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Foliar application of nutrients can cause phytotoxicity, the most common symptoms of which are ‘burning’ or ‘scorching’ of the leaves, i.e., leaf necrosis (Krogmeier 1989; Burkhardt 2010). Although tolerance to foliar-applied urea or KNO3 varies

between different species, concentrations above about 1.2% are generally only

suitable for postharvest or pre-leaf fall applications because of the risk of leaf damage (Weinbaum 1978). Typically young leaves are more susceptible to damage from foliar-applied chemicals so that the risk of foliar damage is greater with early season applications (Weinbaum 1988). Foliar damage is usually related to localised

desiccation of the leaf tissues due to the hydroscopic properties of the foliar-applied solutes and the relatively high concentrations that remain on the leaf surface

(compared to those around the roots in the soil solution) following evaporation of the spray carrier (usually water) (Burkhardt 2010; Fernandez and Eichert 2009; Wojcik 2004). Such phytotoxic effects are related to the salt index of the foliar solute, which could be a practical measure for predicting safe concentrations (Lea-Cox and

Syvertsen 1995). Nevertheless, the range of optimal rates reported is too broad to allow definitive recommendations for optimal concentrations to be made for any particular nutrient form (Fernandez and Eichert 2009). Symptoms of phytotoxicity usually involve marginal and tip chlorosis or necrosis which might reflect the accumulation of the spray solutions at these parts of the leaf which results in much higher residual concentrations following evaporation of the water carrier (Albrigo 2002). Therefore, application techniques and use of adjuvants that reduce runoff and allow more uniform distribution of the spray on the leaves could help to minimise phytotoxic effects. Early morning applications when dew is still present on leaves might be more phytotoxic than those made at midday or during the late afternoon but this probably depends on the prevailing atmospheric conditions (Frageria et al. 2009). ‘Leaf tip yellowing’ with foliar urea applied to citrus during hot weather was

attributed to rapid drying and high concentrations of residual urea remaining on the leaf surface (Kiang 1982).

Urea is generally believed to be less phytotoxic than other N forms used for foliar N sprays (Weinbaum 1978; Klein and Weinbaum 1984; Swietlik and Faust 1984). However, at the same compound concentration, rather than N concentration, the phytotoxicity of urea and inorganic N salts, such as KNO , is similar. For example,

although Furuya and Umemiya (2002) considered KNO3 more phytotoxic than urea,

both compounds have a very similar salt index (73.6 and 75.4 units, respectively), and if solutions of comparable osmotic concentration are used the risk of leaf damage is probably similar for both compounds (Lea-Cox and Syvertsen 1995).

1.4.10.1 Biuret phytotoxicity

Urea can contain biuret, a potentially phytotoxic compound present in fertiliser grade urea. It is created in the high temperatures occurring during urea manufacture by the fusion of two urea molecules (Mikkelsen 2007; Figure 1.6). Modern urea manufacture has been improved to reduce the content of biuret to between 1 and 2% by weight (Mikkelsen 2007).

Figure 1.6 Representations of the molecular structure of biuret (left) and urea (right).

Biuret is not quickly metabolised in the plant and its toxicity has been attributed to disruption of nitrogen metabolism and irreversible damage to chloroplasts (Mikkelsen 2007; Albrigo 2002). Typical foliar symptoms of biuret toxicity include leaf tip chlorosis and necrotic leaf margins and were first reported on pineapple (Sanford et al. 1954). In practice these symptoms are difficult to distinguish from those caused by urea (El-Zeftawi 1974). For use in foliar applications, urea with a biuret content between 0.25 and 0.80% (low biuret urea) is generally recommended to reduce the risk of leaf damage. There are several crops that are exceptionally sensitive to biuret, including citrus, pineapple, potato, and avocado. However, its toxicity to crops in general may have been exaggerated. For example, applications of biuret alone at concentrations considerably above those likely to be encountered in the foliar use of fertiliser grade urea failed to cause phytotoxic symptoms in apple (Khemira et al. 2000). However, there was some phytotoxicity when biuret was combined with urea at concentrations of 2% and higher. Biuret by itself did damage the leaves of young

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H2N – C – NH – C – NH2 H2H – C – NH2

citrus trees (Jones 1954). Obviously sensitivity to biuret varies greatly between species and might reflect the capacity of a species to metabolise it. For example, one study found biuret persisted in orange leaves eight months after foliar urea application (Mikkelsen 2007). Despite their low tolerance to biuret, citrus are relatively tolerant of urea with application rates up to 34 kg N ha-1being reported without damage (Albrigo 2002).

