Concentration effect of prohexadione calciumon on growth of new and superior apple varieties and application suggestions

【Objective】The eastern Gansu region stands as a pivotal hub for premium apple production in China. However, the industry faces systemic challenges collectively referred to as the "four aging" dilemmas: Aging orchards, an aging workforce, obsolete cultivars, and outdated cultivation techniques. Traditional vigorous- rootstock systems exacerbate issues like excessive vegetative growth, low flower bud differentiation rates, and high dependency on labor-intensive practices such as trunk girdling and manual pruning and training. These factors severely hinder the transition to modern, high-density dwarfing cultivation systems. Chemical growth regulation, particularly using plant growth retardants, offers a promising solution to mitigate vegetative vigor, optimize tree architecture, and enhance reproductive efficiency. This study investigated the concentration- and variety-specific responses of four newly introduced elite apple varieties—Qinchui, Jiuyue Miracle, Azhen, and Venus Golden—to prohexadione calciumon (Pro-Ca) applications. The primary objectives were to: (1) quantify the effects of Pro-Ca on leaf physiology, branch morphology, and flower bud differentiation; (2) elucidate varietal sensitivity thresholds; (3) establish tailored chemical regulation protocols to support the modernization and sustainability of apple production in Gansu.【Methods】The experiment was conducted over the 2023—2024 growing seasons. Three- year- old trees of the four test varieties, all grafted onto T337 dwarfing rootstock and planted at 1.5 m × 4.0 m spacing, served as plant material. A randomized complete block design was implemented with four Pro-Ca concentration treatments: 0 mg·L-1 (Control, CK), 250 mg·L-1 , 500 mg·L-1 , and 750 mg ·L-1 . Each treatment included three replicates, with five trees per replicate. Pro-Ca was applied via foliar spraying using a high-pressure sprayer at a volume of 500 mL per tree. Three uniform applications were made at critical growth stages: May 21, June 1, and June 11, 2024. Control trees received equivalent volumes of water. Leaf morphological and physiological parameters including leaf area (cm2 ), leaf length (cm), leaf width (cm), chlorophyll content (SPAD), leaf thickness (mm), petiole length (cm), fresh mass (g), dry mass (g), hundred-leaf mass (g), and leaf water content (%) were measured 30 days after the final spray. Branch morphological parameters including short shoot count, length (cm), diameter (mm), internode count, and internode length (cm) were recorded for current-year shoots. Flower bud differentiation performance parameters including total flower buds, counts of one-year-old branches, long branches (＞30 cm), intermediate branches (15-30 cm), and short shoot (＜15 cm), along with spatial metrics (flower bud intervals on long/intermediate/short branches), were assessed during dormancy.【Results】Pro-Ca significantly suppressed leaf expansion in a concentration- and variety-dependent manner. At 750 mg·L-1 , leaf area reductions were most severe and 38.55%, 34.02%, 31.40% and 29.87% in Qinchui, Venus Golden, Azhen and Jiuyue Miracle, respectively. Dose-dependent declines in leaf length (35.4%-39.6%) and width (10.3%-14.8%) were observed, with Azhen exhibiting peak sensitivity at 500 mg·L-1 . Fresh mass, hundred-leaf mass, and dry mass decreased significantly under 750 mg·L-1 for Jiuyue Miracle, Azhen, and Venus Golden. Chlorophyll content and leaf water status remained statistically unchanged across all treatments and varieties. Qinchui maintained stable leaf thickness and petiole length regardless of concentration, whereas other varieties showed reductions at 750 mg ·L- 1 . Lowdose Pro-Ca (250 mg ·L-1 ) markedly increased short branch density in Qinchui (46.2% higher than CK) and enhanced diameter by 9.6%. Conversely, Azhen required 750 mg ·L-1 to achieve a significant 6.6% increase in short shoot diameter. Concentrations ≥500 mg · L- 1 inhibited shoot elongation. Internode length, total branch length, and node number decreased universally at 750 mg · L- 1 . Venus Golden displayed acute sensitivity, with severe growth inhibition at 750 mg · L- 1 . Qinchui exhibited a 12.5% increase in total flower buds at 250 mg·L-1 , but suffered a 33.3% reduction at 750 mg·L-1 . Venus Golden achieved peak flower bud formation (39.1% above CK) at 500 mg·L-1 . Jiuyue Miracle showed a 53.3% surge in one-year-old branches at 250 mg ·L-1 . Pro-Ca shortened flower bud intervals on long branches across varieties, most notably in Azhen (33.2% reduction at 750 mg·L-1 ) and Venus Golden (43% reduction at 750 mg · L- 1 ). Jiuyue Miracle doubled intermediate- branch flower buds at 750 mg · L- 1 despite overall growth suppression. 750 mg · L- 1 Pro-Ca reduced long-branch counts in Qinchui (71.4% reduction) and Venus Golden (45.5% reduction), promoting a compact canopy structure.【Conclusion】Qinchui and Jiuyue Miracle responded optimally to low doses (250 mg ·L-1 ), where vegetative growth control (short shoot promotion and shoot suppression) and floral enhancement were balanced. In contrast, Azhen and Venus Golden required higher concentrations (≥500 mg · L- 1 ) for effective canopy compaction and flower bud optimization, reflecting fundamental differences in GA metabolism or sink strength. Significant leaf area reduction at 750 mg·L-1 did not impair chlorophyll synthesis or hydration, suggesting Pro-Ca’s action is morphological rather than metabolic. Biomass declines were likely secondary to reduced photosynthetic area rather than direct inhibition of carbon fixation. Pro- Ca’s promotion of short branches and shortened flower bud intervals aligns with the physiological paradigm that compact, fruit-bearing structures enhance light interception and resource allocation efficiency. The doubling of in- termediate-branch flower buds in Jiuyue Miracle at 750 mg·L-1 underscores Pro-Ca’s role in bud initiation beyond mere growth suppression. The recommended concentrations, 250 mg · L- 1 for Qinchui and Jiuyue Miracle, and 500 mg · L- 1 for Azhen and Venus Golden, accelerate yield formation in young dwarf orchards, potentially shorten the non-bearing period by 1-2 years, and enable precise vegetativereproductive balance by enhancing short branch formation, flower bud density, and spatial distribution. This research provides the first comprehensive framework for Pro-Ca application in China’s leading new apple varieties, addressing the core challenges of the“four aging”crisis in Gansu’s apple industry.