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Relationship between the development of hollowing and the separation of vessel sectors in the central region of the root of Japanese radish (Raphanus sativus L.)
SCIENTIA HORTlCULTUiM ELSEVIER
Scientia Horticulturae 59 (1997) 59-72
Relationship between the development of hollowing and the separation of vessel sectors in the central region of the root of Japanese radish ( Ruphanus sativus L.1 Nobuyuki Fukuoka ‘, Yasutaka Kano Ishikawa Agricultural
*
College, Nonoichi, Ishikuwu, 921, Jupun
Accepted 23 August 1996
Abstract The present investigations were carried out to clarify the relationship between the separation of two vessel sectors, especially in the later growth period, and the development of hollowing in the root and the effect of varying the cultural conditions. Roots with hollow cavities were more prevalent in plants whose seeds were sown early in the summer compared with those sown later. The gap between the bifurcated vessel sectors became larger in roots sown earlier. A larger number of hollow roots was observed in the sparsely planted plot than in the high planting density plot, and the gap between the bifurcated vessel sectors became larger in roots at low planting density. Late defoliation, after rapid thickening growth, produced a large number of hollow roots, while early defoliation prevented the root from hollowing due to slower thickening growth. The gap between the bifurcated vessel sectors became wider the later the time of defoliation. A larger number of hollow roots occurred in proportion to increasing amounts of basal fertilizer and larger amounts of basal fertilizer application resulted in a wider space between the two vessel sectors. These facts strongly indicate that the development of hollowing may pass through two indispensable steps. First is the obstruction of cell formation inside the intercellular air space during the middle of the growth period, caused by high soil temperatures. Second is the rapid divergence of bifurcated vessel sectors during the later growth period as a result of enhanced
* Corresponding author.
’ Present address: Ishikawa Sand Dune Agricultural
Experimental
Station, Unoke, Ishikawa 929. I I.
0304-4238/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. F’II SO304-4238(96)00962-4
N. Fukwku,
60
thickening
Y. Kuno /Scientiu
growth. If the first condition
Horticulturue 59 11997) 59-72
is present, the second is the determining factor for the
development of hollowing. 0 Elsevier Science B.V. Keywords:
Ruphunus satiuus L.; Separation of vessel sectors;
0’ Planting density; Defoliation; Basal Hollowin,,
fertilizer
1. Introduction When growing Japanese radish cv. Gensuke during the summer, hollow root, a physiological disorder, which is manifested by a lengthwise hollowing in the central region of the root, is frequently found. Hollowing starts as the intercellular spaces coalesce in the central stele during the first half of the growth period (Kane and Fukuoka, 1992). Although the intercellular spaces are usually filled with large cells, under high soil temperature these cells fail to divide and enlarge enough to prevent the intercellular spaces from developing into a hollow space (Kano and Fukuoka, 1992). High soil temperature triggers the lignification of cells surrounding the coalescing spaces and this biochemical change prevents the cells from intruding into the spaces (Kano and Fukuoka, 1992). The increased injury resulting from hollowness is progressively intensified with rapid thickening growth during the later growth period (Kane and Fukuoka, 1989; Kano and Fukuoka, 1991). The frequency of hollow root is lowered by spraying plants with a high concentration of the sodium salt of cY-naphthaleneacetic acid, mainly due to the severe suppression of the rate of root growth (Kano, 1987). Furthermore, hollowing is liable to develop in the root whose central part has a tendency to expand, while no hollowing occurs in the root whose central part is hindered from expanding (Kano, 1989). These results strongly indicate that the occurrence of hollow root is affected by both the obstruction of cell formation inside the space during the early growth period and the rapid anatomical development during the later growth period. The present investigations were carried out to discover the relationship between root growth, especially during the later growth period, and the development of hollowing as affected by changing cultural conditions such as the sowing date, planting density, defoliation and fertilizer application.
2. Materials
and methods
2.1. Experiment 1 To assess the effect of sowing date on the development of hollowing and the separation of the two vessel sectors in the central region of the root, seeds of Raphanus satiuus L. cv. Gensuke were sown on 1 and 15 July and 1, 15 and 30 August 1990 at a density of six plants m-* in Ishikawa Ag ricultural College Experimental Farm. There was no replication of plots. Fifteen plants were sampled at 10 day intervals from 20 to 60 DAS (days after sowing) and plant growth was measured. To measure the degree of
N. Fukuoka, Y. Kana / Scientia Horticulturae 59 (1997) 59-72
61
Table 1 The amounts of basal fertilizers and the amount and time of the fertilizer application during the growth period Treatment
Basal fertilizer
Top dressing days after sowing 30
40
50
5-5 5-5-5 5-5-5-5 20-5 20-S-5 20-5-5-5
5’ 5 5 20 20 20
5 5 5 5 5 5
0 5 5 0 5 5
0 0 5 0 0 5
Total
10 15 20 25 30 35
* 5 g m-* for each of the following; N, P,O, and K,O.
hollowing, roots were cut lengthwise along the central axis 30, 40, 50 and 60 DAS, and the maximum crossand length-wise dimensions of any cavities were measured and their areas calculated. In all the following experiments, measurement of the degree of hollowing was carried out as in Experiment 1. Any root with a cavity larger than 20 mm* was classed as a hollow root and the average number of hollow roots at each sampling date was calculated for each treatment. For anatomical studies of vessel arrangement, 25-day-old roots, sown on 1 July, 15 and 30 August were sampled. The central region of the mid-portion of the root was dehydrated, embedded in paraffin and then cut transversely into 10 pm sections. The sections were stained with hematoxylin and eosin solutions.
