Zn is an essential micronutrient for plant growth and development. However, in citrus production, Zn deficiency is one of the most damaging and widespread nutritional disorders in both acidic and alkaline soils (Srivastava and Singh, 2009). Zn analyses have shown that over 70% of citrus orchards in China are low in Zn. Globally, Zn is recognized, next to nitrogen, as the most widely deficient nutrient in citrus, thereby limiting citrus yield and quality (Srivastava and Singh, 2005). Typical symptoms of Zn deficiency are well known as “little leaf,” “mottle leaf,” and “rosetting” that are basically characterized by small, narrow leaves, chlorosis between the veins, and crowding along short stems, respectively (Srivastava and Singh, 2005). Apart from these effects, Zn deficiency also reduces tree vigor and photosynthetic activity and lowers fruit set, yield, size, and quality, even in its earliest stages (Chapman, 1968; Fu et al., 2014).
Management of Zn deficiency still relies on two conventionally used methods, soil or foliar fertilization. However, soil-applied Zn is less available to plants due to its low mobility and high soil fixation (Chapman, 1968; Razzaq et al., 2013). In addition, due to its much lower labor and time cost, foliar application of Zn fertilizer has almost completely replaced soil application. The positive effects of Zn sprays on nutrient status, yield, and fruit quality have been observed in mandarin (Razzaq et al., 2013; Srivastava and Singh, 2009), orange (Boaretto et al., 2002; Eman et al., 2007; Hanafy Ahmed et al., 2012; Swietlik and Laduke, 1991), and grapefruit (Swietlik, 2002). Foliar application of Zn is now commonly accepted as a relatively effective method for correcting Zn deficiency in citrus production. However, foliar application still has some limitations. First, utilization efficiency of Zn is very low. Based on our statistical data of fertilization amount and leaf Zn content, the leaves absorbed less than 20% of foliar-applied Zn (unpublished data). Second, it is still difficult to facilitate the recovery of the entire leaf to a green color using foliar application. Recovery of green color is punctiform in the yellow leaf (unpublished data). In addition, necrotic spots often occur on leaves, especially younger ones. This is possibly because the foliar spray often adheres to leaves as droplets. In addition, the Zn poorly transported from the sprayed leaves to the other plant parts, even between the mesophyll cells in one leaf (Boaretto et al., 2002). Boaretto et al. (2002) reported that less than 1% of the applied Zn was transported from sprayed leaves to the other plant organs. As a result, only the locations to which droplets adhered absorbed Zn and increased Zn levels, producing punctiform recovery of green color. Once the sprayed Zn concentration increases to a relatively high value, the locations will very easily accumulate excess Zn, which is poorly transported to other parts of the leaf, finally causing necrotic spots.
In this study, we compared absorption efficiency in citrus leaves of three common Zn fertilizers, ZnSO4.7H2O, ZnCl2, and Zn(NO3)2.6H2O, and their correcting effects on citrus Zn deficiency, to find the best utilization efficiency and correcting effect of chemical Zn. In addition, we tested the roles of agricultural organosilicone surfactant in improving the foliar fertilizer absorption efficiency and correcting effect. Organosilicone surfactant is recognized as an adjuvant with super spreading, wetting, and penetration power for lowering the aqueous surface tension (Knoche, 1994; Stevens, 1993), but literature reporting its applications in citrus is scarce. This study provides useful guidance for citrus Zn deficiency management practice.
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