Plant Water Stress and Irrigation Scheduling3

5.1 Water Stress Indicators

Water is more and more becoming a precious resource and deficit irrigation strategies, such as partial root zone drying (PRD), regulated deficit irrigation (RDI) and others, have been increasingly studied, in different contexts (e.g. Naor, 2006). The starting point to link water use and water stress indicators is the understanding and quantification of stomatal behavior. Quantifying rs is important for ET estimation but also through its relationship with CO2 assimilation rate, affected by water stress mainly due to stomatal closure (Schulze, 1986; Chaves, 1991; Jones, 1992). Besides its interpretative value, rs has a practical value in irrigation scheduling. For practical uses, its direct measure with porometers is not advisable, mainly due to the time required to obtain a representative value. Furthermore, rs changes with rapid environmental changes and it is difficult to establish and use a critical value. Either bulk values for the canopy or the stomatal resistance of leaves in a selected position can be used.

Alternatives have been explored using other water-stress related variables, considered as a cause or/and a consequence of stomatal closure. One of the consequences of stomatal closure is the change in transpirational cooling of leaves resulting in greater leaf temperatures and possibly in greater thermal gradients above the canopy. These variables can be used directly or through more sophisticated approaches as, for instance, the Jackson index (Jackson, 1982). Even for low crops, the measurements require large plots, because of the

The discussion that follows, with a perspective of practical application, is mostly based on already published results.

instability of the observed values in advection conditions (Katerji et al., 1988). These difficulties become more pronounced on woody stands, owing to the higher turbulence in the canopies and consequent instability of leaf temperature.

Among the factors that determine the stomatal behaviour, the soil water potential (ys) and soil water content (9s) have the advantage to be independent from diurnal atmospheric variations. The relationship between rs and those variables is of special value if they reflect the average conditions in the roots zone. This is difficult to achieve with large root systems. The use of a few sensors in a representative point has been often the solution (Isberie, 1992). However, it is difficult to extrapolate the location of the representative point to other soils, irrigation and root systems.

The relationship between the stomatal resistance and the leaf water potential (y) has also been broadly analysed because this variable was commonly considered as the non-climatic factor with more direct influence on stomatal behaviour. However, the large experimental evidence of a good relationship between those variables does not necessarily means a simple causality relation. Water-stress related variations on rs can occur without correspondent changes on y. The stomatal closure is also explained by chemical signals, namely the ABA (Zhang et al., 1987; Davies and Mansfield, 1988; Hartung and Davies, 1989; Correia and Pereira, 1994), as a result of edaphic dryness, a hormonal and a hydraulic regulation simultaneously contributing to a better answer to water shortage.

For operational reasons, y is still commonly used to characterise the leaf water status influence on stomatal behaviour. Precaution is needed when making the interpretation of the experimental results on y, namely following rapid changes in spatial water application in relation to the roots distribution (Ameglio, 1998). For irrigation scheduling purposes a value of y representative of the day is selected.

The value of y measured at predawn, (yp) corresponds to an equilibrium between the soil close to the roots and the plant, which is achieved after some hours without transpiration (night), except if there was not enough time to replenish the water storage in the plants organs (plants with big dimensions, under stress conditions). Other exceptions to this nocturnal equilibrium respect to positive transpiration fluxes during night, owing to the use of heat storage in the soil, air or vegetation. Night transpiration fluxes in close relationship with vapor pressure deficit were observed (not published) in the peach orchard described by Ferreira et al. (1996, 1997b). In most cases, yp is used to represent an integration of soil conditions in the root zone, in respect to water.

The minimum y (1-2 h after solar noon, when T is more intensive) can also be used. Some results suggest that this choice is adequate when the plants tend to an aniso-hydric behaviour. If the plants of a certain species or cultivar behave as iso-hydric, they close stomata so effectively that they avoid an important decrease in noon y (Katerji et al., 1988; Valancogne, 1994). In this case, the difference in yl between irrigated and stressed plants is expected to be higher in the morning that at noon and the use of predawn leaf water potential (yp) can be recommended (Figure 14).

The minimum yl is sometimes measured in previously covered leaves. In this leaves yl equilibrates with the stem water potential (ystem). According to McCutchan and Shackel (1992), ystem is less disturbed by environmental conditions than minimum yl and relates to soil water status in a more clear way. Several studies present encouraging results concerning the use of ystem for orchard irrigation scheduling (e.g., McCutchan and Shackel, 1992; Shackel et al., 1998; Shackel et al., 2000a; Shackel et al., 2000b). Nevertheless, ^stem is influenced by day-to-day variation of VPD, making the establishment of critical thresholds for this indicator difficult (Marsal et al., 2002).

plot (full lines) and stressed plot (dashed lines) for typical isohydric (left) and typical anisohydric (right) plants. The arrows indicate the difference at predawn and at early afternoon (minimum yl) suggesting that, for isohydric plants, it is easier to identify and quantify the water stress using predawn measurements.

plot (full lines) and stressed plot (dashed lines) for typical isohydric (left) and typical anisohydric (right) plants. The arrows indicate the difference at predawn and at early afternoon (minimum yl) suggesting that, for isohydric plants, it is easier to identify and quantify the water stress using predawn measurements.

