Leaf-pack dynamics in a southern African mountain stream

SUMMARY 1. The occurrence, composition and invertebrate fauna of naturally-occurring leaf packs were studied over 24 months in Langrivier, a second-order mountain stream in the south-western Cape, South Africa. Langrivier is shallow and fast-flowing and stores very low levels of allochthonous detritus, although natural leaf packs form an obvious part of the energy base in the stream throughout the year. 2. The occurrence and size of the packs were influenced mainly by stream discharge and by the timing and character of leaf fall from riparian trees. Packs were smallest (minimum dry mass 17 g, minimum volume 1.7–10^?5 m³) in winter when discharge was high, and largest (maximum dry mass 191 g, maximum volume 4.2–10^?3 m³) in spring when discharge decreased and leaf fall from the evergreen riparian trees began. Through the year the packs covered a mean 0.41 % of the stream bed and had a mean abundance of 0.46 packs m^?2 of stream bed. They were ephemeral, lasting on average <1.7 months and yet accounted for 29% of the stored detritus in the system. Wood was the dominant component of packs, and leaves at ali stages of decomposition were present throughout the year. 3. The ratio of numbers of invertebrates in packs: numbers of individuals in the benthos was very low (0.002–0.030), presumably because of the rarity and small size of the packs. Nevertheless, the density of invertebrates per unit area covered by leaf packs was consistently much higher than the density in an equivalent area of the benthos, except during peak leaf fall (October to December). 4. Experiments were undertaken with artificial leaf packs in order to determine the extent to which these simulated natural packs. Although both natural and artificial leaf packs contained a high proportion of Plecoptera (46% and 29% respectively), the natural packs contained high numbers of simuliid larvae (33% of total), whereas artificial packs had a high percentage of chironomid larvae (62%), Several other taxa regularly occurred in both types of pack but in very low numbers. In addition,     

Unsaturated Fatty Acid Synthesis During the Development of Isolated Sunflower (Helianthus annuus L.) Seeds

The effect of temperature on unsaturated fatty acid synthesisin developing sunflower seed embryos (Helianthus annuus L.)has been studied using isolated seeds grown in culture. Variabilitybetween individual embryos in the response to temperature wasalso investigated. Oil and dry matter accumulation in cultured embryos were similarto those of embryos allowed to develop in intact plants, andthe effect of increasing temperature in lowering the amountof linoleic acid in seed oil was reproduced in cultured embryos.The isolated seed culture system, therefore, constitutes a suitablemodel system for studies of oil synthesis in developing sunflowerembryos. The decrease in linoleic acid synthesis in response to highertemperature was detectable after only 18 day-degrees incubation,and the incorporation of labelled substrates suggests that alterationsin the fatty acid composition of seed oil in response to temperatureare produced by an effect on the desaturation of newly synthesizedoleate rather than through turnover of existing lipid. Variation in fatty acid composition between individual embryosgrown at constant temperature was considerable. The detectionof embryos with high linoleic acid levels following growth athigh temperature indicates that potential may exist for theselection of cultivars for temperature-stable fatty acid compositionin sunflower oil. Key words: Fatty acid synthesis, Helianthus annuus, Sunflower seeds     

Influence of temperature and duration of leaf wetness on infection of Acyrthosiphon kondoi with Erynia neoaphidis

Bioassays were carried out to examine the influence of temperature and duration of leaf wetness on the infectivity of an isolate of Erynia neoaphidis for its aphid host Acyrthosiphon kondoi. Preliminary experiments demonstrated that primary spores produced in vitro were as infectious as those formed in vivo. No consistent effect of temperature on infectivity of primary spores could be detected. The time taken to kill an aphid increased as temperature decreased, from 3–5 days at 20 °C to 12–15 days at 8 °C, suggesting a threshold for disease development of 4 °C. Increasing duration of the period of leaf wetness up to 24 h after inoculation increased the final level of infection. At 20 °C, a minimum moisture period of 3 h was required for infection with maximum infection occurring after about 7 h. These times increased slightly at 15 °C but extending to 7 and 16 h respectively at 10 °C. The epizootiological implications of these results are discussed with reference to previously published data on in vivo production of primary spores of E. neoaphidis.     

