interestingly, several studies carried out, show that irrigation events divided into shorter intervals, positively influenced the crop growth and production (Mekonnen, 2012;Adejumobi et al., 2015). However, in a soilless culture system, conflicting results often occur as the efficiency depends greatly on their design (i.e. various types of substrates and different types of growing systems) and how water and nutrients are managed (i.e. open or closed drainage systems; Grewal et al., 2011).
Rose and pelargonium, grown in rockwool and peat pots respectively, increasing the frequency of irrigation positively affected the rate of transpiration, improved water use efficiency and production (Katsoulas et al., 2006;Eiasu et al., 2012). Similarly, Pires et al. (2011) and Rodriguez-Ortega et al. (2016) reported that irrigation events divided into shorter intervals, grown either in cocofibres or perlite substrates, favored tomato fruit yield. In contrast, Tsirogiannis et al. (2010) working with gerbera grown in pots filled with Nisyros pumice, concluded that the yield and quality characteristics (i.e. stem length and flower diameter) were almost equal for the two irrigation frequencies tested. In addition, water use efficiency was higher under the low irrigation frequency trial.
The effect of irrigation frequency, on the amount of nutrient outflow from the greenhouse, was also investigated by many researchers in an attempt to increase the water use efficiency and minimize the harmful effects of high nutrient concentrations discarded into the environment. This does take into account the scarcity of water resources, and local legislation regarding groundwater aquifers which may preexist in multiple countries (Morille et al., 2013;Ferreira et al., 2016). According to Silber et al. (2003), working with lettuce planted in pots filled with perlite, the amount of drainage solution was increased, as the frequency of irrigation decreased. In contrast, Savvas et al. (2007) recorded a restriction of the drainage solution volume of pepper crop, with a low irrigation frequency, in a closed-cycle hydroponic system. Without a doubt, water deficit stress is one of the leading limitations to photosynthesis and plant primary productivity (Elshibli et al., 2016). For that reason, the prompt and timely evaluation of water status in plants through the use of physiological plant measurement sensors (i.e. canopy temperature/reflectance, sap flow, stem variation) has proven useful in irrigation control (Seelig et al., 2009). Therefore, accurate control of the irrigation scheduling, based on a real time plant feedback system -which allows adjustment of frequency and the quantity of irrigation -can lead to optimal water management and increased crop yield, particularly in areas of scarce water resources (Helmer et al., 2005;Steppe et al., 2008;Fernandez et al., 2010). However, even though a significant amount of work is being done in the field of phyto-sensing, to the authors' best knowledge only a few publications relate this technique to hydroponics.
In view of the above, the objectives were to (a) compare the effects of three different irrigation frequencies; fresh and dry weight, crop leaf area and production growth of soilless cucumber crop (cv. "Phenomenon") under Mediterranean climate conditions and (b) to indicate the use of plant based measurement sensors for evaluating physiological plant response, induced by changes in plant water status (e.g. leaf temperature, stem variation) enabling a more efficient management of the water resource, without a significant loss in agricultural productivity.

Greenhouse description
The experiment conducted from April to June 2014 in an East-West, three spans, polyethylene-covered greenhouse at the Agricultural Research Institute, of Cyprus (lat. 33 0 44'N, long. 33 0 19E, alt. five meters) on the coastal area of southern Cyprus. The geometrical characteristics of the greenhouse were as follows: eaves height 3.50 m, ridge height five meters, spans width seven meters, total length 24 m, ground area 504 m 2 , volume 2016 m 3 . The greenhouse was equipped with a single continuous roof vent in the middle span and a side vent at the two walls. The roof vent were 24 m long and one meter wide with a maximum opening area of 24 m 2 , whereas the side vent was 18 m long and 2.20 m wide with a maximum opening area equal to 52.8 m 2 (Fig. 1). Dynamic ventilation was performed by three fans one at each span (air flow rate for each fan was 31500 m 3 h -1 ) when greenhouse temperatures exceeded 25°C and at night, when relative humidity exceeded 75%. The wet evaporative pad system operated when the temperature exceeded 26 °C.

