D.A. Ravetta

Water pulses and biogeochemical cycles in arid and semiarid ecosystems

The episodic nature of water availability in arid and semiarid ecosystems has significant consequences on belowground carbon and nutrient cycling. Pulsed water events directly control belowground processes through soil wet-dry cycles. Rapid soil microbial response to incident moisture availability often results in almost instantaneous C and N mineralization, followed by shifts in C/N of microbially available substrate, and an offset in the balance between nutrient immobilization and mineralization. Nitrogen inputs from biological soil crusts are also highly sensitive to pulsed rain events, and nitrogen losses, particularly gaseous losses due to denitrification and nitrate leaching, are tightly linked to pulses of water availability. The magnitude of the effect of water pulses on carbon and nutrient pools, however, depends on the distribution of resource availability and soil organisms, both of which are strongly affected by the spatial and temporal heterogeneity of vegetation cover, topographic position and soil texture. The ‘inverse texture hypothesis’ for net primary production in water-limited ecosystems suggests that coarse-textured soils have higher NPP than fine-textured soils in very arid zones due to reduced evaporative losses, while NPP is greater in fine-textured soils in higher rainfall ecosystems due to increased water-holding capacity. With respect to belowground processes, fine-textured soils tend to have higher water-holding capacity and labile C and N pools than coarse-textured soils, and often show a much greater flush of N mineralization. The result of the interaction of texture and pulsed rainfall events suggests a corollary hypothesis for nutrient turnover in arid and semiarid ecosystems with a linear increase of N mineralization in coarse-textured soils, but a saturating response for fine-textured soils due to the importance of soil C and N pools. Seasonal distribution of water pulses can lead to the accumulation of mineral N in the dry season, decoupling resource supply and microbial and plant demand, and resulting in increased losses via other pathways and reduction in overall soil nutrient pools. The asynchrony of resource availability, particularly nitrogen versus water due to pulsed water events, may be central to understanding the consequences for ecosystem nutrient retention and long-term effects on carbon and nutrient pools. Finally, global change effects due to changes in the nature and size of pulsed water events and increased asynchrony of water availability and growing season will likely have impacts on biogeochemical cycling in water-limited ecosystems.

Post-damage alkaloid concentration in sweet and bitter lupin varieties and its effect on subsequent herbivory

Abstract:  While most lupin species possess quinolizidine alkaloids, sweet (low alkaloid) varieties are more palatable but at the same time more susceptible to herbivory. Nevertheless, as they are not totally devoid of alkaloids, it may be possible that their alkaloid levels increase after damage. The aim of this study was to compare inductive responses to herbivory in sweet and bitter varieties of Lupinus albus (L.) and Lupinus angustifolius (L.), and to assess if these responses were effective to stop subsequent herbivory. Two experiments were carried out; in the first, Anticarsia gemmatalis (Hübner; Lep., Noctuidae) caterpillars were introduced in field‐growing lupin plants and allowed to feed for 72 h, after which leaves were collected and analysed for alkaloid content and composition. The second experiment was a bioassay, in which leaves collected from experiment 1, from treated and control plants, were offered to another set of Anticarsia caterpillars, and consumption was recorded after 24 h. We found that both L. albus varieties (sweet and bitter) had an increase in their alkaloid concentration after damage, while none of the L. angustifolius varieties had. The sweet L. albus variety, Rumbo, had a greater inductive response than the bitter variety. When leaves were offered to caterpillars (bioassay), this variety showed the greatest difference between consumption of controls and previously eaten leaves, implying that alkaloid levels reached after damage were effective to deter subsequent herbivores as a result of plants probably overcoming a ‘palatability threshold’.

Vegetative propagation of Grindelia chiloensis (Asteraceae).