Boundary Layer Effects in Ion Absorption by Roots and Storage Organs of Plants

In plant nutrition studies with culture solutions it is tacitly assumed that replenishment of ions at the root surface is guaranteed by the turbulence of the stirred solution. That this belief may be erroneous is demonstrated in this study with barley roots and discs of beet roots and potato tubers. At very low concentrations of RbCl (in presence of CaCl₂) the uptake of Rb by roots is strongly controlled by the rate of stirring. The results are interpreted in the light of the Nernst hydrodynamic boundary film which surrounds solids and through which ions must diffuse to reach the root surface. “Film-controlled” and “root-controlled” ion uptake is visualized.     

Weight loss of the female during the first brood as a factor influencing second brood initiation in Great Tits Parus major and Blue Tits P. caeruleus

We tested the hypothesis that the weight lost by female Great and Blue Tits Parus major and P. caeruleus while raising their first brood influences their ability to start a second brood. The evening weight of female parents was recorded when the nestlings were 5 and 13 days old, in different years and habitats. Several predictions were tested: (1) both species lose weight while raising nestlings and Great Tit females which start a second brood lose less weight than females which do not; (2) differences in the average weight lost between years and areas correlate with differences in the proportion of second broods; (3) the relative weight loss in Blue Tits, which only rarely undertake second broods, is higher than in Great Tits in which second broods are more common. Other factors also are related to the probability of undertaking a second brood: more second broods are undertaken by more successful females, adult females and females that lay earlier.
The comparison of Great and Blue Tits suggests that the two species use different reproductive strategies.     

Nesting activity,breeding success and colony size for the Wedge-tailed Shearwater Puffinus pacificus on Heron Island

Abstract The Capricorn Group of islands in Australia's Great Barrier Reef Marine Park sustains one of the world's largest breeding populations of the Wedge-tailed Shearwater Puffinus pacificus. Heron Island, a 13.5 ha coral cay which supports tourist and research station leases as well as a national park, is the third largest nesting island in the group. Sample censuses of breeding burrows were conducted each year between 1985 and 1990 and a further survey was completed in 1993. These returned estimates of between 13 264±1387 and 16 337±1545 active burrows (Y±SE). Burrow densities within each of the habitats monitored showed no significant trends between years, although there were large differences in burrow density between habitats. There were roughly the same number of burrows in the developed (west) and national park (east) halves of the cay. A miniature video camera system (burrowscope), which allowed nesting chambers at the ends of burrows to be inspected, was used in 1989, 1990 and 1993. This demonstrated that around half the burrows were occupied by incubating birds. Variations were found in the distribution of incubating birds between habitats, although this did not remain constant between the years. In the 1993 season, breeding activity was traced from the burrow establishment to fledging stage. Fifty-one per cent of burrows were used for breeding (eggs laid), 77% of eggs hatched and 80% of chicks produced a fledgling. Overall breeding success for the island was estimated at 61%. In 1993 the area designated as Buildings was found to have significantly lower hatching success compared with natural habitats. Most mortality occurred at the egg stage; however, in the Fringe habitat, mortality was highest at the chick stage. Previous surveys have estimated the breeding population from burrow counts. It now appears that only about 30% of such burrows produce fledglings.     

Grain Growth Dynamics in Winter Wheat Crops

A procedure for analysing the dynamics of grain growth is developedand applied to data from winter wheat crops. The algorithm estimatesgrowth rate in relation to local data and preserves a shapethrough the whole growth curve by optimization subject to physiologically-justifiedconstraints: that grain growth rate decreases monotonically,and that there is a discontinuity to zero rate at physiologicalmaturity. It is assumed that effects of temperature can be allowedfor by expressing growth in relation to thermal time, accumulatedtemperature above a base temperature. Conditions are derivedunder which an optimal base temperature for grain growth rateexists under these physiological constraints. When applied todata from crops with different sowing dates and nitrogen treatmentsin different years, weak optimal base temperatures of 2.0 and7.5 °C were obtained for growth rate and duration, respectively.Initial growth rates were similar for all crops, but there wereconsiderable variations in rate later in grain growth. Thesevariations were, in part, related to the overall crop growthrate divided by the number of grains. Grain growth, modelling, winter wheat