1 Jul.
15 Jul. 1 Aug. 15 Aug. 30 Aug. Sowing date
Fig. I. Effect of sowing date on tire occurrence of hollow root. Means followed by the same letter are not significantly different at 5% level @uncan’s multiple range test).
62
N, Fukuoka, Y. Kano / Scientia Horticulturae 59 (1997) 59-72
2.2. Experiment 2 To determine the effect of planting density on the development of hollowing and the separation of the two vessel sectors in the central region of the root, four planting densities were used consisting of 20, ten, six and four plants The required number of seeds of R. satiuus L. cv. Gensuke were sown in areas of 4 m*, 7 m*, 11 m2 and 16 m2, respectively on 12 July 1990 at Ishikawa Agricultural College Experimental Farm. There was no replication of plots. Fifteen plants were sampled at 24,36, 48 and 56 DAS and plant growth was determined. The cavity size in the roots 36, 48 and 56 DAS was measured. Observations of vessel arrangement for the early growth period were made on 30-day-old roots as in Experiment 1. To check on vessel arrangement in the root during the later growth period, the mid-portion of the root was cut away from 48-day-old roots
m-* .
Fig. 2. Effect of sowing date on the separation of vessel sectors in the central region of the root. A, B and C represent cross sections of 25-day-old roots sown on 1 July, 15 and 30 August, respectively. D, E and F are the diagramatic representation of A, B and C. respectively. ‘S’ indicates a space between the bifurcated vessel sectors.
N. Fukuoka, Y. Karw/ Scientia Horticulturae 59 (1997) 59-72
63
Table 2 Effect of sowing date on plant growth Sowing date
1 July
15 July 1 August 15 August 30 August
Root weight (g) days after sowing
Leaf weight (g) days after sowing 30
40
50
60
30
40
50
60
273b * 302a 156c 175c 96d
542a 524a 3881, 23Oc 199c
650a 516b 453bc 35oc 230d
507a 481a 439ab 321~ 346bc
.54a 55a 31b 31b 19c
248a 153b 143b 1Olbc 53c
628a 472b 28Oc 191d 166c
687a 758a 535b 432bc 374c
* Means within a column by the same letter are not significantly different at the 5% level (Duncan’s multiple range test).
and 5 mm thick slices were immersed in phloroglucinol-HCI reagent and photographed immediately. 2.3. Experiment 3 To investigate the effect of defoliation on the development of hollowing and the separation of two vessel sectors in the central region of the root, seeds of R. satiuus L. cv. Gensuke were sown on 28 July 1991 in the field of Ishikawa Sand Dune Agricultural Experimental Station. In the defoliation treatment all expanded leaves were cut in half widthwise beginnidg at either 20, 30, 40 or 50 DAS and continued at 5 day intervals thereafter until harvest. Control plants were not given any treatment over the growth period. Ten plants were sampled from each treatment every 10 days from 30 to 60 DAS
0 20 Planting
10 density
6 4 (plants *m -‘I
Fig. 3. Effect of planting density on the occurrence of hollow root. Means followed by the same letter are not significantly different at 5% level (Duncan’s multiple range test).
64
N. Fukuoka, Y. Kano / Scientia Horriculrurae 59 (1997) 59-72
and plant growth was determined. Hollow size in the roots 40, 50 and 60 DAS was measured. Observation of vessel arrangement during the later growth period was made at 60 DAS using phloroglucinol-HCl reagent as in Experiment 2.
Fig. 4. Effect of planting density on the separation of vessel sectors in the central region of the root. A, B, C and D show cross sections of 30-day-old roots from the plots of 20 plants m-*, ten plants m-*, six plants m-* and four plants m-*, respectively. E, F, G and H are the diagramatic representation of A, B, C and D, respectively. ‘S’ indicates a space between the bifurcated vessel sectors.
Fig. 5. Fffect of planting density on the separation of vessel sectors in the central region of the root. A, B, C and D show cross sections of 48-day-old roots from the plots of 20 plants m-‘, ten plants m-‘, six plants m-’ and four plants m-*, respectively. The distance between arrows in a root show the space between the bifurcated vessel sectors.
66
N, Fukuoka, Y. Kano / Scientia Horticulturae
59 (1997) 59-72
Table 3 Effect of planting density on plant growth Planting density (plant rn-‘)
Leaf weight(g) days after sowing
20 10 6 4
53c 56bc 70a 66ab
24 l
Root weight (g) days after sowing
36
48
56
24
36
48
56
lllc 121c 154b 19Qa
134b 149b 223a 246a
284b 255b 442a 487a
8b 9ab 10a 7b
24b 31b 55a 60a
6@c 87c 124b 205a
143c 128~ 438b 575a
* Means within a column by the same letter are not significant different at the 5% level (Duncan’s multiple range test).
2.4. Experiment 4 To examine the effect of the amount of applied fertilizer and the time of fertilizer application on the development of hollowing and the separation of the two vessel sectors in the central region of the root, seeds of R. satiuus L cv. Gensuke were sown on 28 July 1991 in the field of Ishikawa Sand Dune Agricultural Experimental Station. Six treatments were laid out as shown in Table 1. Ten m* were used for each plot which were separated from each other by a one-metre-wide buffer zone. Ten plants were sampled every, 10 days from 20 to 60 DAS and plant growth was measured. The cavity size in the roots 30, 40, 50 and 60 DAS was measured. Observations of vessel arrangement during the later growth period were made at 60 DAS using phloroglucinolHCl reagent as in Experiment 2.