Plant material, irrigation scheduling and control
Cucumber plants (n=360) (Cucumis sativus L. cv Phenomenon) which had been raised in rockwool started cubes (10 cm x 10 cm x 6.5 cm), were transplanted on 2 nd April 2014, in rockwool slabs (1 m x 0.2 m x 0.075 m) (Grodan Company; Denmark), resulting in a plant density of 1.6 plant m -2 . Three different irrigation doses and application frequency treatments were followed on a number of every 120 plants (Fig. 1). All plants were supported by plastic twine attached 2.2 m above the plant row on a horizontal wire and trained to one stem per plant by pruning all auxiliary shoots and continuous removal of old or damaged leaves. Irrigation control for the first 14 days after transplanting was performed with 0.24 Kg m -2 periodically at fixed time intervals as per usual practices by local growers.
Eventually, the irrigation frequency was based on solar radiation, according to (Katsoulas et al., 2006) and performed whenever accumulated solar radiation energy outside the greenhouse reached 1.3 MJ m -2 (HIF), 1.9 MJ m -2 (MIF) and 3.0 MJ m -2 (LIF). The amount of water applied per irrigation event were 0.192 Kg m -2 (HIF), 0.288 Kg m -2 (MIF) and 0.448 Kg m -2 (LIF). The total daily amount of water applied was equal for the three treatments. Night time irrigation was also performed to avoid substrate dryness. During daytime a safety factor of maximum 180 minutes without irrigation was applied. Complete nutrient solutions were applied in all irrigation events. Nutrient solution compositions were based on recommendations by Savvas et al. (2013).

Crop monitoring
One representative plant in MIF and LIF treatment were monitored by means of a Phyto Sensor system. The plant sensors in each treatment, were a leaf temperature sensor (Model LT-1z), a stem micro-variation sensor (Model SD-5z), and a substrate moisture, EC and temperature sensor (Model SMTE-3z). Data was sent wirelessly to the main system unit, phyto-Logger with micro SD card and received to a PC. Sensors were purchased from Bio Instruments Company, Chisinau, Moldova. Data was collected every 10 minutes.

Data recorder and measurements
Outside greenhouse weather data i.e., air temperature (Ti, °C ), relative humidity (RH, %) (Sensor type PT 100; Galcon, Kfar Blum, Israel) and net solar radiation (R Go , W m -2 ) (Sensor pyranometer type TIR-4P; Bio Instruments Company, Chisinau, Moldova) were recorded. Sensors of the same type were used for monitoring climatic variables within greenhouse. All measurements were recorder on a data logger system (Galileo controller; Galcon, Kfar Blum, Israel). Data was recorder at 30 seconds intervals and a ten minute average was estimated.
Plant transpiration was directly monitored, by a weighting lysimeter consisting of a load cell "S type" (Model 9363; Vishay Precision Group, Malvern, USA) mounded from the greenhouse ceiling to a plant supporting system with a growing media of two plants in MIF and LIF treatments. The cell had a capacity of 50 Kg (±0.02 g). The weight loss measured by the electronic balance was assumed to be equal to crop transpiration. Drainage water was automatically collected and measured from the lysimeters, these determinations were made daily at the same time. The water uptake which was equal to crop transpiration rate was also estimated by using continually measurements of the water volume supplied to the crop and the water collected by the drainage system in each treatment by means of a simplified water balance model (as cited in Tsirogiannis et al., 2010). Water use efficiency was calculated as the ratio of the total yield to the total applied irrigation solution (Meric et al., 2011).
The leaf area index (LAI: m 2 leaf m -2 ground) of the crop was estimated by destructive measurements of the leaf area of sample plants by means of a scanner (F4280; HP, Deskjet, Japan). Leaf area measurements were carried out five times during the experimental period on the 15, 30, 45, 60 and 75 day after transplanting (DAT) and the leaf area, the length, and width of the individual leaves of three plants per treatment and date were measured. To calculate the leaf area, software was applied to leaf scanned images according to Varna and Osuri (2013).
A series of non destructive measurements were made in three labeled plants and on three randomly selected in each irrigation treatment. Measurements of plant height, length and width of each leave; of each plant were carried on 15,30,45,60 and 75 DAT. Harvesting was made during the morning, twice to three times per week and started 23 DAT. The total number of fruit production and total weight in each treatment was measured.
Plant destructive measurements were repeated three times in order to determined fresh and dry weight of different organs (stem, fruit and leaves). Four plants were randomly selected in each irrigation treatment on the 15, 45 and 75 day after transplanting (DAT). Fresh weight was determined by means of a weighting balance (BJ 41000Dd 0.1 gr; Precisa, Swiss Made) and the dry weight after dehumidification at 180°C for 48 hours by means of drying oven (Heraeus t 5050, Germany). All the plants removed for destructive measurements were replaced by back up plants of the same size.

