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The effect of temperature on growth and development of cultivars of radish under winter conditions
Scientia Horticulturae, 5 (1976) 111--118 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
111
THE EFFECT OF TEMPERATURE ON GROWTH AND DEVELOPMENT OF CULTIVARS OF RADISH UNDER WINTER CONDITIONS
M. NIEUWHOF
Institute for Horticultural Plant Breeding (IVT), Wageningen (The Netherlands) (Received 16 October 1975)
ABSTRACT Nieuwhof, M., 1976. 2he effect of temperature on growth and development of cultivars of radish under winter conditions. Scientia Hort., 5 : 1 1 1 - - 1 1 8 . Leaf and root growth and development of 7 early radish cultivars and strains were studied at a series of constant temperatures varying from 10°C to 26°C under winter conditions. Maximum leaf growth was observed at 23°C, maximum root growth initially at 14°C and later on at 10°C. Total dry matter production was highest at 23°C. Between the cultivars/ strains distinct differences in rate of leaf and root growth were noted, but no clear interactions of temperature × cultivar/strain were found. The prospects of shortening the growing-period in winter of early radish by means of regulating environmental factors and plant breeding are discussed.
INTRODUCTION
In The Netherlands radish (Raphanus sativus L. var. radicula Pers.) is a typical spring vegetable, more than 75% being harvested from March to May. In March and April this crop is mainly produced in glasshouses. So far, glasshouse production of radish in autumn and winter has been of minor importance, but interest in growing radish in this period of the year is increasing. As practically no research has been done on the effect of temperature on growth and development of Dutch radish cultivars under winter conditions, an experiment was conducted on this subject. MATERIAL AND METHODS
The experiment included the cultivars 'Champion', 'Cherry Belle', 'Kader', 'Rota' and ' R o n d e Helderrode'. Of the latter, three strains were used, viz. 'Neoro', 'Robijn' and 'Triplo'. 'Champion' is a summer cultivar; the others are mainly used for spring production under glass or in the open. These cultivars form a round bulblet, consisting mainly of a thickened hypocotyl. In this publication the term " r o o t " is used as is generally done in horticultural literature. The experiment was carried o u t in the glasshouses of the IVT p h y t o t r o n
112
(Braak and Smeets, 1956) at 10, 14, 17, 20, 23 and 26°C under natural light conditions, and sowing was done on 9 October 1974. A randomised block design was used with 5 replications, and plots of 1 row with a length of 1 m and a distance between rows of 12 cm. Some days after emergence the seedlings were thinned to approximately 2 cm apart. The dates of seedling emergence (first green cotyledons visible) were recorded On 4 and 18 November and 2, 16 and 30 December at each temperature, one replication of each cultivar/strain was harvested and the rate of r o o t thickening (scale from 0 = no thickening, to 5 = radish of normal size), fresh leaf weight (including cotyledons when still present) and fresh r o o t weight were determined. To prevent misinterpretations, no plants with rotting leaves (which occurred especially after some weeks at higher temperatures) but only healthy plants were harvested. At the first 4 harvest dates more than 30 plants per plot were harvested in most cases and at the fifth harvest date 15--30. At each harvest date 3 plants per temperature were taken at random from 'Cherry Belle', 'Kader' and ' R o t a ' for determinations of dry matter content of leaves and roots. RESULTS
S e e d l i n g e m e r g e n c e . - Increasing temperature resulted in an earlier emergence
