Figures (3)  Tables (1)

    • Figure 1. 

      Hollow heart in some example vegetable crops. (a) Radish[1]. (b) Broccoli[2]. (c) Cucumber[3]. (d) Potato[4]. (e) Watermelon[5].

    • Figure 2. 

      Hypothetical regulatory model of hollow heart formation in vegetable crops. The center picture (hollow heart in cucumber) is a depiction to show the hollow heart trait and does not represent a specific vegetable species. The solid lines indicate the proven regulation, while the dashed lines indicate the assumed regulation. Arrows indicate positive regulation or promotion, while short lines indicate negative regulation or inhibition. The double sided arrow indicates that there is a feedback regulation between the two genes. The sharp triangle represents that there are both positive and negative regulations. The question mark represents the regulation pattern should be further studied. N, nitrogen. S, sulfur. B, boron. P, phosphorus. K, potassium. Ca, calcium. CTK/ZT, cytokinin/zeatin. IBA, 3-indole butyric acid. ABA, abscisic acid. IAA, indole-3-acetic acid. GA, gibberellin. ETH, ethylene. JA, jasmonate. COI1, coronatine insensitive1. EIN3, ethylene insensitive 3. JAH3, jasmonate hypersensitive 3. GID1a, gibberellin insensitive dwarf 1a. GH3, Gretchen Hagen3. ALMT2, aluminum-activated malate transporter 2. sCAX1, H+/Ca2+ antiporter 1 without the autoinhibitory domain from the N-terminal region. WOX1, WUSCHEL-related homeobox1. ALC, ALCATRAZ. SPT, SPATULA.

    • Figure 3. 

      (a)−(c) Acoustic-based, and (d), (e) optics-based detection methods of hollow heart in vegetable crops. (a) Impulse vibration[67]. Left, pressurized-air excitation device system. Right, the effect of hollow volume on normalized second to fourth resonance frequencies. (b) Ultrasonic[68]. Left, the schematic diagram. Right, frequency amplitude spectra of the transmitted signals in hollow and non-hollow potatoes. (c) Helmholtz resonance[70]. Left, schematic diagram. Right, the density distribution of watermelons with internal hollow (with water) or normal. (d) Visible/near-infrared transmittance[73]. Left, measuring system. Right, The range-normalized transmittance spectrum of an intact potato or with hollow heart. (e) Hyperspectral imaging[75]. Left, schematic of transmittance measurement system. Right, mean relative transmittance spectra of normal and defective cucumbers and their standard deviation (STD) spectra after one day of bruising.

    • Exogenous hormone Concentration Application method Plant species Key findings Ref.
      Ethephon (2-chloroethylphosphonic acid) 4 oz·acre−1 One foliar spray at the time of tuber initiation potato Hollow heart was significantly reduced when the largest tuber set was approximately 30 mm in diameter. [41]
      Na-salt of α-naphthaleneacetic acid (NAA) 50 mg·L−1 Foliar spray in every three days from 16 to 30 d after sowing Japanese radish The root thickening rate slowed and lignin formation actived, resulting in a higher density of vessels and hollow heart. [42]
      1-(2-chloro-4-pyridyl)-3-phenylurea (CPPU) 5 mg·L−1 Spray on the leaves every
      3 d from the 11th to the
      40th d after sowing      		 Japanese radish The central region of the root filled with parenchymatous cells, vessels were sparsely arranged, and the hollow heart decreased. [43]
      CPPU 0.1, 1, 10 mg·L−1 Foliar spray at different
      growth stage      		 Japanese radish CPPU or 6-BA resulted in sparsely arranged vessels and fewer hollow roots during the middle of the growth stage, while NAA caused a high density of vessels and more hollow roots during the first half of the growth stage. [44]
      6-benzylaminopurine (6-BA) 1, 10, 100 mg·L−1
      NAA 1, 5 mg·L−1
      2,4-dichlorophenoxy acetic acid 50, 200 mg·L−1 Localized applications
      to ovary      		 Triploid watermelon The treatment increased hollow fruit in a concentration-dependent way. [45]
      CPPU 50, 100, 200 mg·L−1 The treatment did not increase yield or fruit quality but reduced hollow fruit.
      CPPU 5 mg·L−1 Ovary spray on a day
      before flowering, the
      day of flowering, and
      a day after flowering      		 Oriental melon (Cucumis melo L. var. makuwa Makino) Ovary spray on a day before flowering reduced the hollow ratio and substantially increased the proportion of unfertilized seeds. [46]

      Table 1. 

      Role of exogenous hormone on the formation of hollow heart in vegetable crops.