Statistical analysis
Selected data was analyzed and comparisons of means were tested using ANOVA by using a Statistical Package

Greenhouse and crop microclimate
The monthly mean values, for both inside and outside the greenhouse's microclimate during the daylight hours are presented in Table 1. It indicates that was a higher mean indoor air temperature difference of 1.5 °C, compared to the ambient air recorded during the initial phase of the crop, decreased to 0.5 °C as the crop grew.
Matching the leaf temperature-based water stress indices highly correlated with the frequency of irrigation. This was with exception the early morning and late afternoon hours where the leaf temperature of LIF trial was higher and closer to the greenhouse air temperature especially, throughout the midday hours as illustrated in Fig. 2. Between the hours of 10:00 to 16:00, over the course of an eight day representative period (DAT 61-68), a significant higher mean difference in leaf temperature was recorded by 1.23 o C in LIF trial, comparing to MIF. The mean estimated leaf temperature values (± standard deviation) were 29.59(±5.84) for MIF and 30.82(±5.12) for LIF, for a number of observations 300 in both cases (DAT 61-68).
In addition, linear regression modeling developed for leaf temperature as a dependent variable, and the time, the amount and the frequency of irrigation of MIF and LIF, as predictors. All three parameters tested, showed to significantly predict the leaf temperature with a β value for the treatment of -.898, (p=0.002), for irrigation amount 0.021 (p=0.003) and for the time of irrigation 6,569E-5 (p=0,001).
The substrate temperature was also affected by the irrigation frequency, with higher differences between treatments recorded by the end of the experimental period. On a typical day of June (DAT 63), when the maximum temperature of the greenhouse air reached was 32.4 o C at 14:00 h, where the substrate temperature was 34 o C for (LIF) and 31.1 o C for (MIF). As illustrated in Fig. 2, the temperature of the substrate for both treatments was progressively increased after sunrise, followed by a drastic drop in the early afternoon hours. Additionally over the course of the eight day representative period (DAT 61-68), between the hours of 10:00 to 16:00, the mean estimated temperatures in the substrate were 30.97 o C (±3.15) MIF and 31.64 o C (±3.15) LIF (n=300), and differences between   The leaf area S (cm 2 ) of every leaf on each labeled plant were plotted against its length L (cm) and width W (cm) in different treatments, revealing a linear dependence of (S) on (L*W) in all cases. The straight lines obtained by are presented in table 3. The slope of each regression line represents the corresponding leaf area. From the results shown in table 3, a t-test was used, indicating that the slopes of the regression equations are not different, despite the different irrigation frequency applied. Hence, a simple relationship linking the area S (cm 2 ) of a leaf to its length L (cm) and with W (cm) was established with a determinations coefficient R 2 of 0.97.

