Abstract
Crop load has a substantial impact on growth of the aerial and belowground parts of apple trees. Here, we examined the effects of different crop loads on growth and hormone levels in apple roots. A crop load of 1.5 (T1.5) fruits per cm2 trunk cross-sectional area (TCSA) treatment resulted in lower root growth vigor, while non-fruiting (T0) and T0.4 conditions showed higher root growth vigor. In all treatments, dead roots increased in length 90 days after full bloom (DAFB), whereas live roots were more abundant at about 50 and 170 DAFB, showing a bimodal curve. During each root growth peak, levels of cytokinins (CTKs), indole acetic acid (IAA), and gibberellic acid (GA3) were higher. Moreover, hormone levels gradually decreased with increasing crop load within each peak. Root turnover tended to decrease with decreasing crop load. These findings indicate that root growth and hormone contents were positively correlated during the fruit growth phase, and that the negative impact of crop load on root growth may have been caused by hormone level decreases.
Highlights
Alternate bearing is one of the greatest problems in apple production, resulting in unbalanced production and fruit quality
The results showed that root growth peaked at 150 days after full bloom (DAFB) under all crop loads
live root surface area density (LRSAD) and live root volume density (LRVD) at 130 DAFB were significantly higher in T0 and T1.5 showed the lowest levels (p < 0.05)
Summary
Alternate bearing is one of the greatest problems in apple production, resulting in unbalanced production and fruit quality This issue can be alleviated by managing an appropriate crop load, which influences vegetative and productive growth (Smith and Samach, 2013). Studies investigating the effects of fruits on photosynthesis, partitioning of assimilates, and dry matter accumulation have shown higher leaf photosynthetic efficiency in fruiting than in non-fruiting trees (Heim et al, 1979; Negi and Sharma, 2011). Both roots and fruits, non- or lowphotosynthetic organs, act as sinks depending exclusively on photosynthetic products imported from leaves. Despite studies on fruit and shoot growth, the effects of crop load on root growth and development remain unclear
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