Effect of irrigation practices upon yield and fruit quality of four grapefruit ( Citrus paradisi Mac.) cultivars

Two strategies of irrigation were applied during the phase of fruit growth in 30-year-old grapefruit trees ( Citrus paradisi Mac.) of four varieties (cv. Marsh SRA 8, cv. Shambar SRA 22, cv. Frost Marsh, cv. Ruby). For the ﬁ rst strategy (T1), the trees were irrigated every week at 100% crop evapotranspiration (ETc), while for the second strategy (T2) at 60% ETc. At harvest time tree yield was estimated, along with fruit quality and water productivity. T1 strategy signi ﬁ cantly increased the tree yield, the fruit weight and the juice content. Although, an increase in water productivity (crop yield/ total water use) was achieved during the water stress experiment for the three out of four varieties. The main effect of the T2 strategy was a signi ﬁ cant increase of qualitative (commercial and nutritional) attributes like Total Soluble Solids (TSS), citric acid, ascorbic acid (Vitamin C) and phenolic content. In addition, strategy T2 increased the maturation index and the fruits had sweeter ﬂ avor for two varieties. These results lead to the conclusion that in mature grapefruit trees, optimal irrigation practice provided better fruit yield, while water stress practices favor the accumulation of speci ﬁ c nutritional elements and improve speci ﬁ c fruit quality parameters, especially for cv. Ruby. de four


INTRODUCTION
Citrus is one of the most-economically-important crops in the word and they have great value in terms of nutrition. Scarcity of water resources, due to rapidly growing cultivation demands and climate change, is one of the major factors limiting irrigated agriculture (Navarro et al. ).
Globally, agriculture is the largest user of water, with nearly 70% of withdrawals worldwide (FAO ). The limited water supply worldwide and increased irrigation cost, especially in arid and semi-arid areas, demands a more efficient and optimized use of irrigation water. The aim is the maximization of water savings and the improvement of fruit yield and quality. One of the most promising irrigation strategies for accomplishing this objective might be Deficit showed that the application of water stress during flowering, as well as during the initial stage of fruit growth, reduced fruit yield (Romero et al. ; Pérez-Pérez et al. , ; García-Tejero et al. b). Also, when water stress was applied during the stage of fruit growth, it negatively affected fruit size, yield and delayed maturation, while improving some quality parameters (Navarro et  total CaCO 3 and a pH of 7.3. The N-NO 3 was 8.02 mg kg À1 and the phosphorus was 18.28 mg P kg À1 Olsen. Soil water content at field capacity was 300 mm m À1 and the wilting point was 110 mm m À1 , respectively, with an available soilwater content of 190 mm in the root zone (0.6 m from the soil surface). Finally, the bulk density was 1.64 g/cm 3 . Climatic data were provided by the IOSV's meteorological station in Chania (Greece), which is located at a distance of less than 200 m from the experimental plots.
The climate of the area is sub-humid Mediterranean with humid and relatively cold winters, and drywarm summers. The annual rainfall ranges from 300 to 700 mm/year, distributed mainly during late autumn to early spring. The mean annual air temperature ranges around 18 C.

Experimental design and description of irrigation treatments
The experimental design was two randomized complete blocks: each block had four rows with six trees per row (26 trees per block). Two irrigation treatments were applied during this experiment, a control -FI treatment (T1) and a DI treatment (T2). The DI treatments are based on the theory of non-irrigation or irrigation with less water during different phenological stages of fruit growth (Phase I: bloom and cell division; Phase II: cell elongation, rapid fruit growth period; Phase III: ripening and harvest).
Phase I for grapefruits lasts from late April to early June, Phase II from early June to early October and finally Phase III lasts from October until harvest. In Crete, due the local climate conditions, the crop needs at phases I and III are mainly covered by precipitation amounts.
Thus, both T1 and T2 treatments were applied from May to early October, at the fruit growth stage (during the second half of Phase I and during Phase II). T1 was applied in the first block of trees and involved irrigation every week at 100% of the crop needs (ET c ). T2, applied to the second block of trees, also involved irrigation every week at 60% of the crop needs. Irrigation was applied using 12 emitters for each tree, with a discharge rate of 4 L h À1 . Emitters were located in a circle 25 cm away from the tree trunk at an equal distance from each other. The fertilization applied, for both treatments, was according to soil and leaf analysis. The fertilizer applied in the experimental orchards was ammonium nitrate fertilizer -0.5 kg per tree. All optimum agricultural practices, such as pruning, weed control and fertilizer application, were also applied for both irrigation strategies.

Fruit measurements
When fruits had reached commercial maturation, the total number of citrus fruits was recorded in order to estimate the tree yield. Then, ten fruits per variety were selected randomly from a subsample of four trees per treatment. Specifically, five fruits per side (east and west sides) were picked, at the height of the human chest. The net weight of the fruits, the equatorial diameter (mm), the fruit shape, the external fruit color, the rind thickness (mm) and the flesh firmness (Kg) and juice content (%) were measured in the laboratory.
Flesh firmness was quantified by using a 10 mm cylinder Fruit Pressure Tester digital penetrometer and the fruit shape was calculated according to: The external fruit Color Index (Cl) was measured in ten fruits per treatment, using a tristimulus color difference meter (Minolta CR300 colorimeter), at four locations around the fruit. The Hunter parameters a * , b * and L * were reported by the colorimeter, obtaining the CI using the following equation: L * parameter gives a value of the luminance or brightness of the sample; a * parameter indicates the area of variation between red and green spectrum and b * parameter refers to the area of variation between the yellow and blue spectrum.
Subsequently, fruits were cut in the equatorial area and peel thickness was measured at four points.

