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Comparisons of the use of pre-germinated, dry and pelleted seeds for block-raising of glasshouse lettuce
Scientia Horticulturae, 17 (1982) 15--25
15
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
COMPARISONS OF THE USE OF PRE-GERMINATED, DRY AND PELLETED SEEDS FOR BLOCK-RAISING OF GLASSHOUSE LETTUCE
J.R. CHRIMES 1 and D. GRAY 2
Luddington Experimental Horticulture Station, Luddington, Stratford-upon-Avon (Gt. Britain) 2National Vegetable Research Station, Wellesbourne, Warwick (Gt. Britain) (Accepted for publication 29 July 1981)
ABSTRACT Chrimes, J.R. and Gray, D., 1982. Comparisons of the use of pre-germinated, dry and pelleted seeds for block-raising of glasshouse lettuce. Scientia Hortic., 17: 15--25. The use of pre-germinated seeds for establishing glasshouse lettuce in peat blocks was compared with dry and pelleted seeds. Sowing pre-germinated seeds gave between 1 and 6% higher seedling emergence than from pelleted seeds and between 1 and 3% higher emergence than from dry seeds. Seedling emergence was reduced from 100% to 56% when the seeds were covered with 5 m m of silver sand compared with no cover. Seedling emergence from seeds covered with Agrigel (sodium alginate) was reduced by 24% at a concentration of 0.25% and by 62% at 0.7% compared with no cover. Guar (a natural gum) and Laponite (a synthetic clay) gels also reduced emergence on certain occasions but not by as much as Agrigel. Satisfactory levels of emergence were obtained when seeds were sown into blocks having an inverted, cone-shaped depression for locating the seeds, but the seedlings showed poor root-hair development and had elongated hypocotyls. Normal seedling growth and high levels of emergence were obtained from the seeds sown into blocks having cylindrical holes 10 m m in depth and 5 or 10 m m in diameter. Pre-germinated seeds invariably emerged earlier than pelleted or dry seeds. At low propagating-temperatures (12.0°C maximum and 5.0°C minimum), emergence was 4 days earlier from pre-germinated than from dry or pelleted seeds and 2--3 days earlier at high temperatures (21.0°C maximum and 9.0°C minimum). Coefficients of variation of seedling weight at planting were lower from sowing pre-germinated than pelleted or dry seeds, but these differences were not evident at harvest.
INTRODUCTION
Most glasshouse lettuce crops in the U.K. are established from peat blockraised plants. The devices currently used for sowing seeds into blocks are capable of selecting and delivering a single seed per block provided the seeds are approximately spherical in shape. Consequently, most of the lettuce blocks used in commerce are sown using pelleted seeds. Establishment is usually satisfactory, with over 95% of the blocks having plants suitable for transplanting, b u t germination is n o t always completely uniform and there is con-
0304-4238/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company
16
siderable plant-size variation at planting-time. Recent experiments (Gray, 1978a, b) with direct-drilled field-grown lettuce showed that sowing pregerminated seeds gave higher, earlier and more synchronous seedling emergence than dry or pelleted seeds. These results suggest that there could be advantages from the use of pre-germinated seeds for block-raised lettuce in improving plant uniformity, giving earlier establishment particularly at low temperatures and also for overcoming thermodormancy (Gray, 1977}. The recent commercial development of systems for mechanically handling and sowing both dry and pre-germinated seeds prompted, in 1979, an appraisal of the benefits of establishing blocks from pre-germinated or dry seeds compared with split-pill pelleted seeds. In this paper, we describe the techniques necessary for sowing pre-germinated seeds and we report the effects of sowing such seeds, split-pill pellets and dry seeds on seedling establishment and early seedling growth for autumn-, winter- and spring-sown glasshouse lettuce crops. EXPERIMENT 1 and methods. - - Pre-germinated seeds (radicles 1--2 m m long) were sown on 12 December into the conventional cone-shaped hole (15 mm diameter X 15 m m deep) of 43 mm-sized blocks and into the following combinations of sizes of cylindrical-shaped hole: 10 or 15 m m diameter X 5, 10 or 15 m m deep. After sowing a single pre-germinated seed by hand into each of 70 blocks per treatment, the blocks were covered by damp newspaper until the cotyledons appeared. The numbers of seedlings with fully opened cotyledons were counted 8, 9 and 21 days after sowing.
Materials
- - Seedling emergence was more rapid from conventional, invertedcone-shaped holes than from cylindrically-shaped holes, but final seedling emergence was unaffected (Table I). However, root-hair growth was poorer in
Results.
TABLE I Seedling emergence from peat blocks having holes for seed placement of different depth and width (Experiment 1) Type of hole
Conventional "Visser" type (inverted-cone-shape) Cylindrical hole 5-mm depth 10-mm depth 15-ram depth 10-ram diameter 15-mm diameter
Percentage seedling emergence (days after sowing) 8
9
21
53
74
97
6 12 15 7 4
21 35 28 20 53
98 93 97 96 95
17
the blocks with conventional holes and the cotyledons were more elongated and the h y p o c o t y l s frequently coiled. EXPERIMENT 2 and methods. - - Previous work on the direct drilling of lettuce in the field had shown that radicles of 1--2 mm in length gave better establishment than longer or shorter ones (Gray, 1978a). To ascertain if this was so when seeds were sown into peat blocks for plant raising, seeds of cultivar 'Bizet' with radicles just emerging, 1--2 mm long or 3--5 mm long, were sown into peat blocks (70 per treatment). Counts of emerging seedlings were made at regular intervals until no more seedlings emerged.
