Avocado is considered one of the most salt-sensitive crops (Grieve et al., 2012) and one of the highest value crops per acre. The world avocado production in 2013 was 4.72 million tons—led by Mexico with 1.47 million tons, with the United States as the seventh largest producer at 0.175 million tons (FAO, 2018). Avocado production in California (the major U.S. producer) is increasingly affected by the scarcity of freshwater and relies on more saline waters as well as salinization of existing freshwater supplies. It is thus imperative that we develop not only proper irrigation scheduling and salinity monitoring to maintain productivity, but also to use rootstocks that are more tolerant to salinity.
Salinity has a broad range of effects on plants, and therefore there are also many different mechanisms for plants to tolerate this stress. Roy et al. (2014), classified these mechanisms into three main categories: osmotic tolerance, which is regulated by long-distance signals that reduce shoot growth and is triggered before shoot Na+ accumulation; ion exclusion, during which Na+ and Cl– in the roots reduce the accumulation of toxic concentrations of Na+ and Cl– within leaves; and, last, tissue tolerance, in which high salt concentrations are found in leaves but are compartmentalized at the cellular and intracellular levels (especially in the vacuole). Multiple researchers have found citrus and avocado rootstocks to vary in their capability to absorb and transport Na+ and Cl– ions, resulting in different salt tolerances. Oster and Arpaia (1992) showed that the tolerance level of the avocado scion is dependent on the rootstock used.
Normally, plant injury occurs first at the leaf tips (which is common for Cl toxicity) and progresses from the tip back along the edges as severity increases (Mass, 1984). Excessive necrosis is often accompanied by early leaf drop or defoliation. With sensitive crops, these symptoms occur when leaves accumulate from 0.3% to 1.0% Cl on a dry weight basis, but sensitivity varies among these crops. Many tree crops, for example, begin to show injury at more than 0.3% Cl (dry weight) (Ayers and Westcot, 1985).
Bernstein (1965) pointed out that for many fruit crops, plant damage could be related to the concentration of specific ions, such as Cl– or Na+, in the soil solution and/or plant leaves rather than to total soil salinity. Such a tolerance classification is presented in Maas (1984), which shows that, for avocado, the maximum permissible Cl– content in the root zone, expressed as the concentration in the saturation extract (Cl–e), is 7.5, 6, and 5 mmolc·L–1; and in irrigation water (Cl–w) is 5.0, 4.0, and 3.3 mmolc·L–1, respectively, for West Indian, Guatemalan, and Mexican rootstocks. These correspond to the maximum values to avoid leaf injury and reduction in fruit yield. Other researchers have focused on Na+ toxicity of tree crops rather than Cl–. For example, Na in the leaf tissue of tree crops in excess of 0.25% to 0.50% (dry weight basis) is often associated with Na+ toxicity (Ayers and Westcot, 1985). The ability to maintain low leaf Na+ concentration is considered a desirable trait in plants grown under saline conditions.
Previous studies have not established clearly whether avocado production under elevated salinity is impacted by Cl– toxicity, Na+ toxicity, or both. Mickelbart and Arpaia (2002), in an 80-d study with three rootstocks and four salinity levels (EC 1.5–6 dS·m–1), concluded that the “relative tolerance of the various rootstocks appeared due primarily to their ability to exclude Na+ and Cl– from the scion” when expected tolerance was evaluated indirectly by net CO2 assimilation, chlorophyll concentration, and leaf necrosis.
In a more detailed analysis from the same experiment, Mickelbart et al. (2007) found a good correlation between leaf necrosis of older leaves and leaf Cl– concentration, as well as leaf necrosis with an “unbalanced” Na:K ratio in older leaves of two of the three rootstocks. Similarly, in a 130-d greenhouse study with Hass grafted to five rootstocks and an EC = 1.6 dS·m–1 saline treatment, Castro et al. (2009) determined that the rootstock with the greatest CO2 assimilation in the saline treatment was also the one with the lowest Cl– concentration in leaves and the highest Na+ and highest Cl– accumulations in the roots. They concluded that restriction of ion transport (Na+ and Cl–) to the leaves is one of the mechanisms associated with salinity tolerance of avocado. However, they found no interaction between vegetative parameters and saline treatment, perhaps as a result of the short duration of the experiment. Oster et al. (2007), in a field study, found yield loss above ECe = 0.6 dS·m–1, but found no evidence of Cl– or Na+ toxicity.
Water savings have been achieved by successful use of more efficient irrigation systems, such as drip and microsprinkler, to irrigate avocado trees. However, there are limitations to reduced water application, as reduced leaching results in increased soil salinity, which impacts yield adversely. With the expectation that freshwater supplies will continue to be scarce in California, it is essential that we also expand the use of more efficient irrigation practices and lower quality waters (such as recycled water) in irrigated agriculture. The recent 3-year drought from 2014 to 2017 in California aggravated the trend of increasing scarcity and cost of freshwater. As a result of these considerations, California growers in the future will likely have to rely on more saline and/or lower quality waters for irrigation to conserve freshwater (Sheikh, 2017). Thus, our objectives were to screen 13 avocado rootstocks for salinity tolerance and study the relationship between leaf Na+ and Cl– on growth and yield.
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