**3. Water stress indicators and thresholds**

#### **3.1. Water status indicators: use in research and in irrigation scheduling practices**

The use of water status indicators has been enhanced not only by the increasing importance of DI, but also due to the increased possibilities of automatically recording of some of those variables. This requires the selection of the appropriate variables and their threshold values, for different objectives concerning marketable yields. In the perspective of this contribution, the question is how to select a water status variable and how to transform it in a useful stress indicator for DI scheduling. The requirements of a water stress indicator include the consideration of a consistent answer (similar response in similar circumstances), low cost, and easiness of use, reliability with reasonable low sampling, and possibility to define thresholds that facilitate a decision. Above all these requirements, it is necessary to measure or derive an indicator that depends much more on the water stress affecting yield, then on other variables independent from water stress (such as atmospheric demand).

that water deficit limited shoot growth, when shoots and fruits were competing for photo-assimilates. It is important to bear in mind that fruit tree sensitivity to water deficit is not constant during the whole growing season, and a water deficit during a phenological stage less sensitive might benefit WP, as it increases irrigation water savings, and minimizes negative impacts on yield and crop profits [31, 33, 34]. So, when a RDI strategy is applied, it may be necessary to supply full irrigation during the drought sensitive phenological stages and irrigation may be stopped or restricted during the non-critical periods, less sensitive to drought [31, 35]. The crucial constraints of RDI are: (i) difficulty in keeping plant water status within tight limits of water deficit during noncritical phenological periods; (ii) depending on management, unexpected variation in evaporative demand may result in severe losses of yield and fruit [36]; (iii) need to define precise criteria for the water deficits, in different growth conditions, related to species, weather, soil depth, fruit load, and rootstock [37, 38]; and (iv) lack of precise knowledge in the effect of water deficit during

Partial rootzone drying (PRD) is a strategy of DI that consists in irrigating only one half of the rootzone in each irrigation event, while the other half is allowed to dry. For this, both halves are watered alternately [40]. This technique was first developed in Australia for vineyards and relies on root-to-leaf signaling induced by a rootzone that is in a drying process [41], decreasing stomatal aperture and leaf growth, preventing water loss [42, 43] with a little effect on photosynthesis, hence increasing transpiration efficiency [41]. At the same time, the wet portion of the root system receiving water enables the plant to maintain a favorable plant water status, such that yield is not significantly compromised and quality may even improve [42]. The PRD performance is based on the assumption that photosynthesis and fruit growth are less sensitive to water deficit than transpiration, and besides, water deficit induces the production of chemical signals, like ABA in the root, that can be translocated to leaves [44] inducing stomatal closure. As demonstrated in a recent meta-analysis, the advantages of PRD in relation to RDI are highly controversial and also depend on the soil texture, a success or enhanced yield performance with RDI and PRDI occurring most likely in deep and finely

**3.1. Water status indicators: use in research and in irrigation scheduling practices**

The use of water status indicators has been enhanced not only by the increasing importance of DI, but also due to the increased possibilities of automatically recording of some of those variables. This requires the selection of the appropriate variables and their threshold values, for different objectives concerning marketable yields. In the perspective of this contribution, the question is how to select a water status variable and how to transform it in a useful stress indicator for DI scheduling. The requirements of a water stress indicator include the consideration of a consistent answer (similar response in similar circumstances), low cost, and

bud development [38, 39].

54 Irrigation in Agroecosystems

textured soils [45].

**3. Water stress indicators and thresholds**

**2.3. Partial root drying system**

Stomatal conductance (gs ), which decreases as soil water deficit develops, is a primary mechanism in regulation of plant transpiration; therefore, a potential indicator of water stress [46]. Stomatal opening is not only affected by the soil water status, but also by external factors not related to water stress, such as meteorological conditions at leaf level, mainly vapor pressure deficit (VPD) [47]. Consequently, it makes more sense to use g<sup>s</sup> taken in relative, which is the value in a stressed crop divided by the correspondent value in a well-watered one. Such measurements are time consuming, due to the required sampling, consequence of the high scattering in the canopy and instability with clouds or gusts of wind. It is very difficult to automate gs measurements and the sensors used (porometers) are delicate and expensive. Therefore, its use is limited to research.

