**2.2. Regulated deficit irrigation**

To our knowledge, the concept of regulated deficit irrigation (RDI) was first presented in the 1980s [31, 32] with the aim of controlling excessive vegetative growth in peach orchards. They founded 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 bud development [38, 39].

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

nism 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

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

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 ten-

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

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

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 aniso-

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

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

Due to the buffer role of the soil, soil water potential and soil water content (θ<sup>s</sup>

measurements and the sensors used (porometers) are delicate and expensive.

), which decreases as soil water deficit develops, is a primary mecha-

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

(volumetric fraction) have the advantage of being a

taken in relative, which is

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

) have the

55

independent from water stress (such as atmospheric demand).

deficit (VPD) [47]. Consequently, it makes more sense to use g<sup>s</sup>

Stomatal conductance (gs

Therefore, its use is limited to research.

siometers operate well. The changes in θ<sup>s</sup>

been broadly used for irrigation scheduling purposes.

been almost unavoidable in research studies [53, 54].

automate gs

(TAW) [48].

hydric behavior.
