| Some studies concerning the metabolic basis for the
speltoid-compactoid series in Triticum vulgare* S. F. H. THRELKELD Botany School, Cambridge, England It is well known that aneuploids of Triticum vulgare, concerning Chromosome IX (the "speltoid chromosome"), give rise to plants that when mature are of various heights and head types, depending on their chromosome constitution. Thus plants monosomic for Chromosome IX are taller than the normal disomic and have speltoid heads, while plants that are trisomic and tetrasomic are increasingly shorter than the disomic, and have, what may be described as, semi-compactoid and compactoid heads respectively. Heights of field grown plants disomic trisomic, and tetrasomic for Chromosome IX, measured by averaging the heights of the three tallest tillers of each plant, are shown in Table 1. A number of investigations are being conducted in an attempt to determine the metabolic basis for these height differences. Broadly speaking there are two approaches for such investigations. One approach is through the study of differences between the plant types in their respiratory systems and the concomitant production of high energy metabolites; the other is through the study of the better known growth promoting substances. These two approaches differ essentially in that the former tends to seek general information, while the latter seeks specific information. Each approach has its own disadvantages. Thus respiratory studies may provide information that is rather vague, and growth substance studies may result only in negative information. A start has been made on the latter approach. Plant extracts were obtained from disomic and tetrasomic seedlings by the following method. After harvesting, the seedlings were weighed and immediately frozen at -14C. The tissues were then ground to a pulp with a pestle and mortar at 0C. To this pulp was added iced distilled water at the rate of 100 ml per 1 g fresh seedling weight, and the mixture was centrifuged. Following centrifugation the supernatant was dialysed against distilled water. The dialysates were tested for indoleacetic acid (IAA) oxidase activity; the destruction of IAA by the dialysates was measured colorimetrically. A control of boiled dialysate showed no IAA destruction. Table 2 summarises these results. These experiments demonstrated that on a fresh weight basis there is no significant difference in IAA destruction by extracts of disomic and tetrasomic seedlings. Further evidence of this point was obtained by using wheat coleoptiles, both disomic and tetrasomic, in place of Avena coleoptiles in the standard Avena coleoptile test. Known amounts of IAA in agar blocks placed on the prepared wheat coleoptiles gave no significant differences between the resulting curvatures of the disomic and tetrasomic coleoptiles. Disomic and tetrasomic seedlings were grown in solutions of gibberellic acid (GA), kinetin, and water. The seedlings clearly showed a response to GA; however disomic and tetrasomic seedlings seemed to respond by nearly equal amounts. Seedlings grown in kinetin has reduced growth compared with those grown in water, and it appeared that disomic and tetrasomic seedlings had nearly equal growth rates. It may well be that the kinetin solution gave rise to overall limiting factors, quite divorced from the factors causing the dwarfness under study, but such as to prevent the typical development of both disomic and tetrasomic phenotypes. Seedlings were also grown in water under light and dark conditions; no differential responses were observed. These results are summarised in Table 3. |
| * These investigations were carried out at the University of Alberta, Edmonton, Alberta, Canada. They formed part of an M. Sc. thesis which was supported by a bursary from the National Research Council of Canada. |