Biuret can also have beneficial effects on crops. For example, small amounts of biuret applied to the soil increased the growth of Douglas fir seedlings (Xue et al. 2004). According to Xue et al. biuret acts like a plant growth regulator, while also having a priming effect on the mineralisation of nitrogen by soil microorganisms. However, higher rates of biuret applied to the soil resulted in seedling mortality and reduced nitrification. Its relatively slow mineralisation in soil compared to urea gives it some potential for use as a slow release nitrogen fertiliser (Xue et al. 2004).

Because biuret tends to accumulate in plant tissues, repeated applications may have a cumulative effect with subsequent appearance of symptoms of toxicity (Mikkelsen 2007). Albrigo (2002) suggested a biuret limit of 0.34% in urea used for foliar application to avoid phytotoxicity in citrus, but a higher content of up to 0.5% was probably suitable for occasional use providing application rates were about 1168 L ha-1 and supplied not more than 15.7 kg N ha-1.

The pattern of chlorosis and leaf damage at the leaf tips is consistent with spray solution runoff and accumulation at the tips. Therefore, the higher tolerance to biuret found by Albrigo (2002) than was reported in earlier studies might be related to the 4- 8 fold decrease in spray volumes since the time of the earlier studies.

Singh et al. (1979) found that three consecutive urea (2%) foliar sprays increased potato yields by between 15 and 31% in three seasons, but when the urea contained biuret at levels above 0.8%, the yield advantage was steadily decreased with

increasing biuret content. Biuret at 0.35% had no adverse effects on potato yield although symptoms of leaf damage were not reported.

1.4.10.2 Urea phytotoxicity

Apart from biuret, the formation of ammonium cyanate in urea solutions might contribute to phytotoxicity (El-Zeftawi 1974). The formation of this compound in urea solutions increases with time and in conditions of high pH and temperature. Therefore the use of freshly prepared urea solutions would be prudent. Within the plant the enzymatic hydrolysis of urea by urease generates NH3, and both NH3and

NH4+are phytotoxic and capable of disrupting a large range of metabolic processes

within plant cells (Vines and Wedding 1960; Britto and Kronsuker 2002; Fernandes and Rossiello 2011). Effects of foliar-applied urea on increasing flowering and fruit yields of citrus may be due to the slight phytotoxicity of urea and its breakdown product ammonium (Albrigo 2002). Lovatt (1999) also suggested that the increased fruit set following even a single pre-blossom foliar urea application is due to a transient increase in NH3-NH4+content in the treated tissues. Orbovic et al. (2001)

concluded that temporary toxication symptoms caused by foliar urea may be due to ammonia toxicity. Nevertheless, urea itself is considered more phytotoxic than NH3

or NH4+(Orbovic et al. 2001).

In soya beans, foliar urea (presumably laboratory grade) applied at concentrations of 2.5% (w/v) and higher caused leaf necrosis and the damage was associated with the accumulation of urea within the necrotic tissues (Krogmeier et al. 1989).

Accumulation of urea and/or biuret might result when tissues have been severely damaged halting further metabolic activity. Plasmolysis of mesophyll cells in citrus leaves following foliar urea application have been observed (Orbovic et al. 2001) and the chlorotic symptoms of biuret toxicity being characteristically irreversible are indicative of senescence (Achor and Albrigo 2005). In tomatoes, daily foliar urea applications (as the only N source) at concentrations above 0.2% caused

phytotoxicity, also apparently due to urea accumulation in the treated tissues

(Nicoulaud and Bloom 1996). El-Zeftawi (1974) recommended an interval of at least 4 weeks between foliar urea applications to avoid excessive accumulation of urea in the leaf tissue. The accumulation of urea in treated tissues might be a phenomenon peculiar to certain species because generally its high mobility through membranes and within the phloem, and also the likelihood of its rapid hydrolysis by urease makes accumulation less likely (Wang et al. 2008; Swietlik and Faust 1984).

