The effects of three concentrations of sodium chloride (NaC1) on seven citrus rootstocks were studied under greenhouse conditions. Leaf and root mineral concentrations and seedling growth were measured. Sodium chloride was added to the nutrient solution to achieve final osmotic potentials of -0.10, -0.20, and -0.35 MPa. Increasing the concentration of NaC1 in the nutrition solution reduced growth proportionally and altered leaf and root mineral concentrations of all rootstocks. Significant differences in leaf and root mineral concentration among rootstocks were also found under stressed and non-stressed conditions. Salinity caused the greatest growth reduction in Milam lemon and trifoliate orange and the least reduction in sour orange and Cleopatra mandarin. No specific nutrient deficiency was the sole factor reducing growth and causing injury to citrus rootstocks. Sodium chloride sensitivity of citrus rootstocks in terms of leaf burn symptoms and growth reduction could be attributed more to C1 than to Na. Sodium and CI concentrations were greater in the leaves than in the roots, particularly at the medium and high salinity levels. Root C1 was not useful for assessing injury because no differences were found in root C1 concentrations among rootstocks. Increasing salinity level did not affect the level of N and Ca in the roots but did reduce N and Ca levels in the leaves. No relationship in mineral concentration or accumulation seemed to exist between citrus leaves and roots. At the -0.10MPa salinity level, sour orange, rough lemon, and Milam were not able to exclude either Na or C1 from their leaves. Trifoliate orange and its two hybrids (Swingle citrumelo and Carrizo citrange) excluded Na at the lowest salt level used, but were unable to exclude Na at the higher salinity levels. Similarly, Cleopatra mandarin excluded C1 at the lowest salt level, but was not able to exclude C1 at higher salt concentrations. Hence, the ability of citrus rootstocks to exclude Na or C1 breaks down at higher salt concentrations.

WP by increasing yield and/or reducing ET always results in net savings, thus reducing agricultural water requirements. This is a key point in assessing the opportunities for true water savings in horticultural crop cultivation and will be addressed in detail later in this paper.Water productivity in irrigated agriculture varies widely and depends on many biophysical and management factors. Because variations in ET among crops are within an order of magnitude apart, by far the most important factor infl uencing WP is the economic value of the product. Horticultural products are usually high value and thus WP normally exceeds that of fi eld and row (agronomic) crops. For example, using current values for yield and ET characteristic of California agriculture, the WP of corn is about 0.20 $/m 3 , compared to 0.70 $/m 3 for almond, 5.00 $/m 3 for strawberry, and even more for greenhouse and ornamental crops. An extreme example of this occurs with vegetable crops grown under plastic in southeast Spain during the off-season. The combination of high market prices and low ET leads to a WP of about 10 $/m 3 . While impressive, even this value cannot compete with that of industrial and urban uses. Nevertheless, it helps explain the trend of shifting irrigated acreage from low value fi eld and row crops to horticultural crops in many water-scarce areas of the US, a trend that will probably increase worldwide (National Research Council, 1996). Indeed, the WP of an irrigation district in Southern Spain increased over a four year period as the proportion of horticultural crops increased (I. Lorite, L. Mateos, and E. Fereres, unpublished data).This paper describes the evolution of water use as related to productivity with an emphasis on the U.S. experience, analyzes how irrigation systems and management have evolved since the early 1900s, and explores the challenges and opportunities for water conservation in horticulture. Because of the broad scope of the subject and limited space, the paper focuses primarily on tree crops although many principles are applicable to all horticultural crops.

Florida citrus trees must be irrigated to reach maximum production due to low soil water-holding capacity. In a highly urbanizing state with limited water resources, improved understanding of soil water uptake dynamics is needed to optimize irrigation volume and timing. The objectives of this study were: (i) estimate mature citrus daily evapotranspiration (ET c ) from changes in soil water content (u), (ii) calculate citrus crop coefficients (K c ) from ET c and reference evapotranspiration (ET o ), (iii) determine the relationship of soil water stress coefficient (K s ) to u, and (iv) evaluate how ET c was related to root length density. In a 25-mo field study using mature 'Hamlin' orange [Citrus sinensis (L.) Osbeck] trees, ET c averaged 1137 mm yr 21, and estimated K c ranged between 0.7 and 1.1. Day of year explained more than 88% of the variation in K c when u was near field capacity. The value of K s decreased steadily from 1.0 at field capacity (u 5 0.072 cm 3 cm 23 ) to approximately 0.5 at 50% available soil water depletion (u 5 0.045 cm 3 cm 23 ). Roots were concentrated in the top 15 cm of soil under the tree canopy (0.71 to 1.16 cm roots cm 23 soil), where maximum soil water uptake was about 1.3 mm 3 mm root 21 d 21 at field capacity, decreasing quadratically as u decreased. Estimating daily plant water uptake and resulting soil water depletion based on root length density distribution would provide a reasonable basis for a citrus soil water balance model.