Due to the buffer role of the soil, soil water potential and soil water content (θ<sup>s</sup> ) have the advantage of being almost independent from diurnal atmospheric variations. Soil water potential measurements (with tensiometers) are easy and cheap, they can be, in principle, easily automated, but there are limits concerning the range of soil water status in which tensiometers operate well. The changes in θ<sup>s</sup> (volumetric fraction) have the advantage of being a direct component of the soil water balance equation. The relative extractable water (REW) is a very useful concept that relates the actual volume of water available for plants to the total available water capacity, between the so-called field capacity and permanent wilting point (TAW) [48].

Leaf water potential (Ψleaf) is also related to stomatal closure. Even if, for different reasons, reductions in stomatal opening can occur without changes of Ψleaf [47, 49], this indicator has been broadly used for irrigation scheduling purposes.

The use of stem water potential at noon (Ψstem) has the advantage of being less disturbed by environmental conditions than Ψleaf [50] but it loses its relevance in the case of isohydric behavior, as such plants close stomata so effectively that they avoid important decreases in noon Ψleaf [51, 52]. In such cases, the difference between irrigated and stressed plants can be higher at predawn than at noon and predawn leaf water potential (Ψpd), being independent from diurnal oscillations can better represent water status in both cases: isohydric or anisohydric behavior.

The difficulties in finding meaningful correspondence between gas exchange and plant water balances impose limitations on accurate measurement of plant water stress in field conditions. It is largely demonstrated however that, in spite of such limitations, Ψpd or Ψstem are variables considered reliable as water status indicators for irrigation scheduling purposes and have been almost unavoidable in research studies [53, 54].

Several variables have been derived from stem diameter variations (SDVs) [55, 56], with the advantage of being cheap and easily continuously recorded. The most used are the organ (stem or fruits) growth rate (OGR), the daily trunk shrinkage (DTS), or the relative DTS (RDTS), where the relative value of daily amplitude in diameter is divided by the correspondent in well-watered plants, obtaining an indicator practically independent from atmospheric variations, as required. Sometimes, maximum and minimum trunk diameters are used individually (MXTD and MNTD).

Experience and knowledge of varieties, environmental conditions, and technical and financial capabilities of the growers will ultimately determine the most adequate method or combination of methods to use for evaluation of the status of their crops and how to better manage

Deficit Irrigation in Mediterranean Fruit Trees and Grapevines: Water Stress Indicators and Crop…

In general, plant water potential seems to be a better indicator than the SDV-derived variables, when full irrigation scheduling is applied. Moriana et al. [62] suggested that values of

maximum yield was obtained [63]. Pérez-López et al. [64] suggested that a threshold value Ψstem of −2.0 MPa (moderate water deficit) may be used to DI. Nevertheless, Ψstem in DI trees was affected by crop load and environmental conditions. Indeed, Moriana and Fereres [65] reported that VPD produced a variation on Ψstem from −0.8 to around −1.4 MPa in fully irrigated olive trees of different ages and fruit load. A threshold value of Ψpd > −0.9 MPa was

It has been observed that SDVs are affected by seasonal growth patterns, crop load, plant age and size, and other factors, apart from water stress [58]. So, the use of SDV needs expert interpretation, which limits their potential for automating the calculation of irrigation depth (ID). Despite this, they refer that, when combined with aerial or satellite imaging, SDV measurements are useful for scheduling irrigation in large orchards with high crop-water-stress

Alcaras et al. [69] reported that the increase in MXTD showed strong relationships with REW,

and it decreased along with Ψstem until it reached a constant negative growth rate, at Ψstem of −2.7 MPa. In their study, DTS was much less responsive to irrigation than either MXTD or TGR. They suggest the use of automated soil moisture sensors if reliable soil moisture values can be obtained, and indicate that a continuous recording of trunk diameter has some poten-

