Growth schedule of Xanthium canadense: Does it optimize the timing of reproduction?

Summary The effects of nutrition on the timing of reproductive initiation of a short-day annual plant Xanthium canadense (cocklebur) were examined with the following hypotheses in mind: If the plant always follows an optimal growth schedule, low-nutrient plants will initiate reproductive growth earlier than high-nutrient plants. On the other hand, if the plant flowers in response to photoperiodic stimuli, both plants will initiate reproductive growth on the same day. The sand-culture experiment showed that high-nutrient plants flowered earlier than the low-nutrient plants, leading to rejection of the first hypothesis. The predicted optimal flowering time is 2 days later than the actual flowering time in high-nutrient plants and 10 days earlier in low-nutrient plants. These deviations from the optimal times reduced the reproductive yield by 0.1% and 2.3%, respectively. The ratio of the final reproductive yield to the vegetative mass at flower initiation was 1.10 in high-nutrient plants and 0.63 in low-nutrient plants. Since the expected ratio for the optimal growth schedule is 1.0, high-nutrient plants followed the opitmal growth schedule more closely than the low-nutrient plants. Cocklebur is a fast-growing annual which is common in relatively nutrient-rich environments. This study suggests that cocklebur adapts itself to such environments through its photoperiodic response.     

Multiple pathways in diaspore dispersal, exemplified by studies of Noogoora Burr (Xanthium occidentale Bertol., Compositae)

LIDDLE, M. J. & ELGAR, M. A., 1984. Multiple pathways in diaspore dispersal, exemplified by studies of Noogoora Burr ( Xanthium occidmtale Bertol., Compositae). Some of the ways in which the burrs are dispersed and some of the factors that may cause changes between the different dispersal pathways were examined. Populations of burrs were marked on the plants in an ungrazed paddock, a paddock grazed by horses, and a paddock grazed by sheep. Eleven percent of the marked populations were recovered from the hair of horses and 26% from the sheeps' wool. A single flood removed 80% of one population. Burrs moved a mean distance of 30 cm on paths used by horses and 18% were buried in the path soil. Burrs were placed in animals' hair by hand. After 16 days, sheep retained 76% of the burrs in their wool, while only 2% remained in the hair of horses and cattle. An experiment showed that burrs may be catapulted up to a distance of 3.3 m from the plants.
A model is presented which outlines the alternative or sequential pathways that may be followed by the burrs. Occasional events, such as flooding and presence or absence of stock, were the main causes of changes between pathways. In the model, these dispersal mechanisms are considered as release-phase and substrate-phase processes. Each process derives its energy from different sources and may be modified by inputs of energy from other sources.     

Nitrate and reduced-N concentrations in the xylem sap of Stellaria media, Xanthium strumarium and six legume species

Abstract At an applied nitrate concentration of 1 mol m^?3, the proportion of xylem sap nitrogen as nitrate was < 15% for Cajanus cajan, Lupinus albus and Trifolium repens, 33% for Pisum sativum and within the range 57–62% for Glycine max, Phaseolus vulgar is, Stellaria media and Xanthium strumarium. At an applied nitrate concentration of 10 mol m～³ the value had increased to 66% for T. repens while at 20 mol m^?3 nitrate values had increased to 46, 51 and 49% for C. cajan, L. albus and Pisum sativum, respectively, and 89% and 85% for 5. media and X. strumarium, respectively. Glycine max and Phaseolus vulgaris differed from the other species in that the proportion of their xylem sap nitrogen as nitrate remained constant (～ 60%) as applied nitrate concentration increased from 1 to 20 mol m^?3. The proportion of total plant nitrate reductase activity in the shoot of C. cajan, S. media and X. strumarium increased as applied nitrate concentration increased from 1 to 20 mol m^?3. Values at the lower and upper concentrations were, respectively, 26 and 72% for C. cajan. 48 and 80% for X. strumarium and 68 and 87% for S. media. The partitioning of nitrate assimilation between root and shoot in these species is discussed.