Abstract Grindelia chiloensis (Asteraceae) is a shrub native to Patagonia, Argentina, in the process of domestication as a source of resin to complement rosin production by pines. Vegetative propagation to multiply selected genotypes has produced breakthroughs in the cultivation of new crops like jojoba (Simmondsia chinesis). The only available way to propagate Grindelia is by transplanting seedlings or direct seeding. Both alternatives rely on selected varieties, a process that takes several years in an open-pollinated, self-incompatible species. The objective of this study was to generate a protocol for the vegetative propagation of G. chiloensis. Two experiments were carried out to evaluate the effect of the mother plant, the source of the cutting (apical or basal), and the concentration of indole butyric acid (IBA) on rooting success and root and above-ground biomass accumulation. A third study was conducted to evaluate the field survival of the propagated plants. Stems (15 cm) were cut from several mother plants growing in the field and stored in plastic containers with ice for about 4 h. Two types of cuttings were prepared from each stem: apical (upper 6–8 nodes) and basal (lower 6–8 nodes). A commercially available IBA source (Hormex, rooting powder, Brooker Chemical, Hollywood, CA) was used at six concentrations (0.1, 0.3, 0.8, 1.6, 3.0, and 4.5%). The cuttings were dipped in water and in the plant regulator and planted into speedlings filled with peat moss, vermiculite and sand (1:1:1, v/v/v). The speedlings were placed under a mist system in a greenhouse at 25°C. Rooting success and root weight were evaluated 30 days after the experiments were started. The position of the cutting, the IBA concentration, and the mother plant affected rooting success. None of the basal cuttings rooted even with IBA treatment. For the apical cuttings, IBA concentrations between 0.1 and 1.6% resulted in more than 64% rooting and the largest root mass (P

Grindelia chiloensis resin and biomass production in its native environment

Preliminary studies have shown that Grindelia chiloensis has higher resin contents than Grindelia camporum which has been previously evaluated as a new resin crop for arid environments. G. chiloensis may provide an additional source of germplasm variability from which to improve resin content and adaptation to cold, arid environments. A number of studies were conducted to evaluate several native populations of G. chiloensis growing in its native environment in southern Argentina. Three native stands of G. chiloensis from northeastern Patagonia were evaluated. All three sites are located around 42 S latitude, within 200 m from the seacoast, and receive an average precipitation below 160 mm/year. Native cover of G. chiloensis ranged between 8% and 11% and was not significantly different among populations. Average individual plant net primary productivity (NPP) was estimated with a single harvest at the end of the growing season (April 1995) and ranged between 92 g and 168 g dry weight (DW). No significant difference in NPP was found among populations. Plant density was significantly different among populations (P < 0.01) and ranged between 2.8 and 0.16 plants/m2. Crude resin (CR) content of the years biomass (stems, leaves, and flowers) was significantly different among populations (P < 0.01); CR content was 23.4%, 17.4%, and 16.7% for the three populations studied. Refined resin (RR, an estimate of resin acids content) was significantly different among populations and ranged between 16.3% and 11.5% for these samples. The ratio RRCR was around 69% and was not different among populations. Capitula and leaves did not differ in resin CR and RR contents, but these were significantly higher than those found in stems. This contrasts with what has been found in G. camporum in which capitula have significantly higher resin contents than leaves. This characteristic will influence the ideotype used in the selection of high productivity germplasm.

Development of alternative Lesquerella species in Patagonia (Argentina): potential of Lesquerella angustifolia

