1. Introduction
Greenhouse agricultural systems have expanded worldwide, especially in areas with mild winter climates, such as the Mediterranean basin. One of the largest greenhouse areas in the world is located on the SE Spanish Mediterranean coast, where fruit-vegetable crops are usually grown in low-cost structures covered with plastic film, without active climate control systems and with
enarenado (sand-mulched) soils [
1]. In this and other Mediterranean greenhouse areas, crops are intensive and heavily fertigated, the irrigation water is usually scarce and the quality of surface and groundwater sources is deteriorating [
2,
3,
4]. Consequently, there is increasing pressure to optimise irrigation management.
Steps to improve the irrigation water efficiency in Mediterranean greenhouses have included: determining water requirements of greenhouse crops using the K
c−ET
0 method adapted to Mediterranean greenhouses [
5,
6]; development of a simple computer programme for estimating daily irrigation crop water requirements [
7]; and analysis of on-farm irrigation performance [
8]. In most Mediterranean greenhouses from the SE Spanish coast, the conventional recommendation regarding irrigation frequency of soil-grown vegetable crops is to maintain the soil matric water potential (SMP) in the middle of the soil layer where most roots usually grow above or equal to −20 or −30 kPa, depending on soil texture (i.e., to keep the soil water status slightly over or around field capacity). This recommendation is in line with numerous studies aimed to establish the optimum SMP threshold for crop irrigation, although they were carried out under open field conditions [
9,
10,
11]. In greenhouse crops, water deficit has been found when they have been grown with SMP values below −58 kPa for pepper, −35 kPa for melon and −38 to −58 kPa for tomato [
12]. However, the use of high irrigation frequencies to maintain a high soil water availability (SMP values higher than –20 kPa) around the roots of high-value greenhouse crops has been recurrently proposed or questioned [
13], and various high-frequency irrigation systems have been implemented and commercially distributed in the region over recent decades without thorough experimental evaluation. Moreover, a recent study, carried out in a typical greenhouse on the SE Spanish Mediterranean coast with a silty loam soil [
14], is supporting the use of high irrigation frequencies in soil-grown greenhouse crops: a higher biomass production and fruit yield was found in a zucchini crop irrigated at SMP of −10 kPa, compared to that irrigated when the SMP was −25 kPa. A higher soil water availability can theoretically facilitate crop water uptake and improve soil nutrient availability [
15], especially under water stress conditions, but the responses of plant species to water stress or availability significantly depend on the intensity and duration of stress and their stages of development [
16,
17,
18,
19].
Irrigation scheduling and methods can affect root distribution, and water and nutrient availability within the soil. Under drip-irrigation systems, roots grow preferentially around the wetted emitter area and are concentrated within the upper part of the soil profile [
20,
21], but this pattern of root growth and distribution is usually affected by the frequency and rate of irrigation. A higher irrigation frequency or a lower water application rate modified the soil root distribution and, consequently, the plant water uptake across the soil profile [
15].
This work was aimed at analysing the agronomical effects, including the soil root distribution, of modifying the level of soil water availability by using various irrigation frequencies in some of the main soil-grown vegetable crops of a representative Mediterranean greenhouse area.
4. Discussion
A high soil water availability throughout the whole crop cycle, which can theoretically facilitate water and soil nutrient availability and uptake [
15], did not improve the biomass, yield or the physical fruit quality of high-value Mediterranean greenhouse crops irrigated with water of low salinity (EC of 0.4 dS m
−1). Vegetable crops grown under high (between SMP values of −10 and −20 kPa,
Figure 1), conventional (between −20 and −30 kPa) and low (between −30 and −50 kPa) soil water availability (irrigated with practically the same total amount of water,
Table 2) did not present significant differences for the total and marketable yield, the fresh weight of first or second class fruits, or the yield components of marketable fruits (
Table 6). This response could be mainly attributable to the relatively high soil water availability observed in all the irrigation treatments (
Figure 1), since SMP values were generally higher than (H and C treatments) or close to (L treatment) those values at which water stress may occur [
12,
27]. Therefore, fruit-vegetable greenhouse crops can be irrigated under a relatively wide range of SMP values without significantly affecting their yields. This finding can be relevant for implementing new precise automatic irrigation scheduling technologies [
28], which appears to be the best way of optimising water and nutrient use in soil-grown Mediterranean greenhouse crops. These results are still of great agronomical interest since there is an urgent need of improving the irrigation water use in many Mediterranean greenhouse areas due to increasing problems of water scarcity, and water and soil pollution and salinization [
3,
4]. Moreover, in the same area and greenhouse system, [
14] recently found a higher yield and aerial biomass in a zucchini crop irrigated when the SMP reached −10 kPa, compared to that irrigated when the SMP reached −25 kPa or −40 kPa. The result of this study [
14] appears be conflicting with the results of our work (although experiments are not fully comparable) and with previous studies, mostly carried out under open field conditions [
9,
10,
11] and some under greenhouse conditions [
12]. In the six experiments presented in our work, irrigated with water of low salinity (EC of 0.4 dS m
−1), different soil water availability treatments were induced by modifying the irrigation frequency, but the total amount of irrigation water supplied was similar for all the treatments (
Table 2). In the zucchini experiment [
14], irrigated with moderate saline water (1.4 dS m
−1), the total water supply was substantially greater in the crop irrigated when the SMP reached −10 kPa (390 mm) than in those irrigated when the SMP reached −25 kPa (315 mm) or −40 kPa (272 mm). The irrigation of the zucchini crop with moderate saline water may have led to soil salt accumulation, particularly in the treatments less frequently irrigated (C and L), which might have affected the soil water availability for the crop. However, this hypothesis can not be contrasted since measurements of soil water osmotic potential or soil solution EC are not available in this study. Therefore, further and more detailed research is required to optimise the irrigation water use (irrigation frequency and rate) in soil-grown greenhouse crops irrigated with moderate saline waters since the water quality of many greenhouse Mediterranean areas is deteriorating [
3,
4].
