Abstract
N-(2-Chloro-4-pyridyl)-N′-phenylurea (CPPU) is a highly active cytokinin-like plant growth regulator that promotes chlorophyll biosynthesis, cell division, and cell expansion. It also increases fruit set and accelerates fruit enlargement. However, there has been no report about the effect of CPPU on fruit development and its physiological mechanism in macadamia. In this study, we investigated the effect of CPPU treatment at early fruit development via foliar spray or raceme soaking at 20 mg·L-1 on fruit set and related physiology in macadamia. Changes in carbohydrate contents and endogenous hormones in leaves, bearing shoots and fruit were also examined. Results showed that CPPU significantly reduced young fruit drop and delayed the wave of fruit drop by 1–2 weeks. The treatment significantly decreased the contents of total soluble sugars and starch in the leaves, but increased them in the bearing shoots and total soluble sugars in the husk (pericarp) and seeds. These findings suggested that CPPU promoted carbohydrate mobilization from the leaves to the fruit. In addition, CPPU increased the contents of indole-3-acetic acid (IAA), gibberellin acid (GA3), and zeatin riboside (ZR) and decreased the abscisic acid (ABA) in the husk. Therefore, CPPU treatment reduced the early fruit drop by increasing carbohydrate availability and by modifying the balance among endogenous hormones.
Highlights
The cultivated macadamias (Macadamia integrifolia Maiden and Betche, M. tetraphylla Johnson, and hybrids) are evergreen trees originated in the eastern Australia and grown commercially in subtropical regions across the world for their nutritious kernel
Fruit set per raceme was significantly increased by foliar spray of CPPU at 15 days after anthesis, and the average number of fruit per raceme was 0.57 at 65 days after treatment (80 days after anthesis), which was 2.4 folds greater than the control (Fig 1A)
These results suggest that fruit drop mainly occurred during early fruit development
Summary
The cultivated macadamias (Macadamia integrifolia Maiden and Betche, M. tetraphylla Johnson, and hybrids) are evergreen trees originated in the eastern Australia and grown commercially in subtropical regions across the world for their nutritious kernel. Effect of CPPU on Young Macadamia Fruit world’s planting area [1]. Production of macadamia in China was very low due to low tree productivity even in the mature orchards [1]. The excessive fruit drop is a common problem in macadamia orchards across the production regions in China, including Guangdong [9], Yunnan [10], and Guangxi [11]. This phenomenon has posed a major challenge to the development of the macadamia industry
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