Materials
- - Sowing seeds with the radicles just emerging, 1--2 mm or 3--5 mm in length, had no effect on percentage seedling emergence, b u t the seedlings from the seeds with the longest radicles produced plants with coiled and elongated hypocotyls.
Results.
EXPERIMENT 3 and methods. - - Results from seed-box experiments (Darby, 1980) showed that the type of gel used affected seedling emergence, and so the effect of different types and concentrations of gel (see Table II) used to cover the seed after sowing in blocks were compared with a covering of sand and with no cover. Pre-germinated seeds having radicles 1--2 mm long were sown by hand into blocks (100 per treatment). Counts of emerging seedlings were made at regular intervals until no more emerged.
Materials
TABLE II Seedling emergence from blocks using different seed coverings (Experiment 3) Type of cover
Seedling emergence
(%) Guar
0.25% 0.50% 0.75%
100 100 65
Laponite
1.0% 1.5% 2.0%
91 I00 96
Agrigel
0.35% 0.53% 0.70%
Silver sand to 0.5-cm depth No cover
76 44 28 56 100
18 - - Covering the pre-germinated seeds with silver sand and Agrigel at any concentration reduced emergence (Table II) and Agrigel retarded roothair development. Guar and Laponite dispersed rapidly into the blocks at the lowest concentration used, but Agrigel did not disperse and formed a film over the seeds. Guar reduced emergence at the highest concentration and prevented root-hair development. At lower concentrations, growth was unaffected and emergence was similar to uncovered seed. Over the range used, the concentration of Laponite had little effect on emergence. Results.
EXPERIMENT 4 - - Lettuce seeds 'Bizet' were graded using an Ottawa air-blower to remove the lightest 10% of seeds and a proportion of the " h e a v y " seeds were sent to Royal Sluis for pelleting. A germination test, following I.S.T.A. rules (Anon., 1976), was made on samples of dry and pelleted seeds. To determine the coefficient of variation of seed weight, 100 dry seeds and 100 seeds removed from the split-pill pellets were weighed individually. Pre-germinated seeds with radicles 1--2 mm long were prepared by imbibing seeds on moist paper in trays at 20°C for 20 h. Pre-germinated, dry and pelleted seeds of this stock were then sown into 43-mm peat blocks on 10 May. After sowing, the seeds were either covered with 2 ml of 0.25% guar, 2 ml of 1.0% Laponite, or left uncovered. There were 144 peat blocks in each plot o f the 9 treatments, (3 seed types X 3 "covering" methods) and 4 replicates. After sowing and the application of the seed "covering" treatments, the blocks were covered with damp newspaper. This was removed at cotyledon emergence and the blocks were then watered frequently to prevent them drying out. Metalaxyl (0.2 g a.i./l) was applied to the plant and blocks on 17 May for protection against d o w n y m i l d e w ( B r e m i a l a c t u c a e Regel) and a spray of iprodione (0.25 g a.i./1) was applied on 17 May and 15 June for protection against grey mould ( B o t r y t i s c i n e r e a Percs. ex. Fr.). Counts of emerged, healthy seedlings were made on 15 and 17 May on 144 blocks per plot. Sixty-four block-raised plants from each of the 9 treatments were planted on 6 June at a spacing of 20 X 20 cm in a Venlo glasshouse. The treatments were arranged in a randomised block design and there were 4 replicates. At planting, 60 plants per plot were weighed individually. On 11 July, 60 plants from each plot were individually weighed after trimming and placing them in plastic bags.
Materials and methods.
- - Mean seed weights of pelleted and dry seeds were similar at 1.26 + 0.186 mg and 1.24 + 0.180 mg, respectively. Laboratory germination as determined b y the I.S.T.A. test-method was 95% for pelleted seeds and 99% for dry seeds. At the time of sowing, over 95% of the "pre-germinated" sample had germinated. There was no interaction between the t y p e of seed used and the covering-
Results.
19 m e t h o d on the numbers of emerged plants, on plant size at planting or at harvest, and so only the main t r e a t m e n t effects are shown in Table III. Sowing pre-germinated seeds gave significantly more robust plants at the final count compared with pelleted seeds, but there was no improvement compared with dry seeds. Plants emerged more rapidly from blocks sown with pre-germinated than dry seeds, and these in turn emerged more rapidly than plants from pelleted seeds. Covering the seeds with gel reduced emergence compared with no cover (Table III). Pre-germinated seeds gave significantly larger plants at planting than those from dry or pelleted seeds, which each gave plants of a similar size. Analysis of the distributions of individual plant weights showed that t h e y were normally distributed for all the treatments. Therefore, the coefficient of variation (C.V.) of plant weight was used to compare the variability in plant weight between treatments. The C.V. was lower for plants established from pre-germinated or dry seeds than for those from pelleted seeds. Covering had no effect on mean plant weight but it increased the variability of plant weight at planting. At maturity, the same trend in responses was apparent, but none of the effects were significant (Table III). EXPERIMENT 5 Materials and methods. - - This experiment was identical to Experiment 4 except that 'Ravel' was used, sowing t o o k place on 22 August and the blocks were planted on 18 September. Emergence counts were made on 30 August and 4 September, and 60 plants were weighed individually at planting and on 7 November after trimming and placing in polythene bags. Application of metalaxyl (0.2 g a.i./1) and iprodione (0.25 g a.i./1) were made at the time of cotyledon expansion and again on 21 September. - - Laboratory germination in both pelleted and dry seeds was 99%, and at sowing over 95% of the pre-germinated seeds had germinated. There was no significant effect of seed t y p e or covering treatment on the final n u m b e r of healthy seedlings. However, seedlings from pre-germinated seeds emerged earlier than those from d r y or pelleted seeds (Table IV). There was no effect of covering m e t h o d on seedling weight, but seedlings from pregerminated seeds were heavier than those from dry or pelleted seeds. Individual seedling weights were normally distributed and so the C.V. of seedling weight was used to compare the effects of the treatments on plant variability. There was no effect of treatment on C.V. of seedling weight at planting, nor on mean plant weight or C.V. of plant weight at maturity (Table IV). There were no significant interactions between seed and covering-treatments. Results.