The low soil water availability treatment did not significantly affect shoot biomass of cucumber crops (
Table 4), but it produced smaller leaves (data not shown) reducing LAI values significantly (
Figure 2) throughout most of the autumn–winter and spring cycles [
15,
19,
29]. Despite the lower LAI values, fruit yield and physical fruit quality of the cucumber grown under low soil water availability were not significantly affected (
Table 6). The cucumber crops grown under low soil water availability frequently presented SMP values slightly lower than −40 kPa. These values might cause mild water stress [
12,
27], but not so severe as to clearly inhibit stomata conductance and photosynthesis per unit leaf area or to increase root dry matter production [
30,
31]. However, they may reduce leaf growth and its role as a sink for assimilates [
17,
19]. Thus, for high-value greenhouse crops, such as cucumber (about 12 € m
−3 of water-productivity [
8]), maintaining low soil water availability throughout the whole crop cycle may prove risky, especially during periods of high evaporative demand (spring cycles), and it may, therefore, be unadvisable over the whole crop cycle in commercial Mediterranean greenhouses.
The irrigation treatment of low soil water availability did not affect root biomass at the end of the autumn–winter cucumber cycle (
Table 5), but it modified the root distribution across the soil profile. Roots grew preferentially around the wetted emitter area and were concentrated within the upper part of the soil profile in the crops under both irrigation treatments, high and low soil water availability (
Figure 3), which coincided with previous results from [
20,
21]. However, this pattern of root growth was less accentuated for the cucumber under low soil water availability, which showed a more homogeneous root distribution throughout the soil profile (
Figure 3). Although high-value greenhouse crops usually receive abundant water and nutrients [
32], a more homogeneous root distribution may be of interest in commercial greenhouses whether water or nutrient crop requirements are not properly supplied or when soil characteristics might hamper crop water and nutrient uptake.
On the other hand, growing vegetable crops under high soil water availability does not appear to be the best management practice for autumn–winter vegetable cycles in commercial Mediterranean greenhouses. The total shoot and vegetative biomass was significantly lower under high than under conventional soil water availability in two (zucchini and green been) of the three autumn–winter cycles studied (
Table 4), although the marketable fresh fruit weight was not affected (
Table 6). In crops grown at the end of the autumn and winter periods, the greenhouse evaporative demand under the non-controlled climatic conditions of most Mediterranean greenhouses is usually low but variable [
5,
6]. Under these circumstances, the probability of periods with excessive water content limiting the root oxygen supply or enhancing the proliferation of some soil diseases might be greater in crops grown with high irrigation frequency, but further research is required to elucidate this hypothesis.
In substrate-grown greenhouse crops, regulating the level of water availability by varying the irrigation frequency is a usual practice for growing well-balanced plants and other management purposes [
32]. In soil-grown greenhouse crops, regulating the soil water availability without affecting plant photosynthesis by varying the irrigation frequency throughout the crop cycle can be useful for: (i) producing fruit vegetable plants with a better equilibrium between vegetative (source strength) and generative (sink strength) parts [
16], or enhancing fruit quality [
27]; (ii) modifying the soil root distribution in order to increase the potential root access to water and nutrients and to improve the use of these scarce resources; and (iii) controlling or minimising salt accumulation in the root zone. The latter might become of great interest since the irrigation water quality of many greenhouse Mediterranean areas is deteriorating [
3,
4]. Farmers, by varying the irrigation frequency, can maintain relatively low soil water availability when the plant vegetative growth is too vigorous or increase the soil volume explored by the roots, or can maintain relatively high soil water availability over periods under stressful climate conditions, such as periods of hot and dry winds.