2.51
Fig. 6. Effect of defoliation treatment on the occurrence of hollow root. Means followed by the same letter are not significantly different at 5% level @uncan’s multiple range test). ’ Defoliation treatment began from 20 DAS to 60 DAS.
N. Fukuoka, Y. Kane/ Scientia Horticulturae 59 (1997) 59-72
67
3. Results 3.1. Experiment
1
A large number of hollow roots occurred in the early sown plot but a small number in the late sown plot (Fig. 1). Examination of transverse sections of 25-day-old roots at the middle of their growth period are shown in Fig. 2. Vessels, especially in the central region of the pith, diverged into two diametrically opposite groups producing a gap between them regardless of the sowing date. However, the gap between the bifurcated vessel sectors widened further in roots sown early as compared with those sown late. Consequently, the gap in plants sown on 1 July was approximately six times longer than that in plants sown on 30 August. Leaf weight was greater with earlier sowing dates than with later ones (Table 2). Likewise, root weight tended to be heavier the earlier the sowing date; later sowing resulted in a slower daily gain of weight during the growth period. 3.2. Experiment
2
A large number of hollow roots were found in the sparsely planted plot, while fewer were found in the densely planted plots (Fig. 3). Examples of transverse sections of
Fig. 7. Effect of defoliation treatment on the separation of vessel sectors in the central region of the root. A, B, C and D show cross sections of 60-day-old roots from 20, 30, 40 and 50 day treatments, respectively. E represents the root from the control. The distance between arrows in a root shows the space between the bifurcated vessel sectors.
68
N. Fukuoka, Y. Kano/Scientia
Horticulturae 59 (1997) 59-72
Table 4 Effect of defoliation treatment on plant growth Days when defoliation treatment initiated 20 * 30 40 50 Control
Leaf weight(g) days after sowing
Root weight(g) days after sowing
30
40
50
60
30
40
40
60
7Oc’ 137b 169b 134b 192a
123~ 163~ 298ab 263b 34Oa
124c 156bc 203b 321a 306a
123~ 137c 208b 173b 323a
1Oc 27b 37ab 26b 39a
56c 109b 195a 200a 234a
126d 15Od 21Oc 438a 391b
151d 141d 27.5~ 451b 625a
* Defoliation treatment began from 20 DAS to 60 DAS in 5 day intervals.+ Means within a column by the same letter are not significantly different at the 5% level (Duncan’s multiple range test).
30-day-old roots are shown in Fig. 4. The photographs and diagrams revealed that the lower the planting density, the larger was the space between the bifurcated vessel sectors. The space at densities of four and six plants mm2 was two to four times greater than at ten and 20 plants mm2. This contrast was more pronounced in the later growth period, 48 DAS (Fig. 5). Planting density had a significant effect on root growth for leaf and root weights increased faster when planting density was lower (Table 3). 3.3. Experiment 3 A large number of hollow roots was observed in the plots defoliated later in the growth period, but none were observed in the plants defoliated before 30 DAS (Fig. 6).
0 5-5l 5-5-5 5-5-5-5 20-5 Amount and time of application
of tertitlzer
Fig. 8. Effect of amount and time of application of fertilizer on the occurrence of hollow root. Means followed by the same letter are not significantly different at 5% level (Duncan’s multiple range test). ’ See Table 1.
N. Fukuoka, Y. Kam / Scientia Horticulturae 59 (1997) 59-72
69
Fig. 9. Effect of amount and time of application of fertilizer on the separation of vessel sectors in the central region of the root. A, B, C, D, E and F show cross sections of 60-day-old root from the plots of 5-5 ’ , 5-S-5, 5-5-5-5, 20-5, 20-5-5 and 20-5-5-5, respectively. The distance between arrows in a root shows the space between the bifurcated vessel sectors. ’ SeeTable 1.
The photographs made of 60-day-old roots revealed that the gap between the bifurcated vessel sectors was wider the later the time of defoliation (Fig. 7). The space in roots of the plants defoliated at the 30 DAS remained small over the growth period. Leaf and root weights of the undefoliated control were the heaviest (Table 4) and leaf and root weights decreased the earlier the defoliation treatments were started. Late defoliation
Table 5 Effect of amount of fertilizers, and amount and time of top dressing on plant growth Treatment
5-5 5-5-5 5-5-5-5 20-5 20-5-5 20-5-5-5 l
Leaf weight(g) days after sowing
Root weight(g) days after sowing
30
40
50
60
30
40
50
60
109c + 117c 128~ 207b 217ab 236a
246c 287~ 301c 430b 527a 514a
354b 348b 353b 617a 642a 707a
352d 51 lc 448~ 523~ 782a 653b
27b 34b 28b 55a 58a 60a
192b 190b 17lb 347a 325a 319a
467C
741b 764b 792b 1130a 1229a 1246a
484c 46% 760a 760a 714a
* See Table I.+ Means within a column by the same letter are not significantly different at the 5% level (Duncan’s multiple range test).
70
N. Fukuoka, Y. Kano / Scientia Horticulturae 59 (1997) 59-72
resulted in rapid thickening growth and promoted the development of hollowing, while early defoliation generally prevented the root from hollowing as a result of slower thickening growth (Fig. 6). This contrast was clearly noticed during the later growth period. 3.4. Experiment 4 The number of hollow roots became larger in proportion to the amount of total fertilizer applied (Fig. 8). Observation of the vessel arrangement at 60 DAS revealed that an increase in the total fertilizer applied, resulted in a larger space between the two vessel sectors (Fig. 9). The amount of basal fertilizer rather than the time and the amount of top dressing was especially important for enlargement of the roots (Table 5). The larger the basal fertilizer applied, the more vigorous was the root growth.
