No abstract

Incomplete stomatal closure at night can result in substantial water loss at times when photosynthetic carbon gain is not occurring in C3 and C4 plant species. To investigate the magnitude of nighttime water loss for a crop species in the field, measurements of nighttime water loss by tomato (Lycopersicon esculentum Mill. cv. Heinz 8892) were made by three methods: a field-scale lysimeter and two leaf-level instruments, an automated viscous flow porometer and a portable photosynthesis system. The portable photosynthesis system indicated nighttime transpiration of 10% of maximal daytime transpiration and the viscous flow porometer demonstrated partially open stomata. Integrated crop water loss during the dark, non-photosynthetic hours measured on the lysimeter was 3–10.8% of total daily water loss. In the glasshouse, a survey of closely related wild and cultivated tomato species showed that under ambient conditions nighttime transpiration varied within and among species and was 8–33% of maximal daytime transpiration. Implications of such a substantial fraction of total daily crop water use occurring during the night are significant in agronomic, environmental, and economic terms. Further, variation within and among species in nighttime water loss has implications for breeding to improve crop water use efficiency.

Invasion by exotic annual grasses is one of the most significant threats to arid ecosystems in the western USA. Current theories of invasibility predict plant communities become more susceptible to invasion whenever there is an increase in the amount of unused resources. The objective of this field study was to examine how resource pulses and temporal variation in resource demand by the native shrub vegetation influences establishment of the invasive annual grass Schismus arabicus. Water and nitrogen were applied as pulses in early spring, mid-spring, or continuously throughout the growing season to plots containing either Atriplex confertifolia or Atriplex parryi shrubs. The effect of resource pulses on Schismus density and biomass was highly dependent on the seasonal timing of the resource pulses and the identity of the neighbor shrub. When resource pulses coincided with high rates of resource capture and growth of the native vegetation, density and biomass of Schismus was reduced. Schismus establishment was greater under continuous resource supply compared to pulsed resource supply, likely because more soil resources were available at a shallow depth when resources were supplied at a continuous low rate. These results suggest that the establishment of invasive annual grasses in arid systems can be influenced by the magnitude and spatial distribution of resource pulses in addition to the seasonal timing of resource pulses. r