Charles R. Tischler

Woody invasion of grasslands: evidence that CO2 enrichment indirectly promotes establishment of Prosopis glandulosa

Grasslands worldwide have been invaded by woody species during the last200 years. Atmospheric CO2 enrichment may indirectly havefacilitatedinvasion by reducing soil water depletion by grasses. We used a two-stepcorrelative approach to test this hypothesis with the invasive and native shrubhoney mesquite (Prosopis glandulosa Torr. var.glandulosa). 1) Water content to 0.15 m depthwas measured in grassland exposed to a CO2 gradient from 200 to 550μmol/mol to evaluate the prediction that CO2enrichment lessens soil water depletion by grasses. 2) Soil water content andemergence and survival of mesquite seedlings were measured in adjacentgrasslandplots from which grass roots were excluded to 0.15 m depth toreduce water depletion or that were irrigated to increase soil water levels.With these measurements, we tested the hypothesis that mesquite establishmentislimited by water.Excluding grass roots doubled emergence of mesquite and almost tripledthe fraction of emergent seedlings that survived for 12 weeks following thefirst of two plantings. Seedlings were taller, heavier, and had greater leafarea when grown without grass roots. Root exclusion did not measurably affectsoil water during the 3-week period of seedling emergence, but soilwatercontent over the 12 weeks that seedling survival was studied was higher inplotsfrom which grass roots were excluded and following an April than May planting.Survivorship of mesquite seedlings correlated positively with soil watercontent. Percentage survival of seedlings increased from 1.5% to15% and 28% at the soil water content measured in grasslandexposed to CO2 concentrations of 270 (preindustrial), 360 (current),and 550 μmol/mol (future), respectively. We infer thatrecent and projected increases in atmospheric CO2 concentration maybe large enough to increase establishment of invading mesquite seedlings ingrasslands that are severely water-limited.

Links between Transpiration and Plant Nitrogen: Variation with Atmospheric CO2 Concentration and Nitrogen Availability

Transpiration is closely linked to plant nitrogen (N) content, indicating that global or other changes that alter plant N accumulation or the relative requirements of plants for water and N will affect transpiration. We studied effects of N availability and atmospheric CO2 concentration, two components of global biogeochemistry that are changing, on relationships between whole‐plant transpiration and N in two perennial C3 species, Pseudoroegneria spicata (a tussock grass) and Gutierrezia microcephala (a half‐shrub). Two indices of plant N requirement were used: N accretion (N in live and dead tissues) and N loss in litter (N in dead tissues). Transpiration was analyzed as the product of N accretion or loss by plants and the ratio of transpiration to N accretion or loss. The two indices of plant N requirement led to different conclusions as to the effects of N availability on plant use of water relative to N. Transpiration scaled proportionally with N accretion, but transpiration per unit of N loss declined at high N. Carbon dioxide enrichment had little effect on the ratio of transpiration to N accretion and no effect on transpiration per unit of N loss. The two species accumulated similar amounts of N, but the half‐shrub used more than twice as much water as the grass. Nitrogen availability and CO2 concentration influenced whole‐plant transpiration more by changing plant N accumulation than by altering the stoichiometry between transpiration and plant N. Species differences in total water use, by contrast, reflected differences in the scaling of transpiration to plant N. A better understanding of species differences in water and N dynamics may thus be required to predict transpiration reliably.

Evaluating different bahiagrass cytotypes for heat tolerance and leaf epicuticular wax content

SummaryBahiagrass (Paspalum notatum Flugge) is a polymorphic species indigenous to South America which has become naturalized in the southeastern U.S. The most common form in the United States is Pensacola bahiagrass,P. notatum var.saurae Parodi., which is a valuable forage. Pensacola is a sexual diploid, while most other bahiagrasses are apomictic tetraploids. Pensacola bahiagrass is considered to have greater heat tolerance (based on anin vitro heat tolerance test) than a number of otherPaspalum species, but has less leaf epicuticular wax (a drought avoidance mechanism) than other species. Both heat tolerance and leaf epicuticular wax are desirable characteristics for species grown where periodic drought occurs. We measured both characteristics over two years in a collection of 23 bahiagrass accessions, many of which had been collected in South America near the center of origin of the species. The collection included various ploidy levels. For both years, no accessions ranked statistically lower in damage in the heat tolerance test than Pensacola, although eight had significantly higher damage. Two entries in addition to Pensacola had very low damage in the heat tolerance test. Pensacola was high in leaf wax, with 16 accessions rated significantly lower in wax. The accession having the lowest wax content was a triploid, which also exhibited considerable leaf death in the field in response to drought and heat. The diploid entries tended to be higher in leaf wax than the other ploidy levels. This study has identified additional bahiagrass germplasm which may be of use in a breeding program.