4. Discussion
The experimental results clearly revealed that the cultural and environmental conditions had a significant effect on the development of hollowing in the root of radish and that the development of hollowing was closely related to root growth during the maturation period. Favourable conditions for root growth such as sowing earlier in the summer, lower planting density and/or higher rates of fertilizer application resulted in a greater amount of hollowness. Conversely, plants were free from hollowness under conditions that prevented roots from growing well; such as later sowing, higher planting density, earlier defoliation treatment and lower rates of fertilizer application. These results agree with the reports of Toyotomi and Imaizumi (1970), and Inagaki and Imaizumi (1970). Late thinning reduced the size of the central cavity because root growth was restricted during the growth period (Kane and Fukuoka, 1991). Moreover, the faster the root grew, the more likely that hollowing would develop (Kane and Fukuoka, 1989; Tamada and Emura, 1981). Thus, it seems that rapid root thickening during later growth causes a high occurrence of hollow root. Anatomical observations showed that the vessels in the central region of the pith had diverged into two sectors by the middle of the growth period. The gap between the bifurcated vessel sectors was more pronounced when thickening was achieved more rapidly during the later growing season, whereas slower thickening hindered the space from extending and the existing space remained small. Vessels lined up radially in the central region of the pith and divided into two diametrically opposed sectors from the beginning to the middle of the growth period. The separation of these vessel sectors was more pronounced as radial growth was enhanced (Kane and Fukuoka, 1991). Thus, it may be considered that environmental conditions regulate the distance between the two polarized vessel sectors by controlling root growth during the later growth period. A swift separation of the vessel sectors accompanied by radial thickening growth makes the spaces coalesce into a large hollow cavity. Our recent publications (Fukuoka and Kano, 1992; Kano and Fukuoka, 1991; Kano and Fukuoka, 1992) are concerned with anatomical observations of hollowness in the early stages. In general, intercellular space
N. Fukuoka. Y. Kane / Scientia Horticulturae 59 (1997) 59-72
71
appears initially as a result of mechanical strains between the two vessel sectors. Although this space is usually filled with large cells because some previously formed projecting cells inside the space continue cell division and enlargement, under high soil temperature they fail to divide and enlarge enough to prevent the spaces from coalescing into a hollow cavity. The reduction in cell production by these cells lining the space is attributed to the induction of lignin formation on the surfaces of the space. These facts strongly indicate that the development of intercellular space must pass through two indispensable steps. First is the obstruction of cell formation inside the space during the middle of the growth period due to high soil temperatures. Second is the rapid divergence of bifurcated vessel sectors during the later growth period as a result of enhanced thickening growth. If the first condition exists, the second is the determining factor for the occurrence of hollow root. When root growth during this period is enhanced by favourable conditions, cell formation inside the intercellular space does not have enough developmental potential to prevent the space from growing. However, when thickening growth is restricted by various cultural stresses, cell formation inside the space can exceed the enlargement of the cavity area created by the slower separation of the two vessel sectors. Hollowness is then not favoured by slow thickening growth If cell differentiation on the cavity surface is activated by low soil temperatures, the production of cells exceeds the enlargement capability of the hollow cavity. Because of this, the space is filled with parenchymatous cells and does not develop into a large hollow cavity.
Acknowledgements
The authors are grateful to Shuichi Iwahori, Professor of University of Tsukuba for his critical reading of the manuscript. This work was partly supported by a Grant-in-Aid Scientific Research (B) No. 02556005 from Ministry of Education, Science and Culture.
References Fukuoka, N. and Kano, Y., 1992. The difference in the development of hollowness in roots of ‘Gensuke’ radish between the early and late sowing of seeds. J. Jpn. Sot. Hortic. Sci., 60: 881-887. Inagaki, S. and Imaizumi, H., 1970. Experiments for suppressing the occurrence of hollow root in Japanese radishes. File of experimental results on vegetable of Mie Agric. Tech. Center, 1969: 63-67 (in Japanese). Kano, Y., 1987. Effect of auxin treatment on the occurrence of ‘hollow root’ of ‘Gensuke’. Bull. Ishikawa. Agric. Coll., 17: 21-24. Kano, Y., 1989. Effects of root growth - especially of the growth of the central part of the root - on the occurrence of hollow root in Japanese radishes. Bull. Ishikawa. Agric. Coll., 19: 17-23. Kano, Y. and Fukuoka, N., 1989. Studies on the occurrence of hollow root in Japanese radish cv. Gensuke. 7. Relationship between the occurrence of hollow root and the growth rate of root. J. Jpn. Sot. Hortic. Sci. (Suppl. 1): 114- 115 (in Japanese with English title). Kano, Y. and Fukuoka, N., 1991. Effect of planting density on the occurrence of hollow root in Japanese radish cv. Gensuke. J. Jpn. Sot. Hortic. Sci., 60: 379-386. Kano, Y. and Fukuoka, N., 1992. Relationship between the occurrence of hollowing and lignitication of parenchymatous cells in the root of Japanese radish cv. Gensuke. J. Jpn. Sot. Hortic. Sci., 61: 359-366.