III
55 64 87 73 69 64 ±3.7
Pelleted seeds Dry seeds Pre-germinated seeds
Uncovered Covered with 2 ml 0.25% Guar Covered with 2 ml 1% Laponite
SED (Standard error of.a difference between 2 means) _t
±0.196
4.10 3.95 4.14
89 85 83
±1.6
3.77 3.96 4.45
83 86 89
(g)
± 1.09
18.7 21.4 20.3
22.5 18.9 19.0
+ 5.41
199 196 198
193 198 202
± 1.13
14.9 16.3 16.3
17.3 15.2 15.1
C.V. of plant weight (%)
Coefficient of Mean plant variation (C.V.) weight of plant (g) weight (%)
5 days after 7 days after sowing sowing
Mean plant weight
Seedling weights at planting Plant weights at harvest (26 days after sowing)
Percentage of blocks with healthy plants
The effect of sowing pelleted, dry and.pre-germinated seeds of 'Bizet' on the percentage of blocks with healthy plants, and on seedling growth and plant weight at harvest (Experiment 4)
TABLE
t~ O
IV
88 88 85 ± 2.6
Uncovered Covered with 2 ml 0.25% Guar Covered with 2 ml 1% Laponite
SED
± 0.095
1.39 1.29 1.38
95 96 95 ± 1.8
1.16 1.36 1.54
94 95 97
± 2.19
43.0 41.0 41.8
39.8 44.1 41.8
(%)
(g) 81 86 95
C.V. of plant weight
Mean plant weight
8 days after 13 days after sowing sowing
Pelleted seeds Dry seeds Pre-germinated seeds
Plant weights at planting (26 days after sowing)
Percentage of blocks with healthy plants
± 6.75
189 197 194
192 191 198
(g)
Mean plant weight
± 1.196
17.9 16.2 16.9
17.1 17.7 16.7
C.V. of plant weight (%)
Plant weights at harvest
The effect of sowing pelleted, dry and pre-germinated seeds of 'Ravel' on the percentage of blocks with healthy plants, and on seedling growth and plant weight at harvest (Experiment 5)
TABLE
+- 0 . 7 2
SED
1 Bird damage.
99 99 98
20.8
9.7
15.2
15.6
Sown into propagating-house o n 28 M a r c h P e l l e t e d seeds Dry seeds P r e - g e r m i n a t e d seeds
9.0 99 99 99
22.2
8.5
Mean
Sown into propagating-house o n 25 M a r c h Pelleted seeds D r y seeds P r e - g e r m i n a t e d seeds
4.8
Mean min.
Final percentage emergence
96 _1 97
12.3
Mean max.
T e m p e r a t u r e s (o C) f r o m sowing to emergence
S o w n i n t o cold f r a m e s o n 20 March P e l l e t e d seeds Dry seeds P r e - g e r m i n a t e d seeds
Treatment
+ 0.17
5.0 5.0 3.2
5.0 5.0 2.2
10.0 _1 6.3
Days to 50% emergence
+- 0 . 4 5 0
2.47 2.44 3.03
4.63 4.13 4.66
1.45 _1 2.45
Mean plant w e i g h t (g) on 22 April
-+ 2.9
20.2 14.6 12.3
15.7 15.1 12.8
20.6 _1 15.6
C.V. o f p l a n t weight
E f f e c t o f seed t r e a t m e n t a n d t e m p e r a t u r e o f t h e early p r o p a g a t i n g - p h a s e o f g r o w t h o n seedling e m e r g e n c e a n d o n p l a n t w e i g h t s o f ' P l e v a n o s ' ( E x p e r i m e n t 6)
TABLE V
b~ b~
23 EXPERIMENT 6 and methods. - - Previous work on o u t d o o r lettuce (Gray, 1978a) showed that when pre-germinated seeds were sown into soil at 8--10°C, seedlings emerged 8 days earlier than from sowing dry or pelleted seeds under similar conditions. To examine the possible advantages of using pre-germinated seeds for rapid establishment of plants in blocks in the winter, a replicated experiment using 'Plevanos' was conducted, in which pre-germinated, pelleted and dry seeds were sown into blocks and then placed in an unheated Dutchlight frame on 20 March. After emergence was complete, the blocks were transferred from the frames to a cool propagating-house. Further batches of the types of seeds were sown into blocks on 25 March and 28 March and placed in a cool propagating-house. These dates were chosen so that one of them would give plants of the same size as the frame-raised plants at planting-time. There were 160 peat blocks per treatment and 6 replicates of each treatment. Counts of emerged seedlings were made at approximately 2--3 day intervals until no more seedlings emerged. Individual seedling fresh weights on 60 plants per plot were obtained at planting-time. Records of the temperatures in the propagating-areas were obtained from thermographs screened from direct sunlight and placed at the level of the peat blocks.
