Dormancy of Urena lobata L. seeds. I. Development of sulphuric ...

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Percentage Germination of Successive Batches of Urena lobata Seeds Treated for 30 min with Repeatedly Recycled Sulphuric Acid LSD Initial acid volume (dmJ/kg seed) Volume of acid consumed (dmJ/kg seed) Germination (%) I 10.00 1.67 92.4 2 8.33 1.26 93.3 3 7.07 0.87 93.7 4 6.20 0.87 83.4 5 5.33 1.33 64.6 6 4.00 0.87 46.6 7 3.13 0.80 21.5 8 2.33 0.93 16.6 9 1.40 0.60 11.7 10 0.80 0.50 2.7 P = 0.05 7.5 P = 0.01 10.0 Treatment time (months before sowing date) Stored in cloth bags o I 2 3 4 Untreated Stored in sealed glass containers o I 2 3 4 Untreated Germination ("A,) Immediately At sowing after treatment date 97.7 98.2 85.6 91.4 82.5 93.5 100.0 91.4 germmation or viability. Furthermore, no evidence was obtaIned for loss of viability or ability to germinate during storage of seeds scarified with sulphuric acid and returned to either storage condition. Discussion The percentage germination of unscarified seeds was consistently low, the highest recorded percentage in this investigation being 4.2 per cent. The data presented here are thus consistent with previous statements concerning the need for an effective dormancy breaking technique for seeds of this type (Harris, 1981). The results confirm that scarification with sulphuric acid can be a highly successful method of overcoming U. lobata seed dormancy. The most effective treatment tested was to immerse seeds in 18 M sulphuric acid for between 30 and 90 min. This conclusion is similar to those reached by previous workers. Horn & Natal Colon (1942) increased germination from 22 to 87 per cent by a I-h treatment with 96.5 pe cent sulphuric acid. This was considerd the optimum 91.9 100.0 94.5 91.1 86.0 4.2 94.5 94.9 95.0 86.5 96.2 0.8 Viability (%) Immediately At sowing after treatment date 91.7 90.4 89.6 91.7 95.4 89.6 85.0 91.7 92.9 86.7 91.3 88.8 92.1 90.8 90.4 89.6 91.7 93.8 88.3 91.7 soaking time for hulled seeds and germination was less with longer or shorter soaking times. Juillet (1952) suggested a sulphuric acid treatment for
Dormancy of Urena lobata L. seeds hulledseedsforO.75hat30"Corl hat 12-15°C. Such treatments increased germination from 30 to 79.5 per cent in the laboratory and from 40 to a maximum of 65 per cent when used in conjunction with fungicides on seed sown in compost in a greenhouse. Extendingthesoakingtimein 15or I8Msulphuric acid to 180 min reduced germination. This time was probably sufficient for acid not only to increa~e testa permeability but also to reach and damage the embryo. It appears unlikely that a reduction in the concentration of acid used could adequately be compensated for by an extension of the soaking time. With lower concentrations inherent variation in testa permeability may result in dormancy being broken rapidly in some seeds but only slowly in others. A proportion of seeds would thus be damaged owing to acid uptake while others remained unscarified, causing a less than maximum percentage germination. Although temperature affects the response of seeds to sulphuric acid, successful scarification was achieved over a wide range of temperatures with a 30 min treatment. At temperatures likely to be encountered in practice, both 30 and 60 min treatments are appropriate. Although the temperature of sulphuric acid was shown to increase during scarification, control of this environmental parameter should be unnecessary under normal field or laboratory conditions. A minimum volume of sulphuric acid is required to successfully scarify U. lobata seeds. If the volume of acid used is too little, even though it may be sufficient to moisten the entire seed surface, less than complete scarification is obtained. Working on the assumptions ofa suggested plant density for fibre production of 400,000 plants/ha, a dry seed weight of 12 mg/seed, 100 per cent seed viability, and a minimum acid requirement of 0.67 dm 3/kg (seed), arequirementof3.22 dmlfhamay be computed. After appropriate adjustments for the viability of particular seed batches, required plant densities, and allowances for wastage, this value could be used in considering the economics of scarification with regard to local availability and cost of sulphuric acid. Experiments in which the residual sulphuric acid was removed and used to scarify further batches of seeds proved generally unsuccessful. The efficiency of the acid declined rapidly even while the volume present remained well inexcess of the previously determined minimum. Furthermore, the average loss of acid during removal and washing of the seeds exceeded the minimum volume required for scarification when the acid was utilized once only. Loss of viability of U. lobata seeds due to delayed sowing has been reported by Crane &Acuna (1945). The present author found no evidence for viability loss in seeds of U. lobata indigenous to Sierra Leone with a storage time of up to 4 months after harvest (Harris, 1981). The data presented here have further indicate that loss of viability does not occur when seeds are stored from one growing season to the next, even if they are scarified immediately after harvest. There is thus considerable flexibility in the time at which seeds may be scarified. This fact could have important implications if seeds were to be produced and scarified centrally before distribution to individual growers. The results of this study indicate that sulphuric acid scarification of U. lobata seeds is a highly effective method of breaking their dormancy. The technique for scarifying the seeds is relatively simple and may easily be carried out under normal working conditions without strict control of environmental conditions or of the date and duration of treatment. An exposure to 18 M sulphuric acid for 30-60 min followed by thorough washing and air-drying is recommended. A minimum volume of 0.67 dm 3 /kg (seed) should be used and the residual acid discarded.· Against the advantages of the technique must be set disadvantages such as the dangers faced by inexperienced operators handling concentrated sulphuric acid and the recurrent cost involved in seed scarification. Thus, while efforts should be directed towards developing mechanical and low cost treatments and to selecting nondormant seeds, suIphuric acid scarification has a
Page 1 and 2: Dormancy of Urena lobata L. seeds.
Page 3: viable seeds germinating after trea

Dormancy of Urena lobata L. seeds. I. Development of sulphuric ...

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