of the seedlings. At 26°C the first cotyledons were already visible 1--3 days after sowing, and at 10°C 4--5 days later (Table 1). 'Kader' emerged somewhat slower than the other cultivars (Table 2). G r o w t h a n d d e v e l o p m e n t o f leaves. - - At higher temperatures, in particular at 23 and 26°C, plants became lanky and hypocotyls were attenuated. More normal, sturdier plants developed at 10 and 14°C. In the second part of the growing-period, especially at the higher temperatures, a beginning of stem elongation was observed. TABLE
1
Effect of temperature on date of emergence and thickening of roots at 5 successive harvest dates (averages of 7 cultivars/strains). Temperature (°C)
10 14 17 20 23 26
Days from sowing to emergence
Degree of root thickening Nov.4
Nov.18
Dec.2
Dec.16
Dec.30
Average
6.1 4.5 3.9 2.6 2.2 1.6
0 1.7 0.6 1.4 0.4 0
2.0 2.9 2.3 1.9 1.9 0.6
2.7 3.6 2.6 2.6 2.1 1:9
3.9 3.0 2.4 2.9 1.9
4.1 2.6 3.1 1.8 2.6 1.0
2.5 2.8 2.2 2.2 1.8 0.9
113
TABLE 2
Date of emergence and thickening of roots of 7 cultivars/strains at 5 successive harvest dates (averages of 6 temperatures) Cultivar
'Champion' 'Cherry Belle' 'Kader' 'Rota'
Days from sowing
Degree of root thickening
to emergence
Nov. 4
Nov. 18
Dec. 2
Dec. 16
Dec. 3 0
Average
3.0 3.1 4.5 3.2
0.2 0.7 0.7 1.0
1.2 2.0 2.2 2.5
1.5 2.2 2.8 3.5
2.2 2.6 2.6 4.0
1.8 2.5 2.5 3.6
1,4 2,0 2,2 2,9
0.5 1.0 0.7
1.3 2.3 1.8
2.0 3.3 2.7
2.0 3.2 3.0
1.8 3.0 2.8
1.5 2.6 2.2
'Ronde Helderrode' 'Neoro' 'Robijn' 'Triplo'
3.4 3.6 3.4
Senescence of cotyledons started after the first harvest. At the second harvest at 20°C and higher, and at the third harvest at all temperatures, wilting was observed, except at 10°C where it did n o t occur until the fifth harvest. At the first harvest at 10°C, only the cotyledons were visible; at 14°C the 2 first true leaves had almost the same size as the cotyledons, and at higher temperatures 2--4 leaves larger than the cotyledons were present. At later harvests generally 4--8 true leaves were present (at higher temperatures usually a few more than at lower temperatures); leaves were a b o u t 10 cm long at 10°C, a b o u t 15 cm at 14--17°C and 20 cm at 20--26°C. The o p t i m u m temperature for leaf growth was 23°C (Fig.l). Slowest leaf growth was recorded at 10°C, followed by 14 and 17°C. Between 10 and 20°C a steady increase in leaf weight was observed. At 23 and 26°C this was no longer the case after the beginning of December, presumably (partly) due to senescence and decay of the oldest true leaves (Fig.l). 'Champion' and 'Robijn' produced the highest leaf weightsl followed b y 'Cherry Belle' (Table 3). G r o w t h a n d d e v e l o p m e n t o f r o o t s . - - Practically no r o o t thickening was noted at 23°C and 26°C (Fig.2). Until the beginning of December root thickening at 14°C was most pronounced. R o o t thickening at 10°C was initially very slow, but in the latter half of December the highest scores were recorded at this temperature. 'Rota' showed the fastest root thickening, followed by 'Robijn'. 'Cherry Belle', 'Triplo' and 'Kader' were intermediate in this respect. 'Neoro' and the summer cultivar 'Champion' exhibited the slowest r o o t swelling (Table 3} R o o t / l e a f ratio. - - At 10°C this ratio reached the highest values, attaining a maximum at the last harvest (Fig.3). At 23 and 26°C the root/leaf ratio remained low, only increasing slightly when harvesting later, while at the other temperatures intermediate values were found.
114
°"
/X
!.
6OO
soo
? 23 oC~
"~.
•
"
400
~ 300 ~'200
./'
I
Nov.4
I
Nov. 1~
I
I
Dec.2 Dec.16 H a r v e s t date
I
Dec.30
Fig.l, Effect of temperature on fresh leaf weight at 5 successive harvest dates (averages of 7 cultivars/strains).