The experiments about the role of ABA or other chemicals acting as hormonal regulators, led to a renewed attention to the water in the soil (vd Passioura, 1988; Kramer, 1988 and Boyer, 1989). However, due to experimental difficulties with the soil measurements, presents advantages in representing the edaphic conditions, in both cases (iso-hydric or aniso-hydric behaviour). As the soil indicators, it has the advantage to be rather independent from diurnal variations.

The relationship between and noon gs (=1/rs) of selected leaves (sunlit, for instance) often exhibits a change on slope and scattering for a specific value of which can be used as a threshold value for irrigation scheduling: values about -0.4 MPa have been reported for tomato (Katerji et al., 1988), and -0.45 MPa for peach (Ferreira et al., 1996), separating a moderate from a more intensive water stress.

The use of relative conductance (gs of a stressed plot divided by gs of the well irrigated one) requires simultaneous measurements in two treatments but has the advantage of reducing the scattering due to inter-daily variation of air humidity, allowing a better identification of the critical value. In fact, air humidity around the leaves has a detectable influence on relationships with gs (Ferreira, 1993; Granier and Breda, 1994; Ferreira et al., 1996) so care must be taken on its interpretation and extrapolation in respect to the size of the plots and inter-row advection. In a large stressed plot, where the air humidity close to the leaves is lower then in an irrigated plot, stomata react simultaneously to air and soil dryness, specially when stress is moderate and not intensive (Ferreira and Katerji, 1992). In a small stressed plot surrounded by irrigated areas, relative stomatal closure is likely to be mostly related to soil dryness, being easier to identify a threshold value. Even if the last situation is less representative of real situations, it corresponds to experiments performed in small areas.

Besides, it can be an experimental strategy to obtain critical values for practical applications, as suggested in Ferreira et al. (1996).

Another difficulty on comparing results obtained in different experiments comes from discrepancies on the value of gs used to represent the canopy behaviour (leaves or layer where gs is measured, period of the day when measurements are performed). It has been observed (e.g. Katerji et al., 1988) that the maximum difference between gs on well-watered and on water-stressed plants occurs about noon, often about 1 h after solar midday. This specific value of gs is a good indicator when relating stomatal behaviour with other variables, for irrigation scheduling purposes (e.g. Ferreira et al., 1996, Silvestre et al., 1999).

The variations in stem diameter, whose measurement can be easily automated, can also be used in scheduling irrigation, as a good relationship between stem diameter and water stress has been consistently observed (Kozlowski, 1972; Lansberg et al., 1976; Huguet, 1985; Garnier and Berger, 1986; Ameglio and Cruiziat, 1992). Stem diameter variations can be determined by other factors than water stress progression, as for example growth. Therefore, it is necessary to monitor simultaneously stressed and unstressed plants in order to establish reference values for the referred variables (e.g., RDTS). RDTS is the daily magnitude of trunk diameter (maximum minus minimum daily diameter) measured in stressed plants, divided by the correspondent value in well watered plants. A critical level can be determined and used directly, as a threshold value. However they can be highly variable, according to species and trunk dimensions, showing also a great variability among plants of the same population (Katerji, 1997).

5.2 Relative Transpiration

Relative T of stressed plants (RT) is the transpiration of these plants in relation to the transpiration of well-watered ones. Daily RT can be seen as an approximate integrated value of stomatal behaviour response to water stress. Due to SF methods, RT is easier to obtain than a correspondent value of rs. Thus, during the last decade RT, as well as other variables whose measurement can be automated, have been studied for application in scheduling irrigation. The relationship between RT and water-stress indicators as the leaf water potential can also be a useful tool for ET modelling and water stress analysis.

When soil evaporation (Es) is very low compared to total ET, RT corresponds approximately to the coefficient Ks, in the practical equation ETa= ETo.Kc.Ks. If Es is high, Ks can be significantly different from RT and estimated or measured values of Es have to be taken into account in the Ks calculation.

The relationships described by Denmead and Shaw (1962), between ys and RT, for different rates of ETc, correspond approximately to the relationship between RT and as in this last case, is used to represent the soil water status. The consequence of this last approach seems to be that less scattering is expected between the lines correspondent to different ETc rates when using yp, because the relationship - RT is less dependent on the soil hydraulic conductivity, than the relationship ys - RT.

According to Valancogne et al. (1996), one of the parameters in the equation yp -RT can be calculated from the maximal observed value of yp, for the system soil-crop in consideration. For a peach orchard in a sandy soil, in central Portugal the parameters of the relationship experimentally obtained: RT = 1.28 exp (1.474 y ) with r2=0.85 (Ferreira et al., p'

1997b) corresponded to those suggested by that methodology. The relationship RT-yp can be directly used in irrigation scheduling, if can be measured and a specific threshold value of RT is selected. The irrigation should take place when the measured value of correspondent (from the equation) to the selected RT is attained. This relationship can also be useful in ET estimates, if is measured, as it can give a day-by-day approximate value of Ks, allowing the determination of soil water depletion, as in the example given later (5.3).

The relationship between RDTS and RT can also provide the information on Ks needed for daily ET estimates. For instance, in a Prunus persica orchard, the value of 2 for RDTS corresponded to 65% of RT (and 50% of gs), in relation to the well irrigated plot (Ferreira et al., 1996). The analysis of the possibilities of use of an indicator related with stem diameter variations is encouraged by the fact that it provides information for direct use, at the farm scale, that can be continuously recorded and connected to automated irrigation systems. However, results are often of difficult interpretation.

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