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N. Fukuoka, Y. Kano / Scientia Horticulturae 59 (1997) 59-72
Tamada, A. and Emma, M., 1981. Effect of sowing date on the occurrence of hollow root in a Japanese radish, ‘Taibyosobutori’. File of experimental results of Uchino Branch Niigata Hortic. Exp. Stn., 1980: 63-65 (in Japanese). Toyotomi, Y. and Imaizumi, H., 1970. Study on the occurrence of hollow root in a different soil textures and the histological observations of the hollow root. Abstr. Jpn. Sot. Hortic. Sci. Autumn Meet., pp. 114-115 (in Japanese).
Scientia Horticulturae 59 (1997) 59-72
Relationship between the development of hollowing and the separation of vessel sectors in the central region of the root of Japanese radish ( Ruphanus sativus L.1 Nobuyuki Fukuoka ‘, Yasutaka Kano Ishikawa Agricultural
*
College, Nonoichi, Ishikuwu, 921, Jupun
Accepted 23 August 1996
Abstract The present investigations were carried out to clarify the relationship between the separation of two vessel sectors, especially in the later growth period, and the development of hollowing in the root and the effect of varying the cultural conditions. Roots with hollow cavities were more prevalent in plants whose seeds were sown early in the summer compared with those sown later. The gap between the bifurcated vessel sectors became larger in roots sown earlier. A larger number of hollow roots was observed in the sparsely planted plot than in the high planting density plot, and the gap between the bifurcated vessel sectors became larger in roots at low planting density. Late defoliation, after rapid thickening growth, produced a large number of hollow roots, while early defoliation prevented the root from hollowing due to slower thickening growth. The gap between the bifurcated vessel sectors became wider the later the time of defoliation. A larger number of hollow roots occurred in proportion to increasing amounts of basal fertilizer and larger amounts of basal fertilizer application resulted in a wider space between the two vessel sectors. These facts strongly indicate that the development of hollowing may pass through two indispensable steps. First is the obstruction of cell formation inside the intercellular air space during the middle of the growth period, caused by high soil temperatures. Second is the rapid divergence of bifurcated vessel sectors during the later growth period as a result of enhanced
* Corresponding author.
’ Present address: Ishikawa Sand Dune Agricultural
Experimental
Station, Unoke, Ishikawa 929. I I.
0304-4238/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. F’II SO304-4238(96)00962-4
N. Fukwku,
60
thickening
Y. Kuno /Scientiu
growth. If the first condition
Horticulturue 59 11997) 59-72
is present, the second is the determining factor for the
development of hollowing. 0 Elsevier Science B.V. Keywords:
Ruphunus satiuus L.; Separation of vessel sectors;
0’ Planting density; Defoliation; Basal Hollowin,,
fertilizer
1. Introduction When growing Japanese radish cv. Gensuke during the summer, hollow root, a physiological disorder, which is manifested by a lengthwise hollowing in the central region of the root, is frequently found. Hollowing starts as the intercellular spaces coalesce in the central stele during the first half of the growth period (Kane and Fukuoka, 1992). Although the intercellular spaces are usually filled with large cells, under high soil temperature these cells fail to divide and enlarge enough to prevent the intercellular spaces from developing into a hollow space (Kano and Fukuoka, 1992). High soil temperature triggers the lignification of cells surrounding the coalescing spaces and this biochemical change prevents the cells from intruding into the spaces (Kano and Fukuoka, 1992). The increased injury resulting from hollowness is progressively intensified with rapid thickening growth during the later growth period (Kane and Fukuoka, 1989; Kano and Fukuoka, 1991). The frequency of hollow root is lowered by spraying plants with a high concentration of the sodium salt of cY-naphthaleneacetic acid, mainly due to the severe suppression of the rate of root growth (Kano, 1987). Furthermore, hollowing is liable to develop in the root whose central part has a tendency to expand, while no hollowing occurs in the root whose central part is hindered from expanding (Kano, 1989). These results strongly indicate that the occurrence of hollow root is affected by both the obstruction of cell formation inside the space during the early growth period and the rapid anatomical development during the later growth period. The present investigations were carried out to discover the relationship between root growth, especially during the later growth period, and the development of hollowing as affected by changing cultural conditions such as the sowing date, planting density, defoliation and fertilizer application.
2. Materials
and methods
2.1. Experiment 1 To assess the effect of sowing date on the development of hollowing and the separation of the two vessel sectors in the central region of the root, seeds of Raphanus satiuus L. cv. Gensuke were sown on 1 and 15 July and 1, 15 and 30 August 1990 at a density of six plants m-* in Ishikawa Ag ricultural College Experimental Farm. There was no replication of plots. Fifteen plants were sampled at 10 day intervals from 20 to 60 DAS (days after sowing) and plant growth was measured. To measure the degree of
N. Fukuoka, Y. Kana / Scientia Horticulturae 59 (1997) 59-72
61
Table 1 The amounts of basal fertilizers and the amount and time of the fertilizer application during the growth period Treatment
Basal fertilizer
Top dressing days after sowing 30
40
50
5-5 5-5-5 5-5-5-5 20-5 20-S-5 20-5-5-5
5’ 5 5 20 20 20
5 5 5 5 5 5
0 5 5 0 5 5
0 0 5 0 0 5
Total
10 15 20 25 30 35
* 5 g m-* for each of the following; N, P,O, and K,O.
hollowing, roots were cut lengthwise along the central axis 30, 40, 50 and 60 DAS, and the maximum crossand length-wise dimensions of any cavities were measured and their areas calculated. In all the following experiments, measurement of the degree of hollowing was carried out as in Experiment 1. Any root with a cavity larger than 20 mm* was classed as a hollow root and the average number of hollow roots at each sampling date was calculated for each treatment. For anatomical studies of vessel arrangement, 25-day-old roots, sown on 1 July, 15 and 30 August were sampled. The central region of the mid-portion of the root was dehydrated, embedded in paraffin and then cut transversely into 10 pm sections. The sections were stained with hematoxylin and eosin solutions.
