Abstract
Chemical fruit thinning has become a popular practice in modern fruit orchards for achieving high quality fruits, reducing costs of hand thinning and promoting return bloom. However, most of the suggested chemical thinners are often concerned for their detrimental effects and environmental problems. 5-Aminolevulic acid (ALA) is a natural, nontoxic, biodegradable, and environment-friendly plant growth regulator. One of its outstanding roles is improving plant photosynthesis and fruit quality. Here, results showed that applying 100–200 mg/L ALA at full bloom stage significantly reduced pear fruit set. Both in vivo and in vitro studies showed that ALA significantly inhibited pollen germination and tube growth. ALA decreased not only cytosolic Ca2+ concentration ([Ca2+]cyt) but also “tip-focused” [Ca2+]cyt gradient, indicating that ALA inhibited pollen tube growth by down-regulating calcium signaling. ALA drastically enhanced pollen Ca2+-ATPase activity, suggesting that ALA-induced decrease of calcium signaling probably resulted from activating calcium pump. The significant negative correlations between Ca2+-ATPase activity and pollen germination or pollen tube length further demonstrated the critical role of calcium pump in ALA's negative effect on pollen germination. Taken together, our results suggest that ALA at low concentrations is a potential biochemical thinner, and it inhibits pollen germination and tube growth via Ca2+ efflux by activating Ca2+-ATPase, thereby thinning fruits by preventing fertilization.
Highlights
Many species of fruit trees, including pear, often bear an abundance of flowers, and produce a surplus of fruits that the tree is unable to support, resulting in many low quality fruits (Bangerth, 2000)
In order to reduce the economic cost of Aminolevulic acid (ALA) application, in the present study, we first investigated whether exogenously applying ALA at low concentrations at 75% bloom could thin fruits
To determine the optimal time for ALA application, we further investigated the effect of 100 mg/L ALA sprayed at different stages of blossom on fruit set
Summary
Many species of fruit trees, including pear, often bear an abundance of flowers, and produce a surplus of fruits that the tree is unable to support, resulting in many low quality fruits (Bangerth, 2000). Chemical thinning, especially with bioregulators (plant growth regulators as well as endogenous plant hormones), has become a popular practice in many orchards for achieving high quality fruits, ALA, an Ideal Fruit Thinner reducing costs of hand thinning, and promoting return bloom (Gonkiewicz et al, 2011). Several chemical agents, such as NAA and its amide (Stopar, 2002), 6-BA (Bound, 2006), ethephon (Jones et al, 1989), carbaryl (Bound, 2006), and ammonium thiosulfate (Janoudi and Flore, 2005), have been reported to be used in fruit thinning. New effective flower or fruit thinning techniques, which have no detrimental effects and meet modern environmental and food quality guidelines, are required
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