Materials
- - The mean maximum and minimum temperatures in the frames and propagating-house from sowing to the completion of emergence are shown in Table V. Percentage emergence from all treatments was high (Table V), and only pelleted seeds sown into blocks in the cold frame gave significantly lower emergence than the other treatments. In the cold frames, pelleted seeds emerged 3--4 days later than pre-germinated seeds (Table V). Birds damaged the plants from the dry seeds and the data is omitted. In the propagating-house, pre-germinated seeds emerged 2-3 days earlier than both pelleted and dry seeds. At planting, plants from pregerminated seeds raised in the frames and the propagating-house were heavier than those from pelleted and dry seeds, although the difference was not significant for the seeds sown on 25 March. The C.V. of plant weight at planting was lower for pre-germinated than for pelleted or dry seeds for plants from both environments, although this was only significant for the comparison between the t w o treatments sown on 28 March.
Results.
DISCUSSION The results presented in this paper show that it is possible to establish satisfactorily block-raised lettuce plants by sowing pre-germinated rather than pelleted seeds. To obtain good emergence and development, it is necessary to sow seeds having radicles of 1--2 mm in length and to use blocks with cylindrical rather than the inverted-cone-shaped holes that are produced by
24 some commercial block-making machines. Seedling emergence was reduced by covering the seeds with sand, Guar and Agrigel. Provided the blocks were kept moist, it was not found necessary to cover the pre-germinated seeds to conserve moisture around the seed. The percentage of blocks established with a healthy seedling from pre-germinated seeds ranged from 89 to 99%. The greatest increase (6%) from sowing pre-germinated seeds compared with pelleted seeds was with 'Bizet', where the latter gave 83% emergence. However, in the other experiments, the advantage for pre-germinated over pelleted seeds only amounted to 1--2%. In all of the experiments, the use of pre-germinated seeds gave earlier emergence than pelleted or dry seeds. For the summer-sown crops and for those sown in the winter but raised with heat, the difference amounted to about 2 days. Where no heat was provided, the advantage of pre-germinated over dry or pelleted seeds increased to about 4 or 5 days, suggesting that there might be some worthwhile savings in fuel in the early stages of propagation of winter and early spring crops arising from the use of pre-germinated seeds. This can be illustrated from the data in Table V, where seedlings from pre-germinated seeds sown into blocks held at 8.5°C mean temperature emerged only 1 day later than those from dry or pelleted seeds sown into blocks held at 15--16°C, which emerged after 5 days. This suggests that for 1 day of extra propagation time in the cold frames compared with the heated glasshouse, a saving of 5 days of the fuel needed to maintain temperatures 70 C above ambient would be possible. Sowing pre-germinated seeds also produced more uniform plants at planting-time compared with pelleted seeds, but this effect could not be clearly demonstrated at harvest. The C.V. of plant weight at harvest was lower than at planting. This would be expected for plants in a non-competing crop, such as lettuce, which reach a "plateau" weight at the market maturity stage, so allowing the individuals further behind in growth to catch up (Currah, 1978). As a result, the treatment differences which were apparent earlier in growth may have been eroded by harvest time. The results suggest that the sowing of pre-germinated seeds could give some small advantages over the use of pelleted seeds for the block raising of lettuce plants for glasshouse production, and this does not take into account the costadvantage of using dry compared with pelleted seeds, which at present is about 15%. However, further trials using a seed selector capable of handling pregerminated seeds in comparison with commercial machines delivering pelleted seeds, and with dry seeds will be necessary fully to evaluate the systems. ACKNOWLEDGEMENTS We thank Mrs. C.M. Edwards at N.V.R.S. and staff at Luddington E.H.S. for technical assistance, Mr. G.E.L. Morris at N.V.R.S. for statistical advice, Rijk Zwaan for supplying samples of seed and Royal Sluis for pelleting samples of dry seeds.
25 REFERENCES Anonymous, 1976. International rules of seed testing. Seed Sci. Tech., 4: 3--49; Annexes, 1976: 51--177. Currah, I.E., 1978. Plant uniformity at harvest related to variation between emerging seedlings. Acta Hortic., 72: 57--68. Darby, R.J., 1980. Effects of seed carriers on seedling establishment after fluid drilling. Exp. Agric., 16: 153--160. Gray, D., 1977. Temperature sensitive phases in the germination of lettuce (Lactuca sativa) seeds. Ann. Appl. Biol., 86: 77--86. Gray, D., 1978a. The effect of sowing pre-germinated seeds of lettuce (Lactuca sativa) on seedling emerg,~nce. Ann. Appl. Biol., 88: 185--192. Gray, D., 1978b. Comparison of fluid drilling and conventional establishment techniques on seedling emergence and crop uniformity in lettuce. J. Hortic. Sci., 53: 23--30.