For peach, the use of Ψstem for defining thresholds under DI conditions is referred by Girona et al. [70], who found the value −1.5 MPa, the limit over which the impairing of bloom fertility appears. Naor et al. [39] have observed that the value of −2.0 MPa for SWP was a threshold for the occurrence of double fruits, while Lopez et al. [71] suggest a threshold of −1.05 MPa to obtain fruits with positive effects on consumer acceptance, without significant impacts on fruit composition and yield, as they have observed that a threshold of −1.25 MPa would

Other authors, using relative transpiration (RT), have observed that a minimum value of 0.7

Using the relationship between (RT) and Ψpd, it was observed [73] that the Ψpd threshold corresponding to RT equal to 0.7 is −0.33 MPa. Using CWSI, based on the temperature differences

. Trunk growth rate (TGR) showed a very early response to water-withholding

and when Ψstem > −1.8 MPa,

http://dx.doi.org/10.5772/intechopen.80365

57

Ψstem > −1.65 MPa in field conditions provide the maximum g<sup>s</sup>

tial, but further investigation of MXTD and TGR is warranted.

has to be observed to avoid yield and quality losses [72].

reduce fruit size and yield, even if advantageous for consumer acceptance.

them.

**3.2. Olive**

often proposed to FI [66–68].

spatial variability.

Ψstem and gs

**3.3. Peach**

The success of SDV-derived variables depends on plants' behavior. Its application seems to be more successful when applied to conditions of anisohydric behavior [57]. Unfortunately, the outputs often are of difficult interpretation [56, 58], sometimes being the use based on visual and qualitative analysis.

Also, as diameter changes, sap flow rate can be continuously and automatically recorded with high resolution across large temporal scale. Sap flow sensors became popular in last decades, and by measuring fluxes, for the same reasons of independence from atmospheric demand, they only can be directly linked to water status indicators, provided relative transpiration (RT) [48] and the absolute values are not used. The inconvenience of requiring well-watered plants as reference limits its use to research.

As the stomatal conductance is reduced to prevent excessive transpiration, the temperature of leaves and canopy rises. Therefore, the temperature of the canopy in relation to the air is linked to the level of water stress, due to the effect of transpiration evaporative cooling. Several indexes have been proposed and applied in different conditions, space and temporal scales, mainly following the work of Jackson et al. in early 1980s [59], to derive the crop water stress index (CWSI). Measuring canopy temperature is a simple procedure using inexpensive infrared thermometers or any other optical devices that can take many observations rapidly without disturbing the plant. However, canopy temperature is affected by multiple factors, namely VPD, turning it complex to relate with soil water availability.

Overviews and results on remote sensing approaches have been presented [60, 61]. The "advantages and pitfalls" of plant-based methods in the perspective of irrigation scheduling have been discussed by Jones [36]. Fernández [57] recently presented a review of soil or plant water status and other variables used as other water stress indicators for irrigation scheduling. In general, technologies have greatly improved over the years, sensors are more affordable but sampling is still a limitation. In all cases where the relative independence from daily variations in atmospheric demand requires well-watered plants as a reference, this represents a practical disadvantage, limiting its use to the field of research. Unfortunately, these affects many possible indicators and the number of those remaining that are not excessively time consuming, is reduced to a few.

Therefore, the combination of these indicators with models for water balance is advisable [48]. In fact, the most popular variables in irrigation scheduling practices, used at present, either by farmers or enterprises, providing irrigation scheduling services, often include soil moisture quantification, sometimes as a complement to water balance models based on estimated ETc, for example, Ondrasek [1]. This is related to easiness, cost, rapidity to obtain the outputs, simplicity of data treatment/interpretation, and significance. Furthermore, the advantage of directly linking θ<sup>s</sup> with the outputs from water balance is crucial. The problems of spatial heterogeneity and the quality of the measurements are often disregarded, meaning that a qualitative use of these outputs is often accepted and considered useful.

Experience and knowledge of varieties, environmental conditions, and technical and financial capabilities of the growers will ultimately determine the most adequate method or combination of methods to use for evaluation of the status of their crops and how to better manage them.
