Interstock of 'Valencia' Orange Affects the Flooding Tolerance in 'Verna' Lemon Trees

Previous work on citrus trees has shown that an interstock, grafted between the rootstock and scion combination, not only can improve tree growth, longevity, fruit production, and quality, but also can increase salinity tolerance. This research was designed to evaluate flooding responses of2-year-old 'Verna' lemon trees [Citrus limon (L.) Burm.; VL] either grafted on 'Sour' orange (C. aurantium L.; SO) rootstock without an interstock (VL/SO) or interstocked with 'Valencia' orange (C. sinensis Osbeck;VLN/SO) or with 'Castellano' orange (C. sinensis Osbeck; VL/C/SO). Well-watered and fertilized trees were grown under greenhouse conditions and half were flooded for 9 days. At the end ofthe flooded period, leaf water relations, leaf gas exchange, chlorophyll fluorescence parameters, mineral nutrition, organic solutes, and carbohydrate concentrations were measured. Leaf water potential ('I' w), relative water content (RWC), net C02 assimilation rate (Ac02), and stomatal conductance (g5) were decreased by flooding in all the trees but the greatest decreases occurred in VLN/SO. The C;/C, (leaf internal C02 to ambient C02 ratio), F)F" (potential activity of PSII) and FviF, (maximum quantum efficiency) ratios were similar in flooded and non-flooded VL/SO and VL/C/SO trees but were decreased in VLN/SO trees by flooding. Regardless of interstock, flooding increased root calcium (Ca), iron (Fe), copper (Cu), and manganese (Mn) concentration but decreased nitrogen (N) and potassium (K) concentration. Based on the leaf water relations, gas exchange, and chlorophyll parameters, 'Verna' lemon trees interstocked with 'Valencia' orange had the least flooding tolerance. Regardless of interstock, the detrimental effect of flooding in 'Verna' lemon trees was the leaf dehydration which decreased Ac02 as a result of non-stomatal factors. Lowered Ac02 did not decrease ·the leaf carbohydrate concentration. Flooding decreased root starch in all trees but more so in VLN/SO trees. Sugars were decreased by flooding in roots ofinterstocked trees but were increased by flooding in VL/SO roots suggesting that the translocation of carbohydrates from shoots to roots under flooded condition was impaired in interstocked trees. Spain is the second highest lemon fruitproducing country in the world and the greatest exporter. Approximately 80% of the Spanish lemon production is located in the arid southeast where drought and salinity stress are common. However, very heavy rainfall frequently occurs in spring and fall, HoRTScrENCE VoL. 47(3) MARCH 2012 which can induce flooding, especially in areas with poorly drained soils (Kijne, 2006). Flooding affects soils by altering soil structure, depleting 0 2, accumulating C02, inducing anaerobic decomposition of organic matter, and reducing Fe and Mn (Kozlowski, 1997). The main direct effect of flooding stress is the limiting availability of oxygen to the roots resulting from different chemical and biochemical reactions (Domingo et a!. , 2002; Kozlowski, 1997). Such changes can cause citrus trees to respond to flooding by reducing leaf water potential and g5 (Islam eta!., 2003; Li et a!., 2007; Ruiz-Sanchez et a!. , 1996). When flooding is prolonged, the Ac02 can also be reduced (Vu and Yelenosky, 1991). A decrease in root hydraulic conductivity could account for decreased water uptake (RuizSanchez eta!., 1996), decreased growth, leaf wilting, and chlorosis (Y elenoski eta!., 1995). Root growth typically is reduced by flooding more than shoot growth (Kozlowski, 1997). The timing of the physiological changes that take place under flooding conditions can also depend on plant size and age (Kozlowski, 1997). In potted 2-year-old 'Vema' lemon trees, Ortuii.o et a!. (2007) observed that leaf water relations were drastically affected after 3 d under flooding conditions but Garcia-Sanchez et a!. (2007) reported that leaf water relations were unaffected during a flooding period of 9 d in 6-month-old seedlings of 'Carrizo' citrange. Interactions among morphological , anatomical, and physiological factors are very important in plants subjected to flooding (Kozlowski, 1997). Factors influencing tolerance to flooding can include reopening of stomata after soil flooding, rapid recovery of leaf water potential (Li et a!., 2004), and an independence of Ac02 and gs during flooding (Fernandez et a!., 1999) enabling the maintenance of high photosynthetic rates during flooding (Nabben et a!., 1999). Under flooding conditions, an energy shortage can occur so remobilization of stored carbohydrates can play a vital role in providing energy needed for cell and organ maintenance as well as for stress responses (Greenway and Gibbs, 2003). For example, anoxia-tolerant plants like rice are able to degrade starch under low 0 2 conditions (Dixon et a!., 2006; Magneschi and Perala, 2009). Increased superoxide dismutase activity and elevated catalase action can also play an important role in keeping the concentration of reactive oxygen species low under flooded conditions (Hossain et a!., 2009). Citrus trees are considered to be flooding-sensitive because citrus does not possess many physiological or anatomical adaptations to flooding, but important differences in tolerance among citrus genotypes have been reported (Arbona eta!. , 2008). The rootstocks 'Cleopatra' mandarin and citrus Macrophylla are floodingsensitive when compared with SO, 'Citrumelo' swingle, and 'Carrizo' citrange (Agusti eta!. , 2003; Ruiz-Sanchez eta!., 1996; Yelenoski eta!., 1995). Arbona eta!. (2008) tested all of these rootstocks under flooded conditions and observed that the rootstocks considered floodtolerant had higher antioxidant defences than those considered to be flood-sensitive . Citrus tolerance to the abiotic stresses can depend on several factors including type of rootstock, N fertilization, soil, and climate conditions (Garcia-Sanchez and Syvetisen, 2009; Gimeno et a!., 2009a; Saleh et a!., 2008). In addition, an interstock, grafted