15
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
COMPARISONS OF THE USE OF PRE-GERMINATED, DRY AND PELLETED SEEDS FOR BLOCK-RAISING OF GLASSHOUSE LETTUCE
J.R. CHRIMES 1 and D. GRAY 2
Luddington Experimental Horticulture Station, Luddington, Stratford-upon-Avon (Gt. Britain) 2National Vegetable Research Station, Wellesbourne, Warwick (Gt. Britain) (Accepted for publication 29 July 1981)
ABSTRACT Chrimes, J.R. and Gray, D., 1982. Comparisons of the use of pre-germinated, dry and pelleted seeds for block-raising of glasshouse lettuce. Scientia Hortic., 17: 15--25. The use of pre-germinated seeds for establishing glasshouse lettuce in peat blocks was compared with dry and pelleted seeds. Sowing pre-germinated seeds gave between 1 and 6% higher seedling emergence than from pelleted seeds and between 1 and 3% higher emergence than from dry seeds. Seedling emergence was reduced from 100% to 56% when the seeds were covered with 5 m m of silver sand compared with no cover. Seedling emergence from seeds covered with Agrigel (sodium alginate) was reduced by 24% at a concentration of 0.25% and by 62% at 0.7% compared with no cover. Guar (a natural gum) and Laponite (a synthetic clay) gels also reduced emergence on certain occasions but not by as much as Agrigel. Satisfactory levels of emergence were obtained when seeds were sown into blocks having an inverted, cone-shaped depression for locating the seeds, but the seedlings showed poor root-hair development and had elongated hypocotyls. Normal seedling growth and high levels of emergence were obtained from the seeds sown into blocks having cylindrical holes 10 m m in depth and 5 or 10 m m in diameter. Pre-germinated seeds invariably emerged earlier than pelleted or dry seeds. At low propagating-temperatures (12.0°C maximum and 5.0°C minimum), emergence was 4 days earlier from pre-germinated than from dry or pelleted seeds and 2--3 days earlier at high temperatures (21.0°C maximum and 9.0°C minimum). Coefficients of variation of seedling weight at planting were lower from sowing pre-germinated than pelleted or dry seeds, but these differences were not evident at harvest.
INTRODUCTION
Most glasshouse lettuce crops in the U.K. are established from peat blockraised plants. The devices currently used for sowing seeds into blocks are capable of selecting and delivering a single seed per block provided the seeds are approximately spherical in shape. Consequently, most of the lettuce blocks used in commerce are sown using pelleted seeds. Establishment is usually satisfactory, with over 95% of the blocks having plants suitable for transplanting, b u t germination is n o t always completely uniform and there is con-
0304-4238/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company
16
siderable plant-size variation at planting-time. Recent experiments (Gray, 1978a, b) with direct-drilled field-grown lettuce showed that sowing pregerminated seeds gave higher, earlier and more synchronous seedling emergence than dry or pelleted seeds. These results suggest that there could be advantages from the use of pre-germinated seeds for block-raised lettuce in improving plant uniformity, giving earlier establishment particularly at low temperatures and also for overcoming thermodormancy (Gray, 1977}. The recent commercial development of systems for mechanically handling and sowing both dry and pre-germinated seeds prompted, in 1979, an appraisal of the benefits of establishing blocks from pre-germinated or dry seeds compared with split-pill pelleted seeds. In this paper, we describe the techniques necessary for sowing pre-germinated seeds and we report the effects of sowing such seeds, split-pill pellets and dry seeds on seedling establishment and early seedling growth for autumn-, winter- and spring-sown glasshouse lettuce crops. EXPERIMENT 1 and methods. - - Pre-germinated seeds (radicles 1--2 m m long) were sown on 12 December into the conventional cone-shaped hole (15 mm diameter X 15 m m deep) of 43 mm-sized blocks and into the following combinations of sizes of cylindrical-shaped hole: 10 or 15 m m diameter X 5, 10 or 15 m m deep. After sowing a single pre-germinated seed by hand into each of 70 blocks per treatment, the blocks were covered by damp newspaper until the cotyledons appeared. The numbers of seedlings with fully opened cotyledons were counted 8, 9 and 21 days after sowing.
Materials
- - Seedling emergence was more rapid from conventional, invertedcone-shaped holes than from cylindrically-shaped holes, but final seedling emergence was unaffected (Table I). However, root-hair growth was poorer in
Results.
TABLE I Seedling emergence from peat blocks having holes for seed placement of different depth and width (Experiment 1) Type of hole
Conventional "Visser" type (inverted-cone-shape) Cylindrical hole 5-mm depth 10-mm depth 15-ram depth 10-ram diameter 15-mm diameter
Percentage seedling emergence (days after sowing) 8
9
21
53
74
97
6 12 15 7 4
21 35 28 20 53
98 93 97 96 95
17
the blocks with conventional holes and the cotyledons were more elongated and the h y p o c o t y l s frequently coiled. EXPERIMENT 2 and methods. - - Previous work on the direct drilling of lettuce in the field had shown that radicles of 1--2 mm in length gave better establishment than longer or shorter ones (Gray, 1978a). To ascertain if this was so when seeds were sown into peat blocks for plant raising, seeds of cultivar 'Bizet' with radicles just emerging, 1--2 mm long or 3--5 mm long, were sown into peat blocks (70 per treatment). Counts of emerging seedlings were made at regular intervals until no more seedlings emerged.
