Methods for regulating crop load to commercially acceptable levels that mitigate the need for hand-thinning remain a key challenge in apple (Malus domestica Borkh.) and peach (Prunus persica Batsch.) production systems worldwide. This challenge has become more acute given the uncertainty of the availability and cost of agricultural labor in the future and the increasing regulatory attention focused on products used for thinning of apples. Products currently registered for fruit thinning of apple in the United States include the carbamate insecticide, 1-naphthyl methylcarbamate (carbaryl), the ethylene releasing agent, 2-chloroethylphosphonic acid (ethrel, ethephon), the cytokinin, 6-benzyladenine (6-BA), and the synthetic auxins, 1-naphthaleneacetic acid (NAA) and naphthaleneacetamide. These products are often applied in different combinations and at different times during the 3–4 weeks after bloom to achieve more aggressive fruit abscission when compared with the application of any single product alone. Several of these compounds, notably carbaryl and ethrel, are coming under increasingly stringent regulatory pressures worldwide (Anon, 2006, 2009). The potential for loss of existing fruit thinning products, together with uncertainties about the cost and availability of agricultural labor for hand-thinning in the future, has provided focus for renewed efforts to identify alternative thinning materials for apple.
The imposition of shade treatments during or shortly after bloom stimulates fruit abscission in several crops including apples (Byers et al., 1985, 1990, 1991; McArtney et al., 2004; Zibordi et al., 2009), peaches (Byers et al., 1984), and grapes (Vitis vinifera L.) (Ferree et al., 2001). Shade treatments are presumed to create a transient reduction in the supply of carbohydrates to developing fruit during a period when the fruit are sensitive to such a stress. In apple, shoot growth has priority over fruit growth for carbohydrate partitioning when light levels during the first 40 d after bloom are limiting (Bepete and Lakso, 1998). A carbon balance modeling approach was used to identify a high probability of fruit production being limited by the development of a carbohydrate deficit in the tree during the 2- to 3-week period after bloom (Lakso et al., 1999). Furthermore, application of the fruit thinner 6-BA to apple trees was recently shown to result in a carbohydrate deficit in the tree that was rapidly perceived in the fruit cortex (Botton et al., 2011). From gene expression studies it was hypothesized that embryo development was blocked by the severe carbohydrate deficit after 6-BA application, resulting in reduced polar auxin transport across the fruit pedicel and enhanced sensitivity of the abscission zone to ethylene, eventually leading to activation of the abscission zone (Botton et al., 2011).
Foliar application of photosynthetic inhibitors has been used to stimulate fruit abscission in fruit crops, although none are currently registered for this purpose. Lime sulfur reduced leaf photosynthesis (Hoffman, 1935; Hyre, 1939; Palmer et al., 2003) and fruit set (McArtney et al., 2006) in apple. The PSII inhibitor terbacil reduced fruit set of peaches (Byers et al.,1984; Del Valle et al., 1985), apples (Byers et al., 1985, 1990), and grapes (Lopez et al., 2004). More recently, the PSII inhibitor metamitron has been shown to reduce fruit set in apples (Clever, 2007; Deckers et al., 2010; Dorigoni and Lexxer, 2007; Lafer, 2010). Photosynthetic inhibitors might also be used to enhance the activity of existing chemical thinning agents in apples (Byers et al., 1984). Before adopting such an approach, it may be necessary to account for increased activity of PSII inhibitors if they are applied in combination with commercial formulations of existing thinning chemicals that include a surfactant.
The triazinone herbicide metamitron is a systemic, xylem-translocated PSII inhibitor that acts by blocking electron transfer between the primary and secondary quinones of PSII (see Abbaspoor et al., 2006, and references cited therein). Interruption of photosynthetic electron transport inhibits adenosine 5′-triphosphate production and carbon fixation. If this interruption is permanent, plant death is caused by lipid peroxidation and proteolysis and dissociation of the protein-pigment complexes of PSII as a result of light-induced oxidative stress (Abbaspoor et al., 2006). The photosynthetic response to metamitron was described in a number of plant species (Van Oorschot and Van Leeuwen, 1979). Complete recovery of photosynthesis in sugar beet (Beta vulgaris L.) occurred within 2 h after a spray application to the leaves or withdrawal of metamitron from the rooting medium. Recovery of photosynthesis was slower and incomplete in perennial ryegrass (Lolium perenne L.) and undetectable in maize (Zea mays L.) and Portulaca oleracea L. Differences in the rate of photosynthetic recovery of different plant species after exposure to metamitron are the result of the rate of inactivation through an enzymatic, light-independent deamination (Schmidt and Fedtke, 1977). The effects of metamitron on the photosynthetic activity of tree fruits such as apple and peach have not been described.
Photochemistry, chlorophyll fluorescence, and heat dissipation represent three competing de-excitation pathways for the light energy absorbed by chlorophyll in plant leaves (Maxwell and Johnson, 2000). A reduction in the efficiency of photochemistry can be measured as an increase in chlorophyll fluorescence or heat dissipation. The normalized ratio of variable fluorescence to maximum fluorescence represents the maximum potential quantum efficiency of PSII if all capable reaction centers are open. Changes in Fv/Fm were used to study the recovery process after the addition of root-absorbed PSII inhibitors to the nutrient solution of sugar beets growing in hydroponic culture (Abbaspoor et al., 2006).
The objectives of the present study were to 1) compare the thinning responses of apple and peach to different concentrations of foliar-applied metamitron; 2) use the dark-adapted chlorophyll fluorescence parameter Fv/Fm to describe the effects of this PSII inhibitor on the photosynthetic apparatus in these two species; and 3) use various chlorophyll fluorescence parameters to describe the effects of a non-ionic surfactant on the activity of metamitron in apple.
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