Effects of invasion of fire‐free arid shrublands by a fire‐promoting invasive alien grass (Pennisetum setaceum) in South Africa

Arid shrublands in the Karoo (South Africa) seldom accumulate sufficient combustible fuel to support fire. However, as a result of invasion by an alien perennial grass (Pennisetum setaceum), they could become flammable. This paper reports on an experiment to assess the effects of fire following invasion by P. setaceum. We established 10 plots (5 × 10 m) separated by 2.5 m, and added grass fuel to five plots (5 and 10 tons ha^?1 to alternate halves of the plot) leaving the remaining five plots as interspersed controls. Plots with fuel added were burnt, and fire behaviour was measured during the burns. Rates of fire spread were generally low (0.01–0.07 m s^?1) and did not differ significantly between burn treatments. Mean fireline intensities were higher in the high compared with the low fuel treatments (894 and 427 kW m^?1, respectively). We recorded plant species and their cover before and after burning on each of the plots. After 15 months of follow‐up monitoring in the burn plots, only two species, the dwarf shrub (Tripteris sinuata) and the perennial herb (Gazania krebsiana) resprouted. Most individuals of other species were killed and did not reseed during the 15‐month study. The mass of added fuel load (high or low) did not influence vegetation recovery rates after fire. Should future invasions by P. setaceum lead to similar fuel loads in these shrublands, inevitable fires could change the vegetation and may favour spread of the flammable grass. Our results have important implications for predicting the effects of invasive alien plants (especially grasses) on fire‐free ecosystems elsewhere. The predicted impacts of fire may alter species composition, ultimately affecting core natural resources that support the Karoo economy.     

Factors affecting 14C-sucrose uptake by single-node explants of rose flower stems

Single node explants of ‘Baccara’ rose were fed through the basal cut surface with ¹⁴C-sucrose in agar. Accumulation of ¹⁴C-metabolites by buds is a steady process, while a quick uptake occurs in the stem piece during the initial stage of incubation suggesting a passive uptake at this stage. The uppermost 5-leaflet node buds had an initially higher ¹⁴C-uptake than the lowest bud. This difference disappeared after 2–3 days when growth started in both buds. Light enhanced uptake of ¹⁴C-sucrose by both stem pieces and buds. Accumulation of ¹⁴C-metabolites by buds was in direct relationship to buds deviation from the direction of gravity. This phenomenon was not evident in the stem part of the explant.     

ANTIBACTERIAL SUBSTANCES IN SEED COATS AND THEIR ROLE IN THE INFECTION OF SWEET PEAS BY CORYNEBACTERIUM FASCIANS (TILFORD) DOWSON

Antibacterial substances active against Corynebacterium fascians have been shown to be present in the coats of various seeds. There were considerable differences in activity between the seeds of different genera and species and between individual seeds of a single species.
C. fascians also showed strain variation in sensitivity to the antibacterial substances of the seeds of any one species. An inverse relationship was demonstrated between the sensitivity of a strain to the sweet-pea seed substances and its ability to infect sweet-pea seedlings under laboratory conditions.