Materials
- - Sowing seeds with the radicles just emerging, 1--2 mm or 3--5 mm in length, had no effect on percentage seedling emergence, b u t the seedlings from the seeds with the longest radicles produced plants with coiled and elongated hypocotyls.
Results.
EXPERIMENT 3 and methods. - - Results from seed-box experiments (Darby, 1980) showed that the type of gel used affected seedling emergence, and so the effect of different types and concentrations of gel (see Table II) used to cover the seed after sowing in blocks were compared with a covering of sand and with no cover. Pre-germinated seeds having radicles 1--2 mm long were sown by hand into blocks (100 per treatment). Counts of emerging seedlings were made at regular intervals until no more emerged.
Materials
TABLE II Seedling emergence from blocks using different seed coverings (Experiment 3) Type of cover
Seedling emergence
(%) Guar
0.25% 0.50% 0.75%
100 100 65
Laponite
1.0% 1.5% 2.0%
91 I00 96
Agrigel
0.35% 0.53% 0.70%
Silver sand to 0.5-cm depth No cover
76 44 28 56 100
18 - - Covering the pre-germinated seeds with silver sand and Agrigel at any concentration reduced emergence (Table II) and Agrigel retarded roothair development. Guar and Laponite dispersed rapidly into the blocks at the lowest concentration used, but Agrigel did not disperse and formed a film over the seeds. Guar reduced emergence at the highest concentration and prevented root-hair development. At lower concentrations, growth was unaffected and emergence was similar to uncovered seed. Over the range used, the concentration of Laponite had little effect on emergence. Results.
EXPERIMENT 4 - - Lettuce seeds 'Bizet' were graded using an Ottawa air-blower to remove the lightest 10% of seeds and a proportion of the " h e a v y " seeds were sent to Royal Sluis for pelleting. A germination test, following I.S.T.A. rules (Anon., 1976), was made on samples of dry and pelleted seeds. To determine the coefficient of variation of seed weight, 100 dry seeds and 100 seeds removed from the split-pill pellets were weighed individually. Pre-germinated seeds with radicles 1--2 mm long were prepared by imbibing seeds on moist paper in trays at 20°C for 20 h. Pre-germinated, dry and pelleted seeds of this stock were then sown into 43-mm peat blocks on 10 May. After sowing, the seeds were either covered with 2 ml of 0.25% guar, 2 ml of 1.0% Laponite, or left uncovered. There were 144 peat blocks in each plot o f the 9 treatments, (3 seed types X 3 "covering" methods) and 4 replicates. After sowing and the application of the seed "covering" treatments, the blocks were covered with damp newspaper. This was removed at cotyledon emergence and the blocks were then watered frequently to prevent them drying out. Metalaxyl (0.2 g a.i./l) was applied to the plant and blocks on 17 May for protection against d o w n y m i l d e w ( B r e m i a l a c t u c a e Regel) and a spray of iprodione (0.25 g a.i./1) was applied on 17 May and 15 June for protection against grey mould ( B o t r y t i s c i n e r e a Percs. ex. Fr.). Counts of emerged, healthy seedlings were made on 15 and 17 May on 144 blocks per plot. Sixty-four block-raised plants from each of the 9 treatments were planted on 6 June at a spacing of 20 X 20 cm in a Venlo glasshouse. The treatments were arranged in a randomised block design and there were 4 replicates. At planting, 60 plants per plot were weighed individually. On 11 July, 60 plants from each plot were individually weighed after trimming and placing them in plastic bags.
Materials and methods.
- - Mean seed weights of pelleted and dry seeds were similar at 1.26 + 0.186 mg and 1.24 + 0.180 mg, respectively. Laboratory germination as determined b y the I.S.T.A. test-method was 95% for pelleted seeds and 99% for dry seeds. At the time of sowing, over 95% of the "pre-germinated" sample had germinated. There was no interaction between the t y p e of seed used and the covering-
Results.
19 m e t h o d on the numbers of emerged plants, on plant size at planting or at harvest, and so only the main t r e a t m e n t effects are shown in Table III. Sowing pre-germinated seeds gave significantly more robust plants at the final count compared with pelleted seeds, but there was no improvement compared with dry seeds. Plants emerged more rapidly from blocks sown with pre-germinated than dry seeds, and these in turn emerged more rapidly than plants from pelleted seeds. Covering the seeds with gel reduced emergence compared with no cover (Table III). Pre-germinated seeds gave significantly larger plants at planting than those from dry or pelleted seeds, which each gave plants of a similar size. Analysis of the distributions of individual plant weights showed that t h e y were normally distributed for all the treatments. Therefore, the coefficient of variation (C.V.) of plant weight was used to compare the variability in plant weight between treatments. The C.V. was lower for plants established from pre-germinated or dry seeds than for those from pelleted seeds. Covering had no effect on mean plant weight but it increased the variability of plant weight at planting. At maturity, the same trend in responses was apparent, but none of the effects were significant (Table III). EXPERIMENT 5 Materials and methods. - - This experiment was identical to Experiment 4 except that 'Ravel' was used, sowing t o o k place on 22 August and the blocks were planted on 18 September. Emergence counts were made on 30 August and 4 September, and 60 plants were weighed individually at planting and on 7 November after trimming and placing in polythene bags. Application of metalaxyl (0.2 g a.i./1) and iprodione (0.25 g a.i./1) were made at the time of cotyledon expansion and again on 21 September. - - Laboratory germination in both pelleted and dry seeds was 99%, and at sowing over 95% of the pre-germinated seeds had germinated. There was no significant effect of seed t y p e or covering treatment on the final n u m b e r of healthy seedlings. However, seedlings from pre-germinated seeds emerged earlier than those from d r y or pelleted seeds (Table IV). There was no effect of covering m e t h o d on seedling weight, but seedlings from pregerminated seeds were heavier than those from dry or pelleted seeds. Individual seedling weights were normally distributed and so the C.V. of seedling weight was used to compare the effects of the treatments on plant variability. There was no effect of treatment on C.V. of seedling weight at planting, nor on mean plant weight or C.V. of plant weight at maturity (Table IV). There were no significant interactions between seed and covering-treatments. Results.
