Growth and curvature in seedlings of Pisum sativum and an ageotropic mutant [Pisum sativum cv. Sabel, Pisum sativum ageotropum, ageotropic roots, pea]

In an attempt to explain the influence of gravity on the behaviour of ageotropic plant organs, a pea mutant (Pisum sativum ageotropum) and normal pea (Pisum sativum cv. Sabel) were examined. The mutant has a significantly lower germination rate (large seeds: 25 per cent, small seeds: 10 per cent) than normal pea seeds (55 per cent). Removal of testa increased germination dramatically, the values obtained were 63 and 89 per cent, respectively. Immediately after imbibition the mutant from which the testa had been removed, developed more slowly than normal pea seeds; after 28 h the difference in elongation rate between the two types was reversed. When continuously stimulated geotropically in the horizontal position the elongation in the mutant is larger than in the normal pea roots kept in the same position. During a 24 h period starting 48 h after imbibition the mutant root elongated 45.0 mm while the value for the normal pea root was 11.5 mm. The course of the geotropic curvature in roots of the two types has been followed during a period of 24 h. Normal pea roots develop an asymmetry in the extreme root tip region after 30 min of horizontal stimulation. After prolonged stimulation (exceeding 2 h) the asymmetry has disappeared and the curvature is distributed over the entire growth region. When roots of normal pea are stimulated continuously at various angles, the optimum angle of geotropic response is 90 degree with decreasing responses in the order 135 degree (i.e. the root tip is pointing obliquely upward) and 45 degrees. The presumed ageotropic behaviour of the mutant has only to a certain extent been confirmed in the present study. When stimulated at 135 degree a slight positive curvature developed; stimulation at 90 degrees and 45 degrees gave a slight negative curvature