Fine sand soils important to Florida agriculture have volumetric soil water content values (θv) of <0.10 cm3 cm−3 after drainage due to gravity has ceased. Small changes in θv in the range of 0.02 to 0.08 cm3 cm−3 can greatly affect plant available water and, therefore, good calibration of soil water content sensors is necessary. The EnviroSCAN (Sentek Pty. Ltd., South Australia) is a multiple sensor capacitance probe capable of continuous measurement of soil water content by volume (θv). Many fine sand soils in Florida have plant available θv values of ≤0.08 cm3 cm−3 The manufacturer's calibration curve has very few data points <0.10 cm3 cm−3 θv and no data in the 0.02 to 0.04 cm3 cm−3 θv range. Because of the lack of data in this range, a calibration curve from 0.02 to 0.08 cm3 cm−3 θv was developed for Candler fine sand (hyperthermic, uncoated Typic Quartzipsamments), Apopka fine sand (loamy, siliceous, hyperthermic Grossarenic Paleudults), and Immokalee fine sand (sandy, siliceous, hyperthermic Arenic Alaquods) in two locations in Florida. Since calibration curves for the three soils did not differ significantly, data from the three soils were combined. An exponential calibration curve was developed This equation provides substantially different estimates of water content in the 0.02 to 0.08 range than values obtained from the manufacturer's calibration. This improved calibration extends the useful range of the EnviroSCAN to include an important group of soils with very low water holding capacity.

The occurrence of diurnal changes in root resistance of cotton was studied by measuring the flow of water through 35‐to70‐day‐old root systems under a pressure of 3.10 bars or a vacuum of 0.88 bar. The volume of exudate obtained under constant pressure or constant vacuum was 2 to 3 times greater near midday than near midnight indicating that the root resistance apparently was 2 to 3 times greater at night than during the day. The salt concentration of the exudate also cycled; the concentration was lowest at midday and highest at night, hence there was little diurnal variation in the total amount of salt moved per hour. The cycle for volume of exduate, salt concentration, and apparent root resistance had a period of 22 to 26 hours at 24°C. The cycle gradually died away 2 to 3 days after removal of the shoots. The diurnal variations appeared to be controlled by signals from the shoots because the phase of the cycles could be reset by changing the light‐dark cycle under which the plants were grown. Cycling was eliminated by exposure to 8 or more days of continuous light before removing the shoots, and cycling could not be entrained by a 6 hour light‐6hour dark cycle. Bubbling nitrogen gas through the nutrient medium stopped cycling. A possible role of ion or growth regulator action is discussed.

Reclaimed water has been safely and successfully used for more than 40 years in Florida and California. Reclaimed water in these states is regulated with restrictions more stringent than World Health Organization guidelines. In the United States, Florida is currently the largest producer and California is the second largest producer of reclaimed water. Reclaimed water is more highly tested than other sources of irrigation water, and the safety of this water has been demonstrated in these and other states. Very high application rates of reclaimed water to citrus on well-drained Florida sands increased tree growth and fruit production. Although reclaimed water contains some nutrient elements, there is usually insufficient macronutrient content to meet plant nutritional requirements. Most reclaimed waters do not have high salinity levels although they are slightly more salty than the potable waters from which they originated. With an adequate leaching fraction, salts in reclaimed water can be handled with appropriate irrigation management. Use of reclaimed water has steadily increased in Florida since 1992, but other entities besides agricultural irrigation are now competing for its use. Public acceptance of reclaimed water has also increased, and crops grown with reclaimed water in Florida and California have been marketed without a negative public reaction. Recent issues of food safety have caused some to question reclaimed water, but there is no evidence of food safety problems with its use. Although reclaimed water in Florida was initially promoted as a way to improve surface water quality, it has now become an important alternate source of water to help meet water shortages and urban demand. In California, reclaimed water has become a necessary part of statewide water management.

The water relations responses to salt of several important citrus rootstocks such as Swingle citrumelo, sour orange, and Milam lemon have not been studied in detail before. Studies were set up to compare growth and root hydraulic properties of these rootstocks to other citrus rootstocks by exposing them to NaCl and polyethylene glycol (PEG) stresses. Seedlings of 7 citrus rootstocks were irrigated for 5 months with nutrient solutions containing NaCl or PEG that had been adjusted to osmotic potentials of ‐0.10, ‐0.20 or ‐0.35 MPa. The 7 rootstocks studied were sour orange (Citrus aurantium), Cleopatra mandarin (Citrus reticulata Blanco), Swingle citrumelo (C. paradisi x P. trifoliata), Carrizo citrange (C. sinensis x P. trifoliata), rough lemon (Citrus jambhiri Lush), Milam lemon (C. jambhiri hybrid), and trifoliate orange (Poncirus trifoliata [L.] Raf.). In both shoot and root growth, Cleopatra mandarin and sour orange were the least sensitive to salt, Milam and trifoliate orange were the most sensitive, and rough lemon, Swingle, and Carrizo were intermediate in sensitivity. Even though the roots were exposed to solutions of equal osmotic potentials, plant growth and root conductivity were reduced more by the PEG treatments than the corresponding NaCl treatments. At ‐0.10 and ‐0.20 MPa, shoot and root dry weights were reduced 16 to 55% by NaCl and 24 to 68% by PEG. Shoot root ratio was lowered at the higher concentrations, particularly by PEG. There was a major decrease in root conductivity caused by NaCl at ‐0.10 MPa (19 to 30% in sour orange and Cleopatra mandarin and 78 to 85% in trifoliate orange and Milam). Conductivity decreased more at ‐0.20 and ‐0.35 MPa, but not proportionally as much as at ‐0.10 MPa. Root weight per unit length increased at the higher salt levels, particularly in trifoliate orange. Water flow rate through root systems followed the same trend as root conductivity; salt affected sour orange and Cleopatra mandarin the least and trifoliate orange and Milam the most. However, reductions in fibrous root length by salt treatment differed. Root lengths of Swingle and Carrizo were least affected by salt while sour orange. Milam, and rough lemon were the most affected. Hence, even though sour orange and Cleopatra mandarin were more tolerant than the other rootstocks in terms of water flow rate or root conductivity, these 2 rootstocks showed a proportionally greater decrease in root length than Carrizo, Swingle, or trifoliate orange.

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