III
55 64 87 73 69 64 ±3.7
Pelleted seeds Dry seeds Pre-germinated seeds
Uncovered Covered with 2 ml 0.25% Guar Covered with 2 ml 1% Laponite
SED (Standard error of.a difference between 2 means) _t
±0.196
4.10 3.95 4.14
89 85 83
±1.6
3.77 3.96 4.45
83 86 89
(g)
± 1.09
18.7 21.4 20.3
22.5 18.9 19.0
+ 5.41
199 196 198
193 198 202
± 1.13
14.9 16.3 16.3
17.3 15.2 15.1
C.V. of plant weight (%)
Coefficient of Mean plant variation (C.V.) weight of plant (g) weight (%)
5 days after 7 days after sowing sowing
Mean plant weight
Seedling weights at planting Plant weights at harvest (26 days after sowing)
Percentage of blocks with healthy plants
The effect of sowing pelleted, dry and.pre-germinated seeds of 'Bizet' on the percentage of blocks with healthy plants, and on seedling growth and plant weight at harvest (Experiment 4)
TABLE
t~ O
IV
88 88 85 ± 2.6
Uncovered Covered with 2 ml 0.25% Guar Covered with 2 ml 1% Laponite
SED
± 0.095
1.39 1.29 1.38
95 96 95 ± 1.8
1.16 1.36 1.54
94 95 97
± 2.19
43.0 41.0 41.8
39.8 44.1 41.8
(%)
(g) 81 86 95
C.V. of plant weight
Mean plant weight
8 days after 13 days after sowing sowing
Pelleted seeds Dry seeds Pre-germinated seeds
Plant weights at planting (26 days after sowing)
Percentage of blocks with healthy plants
± 6.75
189 197 194
192 191 198
(g)
Mean plant weight
± 1.196
17.9 16.2 16.9
17.1 17.7 16.7
C.V. of plant weight (%)
Plant weights at harvest
The effect of sowing pelleted, dry and pre-germinated seeds of 'Ravel' on the percentage of blocks with healthy plants, and on seedling growth and plant weight at harvest (Experiment 5)
TABLE
+- 0 . 7 2
SED
1 Bird damage.
99 99 98
20.8
9.7
15.2
15.6
Sown into propagating-house o n 28 M a r c h P e l l e t e d seeds Dry seeds P r e - g e r m i n a t e d seeds
9.0 99 99 99
22.2
8.5
Mean
Sown into propagating-house o n 25 M a r c h Pelleted seeds D r y seeds P r e - g e r m i n a t e d seeds
4.8
Mean min.
Final percentage emergence
96 _1 97
12.3
Mean max.
T e m p e r a t u r e s (o C) f r o m sowing to emergence
S o w n i n t o cold f r a m e s o n 20 March P e l l e t e d seeds Dry seeds P r e - g e r m i n a t e d seeds
Treatment
+ 0.17
5.0 5.0 3.2
5.0 5.0 2.2
10.0 _1 6.3
Days to 50% emergence
+- 0 . 4 5 0
2.47 2.44 3.03
4.63 4.13 4.66
1.45 _1 2.45
Mean plant w e i g h t (g) on 22 April
-+ 2.9
20.2 14.6 12.3
15.7 15.1 12.8
20.6 _1 15.6
C.V. o f p l a n t weight
E f f e c t o f seed t r e a t m e n t a n d t e m p e r a t u r e o f t h e early p r o p a g a t i n g - p h a s e o f g r o w t h o n seedling e m e r g e n c e a n d o n p l a n t w e i g h t s o f ' P l e v a n o s ' ( E x p e r i m e n t 6)
TABLE V
b~ b~
23 EXPERIMENT 6 and methods. - - Previous work on o u t d o o r lettuce (Gray, 1978a) showed that when pre-germinated seeds were sown into soil at 8--10°C, seedlings emerged 8 days earlier than from sowing dry or pelleted seeds under similar conditions. To examine the possible advantages of using pre-germinated seeds for rapid establishment of plants in blocks in the winter, a replicated experiment using 'Plevanos' was conducted, in which pre-germinated, pelleted and dry seeds were sown into blocks and then placed in an unheated Dutchlight frame on 20 March. After emergence was complete, the blocks were transferred from the frames to a cool propagating-house. Further batches of the types of seeds were sown into blocks on 25 March and 28 March and placed in a cool propagating-house. These dates were chosen so that one of them would give plants of the same size as the frame-raised plants at planting-time. There were 160 peat blocks per treatment and 6 replicates of each treatment. Counts of emerged seedlings were made at approximately 2--3 day intervals until no more seedlings emerged. Individual seedling fresh weights on 60 plants per plot were obtained at planting-time. Records of the temperatures in the propagating-areas were obtained from thermographs screened from direct sunlight and placed at the level of the peat blocks.