S=1.629L * W (1)
With respect to fresh and dry weight of leaves, stem and fruit, significant differences between treatments were recorded only at 75 DAT (Table 4). The results indicated a higher total fresh weight of LIF trial by 55.79% and 34.40% comparing with HIF and MIF treatment respectively (75 DAT). Similarly, a higher total of dry biomass production in favor of LIF trial by 42.82% (HIF) and 25.68% (MIF) was recorded.
The number of fruit related to their weight is the primary indicators of the market value of the cucumber production.

Plant transpiration and water use efficiency
The daily mean variation of the canopy transpiration rate (W m -2 ) in MIF and LIF treatments, as measured with the use of lysimeters (13-67 DAT), is presented in Figs. 2 and 3. Differences in transpiration rates between treatments were significant, with mean estimated values (± standard deviation) of 147.79 W m -2 (± 60.32) for MIF and 139.36 W m -2 (± 51.81) for LIF. From Figs. 2 and 3, it can be clearly seen that higher transpiration rate differences   Statistically, significant interactions between irrigation frequency and the percentage of drainage solution were observed as was expected, resulting from differences in transpiration rates between treatments. The mean daily values, estimating the nutrient solution outflow from the greenhouse to the environment were 40.15% (± 12.56) for HIF, 42.62% (± 13.19) for MIF and 48.85% (± 13.34) for LIF. Additionally the first drain in the morning delayed, as the frequency of irrigation decreased. For the LIF trial, the first drain was recorded approximately two hours later, between 10.00 to10.30 h., compared to the first drain of the HIF treatment (data not show).
In regards to the water use efficiency (WUE), roughly similar values were obtained between treatments. As the total volume of irrigation applied in each treatment for a ten week period was 180 Kg m -2 , and the total production values were 11.97 kg m -2 (HIF), 11.79 kg m -2 (MIF), 12.27 kg m -2 (LIF), the water use efficiency values estimated were 66.45 kg m -3 for (HIF), 65.45 kg m -3 (MIF) and 68.12 kg m -3 (LIF).

Plant water status
Plant indicators of relative water loss and growth rate as estimated from stem micro variation measurements, presented a good association within different irrigation treatments. The diurnal variation in the stem was associated with the diurnal variation in VPD (Fig. 4), agreeing with previous experimental work of several authors (Lee and Shin, 1999;Galardo et. al., 2006;De Swaef and Steppe, 2010). From curves to figures, it can be observed that considerably less explicit trunk depression occurred under the high irrigation frequency treatment. Similarly the daily diameter increment (i.e. general growth rate of a plant) for a four day representative period (DAT 25-28) indicated a significant increase as the irrigation events were divided into shorter intervals, with an estimated stem growth rate of 0.705 mm in (LIF) and 1.154 mm in (MIF).