'Champion' always had very low root/leaf ratios and ' R o t a ' had the highest (Table 3). D r y m a t t e r c o n t e n t . -- Dry matter content of the leaves was not clearly affected by temperature, the averages at the different temperatures varying from 6.2 to 6.8%. At the first harvest, the dry matter content of the foliage was 5.7%. Afterwards it steadily increased, until it reached 7.3% at the last harvest. Dry matter content of the roots tended to be higher at higher temperatures; the average values at the successive temperatures (10 ........ ,26°C) were 5.2, 4.9, 5.3, 5.4, 5.8 and 7.1%, respectively. Dry matter content of the roots was not distinctly influenced by the harvest date; the average values at the different dates varied from 5.4 to 5.8%. Dry matter content of leaves and roots of 'Cherry Belle' were highest, viz. 6.9 and 6.3% respectively. The corresponding values for 'Kader' were 6.6 and 5.2% and for 'Rota' 6.2 and 5.4%. I n t e r a c t i o n s . - - No distinct interactions between cultivars and temperatures were observed for the characteristics studied. DISCUSSION
Leaf growth was dominant at high temperatures, with 23°C as optimum, and root growth at lower temperatures, with the best results at 10°C as optimum, and light conditions the o p t i m u m for r o o t growth of radish lies at a higher temperature, as is demonstrated by the results of other workers. Banga and Van Bennekom (1962) found, at light intensities of 20.000, 10.000 and 5.000 lx, that
115
X
0
Z~ Z~
0
~8 0
c~
r~
116
o
200
S 1 °C
//:
~100
~, r Nov.4
/V
L
#.c
~
Nov.+8
Dec. 2
J
i
Dec.16
Dec.30
Harvest date
Fig.2. Effect of t e m p e r a t u r e on fresh r o o t weight at 5 successive harvest dates (averages of 7 cultivars/strains). 100 o o
/•°C
X
Q)
e 50 0 0 ¢.y
.// I Nov.4
t
I
Nov. 18 Dec. 2 Harvest date
t
D e c 16
I
Dec 30
Fig.3. Effect of t e m p e r a t u r e on r o o t / l e a f ratio at 5 successive harvest dates (averages o f 7 cultivars/strains).
20 > 14 > 8°C, 20 = 14 > 8°C and 14 > 20 > 8°C, respectively. When sowing radish in December or January and harvesting in February or later, Angell and Hillyer (1962} f o u n d 16--18 > 21--23 > 10--13°C. Mehwald (1973) obtained better results in this period at a soil temperature of 12--18°C than at lower and higher soil temperatures. Total fresh leaf + root weight was highest at high temperatures. As there was no influence of temperature on the dry matter c o n t e n t of the leaves and only a slight influence on the dry matter c o n t e n t of the roots, dry m a t t e r weight of leaves + roots was also highest at high temperatures. Thus the poor root growth at high temperatures is not caused by an insufficient net production of dry matter under poor light conditions at these temperatures. This is, for instance, also clearly demonstrated by the fact that at 23°C at the second harvest, 40 days after sowing, the weight of leaves + roots was about as high as the production of leaves + roots at 10°C at the last harvest, 82 days after sowing.
117 Thus the p o o r r o o t formation at high temperatures is caused by the less efficient distribution of dry matter among leaves and roots. At 10°C this distribution is more harmonious. At this temperature the r o o t / l e a f ratio steadily increases, while at higher temperatures, especially at 23 and 26°C, leaf and stem growth is stimulated at the expense of r o o t thickening. Banga and Smeets (1956) reported that small roots were the result of early stem elongation, as occurred at the higher temperatures in m y experiment. Even at 10°C r o o t thickening was slow compared with the rate of root thickening in spring and summer when early radish cultivars can be harvested a b o u t 6 weeks after sowing. At 10°C, more than 11 weeks after sowing, the roots of 'Rota' had nearly reached a marketable size. As a short growing-period is important for glasshouse production, the question arises whether the rate of r o o t thickening can be p r o m o t e d by regulating environmental factors or whether breeding is preferable. Banga and Van Bennekom (1962) found that wide planting distance and a high light intensity had a favourable influence on r o o t thickening, b u t that day length had no influence. However, to obtain satisfactory yields, larger planting distances than the one used in this experiment would be unacceptable. The economical possibilities of reducing the growing period by increasing the light intensity by using artificial lighting are also limited. As r o o t growth was most advanced at 14°C in November, the maintenance of 14°C in the first half of the growing period and 10°C later on, may offer practical possibilities. By using fractionated seed from a vigorous strain the growing period can also be shortened by a few days (Schwanitz, 1950; Lange, 1965; Chen et al., 1972; Kubka et al., 1974). By a combination of both measures a shortening of the whole growing period by a b o u t 2 weeks seems possible. Does plant breeding open up prospects of promoting r o o t growth? With the introduction of 'Rota' nearly 10 years ago it was demonstrated that plant breeding can contribute towards this goal. It was found in m y experiment that a small number of plants formed thickened roots at an earlier date than the other plants at high temperatures. This m a y indicate that further possibilities exist to improve fast growth by breeding. This aspect will be investigated further. REFERENCES
Angell, F.F. and HiUyer, I.G., 1962. Cultural and environmental conditions affecting radish (Raphanus sativus L.) root--hypocotyl development. Proc. Am. Soc. Hort. Sci., 81 : 402--407. Banga, O. and Bennekom, J.L. Van, 1962. Breeding radish for winter production under glass. Euphytica, 11: 311--326. Banga, O. and Smeets, L., 1956. Some effects of the photoperiod on growth and pithiness of radishes. Euphytica, 5: 196--204. Braak, J.P. and Smeets, L., 1956. The phytotron of the Institute of Horticultural Plant Breeding at Wageningen, The Netherlands. Euphytica, 5: 205--221.