1 Jul.
15 Jul. 1 Aug. 15 Aug. 30 Aug. Sowing date
Fig. I. Effect of sowing date on tire occurrence of hollow root. Means followed by the same letter are not significantly different at 5% level @uncan’s multiple range test).
62
N, Fukuoka, Y. Kano / Scientia Horticulturae 59 (1997) 59-72
2.2. Experiment 2 To determine the effect of planting density on the development of hollowing and the separation of the two vessel sectors in the central region of the root, four planting densities were used consisting of 20, ten, six and four plants The required number of seeds of R. satiuus L. cv. Gensuke were sown in areas of 4 m*, 7 m*, 11 m2 and 16 m2, respectively on 12 July 1990 at Ishikawa Agricultural College Experimental Farm. There was no replication of plots. Fifteen plants were sampled at 24,36, 48 and 56 DAS and plant growth was determined. The cavity size in the roots 36, 48 and 56 DAS was measured. Observations of vessel arrangement for the early growth period were made on 30-day-old roots as in Experiment 1. To check on vessel arrangement in the root during the later growth period, the mid-portion of the root was cut away from 48-day-old roots
m-* .
Fig. 2. Effect of sowing date on the separation of vessel sectors in the central region of the root. A, B and C represent cross sections of 25-day-old roots sown on 1 July, 15 and 30 August, respectively. D, E and F are the diagramatic representation of A, B and C. respectively. ‘S’ indicates a space between the bifurcated vessel sectors.
N. Fukuoka, Y. Karw/ Scientia Horticulturae 59 (1997) 59-72
63
Table 2 Effect of sowing date on plant growth Sowing date
1 July
15 July 1 August 15 August 30 August
Root weight (g) days after sowing
Leaf weight (g) days after sowing 30
40
50
60
30
40
50
60
273b * 302a 156c 175c 96d
542a 524a 3881, 23Oc 199c
650a 516b 453bc 35oc 230d
507a 481a 439ab 321~ 346bc
.54a 55a 31b 31b 19c
248a 153b 143b 1Olbc 53c
628a 472b 28Oc 191d 166c
687a 758a 535b 432bc 374c
* Means within a column by the same letter are not significantly different at the 5% level (Duncan’s multiple range test).
and 5 mm thick slices were immersed in phloroglucinol-HCI reagent and photographed immediately. 2.3. Experiment 3 To investigate the effect of defoliation on the development of hollowing and the separation of two vessel sectors in the central region of the root, seeds of R. satiuus L. cv. Gensuke were sown on 28 July 1991 in the field of Ishikawa Sand Dune Agricultural Experimental Station. In the defoliation treatment all expanded leaves were cut in half widthwise beginnidg at either 20, 30, 40 or 50 DAS and continued at 5 day intervals thereafter until harvest. Control plants were not given any treatment over the growth period. Ten plants were sampled from each treatment every 10 days from 30 to 60 DAS
0 20 Planting
10 density
6 4 (plants *m -‘I
Fig. 3. Effect of planting density on the occurrence of hollow root. Means followed by the same letter are not significantly different at 5% level (Duncan’s multiple range test).
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and plant growth was determined. Hollow size in the roots 40, 50 and 60 DAS was measured. Observation of vessel arrangement during the later growth period was made at 60 DAS using phloroglucinol-HCl reagent as in Experiment 2.
Fig. 4. Effect of planting density on the separation of vessel sectors in the central region of the root. A, B, C and D show cross sections of 30-day-old roots from the plots of 20 plants m-*, ten plants m-*, six plants m-* and four plants m-*, respectively. E, F, G and H are the diagramatic representation of A, B, C and D, respectively. ‘S’ indicates a space between the bifurcated vessel sectors.
Fig. 5. Fffect of planting density on the separation of vessel sectors in the central region of the root. A, B, C and D show cross sections of 48-day-old roots from the plots of 20 plants m-‘, ten plants m-‘, six plants m-’ and four plants m-*, respectively. The distance between arrows in a root show the space between the bifurcated vessel sectors.
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Table 3 Effect of planting density on plant growth Planting density (plant rn-‘)
Leaf weight(g) days after sowing
20 10 6 4
53c 56bc 70a 66ab
24 l
Root weight (g) days after sowing
36
48
56
24
36
48
56
lllc 121c 154b 19Qa
134b 149b 223a 246a
284b 255b 442a 487a
8b 9ab 10a 7b
24b 31b 55a 60a
6@c 87c 124b 205a
143c 128~ 438b 575a
* Means within a column by the same letter are not significant different at the 5% level (Duncan’s multiple range test).
2.4. Experiment 4 To examine the effect of the amount of applied fertilizer and the time of fertilizer application on the development of hollowing and the separation of the two vessel sectors in the central region of the root, seeds of R. satiuus L cv. Gensuke were sown on 28 July 1991 in the field of Ishikawa Sand Dune Agricultural Experimental Station. Six treatments were laid out as shown in Table 1. Ten m* were used for each plot which were separated from each other by a one-metre-wide buffer zone. Ten plants were sampled every, 10 days from 20 to 60 DAS and plant growth was measured. The cavity size in the roots 30, 40, 50 and 60 DAS was measured. Observations of vessel arrangement during the later growth period were made at 60 DAS using phloroglucinolHCl reagent as in Experiment 2.
