The Genus Citrus

An efficient water management for Citrus spp. trees in any cropping situation requires accurate quantitative information on water use. Interpretation of the water relations of most Citrus spp. cultivars is difficult due to the occurrence of stomatal oscillations whose origin is not well known and which cause sampling problems in irrigation management (Dzikiti et al. 2006, 2008; Wright 2008).

Vegetative growth, and particularly leaf development and stem diameter, of orange trees (Citrus sinensis (L.) Osbeck) is particularly susceptible to water scarcity (Dzikiti et al. 2006; Aiyelaagbe and Orodele 2007), and plants respond to drought by changes in gas exchange, phytohormonal balance and polyamine contents (Wang and Liu 2009). Moreover, internal water storage contributed significantly to the daily total leaf transpiration in the species (Dzikiti et al. 2006, 2008). García Petillo et al. (2004) compared the effects of different irrigation volumes on "Washington Navel" orange yields during a 5-year period (0, 50%, 100% and 150% ETc). To apply these treatments, one irrigation drip line per tree row, with drippers, of 2, 4 and 6 L h" 1 capacity, separated 1 m apart were used for the 50%, 100% and 150% ET treatments.

Another treatment received the same amount of water as 100% ETc, but with two drip lines spaced 1 m apart per tree row and 2 L h-1 drippers and showed significant increases in total fruit yield and fruit size if compared to 100% ETc. The application of the PRD irrigation method (50 and 100% ETc) to orange trees was evaluated over two growing seasons by Dzikiti et al. (2008b). The authors found that stomatal conductance in the PRD treatments was lower than in the control fully-watered treatment but no significant changes in average fruit yield were found between the two PRD treatments and the control plants. Regarding RDI, it has been demonstrated that the irrigation cut-off during the final fruit growth and maturity process (phase III) in orange (cv. "Lane late" grafted on "Carrizo" citrange) reduces midday SWP, does not reduce fruit yield and increases total soluble solids and titrable acidity, without altering fruit quality and the final maturity index (Pérez-Pérez et al. 2009). On the contrary, García-Tejero et al. (2010) found that RDI applied during the flowering and early fruit-growth phases (cv. "Navelina" grafted onto "Carrizo" citrange), both yield and fruit quality (in terms of total soluble solids and titrable acidity) were negatively affected.

Fig. 6.2 Comparison between a sustainable and a conventional management of a mature olive orchard (Olea europaea L., cv. "Maiatica") located in Southern Italy,

under a semi-arid climate with an average annual rainfall of 561 mm. Olive trees were vase trained (156 plants ha-1) (data from Palese et al. 2008, 2009b)

Other Citrus species showed responses against drought stress similar to those found in orange. Huang et al. (2000) examined the growth changes generated by mild drought stress on potted tangerine trees (Citrus reticulata Blanco, cv. "Zhuju") during early juice sac expansion stage. They observed that fruit growth was inhibited by drought stress but a greater water uptake was caused by a lower water potential in fruits of stressed plants, likely due to a higher water loss from fruit to transpiring leaves during water shortage and some active adaptive physiological responses (osmotic adjustment and cell wall loosening) of fruit to this stress. In satsuma mandarin trees (Citrus unshiu Marc.), a positive relationship between flower-bud induction and the level of endogenous plant hormones was found as a result of the application of mild (predawn LWP=from -0.5 to -1.0 MPa) and moderate drought stress (predawn LWP=from -1.5 to -2.0 MPa) (Yoshita and Takahara 2004). The results indicate that gibberellin levels were enhanced by severe drought stress, higher in the leaves from the branches that produce fewer flowers during flower-bud induction periods, whereas the levels of indole-3-acetic acid were higher in the leaves from the branches that produced more flowers during the season when flower-buds develop. As in C. sinensis, the measurement of maximum daily trunk shrinkage is a suitable and reliable indicator of the level water deficit reached by plants of other Citrus species, such as lemon (C. limon (L.) Burm. fil.) and sour orange (C. aurantium L.) (Ortuno et al. 2009).

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