Effect of Water Potential and Temperature on the Germination of Four Species of African Savanna Trees

The germination responses of seeds from the African tree speciesColophospermum mopane, Combretum apiculatum, Acacia tortilisand Acacia karroo under varying regimes of temperature and waterstress (induced by incubation in PEG 8000) are reported Withthe exception of Combretum (at –0.14 and –0.29 MPa)and Colophospermum (at –0.29 MPa), incubation in PEG decreasedthe maximum achieved germination percentage (90–100% forall species), but did not extend the germination lag (exceptin Combretum) or affect the time required to reach maximum germinationCombretum and Colophospermum were found to germinate under thewidest range of temperatures and water potentials, for example,as strongly negative as –1.0 MPa at 20 and 30 °C,respectively These seeds also showed greater or equivalent hypocotylelongation in PEG solutions creating potentials of –0.14,–0.29 or –0.51 MPa when compared with seeds germinatedin water, indicating an additional stress adaptation Acaciaspecies showed progressive reduction in germination rates andradicle elongation in response to decreasing water potentialExperiments giving pre-imbibition treatments in water priorto transfer to PEG solutions showed that both Acacia speciesgerminated at approximately 90% if given such pre-treatmentand less than 10% if transferred directly to PEG It is concludedthat the most stress-adapted species studied are Colophospermummopane and Combretum apiculatum, a finding generally correlatedwith the growth habit of these trees Colophospermum mopane, Combretum apiculatum, Acacia tortilis, Acacia karroo, germination, water stress, Zimbabwe     

Comparative Photosynthesis in Developing Leaves of Brachystegia spiciformis Benth

Photosynthesis and dark respiration were studied during andafter the spring flush in Brachystegia spiciformis Benth. Variousparameters were examined including anthocyanin and chlorophyllcontent, Hill reaction activity, and gas exchange. Total chlorophyllcontent steadily increased reaching a constant value in fullyexpanded (25-d-old) leaves, whereas anthocyanin concentrationincreased as the chlorophylls but rapidly declined after 23d. Hill reaction measurements confirmed that leaf material fromevery stage of the flush (bud burst to mature leaves) was photosyntheticallyfunctional. The activity was low in flushing leaves with highanthocyanin content and then dramatically increased as leafanthocyanin content declined. Oxygen exchange measurements showedflushing leaves to have lower photosynthetic rates but higherrespiratory activity than mature leaves (60% and 120%, respectively).Gas exchange and in vitro electron transport were also generallycorrelated; Hill reaction activity was 128% of gas exchangein mature leaves and 92% in flushing leaves. It is concludedthat although photosynthetic rates are lower and respirationrates are higher in flushing leaves than in mature, fully expandedleaves, flushing leaves are fully photosynthetically competentand apparently require no net input of carbon for growth anddevelopment. Key words: Brachystegia, Hill reaction, photosynthesis, Zimbabwe