2.51
Fig. 6. Effect of defoliation treatment on the occurrence of hollow root. Means followed by the same letter are not significantly different at 5% level @uncan’s multiple range test). ’ Defoliation treatment began from 20 DAS to 60 DAS.
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67
3. Results 3.1. Experiment
1
A large number of hollow roots occurred in the early sown plot but a small number in the late sown plot (Fig. 1). Examination of transverse sections of 25-day-old roots at the middle of their growth period are shown in Fig. 2. Vessels, especially in the central region of the pith, diverged into two diametrically opposite groups producing a gap between them regardless of the sowing date. However, the gap between the bifurcated vessel sectors widened further in roots sown early as compared with those sown late. Consequently, the gap in plants sown on 1 July was approximately six times longer than that in plants sown on 30 August. Leaf weight was greater with earlier sowing dates than with later ones (Table 2). Likewise, root weight tended to be heavier the earlier the sowing date; later sowing resulted in a slower daily gain of weight during the growth period. 3.2. Experiment
2
A large number of hollow roots were found in the sparsely planted plot, while fewer were found in the densely planted plots (Fig. 3). Examples of transverse sections of
Fig. 7. Effect of defoliation treatment on the separation of vessel sectors in the central region of the root. A, B, C and D show cross sections of 60-day-old roots from 20, 30, 40 and 50 day treatments, respectively. E represents the root from the control. The distance between arrows in a root shows the space between the bifurcated vessel sectors.
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Table 4 Effect of defoliation treatment on plant growth Days when defoliation treatment initiated 20 * 30 40 50 Control
Leaf weight(g) days after sowing
Root weight(g) days after sowing
30
40
50
60
30
40
40
60
7Oc’ 137b 169b 134b 192a
123~ 163~ 298ab 263b 34Oa
124c 156bc 203b 321a 306a
123~ 137c 208b 173b 323a
1Oc 27b 37ab 26b 39a
56c 109b 195a 200a 234a
126d 15Od 21Oc 438a 391b
151d 141d 27.5~ 451b 625a
* Defoliation treatment began from 20 DAS to 60 DAS in 5 day intervals.+ Means within a column by the same letter are not significantly different at the 5% level (Duncan’s multiple range test).
30-day-old roots are shown in Fig. 4. The photographs and diagrams revealed that the lower the planting density, the larger was the space between the bifurcated vessel sectors. The space at densities of four and six plants mm2 was two to four times greater than at ten and 20 plants mm2. This contrast was more pronounced in the later growth period, 48 DAS (Fig. 5). Planting density had a significant effect on root growth for leaf and root weights increased faster when planting density was lower (Table 3). 3.3. Experiment 3 A large number of hollow roots was observed in the plots defoliated later in the growth period, but none were observed in the plants defoliated before 30 DAS (Fig. 6).
0 5-5l 5-5-5 5-5-5-5 20-5 Amount and time of application
of tertitlzer
Fig. 8. Effect of amount and time of application of fertilizer on the occurrence of hollow root. Means followed by the same letter are not significantly different at 5% level (Duncan’s multiple range test). ’ See Table 1.
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69
Fig. 9. Effect of amount and time of application of fertilizer on the separation of vessel sectors in the central region of the root. A, B, C, D, E and F show cross sections of 60-day-old root from the plots of 5-5 ’ , 5-S-5, 5-5-5-5, 20-5, 20-5-5 and 20-5-5-5, respectively. The distance between arrows in a root shows the space between the bifurcated vessel sectors. ’ SeeTable 1.
The photographs made of 60-day-old roots revealed that the gap between the bifurcated vessel sectors was wider the later the time of defoliation (Fig. 7). The space in roots of the plants defoliated at the 30 DAS remained small over the growth period. Leaf and root weights of the undefoliated control were the heaviest (Table 4) and leaf and root weights decreased the earlier the defoliation treatments were started. Late defoliation
Table 5 Effect of amount of fertilizers, and amount and time of top dressing on plant growth Treatment
5-5 5-5-5 5-5-5-5 20-5 20-5-5 20-5-5-5 l
Leaf weight(g) days after sowing
Root weight(g) days after sowing
30
40
50
60
30
40
50
60
109c + 117c 128~ 207b 217ab 236a
246c 287~ 301c 430b 527a 514a
354b 348b 353b 617a 642a 707a
352d 51 lc 448~ 523~ 782a 653b
27b 34b 28b 55a 58a 60a
192b 190b 17lb 347a 325a 319a
467C
741b 764b 792b 1130a 1229a 1246a
484c 46% 760a 760a 714a
* See Table I.+ Means within a column by the same letter are not significantly different at the 5% level (Duncan’s multiple range test).
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resulted in rapid thickening growth and promoted the development of hollowing, while early defoliation generally prevented the root from hollowing as a result of slower thickening growth (Fig. 6). This contrast was clearly noticed during the later growth period. 3.4. Experiment 4 The number of hollow roots became larger in proportion to the amount of total fertilizer applied (Fig. 8). Observation of the vessel arrangement at 60 DAS revealed that an increase in the total fertilizer applied, resulted in a larger space between the two vessel sectors (Fig. 9). The amount of basal fertilizer rather than the time and the amount of top dressing was especially important for enlargement of the roots (Table 5). The larger the basal fertilizer applied, the more vigorous was the root growth.
