The effects of acute and chronic gamma irradiation on Lupinus albus L.—I. Effects of acute irradiation on the vegetative shoot apex and general morphology

Abstract A Plastochron Index (P.I.) was established for lupine that provided a baseline from which dose rate and total dose effects could be evaluated. The P.I. was calculated to be 3.56 days and each control leaf elongated exponentially for about 10 days at approximately equal plastochron intervals. In plants irradiated with 3.3–16 kR, the plastochron interval tended to lengthen slightly during irradiation but returned to control values in 11 days. Statistically significant ( p ≦ 0.01; t test for correlated samples) depressions were observed in the slopes of growth curves at all dosages for leaves growing exponentially at the time of irradiation. Using divergence angle and plastochron ratio it was shown that phyllotaxis in lupine was not changed after irradiation (6.6–32 kR) from the 2,3 arc. An increase in divergence angle did occur, however, after irradiation, indicating that the positions of leaf primordia were moved laterally, but only a change in the radial rates of leaf displacement would elicit a change in phyllotactic pattern. Lupine apices given acute doses of 6.6–32 kR showed both scattered and specific damage. Scattered damage was limited and unimportant since it induced no developmental changes. Specific damage was reflected in an increasing percentage of apices which showed tunica-corpus damage at increasing dosages at or above a threshold dose of 8.5 kR, but no one apex showed all stages in the whole range of localized necrotic damage. Therefore, only the percentage of apices damaged was dose dependent, and the induced cellular changes, from initial vacuolation to complete cell necrosis followed by post-mortem changes, were not dose dependent. At 6.6 kR there was no localized damage to the second tunica layer common to higher dosages, but only slight vacuolation of some apical zone cells (initials and central mother cells). Vacuolation is discussed as the earliest and most obvious radiation response and can best be explained by the passive osmotic theory of Vasil'ev. Localized damage in the second tunica layer, beginning in the initial zone and extending down to and including the region of leaf initiation, could suppress the initiation of a leaf primordium. Corpus cells subjacent to the collapsed second tunica layer then divided to form files of cells which enlarged by vacuolation and then collapsed to obliterate the apical meristem. Primary tissues derived from the apical meristem showed varying radiosensitivities—the pith was most sensitive and then the primary vascular tissues, while the epidermis and cortex showed only slight vacuolation. Other general morphological changes were a fusion of folioles and the occasional (5 of 26) elimination of leaves. The main vegetative radiation response in lupine was suppression of a median leaf bundle and subsequent failure of the leaf to be initiated after doses of 6.6–32 kR and 3–14 days recovery (to allow the apex to continue to produce leaves). This experimental work supports the view of longitudinal induction of patterning of primary vascular differentiation and the associated positioning of putative leaves. Data from the present study support the concept that the phyllotactic pattern of putative leaves found on the apex is a result of the apex and its derivatives interacting with more mature subjacent tissues in a correlative fashion.