The genus Lesquerella is a promising source of hydroxy fatty acid (HFA). Within this genus, Lesquerella fendleri is a good candidate for domestication in the arid southwestern United States because it has high seed and oil yields, low seed dormancy, and low fruit dehiscence. Other species of Lesquerella, however, could be developed and expanded for other regions of the world with different climatic and environmental conditions. The objective of this study was to test the yield and oil content of several Lesquerella species in the Chubut River Valley, Patagonia, Argentina. The area is characterized by low temperatures during winter and high temperatures and radiation during summer. Two experiments were conducted for two consecutive years. Seed yield and oil content were determined for seven species of Lesquerella. In the first year, both fall and spring sowings were measured for all tested species. During the second year, Lesquerella angustifolia performance was evaluated together with three different accessions of L. fendleri. Spring sowings reduced seed and oil yield about 80% in L. fendleri and L. gordonii (average of the two species, P<0.05) compared with the fall sowings. These results were presumably caused by the longer daylength period during the summer at these latitudes compared with other sites. Yield was higher in L. angustifolia than other species and three times higher than L. fendleri, which was sown at the same time. The higher yield obtained with L. angustifolia was the result of the combination of both high seed weight and seed number per plant. When L. angustifolia was compared with L. fendleri germplasm originated from arid environments and with two accessions of the same species collected from high elevations, the better performance of the former was confirmed. These results clearly demonstrate that L. angustifolia performs better than L. fendleri in the irrigated valleys of Patagonia, and also that L. fendleri yields would be low under environments with very cold winters and high latitudes. Thus, fall plantings of L. angustifolia could be considered as an alternative source of HFA at this site and other similar high-latitude environments.

Guayule rubber and latex content — seasonal variations over time in Argentina

Abstract Rubber from guayule ( Parthenium argentatum Gray, Asteraceae) is essentially equivalent to that from the rubber tree ( Hevea brasiliensis ). Increased incidents of ‘latex allergies’, especially among health care workers, have resulted in a renewed interest in guayule, because it has been shown that individuals sensitive to latex allergies do not react to guayule latex. Little work documenting changes in guayule latex content with plant age, from season to season, or among lines has been reported. To provide some of this information and hence to assist with the commercialization of guayule, a project was initiated to determine rubber and latex changes over time in guayule plants growing in Catamarca, Argentina. Lines G7-14, N-565 and 11591 were sampled every 2 months, whereas lines AZ-5, AZ-3, P2-17, P1-12, P3-11, G10-130 and G1-16 were sampled every 6 months over a 3 year period. Statistically significant differences in plant weight, and in rubber and latex contents, were found among some lines. Plant weights were positively correlated with age. Latex and rubber contents showed little correlation with age. The latex and rubber contents were well correlated (0.73 for all lines). Latex and rubber production among some lines was statistically different. Seasonal effects on latex and rubber percentages were significant only for lines G7-14 and 11591, whereas seasonal effects on latex and rubber production were significant only for lines G7-14 and N-565. The results for the various lines indicates that time of harvest may not be critical to maximize yield. Harvesting the crop throughout the year would require a smaller capacity processing facility than seasonal harvesting and reduce the cost of a processing facility and production.

Tolerance of Grindelia chiloensis (Asteraceae) to pre- and postemergence herbicides

Abstract Several species of Grindelia (Asteraceae) are known to produce diterpene resin acids with potential uses in the naval stores industry. In recent years, the focus has changed from G. camporum to Grindelia chiloensis as the main candidate for domestication, because of its significantly higher resin content. Most agronomic technology developed for G. camporum has been adapted to G. chiloensis , although weed competition continues to be a problem during establishment, resulting in reductions in plant stands. The objective of this work was to evaluate pre- and postemergence herbicides for weed control in irrigated stands of G. chiloensis . Two greenhouse and field experiments were conducted to evaluate the effect of commercially available broad-spectrum herbicides on biomass production and resin content. Only Dual 8E, Karmex 80DF, AAtrex, and Caparol 4L at intermediate to low doses can be used safely as preemergent herbicides. Once the stand has been established, either by direct seeding or by clone transplanting, pendimenthalin, linuron, propaquazifop, haloxifop- R -methyl, or fluazifop- p -butyl, can be used, because, biomass accumulation was not affected by these herbicides. MCPA when applied at the six to ten leaf stage caused severe damage and plant mortality. Resin content ranged between 22 to 25% (dry weight basis) and no differences were present between the treated and control plants.