Nickle (Ni) is an micronutrient essential for the urease activity and accumulation of urea and associated symptoms of phytotoxicity can be caused by Ni deficiency (Wood et al. 2006). Inclusion of Ni with foliar urea can reduce phytotoxicity of foliar urea sprays (Kutman et al. 2013). Nickle deficiency could be induced in fruit trees by excessive or unbalanced fertilisation practices that lead to the accumulation of elements such as Zn, Cu, Mn, Mg, Fe, and Ca that compete or otherwise inhibit Ni uptake (Wood et al. 2006). Foliar symptoms of Ni deficiency were induced in citrus growing on soils high in Ca and Mg by foliar urea applications (Wood et al. 2006).

Also working with tomato Tan et al. (1999) found slight marginal scorching two days after a single application of urea at 2.2% but not with concentrations of 1.1% or lower. Phytotoxicity symptoms appeared with solutions of 1% (NH4)2SO4but not at

0.5% and with 3% NaNO3 but did not 1.2% or lower. Raising the solution pH

lessened the severity of the symptoms in the case of NaNO3 but also reduced its

uptake; higher pH had no effect on phytotoxicity caused by urea or on its uptake; but worsened the symptoms in the case of (NH4)2SO4and increased its uptake. The non-

ionic hydrocarbon surfactant ‘Tween’ (0.1% v/v) was used in this experiment (Tan et al. 1999). Guvenc and Badem (2002) reported some foliar damage following five applications of 1.1% KNO3on greenhouse grown tomato but no phytotoxic

symptoms apparently appeared with the same number of applications of fertiliser grade urea (biuret content unknown) at concentrations up to 0.8% .

In ‘Golden Delicious’ apples, two foliar sprays of urea at concentrations between 0.4 and 4% in early summer caused no leaf damage or fruit russetting but the quality of the urea used (e.g., its biuret content) was not reported (Thalheimer and Paoli 2002). Weinbaum (1988) suggests phytotoxicity occurs in apples with springtime application of urea at >0.5% concentration but concentrations of 4 to 5% in autumn were possible because leaf damage at this time would not adversely affect tree productivity.

However, Wood and Beresford (2000) reported severe bud damage in apples with autumn applications of foliar urea at concentrations above 5%.

In citrus, single applications of urea (biuret content 0.01% w/w) at concentrations above 1.8% w/v caused foliar burn on young grapefruit leaves and leaf abscission on older leaves but older leaves were undamaged by three applications at 1.3% (Lea-Cox

and Syvertsen 1995). No foliar burn was found in young or old leaves with urea concentrations between 0.5 to 0.9% urea (Lea-Cox and Syvertsen 1995). The sprays were applied with the non-ionic hydrocarbon surfactant Triton X-77 (0.1% v/v). Page et al. (1963) found only minor foliar damage on orange trees sprayed up to six times with KNO3 solutions between 2.5 and 5%. The alkyl resin adhesive surfactant Triton

B1956 (0.25% v/v) was used in the sprays. In prune, six high volume spray

applications during spring of 1.35% w/v urea (<0.4% biuret) with organosilicon or hydrocarbon surfactants and caused slight marginal and tip necrosis on leaves (Leece and Dirou 1979). ‘Partial blights of the leaf margins’ were reported with 3% KNO3 on

grapes (Altindisli et al. 1999). Calvert (1969) found only slight leaf burn in oranges following four applications of 2.4% KNO3, but damage was more severe with

concentrations of 4.8% and 7.2%. Triton B1956 (0.25% v/v) was added to the foliar treatments.