118 Chen, C.C., Andrews, C.H., Baskin, C.C. and Delouche, J.C., 1972. Influence of quality of seed on growth, development and productivity of some horticultural crops. Proc. Int. Seed Test. Assoc., 37: 923--939. Kubka, T., Hortynski, J. and Hulewicz, T., 1974. Influence of seed size on some characters of radish (Raphanus sativus L.), correlations and heritability. Z. Pflanzenzuecht., 71: 208--221. Lange~ H., 1965. lJ'ber den Einfluss der Anbaustufen des Saatgutes auf F ~ h z e i t i g k e i t und Ertragsleistung bei Radies (Raphanus sativus L.) im Unterglasanbau. Zuechter, 35: 46-49. Mehwald, G., 1973. Der Anbau yon Radies im Gev~'chshaus hei unterschiedlichen Bodentemperaturen. Gem~se, 9: 5--6. Schwanitz, E., 1950. Grosz.~amigkeit als Zuchtziel bei Gem~ise mit kurzer Entwicklungsdauer. Zuechter, 20: 37--38.
111
THE EFFECT OF TEMPERATURE ON GROWTH AND DEVELOPMENT OF CULTIVARS OF RADISH UNDER WINTER CONDITIONS
M. NIEUWHOF
Institute for Horticultural Plant Breeding (IVT), Wageningen (The Netherlands) (Received 16 October 1975)
ABSTRACT Nieuwhof, M., 1976. 2he effect of temperature on growth and development of cultivars of radish under winter conditions. Scientia Hort., 5 : 1 1 1 - - 1 1 8 . Leaf and root growth and development of 7 early radish cultivars and strains were studied at a series of constant temperatures varying from 10°C to 26°C under winter conditions. Maximum leaf growth was observed at 23°C, maximum root growth initially at 14°C and later on at 10°C. Total dry matter production was highest at 23°C. Between the cultivars/ strains distinct differences in rate of leaf and root growth were noted, but no clear interactions of temperature × cultivar/strain were found. The prospects of shortening the growing-period in winter of early radish by means of regulating environmental factors and plant breeding are discussed.
