Tristyly in Pontederia cordata (Pontederiaceae)

Pontederia cordata L. (Pontederiaceae) is a perennial, entomophilous, emergent aquatic and possesses a tristylous breeding system. A survey of 74 North American populations indicates that populations usually contain the three floral morphs. A strong pollen trimorphism is associated with differences in stamen and style length among the morphs. Pollen size is proportional to stamen height, while pollen production is inversely proportional. There are two size classes of anthers in P. cordata. Long-level anthers of mid- and short-styled plants and mid-level anthers of the short-styled morph are significantly larger than short-level anthers of long- and mid-styled plants and the mid-level anthers of the long-styled morph. The larger mid-level anthers of the short-styled morph produce nearly twice as many pollen grains as the equivalent level in the long-styled morph. It is suggested that this difference may be associated with varying patterns of stamen insertion and development in the floral morphs.Comparisons of various reproductive parameters among the floral morphs in natural populations suggest that there are no significant differences in flowering patterns, inflorescence and flower production, seed production, and germination levels. In addition, bumble bees show no apparent preferences among the floral morphs during foraging activity. Tristyly appears to be a relatively stable breeding system in P. cordata in comparison with tristylous species of the related Eichhornia.

Colonizing ability in the Echinochloa crus-galli complex (barnyard grass). I. Variation in life history

A comparison of life history traits in four taxa of the Echinochloa crus-galli complex (barnyard grass) which differ in colonizing ability and weediness was made under various environmental conditions. The taxa were the alien var. crus-galli, a cosmopolitan weed; var. oryzicola, a crop-mimic restricted to rice fields; var. frumentacea, a crop domesticate; and E. muricata, a native of wetland habitats. Populations studied were from the Central Valley of California where the four taxa are sympatric but ecologically differentiated. All comparisons were made under uniform glasshouse conditions to isolate the genetic component of life history variation. Measurements of the patterns of dry weight allocation, time to flowering, reproductive effort, and seed production were made on individuals grown during different periods of the year under "stress" and "nonstress" conditions utilizing randomized multi-harvest designs.Developmental plasticity in allocation patterns and reproductive phenology occurred in all taxa in response to seasonality and nutrient stress although there were significant differences among taxa in the form of the response. Individuals germinating in August yielded less total biomass and allocated a smaller proportion to roots and a larger proportion to secondary tillers and seed than individuals germinating in April. In all taxa, except E. crus-galli var. frumentacea, a delay in flowering under long days resulted in larger vegetative biomass, lower reproductive effort, and where nutrients were limiting, inhibition of secondary tillers. Nutrient stress resulted in a delay in flowering, increased senescence rates, and a reduction in total biomass and reproductive effort. Although each taxon displayed a wide range of tactics, certain differences in life history strategy among the taxa were maintained. In all regimes E. crus-galli var. crus-galli flowered earlier, and exhibited a greater seed production and reproductive effort than var. oryzicola. In general, E. crus-galli var. frumentacea and E. muricata were intermediate in behaviour.Interpopulation variability in the life history traits of E. crus-galli var. crus-galli and E. muricata was measured in a single-harvest, completely randomized design using 10 populations of each taxon. Significant interpopulation variation was recorded within taxa in tiller height and number, aboveground vegetative biomass, time to anthesis, reproductive biomass, harvest index, seed production, and seed weight. Averaged over 10 populations, E. crus-galli var. crus-galli was taller during vegetative growth, flowered more rapidly, allocated a greater proportion of aboveground biomass to reproduction, and produced a greater number of seeds than E. muricata.Variation in life history parameters among barnyard grass taxa may explain differences in colonizing potential. In particular, the failure of E. muricata and E. crus-galli var. oryzicola to colonize open, seasonally moist sites in California where E. crus-galli var. crus-galli flourishes, may be due to their inability to reach reproductive maturity before the onset of summer drought.