Materials
- - The mean maximum and minimum temperatures in the frames and propagating-house from sowing to the completion of emergence are shown in Table V. Percentage emergence from all treatments was high (Table V), and only pelleted seeds sown into blocks in the cold frame gave significantly lower emergence than the other treatments. In the cold frames, pelleted seeds emerged 3--4 days later than pre-germinated seeds (Table V). Birds damaged the plants from the dry seeds and the data is omitted. In the propagating-house, pre-germinated seeds emerged 2-3 days earlier than both pelleted and dry seeds. At planting, plants from pregerminated seeds raised in the frames and the propagating-house were heavier than those from pelleted and dry seeds, although the difference was not significant for the seeds sown on 25 March. The C.V. of plant weight at planting was lower for pre-germinated than for pelleted or dry seeds for plants from both environments, although this was only significant for the comparison between the t w o treatments sown on 28 March.
Results.
DISCUSSION The results presented in this paper show that it is possible to establish satisfactorily block-raised lettuce plants by sowing pre-germinated rather than pelleted seeds. To obtain good emergence and development, it is necessary to sow seeds having radicles of 1--2 mm in length and to use blocks with cylindrical rather than the inverted-cone-shaped holes that are produced by
24 some commercial block-making machines. Seedling emergence was reduced by covering the seeds with sand, Guar and Agrigel. Provided the blocks were kept moist, it was not found necessary to cover the pre-germinated seeds to conserve moisture around the seed. The percentage of blocks established with a healthy seedling from pre-germinated seeds ranged from 89 to 99%. The greatest increase (6%) from sowing pre-germinated seeds compared with pelleted seeds was with 'Bizet', where the latter gave 83% emergence. However, in the other experiments, the advantage for pre-germinated over pelleted seeds only amounted to 1--2%. In all of the experiments, the use of pre-germinated seeds gave earlier emergence than pelleted or dry seeds. For the summer-sown crops and for those sown in the winter but raised with heat, the difference amounted to about 2 days. Where no heat was provided, the advantage of pre-germinated over dry or pelleted seeds increased to about 4 or 5 days, suggesting that there might be some worthwhile savings in fuel in the early stages of propagation of winter and early spring crops arising from the use of pre-germinated seeds. This can be illustrated from the data in Table V, where seedlings from pre-germinated seeds sown into blocks held at 8.5°C mean temperature emerged only 1 day later than those from dry or pelleted seeds sown into blocks held at 15--16°C, which emerged after 5 days. This suggests that for 1 day of extra propagation time in the cold frames compared with the heated glasshouse, a saving of 5 days of the fuel needed to maintain temperatures 70 C above ambient would be possible. Sowing pre-germinated seeds also produced more uniform plants at planting-time compared with pelleted seeds, but this effect could not be clearly demonstrated at harvest. The C.V. of plant weight at harvest was lower than at planting. This would be expected for plants in a non-competing crop, such as lettuce, which reach a "plateau" weight at the market maturity stage, so allowing the individuals further behind in growth to catch up (Currah, 1978). As a result, the treatment differences which were apparent earlier in growth may have been eroded by harvest time. The results suggest that the sowing of pre-germinated seeds could give some small advantages over the use of pelleted seeds for the block raising of lettuce plants for glasshouse production, and this does not take into account the costadvantage of using dry compared with pelleted seeds, which at present is about 15%. However, further trials using a seed selector capable of handling pregerminated seeds in comparison with commercial machines delivering pelleted seeds, and with dry seeds will be necessary fully to evaluate the systems. ACKNOWLEDGEMENTS We thank Mrs. C.M. Edwards at N.V.R.S. and staff at Luddington E.H.S. for technical assistance, Mr. G.E.L. Morris at N.V.R.S. for statistical advice, Rijk Zwaan for supplying samples of seed and Royal Sluis for pelleting samples of dry seeds.
25 REFERENCES Anonymous, 1976. International rules of seed testing. Seed Sci. Tech., 4: 3--49; Annexes, 1976: 51--177. Currah, I.E., 1978. Plant uniformity at harvest related to variation between emerging seedlings. Acta Hortic., 72: 57--68. Darby, R.J., 1980. Effects of seed carriers on seedling establishment after fluid drilling. Exp. Agric., 16: 153--160. Gray, D., 1977. Temperature sensitive phases in the germination of lettuce (Lactuca sativa) seeds. Ann. Appl. Biol., 86: 77--86. Gray, D., 1978a. The effect of sowing pre-germinated seeds of lettuce (Lactuca sativa) on seedling emerg,~nce. Ann. Appl. Biol., 88: 185--192. Gray, D., 1978b. Comparison of fluid drilling and conventional establishment techniques on seedling emergence and crop uniformity in lettuce. J. Hortic. Sci., 53: 23--30.