INTRODUCTION
In The Netherlands radish (Raphanus sativus L. var. radicula Pers.) is a typical spring vegetable, more than 75% being harvested from March to May. In March and April this crop is mainly produced in glasshouses. So far, glasshouse production of radish in autumn and winter has been of minor importance, but interest in growing radish in this period of the year is increasing. As practically no research has been done on the effect of temperature on growth and development of Dutch radish cultivars under winter conditions, an experiment was conducted on this subject. MATERIAL AND METHODS
The experiment included the cultivars 'Champion', 'Cherry Belle', 'Kader', 'Rota' and ' R o n d e Helderrode'. Of the latter, three strains were used, viz. 'Neoro', 'Robijn' and 'Triplo'. 'Champion' is a summer cultivar; the others are mainly used for spring production under glass or in the open. These cultivars form a round bulblet, consisting mainly of a thickened hypocotyl. In this publication the term " r o o t " is used as is generally done in horticultural literature. The experiment was carried o u t in the glasshouses of the IVT p h y t o t r o n
112
(Braak and Smeets, 1956) at 10, 14, 17, 20, 23 and 26°C under natural light conditions, and sowing was done on 9 October 1974. A randomised block design was used with 5 replications, and plots of 1 row with a length of 1 m and a distance between rows of 12 cm. Some days after emergence the seedlings were thinned to approximately 2 cm apart. The dates of seedling emergence (first green cotyledons visible) were recorded On 4 and 18 November and 2, 16 and 30 December at each temperature, one replication of each cultivar/strain was harvested and the rate of r o o t thickening (scale from 0 = no thickening, to 5 = radish of normal size), fresh leaf weight (including cotyledons when still present) and fresh r o o t weight were determined. To prevent misinterpretations, no plants with rotting leaves (which occurred especially after some weeks at higher temperatures) but only healthy plants were harvested. At the first 4 harvest dates more than 30 plants per plot were harvested in most cases and at the fifth harvest date 15--30. At each harvest date 3 plants per temperature were taken at random from 'Cherry Belle', 'Kader' and ' R o t a ' for determinations of dry matter content of leaves and roots. RESULTS
S e e d l i n g e m e r g e n c e . - Increasing temperature resulted in an earlier emergence
of the seedlings. At 26°C the first cotyledons were already visible 1--3 days after sowing, and at 10°C 4--5 days later (Table 1). 'Kader' emerged somewhat slower than the other cultivars (Table 2). G r o w t h a n d d e v e l o p m e n t o f leaves. - - At higher temperatures, in particular at 23 and 26°C, plants became lanky and hypocotyls were attenuated. More normal, sturdier plants developed at 10 and 14°C. In the second part of the growing-period, especially at the higher temperatures, a beginning of stem elongation was observed. TABLE
1
Effect of temperature on date of emergence and thickening of roots at 5 successive harvest dates (averages of 7 cultivars/strains). Temperature (°C)
10 14 17 20 23 26
Days from sowing to emergence
Degree of root thickening Nov.4
Nov.18
Dec.2
Dec.16
Dec.30
Average
6.1 4.5 3.9 2.6 2.2 1.6
0 1.7 0.6 1.4 0.4 0
2.0 2.9 2.3 1.9 1.9 0.6
2.7 3.6 2.6 2.6 2.1 1:9
3.9 3.0 2.4 2.9 1.9
4.1 2.6 3.1 1.8 2.6 1.0
2.5 2.8 2.2 2.2 1.8 0.9
113
TABLE 2
Date of emergence and thickening of roots of 7 cultivars/strains at 5 successive harvest dates (averages of 6 temperatures) Cultivar
'Champion' 'Cherry Belle' 'Kader' 'Rota'
Days from sowing
Degree of root thickening
to emergence
Nov. 4
Nov. 18
Dec. 2
Dec. 16
Dec. 3 0
Average
3.0 3.1 4.5 3.2
0.2 0.7 0.7 1.0
1.2 2.0 2.2 2.5
1.5 2.2 2.8 3.5
2.2 2.6 2.6 4.0
1.8 2.5 2.5 3.6
1,4 2,0 2,2 2,9
0.5 1.0 0.7
1.3 2.3 1.8
2.0 3.3 2.7
2.0 3.2 3.0
1.8 3.0 2.8
1.5 2.6 2.2
'Ronde Helderrode' 'Neoro' 'Robijn' 'Triplo'
3.4 3.6 3.4
Senescence of cotyledons started after the first harvest. At the second harvest at 20°C and higher, and at the third harvest at all temperatures, wilting was observed, except at 10°C where it did n o t occur until the fifth harvest. At the first harvest at 10°C, only the cotyledons were visible; at 14°C the 2 first true leaves had almost the same size as the cotyledons, and at higher temperatures 2--4 leaves larger than the cotyledons were present. At later harvests generally 4--8 true leaves were present (at higher temperatures usually a few more than at lower temperatures); leaves were a b o u t 10 cm long at 10°C, a b o u t 15 cm at 14--17°C and 20 cm at 20--26°C. The o p t i m u m temperature for leaf growth was 23°C (Fig.l). Slowest leaf growth was recorded at 10°C, followed by 14 and 17°C. Between 10 and 20°C a steady increase in leaf weight was observed. At 23 and 26°C this was no longer the case after the beginning of December, presumably (partly) due to senescence and decay of the oldest true leaves (Fig.l). 'Champion' and 'Robijn' produced the highest leaf weightsl followed b y 'Cherry Belle' (Table 3). G r o w t h a n d d e v e l o p m e n t o f r o o t s . - - Practically no r o o t thickening was noted at 23°C and 26°C (Fig.2). Until the beginning of December root thickening at 14°C was most pronounced. R o o t thickening at 10°C was initially very slow, but in the latter half of December the highest scores were recorded at this temperature. 'Rota' showed the fastest root thickening, followed by 'Robijn'. 'Cherry Belle', 'Triplo' and 'Kader' were intermediate in this respect. 'Neoro' and the summer cultivar 'Champion' exhibited the slowest r o o t swelling (Table 3} R o o t / l e a f ratio. - - At 10°C this ratio reached the highest values, attaining a maximum at the last harvest (Fig.3). At 23 and 26°C the root/leaf ratio remained low, only increasing slightly when harvesting later, while at the other temperatures intermediate values were found.