Elevated Atmospheric CO2 Magnifies Intra-specific Variation in Seedling Growth of Honey Mesquite: An Assessment of Relative Growth Rates

Abstract The shrub honey mesquite (Prosopis glandulosa var. glandulosa Torr.) readily encroaches into rangelands in the southwestern United States that vary in annual rainfall by almost a factor of 5 (200 to 1 000 mm). This occurs partly because mesquite seedlings grow rapidly and become uncoupled from competition with established herbaceous vegetation. Species that occupy such a wide precipitation gradient frequently include plants that differ genetically in seedling growth rate. Whether atmospheric carbon dioxide (CO2) enrichment affects seedling biomass uniformly across mesquite genotypes or magnifies the expression of genetic variation in seedling growth remains unresolved. We measured the effects of CO2 enrichment on growth of seedlings derived from 14 adult plants (open-pollinated families), 7 plants each from rangelands located near extremes of the rainfall gradient occupied by the shrub (arid southeastern New Mexico vs. mesic central Texas). Growth was measured over days 10 to 30 following emergence on well-watered seedlings in glasshouses at ambient and elevated CO2 concentrations (391 and 706 μmol·mol−1, respectively). Proportional responses of biomass (day 30) to CO2 enrichment varied from 1.03 to 1.74 among families. CO2 enrichment did not consistently favor the largest or fastest-growing families at ambient CO2. Rather, proportional responses of biomass to elevated CO2 were highly correlated across families with the stimulation of relative growth rate (RGR) at elevated CO2. Biomass at ambient CO2 was 19% greater, on average, in families from mesic rangeland than from arid rangeland, but families from extremes of the precipitation gradient did not diverge by seedling size or response to CO2. Selection for greater RGR could augment the mean growth response of mesquite seedlings to CO2. Even in the absence of selection, CO2 enrichment could increase mesquite establishment by enhancing seedling growth and thereby exacerbate the management challenge of minimizing woody encroachment.

Evaluation of Paspalum germplasm for variation in leaf wax and heat tolerance

SummaryThe genus Paspalum contains a number of important forage grasses, including P. dilatatum Poir. (dallisgrass) and P. notatum Flugge (bahiagrass). Even though many Paspalum species are adapted to moist, humid areas, they frequently are subjected to extended periods of drought stress which reduces forage productivity. This study was initiated to determine the range of variation for different indicators of drought tolerance in several Paspalum species. Fifteen species representing a broad cross section of germplasm were grown in a replicated field nursery and sampled for leaf epicuticular wax content on four dates over three years. Wax was extracted with chloroform and quantified by the dichromate oxidation technique. Heat tolerance (solute leakage technique) was also determined by calculating the ratio of the conductivity of the external bathing solution of leaves following a heat treatment to the conductivity of the bathing solution after leaves were boiled. Wax values of the entries ranged from 5.7 to 0.88 mg/dm2. Both relative rankings and absolute wax values were very consistent across all sampling dates. The accessions of Paspalum nicorae Parodi and P. quandrifarum Lam. were consistently high in wax content, while the accessions of P. commune Lillo and P. unispicatum (Scribn. et Merr.) Nash were consistently low. The heat tolerance testing demonstrated that the accession of P. notatum var. saurae Parodi was the most heat tolerant of the entries. Results from this investigation demonstrate the wide range of diversity that exists for both epicuticular wax and heat tolerance within the genus Paspalum.

Regeneration and somaclonal variation in apomictic Paspalum dilatatum Poir

SummaryIn an attempt to incorporate variation into a uniform obligate apomict, plants of apomictic common dallisgrass, Paspalum dilatatum Poir., were regenerated from callus derived from immature inflorescences. Plants developed through both organogenesis and embryogenesis. A total of 682 regenerants were produced and more than 400 were transplanted into a field nursery and screened for somaclonal variation. Eventually 20 regenerants were selected, increased, and planted into a replicated nursery along with normal common dallisgrass. The characteristics examined were maturity date, plant height, number of racemes per inflorescence, number of spikelets per raceme, pubescence, stigma and anther color, ergot resistance, seed germination, seed set, pollen stainability, method of reproduction, and chromosome number. There were differences among the regenerants and between them and common dallisgrass for all traits except chromosome number, stigma and anther color, and ergot resistance. One of the more important regenerants had significantly higher seed set than common dallisgrass. All regenerants reproduced by aposporous apomixis but some exhibited a high degree of abortion while others had more aposporous embryo sacs per ovule than common dallisgrass. These findings demonstrate that common dallisgrass can be regenerated through tissue culture and that somaclonal variation is expressed in some of the regenerants, even though some of the altered traits are deleterious.

SEEDLING RESPONSE TO ELEVATED CO2 IN FIVE EPIGEAL SPECIES

Evidence from numerous sources indicates that atmospheric CO2 concentration has increased over the last 200 yr and will continue to increase in the foreseeable future. Much research is directed toward predicting the effects of this change on terrestrial vegetation. We quantified the effects of elevated CO2 (700 &mgr;L L−1) on the seedling growth characteristics of five C3 species. These species all had epigeal germination morphology but differed by a factor of 20 in seed mass. The species and their respective seed‐mass ranges were as follows: bagpod sesbania (Sesbania vesicaria), 0.23–0.26 g; cotton (Gossypium hirsutum var. Coker 317), 0.12–0.13 g; mesquite (Prosopis glandulosa), 0.041–0.048 g; cucumber (Cucumis sativus L. cv. Straight‐8), 0.026–0.030 g; and hemp sesbania (Sesbania exaltata), 0.012–0.013 g. We measured increased total biomass in the elevated CO2 environment for all species within 3 d of emergence. This rapid response contrasts with published reports that more time was required for a detectable response of seedlings to elevated CO2. Growth analyses indicated that no single growth parameter predicted the responses of these species to elevated CO2. However, maximum area of cotyledons was a better predictor of total biomass (at day 16) than was initial (day 0) seedling mass. Our data indicate that future increases in atmospheric CO2 will greatly improve the competitive ability of C3 epigeal species during the earliest growth stage in which cotyledons are still expanding.