Juice measurements
The fruits were squeezed with an electric citrus fruit-juicer and the juice was strained through a 1-mm-mesh sieve.
The juice from three fruits were combined and considered as a single sample for further chemical analysis. Thus, there were three (3)  The total phenolics were determined by the Singleton et al. () method. According to this method, 1 mL of fruit juice was extracted with 9 mL of 80% methanol for 30 min at room temperature. After centrifugation at 5,000 rpm for 10 min, the supernatant was taken for the determination of total phenolics by the Folin-Ciocalteu method. A reagent blank using H 2 O was prepared. Folin-Ciocalteu phenol reagent (0.5 mL) was added to the mixture and it was shaken vigorously. After 5 min, 5 mL of 5% Na 2 CO 3 solution were added, mixing at the same time. The solution was immediately diluted to 25 mL with distilled water, mixed thoroughly and then allowed to stand for 60 min before measurement: the absorbance was measured at 750 nm versus the prepared blank. The total phenolic content of the sample (three replicates per treatment) was expressed as mg mL À1 of Gallic Acid Equivalent (GAE).

Water productivity
Water productivity in agriculture and landscape irrigation may be generally defined as the ratio between the actual crop yield achieved (Y a ) and the total water use (TWU ¼ irri- WP indicator express the benefit derived from the consumption of water and can be used for assessing the impact of on-farm strategies under water-scarce conditions. It provides a vision of the water that could be saved with simultaneous increase in crop yield (Singh et al. ).

Statistical analysis
Statistical analyses were performed using the SPSSv17

RESULTS AND DISCUSSION
Yield and water productivity The application of water stress treatment during Phase II (T2) decreased tree fruit yield compared with the control treatment (T1), in all grapefruit varieties except for cv.
Frost Marsh (Table 1). Under water stress, the overall tree yield was reduced significantly by 14, 23 and 56% in cv.
Marsh SRA 8, Shambar SRA 22 and Ruby, respectively, while cv. Frost Marsh remained unaffected. This is mainly due to the reduction of fruit weight in all the grapefruit varieties, since fruit load was not significantly affected by the water stress (Table 1). Although a decrease in tree yield was observed, a significant increase in water productivity was achieved during the experiment for the three out of four varieties, namely Marsh SRA 8, Shambar SRA 22 and cv. Frost Marsh (Table 1). In the case of the Ruby cultivar, the lower water productivity during DI T2 treatment is attributed to the significant decrease of both fruit yield and fruit weight. Increasing water productivity may be a means of achieving efficient and effective water use. In agriculture, the interest is to have higher yield with less water, due to water scarcity and the necessity to use water in the most optimum way during agricultural practices (Navarro et al. In this work, fruit quality was affected by the water stress treatment during this experiment. DI significantly reduced the equatorial diameter of all the tested grapefruit cultivars ( Table 2); but at the same time, the market-desired commercial equatorial diameter of near 10 cm was retained for both strategies (T1 and T2). The reduction of equatorial diameter by water stress is in line with the lower fruit weight. The fruit shape, which is the ratio between the equatorial and polar diameter, was not clearly affected  Values are the mean of three replicates per variety. Values followed by different letters within a column are significantly different (sd) and values followed by the same letter are not significantly different (nsd) at the 0.05 level of probability, according to Duncan's test. Values are the mean of three replicates, bars with the same letter are not significantly different at the 0.05 level of probability, according to Duncan's test.

).
Phase II significantly decreases the fruit size (Navarro et al.

).
It is well documented that the reduction of fruit size in drought-stressed grapefruit trees is due to the fact that waterstressed fruits accumulate less dry matter than non-stressed citrus fruits due to active competition among the fruit tissue and other sink organs of the tree structure (Cohen & Goell ).
The external citrus fruit color index (CI) was also measured during this study. The obtained results did not show significant differences between irrigation treatments (T1 and T2), except for cv. Frost Marsh, which exhibited a slightly more yellowish color under DI treatment ( Figure 1).    (Navarro et al. ). It has been proposed that the increased levels of TA during DI regimes may be due to de novo biosynthesis of organic acids in an overall attempt to achieve osmotic adjustment in the fruit matrix (Navarro et al. Vitamin C has a beneficial role in human nutrition and the juice of citrus fruits provides an important source of ascorbic acid, thus is a key parameter for the chemical analysis of juice. In the present work, the ascorbic acid content of the fruit juice of all the grapefruits varieties was also affected by the water stress treatment (Table 3; Figure 3). In detail, under the applied DI strategy, the vitamin C content (ascorbic acid) was increased by 9.1%, 13.8%, 9% and 17.8%

CONCLUSIONS
Lately grapefruit (Citrus paradisi Macf.) have been proposed as a very profitable alternative citrus cultivation compared to traditional ones that are widely cultivated. Grapefruit are characterized by increased irrigation demands for the production of quality fresh fruits. This fact drives most farmers to seek methods and implement agricultural practices that limit the applied irrigation water. Thus, there is a specific need to investigate and evaluate the impact of DI farming practices upon the most important fruit nutritional quality attributes that provide added value to the consumed fruits.
The implementation of DI during Phase II of grapefruit development strongly affects qualitative parameters in several ways. The fact that under DI cv. Ruby increased significant quality criteria like TSS, TA, ascorbic acid content and total phenolics, with respect to cv. Frost Marsh and cv. Marsh SRA 8, clearly highlights the fact that the response is tightly dependent on the cultivar genotype.
Further work should be done, via the use of -omic technologies (genomic, metabolic and transcriptomic analysis) so as to investigate and pinpoint the genes and cascades that participate in the enhancement of quality parameters in grapefruit during drought stress.