114
°"
/X
!.
6OO
soo
? 23 oC~
"~.
•
"
400
~ 300 ~'200
./'
I
Nov.4
I
Nov. 1~
I
I
Dec.2 Dec.16 H a r v e s t date
I
Dec.30
Fig.l, Effect of temperature on fresh leaf weight at 5 successive harvest dates (averages of 7 cultivars/strains).
'Champion' always had very low root/leaf ratios and ' R o t a ' had the highest (Table 3). D r y m a t t e r c o n t e n t . -- Dry matter content of the leaves was not clearly affected by temperature, the averages at the different temperatures varying from 6.2 to 6.8%. At the first harvest, the dry matter content of the foliage was 5.7%. Afterwards it steadily increased, until it reached 7.3% at the last harvest. Dry matter content of the roots tended to be higher at higher temperatures; the average values at the successive temperatures (10 ........ ,26°C) were 5.2, 4.9, 5.3, 5.4, 5.8 and 7.1%, respectively. Dry matter content of the roots was not distinctly influenced by the harvest date; the average values at the different dates varied from 5.4 to 5.8%. Dry matter content of leaves and roots of 'Cherry Belle' were highest, viz. 6.9 and 6.3% respectively. The corresponding values for 'Kader' were 6.6 and 5.2% and for 'Rota' 6.2 and 5.4%. I n t e r a c t i o n s . - - No distinct interactions between cultivars and temperatures were observed for the characteristics studied. DISCUSSION
Leaf growth was dominant at high temperatures, with 23°C as optimum, and root growth at lower temperatures, with the best results at 10°C as optimum, and light conditions the o p t i m u m for r o o t growth of radish lies at a higher temperature, as is demonstrated by the results of other workers. Banga and Van Bennekom (1962) found, at light intensities of 20.000, 10.000 and 5.000 lx, that
115
X
0
Z~ Z~
0
~8 0
c~
r~
116
o
200
S 1 °C
//:
~100
~, r Nov.4
/V
L
#.c
~
Nov.+8
Dec. 2
J
i
Dec.16
Dec.30
Harvest date
Fig.2. Effect of t e m p e r a t u r e on fresh r o o t weight at 5 successive harvest dates (averages of 7 cultivars/strains). 100 o o
/•°C
X
Q)
e 50 0 0 ¢.y
.// I Nov.4
t
I
Nov. 18 Dec. 2 Harvest date
t
D e c 16
I
Dec 30
Fig.3. Effect of t e m p e r a t u r e on r o o t / l e a f ratio at 5 successive harvest dates (averages o f 7 cultivars/strains).
20 > 14 > 8°C, 20 = 14 > 8°C and 14 > 20 > 8°C, respectively. When sowing radish in December or January and harvesting in February or later, Angell and Hillyer (1962} f o u n d 16--18 > 21--23 > 10--13°C. Mehwald (1973) obtained better results in this period at a soil temperature of 12--18°C than at lower and higher soil temperatures. Total fresh leaf + root weight was highest at high temperatures. As there was no influence of temperature on the dry matter c o n t e n t of the leaves and only a slight influence on the dry matter c o n t e n t of the roots, dry m a t t e r weight of leaves + roots was also highest at high temperatures. Thus the poor root growth at high temperatures is not caused by an insufficient net production of dry matter under poor light conditions at these temperatures. This is, for instance, also clearly demonstrated by the fact that at 23°C at the second harvest, 40 days after sowing, the weight of leaves + roots was about as high as the production of leaves + roots at 10°C at the last harvest, 82 days after sowing.
