Phenological responses to temperature of an annual and a perennial Lesquerella species.

BACKGROUND AND AIMS: The annual Lesquerella fendleri, native to the south-western desert of United States and Mexico, and the perennial L. mendocina, native to Argentina, may have potential as new crops for cold-arid environments. The introduction of a new crop requires an understanding of environmental influences on growth and development, particularly temperature, which has been recognized as the main factor affecting the rate of development in crops. The objective of this study was to examine differences in the phenology of L. fendleri and L. mendocina and in the response to temperature in both vegetative and reproductive phases. METHODS: Plants of each species were grown at a range of constant temperatures under controlled conditions and developmental responses were analysed and quantified. KEY RESULTS: The rate of development of L. fendleri increased linearly with temperature in the phase from emergence (EM) to floral bud appearance (FBA) over the range 9-20 degrees C, and for the phase from FBA to first flower open (FL) over the range 9-24 degrees C. In contrast, the rate of development of L. mendocina was insensitive to temperature during the phase EM to FBA. In the phase FBA to FL, L. mendocina had a lower sensitivity to temperature than L. fendleri. In addition, L. fendleri exhibited a quantitative response to supra-optimal temperatures (reducing rate of development with further increases in temperature) whereas L. mendocina showed a qualitative response, with development ceasing to progress at temperatures above the optimum. CONCLUSIONS: This differential behaviour at high temperatures could explain the biennial habit found for L. mendocina sown during late spring under field conditions, whereas it behaves as an annual when sown in autumn-winter. The possibility is discussed of using this information for establishing the coincidence of critical stages with environmental conditions that can limit potential and actual yield through agronomic practices.

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.

PHOTOSYNTHETIC PATHWAYS OF HESPERALOE FUNIFERA AND H. NOCTURNA (AGAYACEAE): NOVEL SOURCES OF SPECIALTY FIBERS.

Hesperaloe funifera and H. nocturna are currently being studied as potential new sources of fibers for specialty papers. This study investigated canopy architecture and light interception in H. funifera, and gas exchange in both species. H. funifera is an acaulescent rosette species with stiff, upright leaves. Mean leaf angle for 3-year-old plants was 70° from horizontal, and more than 90% of the leaf surface was at angles greater than 50°. Vertical orientation of leaves reduced seasonal variation in light interception and midday light interception during summer months. High leaf angles are interpreted as an adaptation to arid habitats that could reduce this species' suitability for cultivation in more humid areas. Both H. funifera and H. nocturna had leaf-tissue water contents and mesophyll-succulence values intermediate between previously investigated Agavaceae known to be either C3 or Crassulacean acid metabolism (CAM) plants. Both species proved to have CAM, however. Gas exchange characteristics varied with leaf age, with older leaves having higher assimilation rates, greater water-use efficiency, and a higher proportion of nighttime CO2 uptake. Interestingly, these older leaves had mesophyll succulence values closer to those of typical C3 species. These Hesperaloe species can thus be characterized as nonsucculent CAM plants. Both species showed CO2 uptake rates of 5-8 µmol m-2 sec-1 expressed on a total-surface-area basis and 10-18 µmol m-2 sec-1 expressed on a projected-leaf-area basis. Expanded cultivation of species possessing CAM in marginal areas has been recommended recently; the physiological studies reported here along with previous studies of their economic botany identify these Hesperaloe species as good crop candidates for dry regions.