DISCUSSION
In the experiment, it was determined the effect of three different irrigation frequencies in soilless cucumber growth and production. The prevailing assumption, before the start of the experiment, was that the irrigation frequency affects the crop growth rate, the production and the plant physiological responses. Therefore, proper irrigation scheduling should be implemented for maintaining crop productivity, with minimal nutrient losses to the environment, without plants having to face any water stress conditions. Usually there are compromises with irrigation by slowing the submission of watering or watering with smaller sized irrigation norms. As a result, plants experiencing water stress, which react differently depending on plant development (Kalaydjieva et al., 2015).
The data from this study confirms that cucumber evapotranspiration increased, as the daily irrigation frequency increased from 6 events to 12 and 16, resulting in a lower drain, and thus less water and nutrient outflow from the greenhouse into the environment. Differences in transpiration could probably be ascribed to easier water uptake from the substrate, as shortening of the irrigation intervals could lead to a constant availability of water and nutrients, decreasing the matric and osmotic potentials on the rhizoplane (Raviv et al., 2001).
On the other hand, statistical analysis indicated that cucumber growth parameters (i.e. height, number of leaves, LAI) and production, were unaffected by the irrigation frequency, even though plants under a low irrigation frequency scheduling, were higher than those in the medium or high irrigation frequency, with maximum differences in height between treatments recorded by the end of the experimental period. Significantly higher differences in the fresh and dry matter weight were observed in favor of LIF trial, also at 75 DAT. Those differences are most likely attributed to higher air temperatures. Similarly, Rodriguez-Ortega et al. (2016) reported greatest vegetative growth in soilless tomatoes plants in low irrigation frequency trial at 32 ˚C, but no differences observed between different irrigation frequencies treatments at 26 ˚C. From these results it could be concluded that there would be of great value in investigating the effects of different irrigation frequencies in cucumber crop growth and production, as affected by higher ambient air temperatures.
In this study, data from phyto-sensing (i.e. leaf temperature, stem variation), rule out that plants following a low irrigation frequency schedule (0.448 Kg m -2 ) were facing water stress conditions. During the midday hours, as the irrigation interval time was greater than one hour under the LIF trial (data not shown), there was a time lack and a difficulty of irrigation supply to meet the demand of transpiration rates, resulting in a closure of leaf stomata and an increase of the leaf temperature. At that time the stem contraction amplitude (i.e. relative water loss) as estimated through the use of a stem variation sensor, was higher during the LIF trial, consistent with the notion that the water moves from stem to fruits -resulting in a negative pressure in the xylem.
The timing and volume of irrigation, strongly correlated with leaf and substrate temperature, and was negatively affected as the irrigation frequency decreased. The preexisting strong correlation, between leaf temperature and the timing of irrigation also reported by Naeeni et al, (2014), concluded that a drop of leaf temperature of soilless cucumber detected 5 minutes even after irrigation. Similarly a well-watered crop is expected to have cooler leaves as it affected by higher values of transpiration observed (Abraham et al., 2000). Regarding high recorded substrate temperatures, past studies have pointed out that it is common in for hydroponic cultures for the temperature of the substrate to exceed 30 o C or even 35 o C, negatively affecting the growth of the crop (Du et al., 1994;Klock, 1995).
From these results, it could be drawn that unnecessary water loss and plant stress could be reduced by applying an adequate irrigation strategy (Juarez-Maldonado et al., 2014;Shun and Son, 2015). However, the results observed in this study could not be simply generalized for other crops. Further research is still needed under different prevailing greenhouse environmental conditions, with emphasis on to the temperatures of the greenhouse, continuing the measurements for a longer period of time, in order to clearly discriminate and strongly support the above mentioned results.

CONCLUSION
In the study, three different irrigation frequencies were tested in order to evaluate the effect of cucumber water consumption, crop fresh and dry weight, leaf area and production. The findings support that the irrigation frequency may not affect cucumber growth rate or production, but it significantly affects the rate of transpiration and the amount of water and nutrients that outflow from the greenhouse into the environment. Therefore, the irrigation scheduling of soilless cucumber, should be based on higher irrigation intervals rates (HIF treatment; 0.192 Kg m -2 ), as it proved to minimized the drainage amount by 24%, in comparison to lower irrigation intervals rates (LIF treatment; 0.448 Kg m -2 ). Lastly low irrigation frequency scheduling, is best avoided, as plants faced water stress conditions, as estimated from leaf temperature and stem variation measurements. In conclusion, this study demonstrates that the amount and the timing of irrigation, can even show to be significant predictors for leaf and substrate temperatures, therefore sensing technology should be implemented as a tool for irrigation monitoring. The findings of the study highlight a promising irrigation strategy for cucumber soilless culture, with the aim of ensuring a more efficient water management strategy, especially in environmentally sensitive zones.

Author's contributions
The work has been completed under the supervision of C.K, N.K, D.N.; C.K and N.K designed the research plan, organized the study. G.N.: performed research, performed statistical analysis and data interpretation and in assistance with D.N wrote the manuscript which was read and approved by all authors.