4. Discussion
The experimental results clearly revealed that the cultural and environmental conditions had a significant effect on the development of hollowing in the root of radish and that the development of hollowing was closely related to root growth during the maturation period. Favourable conditions for root growth such as sowing earlier in the summer, lower planting density and/or higher rates of fertilizer application resulted in a greater amount of hollowness. Conversely, plants were free from hollowness under conditions that prevented roots from growing well; such as later sowing, higher planting density, earlier defoliation treatment and lower rates of fertilizer application. These results agree with the reports of Toyotomi and Imaizumi (1970), and Inagaki and Imaizumi (1970). Late thinning reduced the size of the central cavity because root growth was restricted during the growth period (Kane and Fukuoka, 1991). Moreover, the faster the root grew, the more likely that hollowing would develop (Kane and Fukuoka, 1989; Tamada and Emura, 1981). Thus, it seems that rapid root thickening during later growth causes a high occurrence of hollow root. Anatomical observations showed that the vessels in the central region of the pith had diverged into two sectors by the middle of the growth period. The gap between the bifurcated vessel sectors was more pronounced when thickening was achieved more rapidly during the later growing season, whereas slower thickening hindered the space from extending and the existing space remained small. Vessels lined up radially in the central region of the pith and divided into two diametrically opposed sectors from the beginning to the middle of the growth period. The separation of these vessel sectors was more pronounced as radial growth was enhanced (Kane and Fukuoka, 1991). Thus, it may be considered that environmental conditions regulate the distance between the two polarized vessel sectors by controlling root growth during the later growth period. A swift separation of the vessel sectors accompanied by radial thickening growth makes the spaces coalesce into a large hollow cavity. Our recent publications (Fukuoka and Kano, 1992; Kano and Fukuoka, 1991; Kano and Fukuoka, 1992) are concerned with anatomical observations of hollowness in the early stages. In general, intercellular space
N. Fukuoka. Y. Kane / Scientia Horticulturae 59 (1997) 59-72
71
appears initially as a result of mechanical strains between the two vessel sectors. Although this space is usually filled with large cells because some previously formed projecting cells inside the space continue cell division and enlargement, under high soil temperature they fail to divide and enlarge enough to prevent the spaces from coalescing into a hollow cavity. The reduction in cell production by these cells lining the space is attributed to the induction of lignin formation on the surfaces of the space. These facts strongly indicate that the development of intercellular space must pass through two indispensable steps. First is the obstruction of cell formation inside the space during the middle of the growth period due to high soil temperatures. Second is the rapid divergence of bifurcated vessel sectors during the later growth period as a result of enhanced thickening growth. If the first condition exists, the second is the determining factor for the occurrence of hollow root. When root growth during this period is enhanced by favourable conditions, cell formation inside the intercellular space does not have enough developmental potential to prevent the space from growing. However, when thickening growth is restricted by various cultural stresses, cell formation inside the space can exceed the enlargement of the cavity area created by the slower separation of the two vessel sectors. Hollowness is then not favoured by slow thickening growth If cell differentiation on the cavity surface is activated by low soil temperatures, the production of cells exceeds the enlargement capability of the hollow cavity. Because of this, the space is filled with parenchymatous cells and does not develop into a large hollow cavity.
Acknowledgements
The authors are grateful to Shuichi Iwahori, Professor of University of Tsukuba for his critical reading of the manuscript. This work was partly supported by a Grant-in-Aid Scientific Research (B) No. 02556005 from Ministry of Education, Science and Culture.
References Fukuoka, N. and Kano, Y., 1992. The difference in the development of hollowness in roots of ‘Gensuke’ radish between the early and late sowing of seeds. J. Jpn. Sot. Hortic. Sci., 60: 881-887. Inagaki, S. and Imaizumi, H., 1970. Experiments for suppressing the occurrence of hollow root in Japanese radishes. File of experimental results on vegetable of Mie Agric. Tech. Center, 1969: 63-67 (in Japanese). Kano, Y., 1987. Effect of auxin treatment on the occurrence of ‘hollow root’ of ‘Gensuke’. Bull. Ishikawa. Agric. Coll., 17: 21-24. Kano, Y., 1989. Effects of root growth - especially of the growth of the central part of the root - on the occurrence of hollow root in Japanese radishes. Bull. Ishikawa. Agric. Coll., 19: 17-23. Kano, Y. and Fukuoka, N., 1989. Studies on the occurrence of hollow root in Japanese radish cv. Gensuke. 7. Relationship between the occurrence of hollow root and the growth rate of root. J. Jpn. Sot. Hortic. Sci. (Suppl. 1): 114- 115 (in Japanese with English title). Kano, Y. and Fukuoka, N., 1991. Effect of planting density on the occurrence of hollow root in Japanese radish cv. Gensuke. J. Jpn. Sot. Hortic. Sci., 60: 379-386. Kano, Y. and Fukuoka, N., 1992. Relationship between the occurrence of hollowing and lignitication of parenchymatous cells in the root of Japanese radish cv. Gensuke. J. Jpn. Sot. Hortic. Sci., 61: 359-366.
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Tamada, A. and Emma, M., 1981. Effect of sowing date on the occurrence of hollow root in a Japanese radish, ‘Taibyosobutori’. File of experimental results of Uchino Branch Niigata Hortic. Exp. Stn., 1980: 63-65 (in Japanese). Toyotomi, Y. and Imaizumi, H., 1970. Study on the occurrence of hollow root in a different soil textures and the histological observations of the hollow root. Abstr. Jpn. Sot. Hortic. Sci. Autumn Meet., pp. 114-115 (in Japanese).