117 Thus the p o o r r o o t formation at high temperatures is caused by the less efficient distribution of dry matter among leaves and roots. At 10°C this distribution is more harmonious. At this temperature the r o o t / l e a f ratio steadily increases, while at higher temperatures, especially at 23 and 26°C, leaf and stem growth is stimulated at the expense of r o o t thickening. Banga and Smeets (1956) reported that small roots were the result of early stem elongation, as occurred at the higher temperatures in m y experiment. Even at 10°C r o o t thickening was slow compared with the rate of root thickening in spring and summer when early radish cultivars can be harvested a b o u t 6 weeks after sowing. At 10°C, more than 11 weeks after sowing, the roots of 'Rota' had nearly reached a marketable size. As a short growing-period is important for glasshouse production, the question arises whether the rate of r o o t thickening can be p r o m o t e d by regulating environmental factors or whether breeding is preferable. Banga and Van Bennekom (1962) found that wide planting distance and a high light intensity had a favourable influence on r o o t thickening, b u t that day length had no influence. However, to obtain satisfactory yields, larger planting distances than the one used in this experiment would be unacceptable. The economical possibilities of reducing the growing period by increasing the light intensity by using artificial lighting are also limited. As r o o t growth was most advanced at 14°C in November, the maintenance of 14°C in the first half of the growing period and 10°C later on, may offer practical possibilities. By using fractionated seed from a vigorous strain the growing period can also be shortened by a few days (Schwanitz, 1950; Lange, 1965; Chen et al., 1972; Kubka et al., 1974). By a combination of both measures a shortening of the whole growing period by a b o u t 2 weeks seems possible. Does plant breeding open up prospects of promoting r o o t growth? With the introduction of 'Rota' nearly 10 years ago it was demonstrated that plant breeding can contribute towards this goal. It was found in m y experiment that a small number of plants formed thickened roots at an earlier date than the other plants at high temperatures. This m a y indicate that further possibilities exist to improve fast growth by breeding. This aspect will be investigated further. REFERENCES
Angell, F.F. and HiUyer, I.G., 1962. Cultural and environmental conditions affecting radish (Raphanus sativus L.) root--hypocotyl development. Proc. Am. Soc. Hort. Sci., 81 : 402--407. Banga, O. and Bennekom, J.L. Van, 1962. Breeding radish for winter production under glass. Euphytica, 11: 311--326. Banga, O. and Smeets, L., 1956. Some effects of the photoperiod on growth and pithiness of radishes. Euphytica, 5: 196--204. Braak, J.P. and Smeets, L., 1956. The phytotron of the Institute of Horticultural Plant Breeding at Wageningen, The Netherlands. Euphytica, 5: 205--221.
118 Chen, C.C., Andrews, C.H., Baskin, C.C. and Delouche, J.C., 1972. Influence of quality of seed on growth, development and productivity of some horticultural crops. Proc. Int. Seed Test. Assoc., 37: 923--939. Kubka, T., Hortynski, J. and Hulewicz, T., 1974. Influence of seed size on some characters of radish (Raphanus sativus L.), correlations and heritability. Z. Pflanzenzuecht., 71: 208--221. Lange~ H., 1965. lJ'ber den Einfluss der Anbaustufen des Saatgutes auf F ~ h z e i t i g k e i t und Ertragsleistung bei Radies (Raphanus sativus L.) im Unterglasanbau. Zuechter, 35: 46-49. Mehwald, G., 1973. Der Anbau yon Radies im Gev~'chshaus hei unterschiedlichen Bodentemperaturen. Gem~se, 9: 5--6. Schwanitz, E., 1950. Grosz.~amigkeit als Zuchtziel bei Gem~ise mit kurzer Entwicklungsdauer. Zuechter, 20: 37--38.