QUANTITATIVE CHANGES IN GIBBERELLIN AND RNA CORRELATED WITH SENESCENCE OF THE SHOOT APEX IN THE ‘ALASKA’ PEA

Senescence of shoot apices of Pisum sativum L. ‘Alaska’ as measured by cessation of stem elongation was delayed by removal of flowers and by treatment with gibberellin A₃ and was hastened by treatment with AMO-1618 (2 isopropyl-4-dimethylamino-5-methylphenyl-1-piperi-dinecarboxylate methyl chloride). Ontogenetic changes in relative endogenous gibberellin levels and in capability of gibberellin biosynthesis in deflowered and control plants were determined indirectly by studying time-course changes in the sensitivity, as indicated by the growth response, of these plants to applied gibberellin and AMO-1618. The results of these experiments suggest that the endogenous gibberellin level varies directly with the growth rate. Analyses of total RNA and protein in shoot tips of deflowered and control plants revealed that the levels of these substances also vary directly with growth rate throughout ontogeny. It is concluded that decreases in endogenous gibberellin, RNA and protein are factors correlated with senescence of the shoot apex.     

THE DISTRIBUTION AND ACCUMULATION OF SELENIUM IN ROOTS AND SHOOTS OF PLANTS NATURALLY GROWN IN THE SOILS OF KEBAN’S PB-ZN-F MINING AREA,TURKEY

The objective of this study is to determine the distribution factors and enrichment coefficients between soil and plant parts by studying the accumulation and distribution of selenium (Se) in the roots and shoots of different plants. The plants (9 samples of Euphorbia macroclada, 5 samples of Verbascum cheiranthifolium, 8 samples of Astragalus gummifer) and their associated soil samples were collected from the Keban mining area. The roots and the shoots of these plants, together with the associated soils, were analyzed by ICP-MS. The mean Se value of the contaminated surface soils was found to be two to five times higher than those of previously studied uncontaminated surface soils. Se concentrations of the plant parts were lower than those in their associated alkaline soils, where the plants were grown, except for in the shoots of A. gummifer. Mean Se concentrations in the roots of E. macroclada, V. cheiranthifolium, and A. gummifer were 0.82, 0.22, and 0.47 mg kg^?1 on a dry weight basis, respectively, while Se concentrations were 0.29, 0.26, and 2.66 mg kg^?1 in the shoots on a dry weight basis, respectively. The enrichment coefficients and the distribution factors of those plants were lower than 1, except for the distribution factors of V. cheiranthifolium and A. gummifer plants. Thus, it appeared that the shoots of these plants could make them efficient bioaccumulator plants for Se because of high distribution factors and enrichment coefficients. Due to such factors, they can also be used to clean or rehabilitate soils and areas contaminated with Se.     

CULTURE STUDIES ON THE MARINE GREEN ALGA HALICYSTIS PARVULA—DERBESIA TENUISSIMA. II. SYNCHRONY AND PERIODICITY IN GAMETE FORMATION AND RELEASE

Unialgal cultures of the macroscopic, vesicular, coenocytic gametophyte (Halicystis parvula Schmitz) of Derbesia tenuissima (DeNotaris) Crouan fr. were grown under various environmental regimes to elucidate the cytology of gamete formation and the factors controlling synchronous gamete formation and release. No synchrony of nuclear division was observed in vegetative plants or during the early stages of gamete formation. In the later stages of gamete formation in plants in a light-dark cycle, nuclear divisions within any gametangium were synchronous, and the stages of gamete formation were synchronous for the population. This synchrony was not as great for plants in continuous light. Gametes of plants in a light-dark cycle were released explosively immediately following the dark-to-light transition. Release was random and much less forceful for plants in continuous light. After a certain stage of gamete formation, gamete release was timed to occur after a particular interval of darkness, but release could be triggered by light during the last portion of this interval. The length of the dark interval was shorter for male plants than for females, but the period of light sensitivity was longer for females. Formation of gametangia by series of isolated plants was also synchronous and sometimes periodic under certain conditions. Intervals between gametangia on the same plant varied from 2 to 14 days but were usually 4 or 5 days (unlike plants in nature, which show a bi- or tri-weekly periodicity). Male and female plants did not differ in synchrony or periodicity. Different media affected the number of gametangia formed over a period of time but not the synchrony of formation. Under some conditions changing the medium had a stimulating or synchronizing effect. Non-repeated temperature changes also synchronized gamete formation. Optimum temperature for continued gamete formation was about 21 C. Regular daily light and temperature variation together maintained synchronous and periodic gamete formation in populations of isolated plants. Reproduction diminished and became less synchronous at constant temperature either in continuous light or under a light-dark schedule, although in the light-dark regime steps in the formation of any given gametangium remained synchronous with the light-dark cycle. Length of times between gametangial formation on individual plants showed a tendency to occur in multiples of the usual period lengths; e.g., plants sometimes tend to “skip” intervals, thus maintaining the synchrony of the population. These results suggest that interaction between daily environmental cycles and an endogenous physiological cycle may maintain periodic reproduction.