Water Costs of Gas Exchange by a Speckled Cockroach and a Darkling Beetle

Respiratory water loss during metabolic gas exchange is an unavoidable cost of living for terrestrial insects. It has been suggested to depend on several factors, such as the mode of gas exchange (convective vs. diffusive), species habitat (aridity), body size and measurement conditions (temperature). We measured this cost in terms of respiratory water loss relative to metabolic rate (respiratory water cost of gas exchange; RWL/V˙CO2) for adults of two insect species, the speckled cockroach (Nauphoeta cinerea) and the darkling beetle (Zophobas morio), which are similar in their mode of gas exchange (dominantly convective), habitat (mesic), body size and measurement conditions, by measuring gas exchange patterns using flow-through respirometry. The speckled cockroaches showed both continuous and discontinuous gas exchange patterns, which had significantly a different metabolic rate and respiratory water loss but the same respiratory water cost of gas exchange. The darkling beetles showed continuous gas exchange pattern only, and their metabolic rate, respiratory water loss and respiratory cost of gas exchange were equivalent to those cockroaches using continuous gas exchange. This outcome from our study highlights that the respiratory water cost of gas exchange is similar between species, regardless of gas exchange pattern used, when the confounding factors affecting this cost are controlled. However, the total evaporative water cost of gas exchange is much higher than the respiratory cost because cuticular water loss contributes considerably more to the overall evaporative water loss than respiratory water. We suggest that the total water cost of gas exchange is likely to be a more useful index of environmental adaptation (e.g., aridity) than just the respiratory water cost.

Respiratory cost of leaf growth and maintenance in white oak saplings exposed to atmospheric CO2 enrichment

Atmospheric CO2 enrichment reportedly reduces respiration of mature leaves in a number of woody and herbaceous perennials. It has yet to be determined, however, whether these reductions reflect changes in maintenance respiration alone or whether CO2 might affect growth respiration as well. This possibility was examined in white oak (Quercusalba L.) seedlings that had been planted directly into the ground within open-top chambers and exposed to ambient, ambient +150 μL•L−1, and ambient +300 μL•L−1 CO2 concentrations over a 3-year period. In the spring of 1992, respiration rates were measured repeatedly during leaf expansion, and the growth and maintenance coefficients were determined using a two-component model. Specific respiration rates (mg CO2•g−1•h−1) were consistently lower for leaves of CO2-enriched saplings than for leaves of ambient-grown saplings. Partitioning these reductions in leaf respiration to either the growth or maintenance coefficients indicated a strong effect of CO2 on both components. The growth coefficient for leaves exposed to the ambient CO2 treatment was 964 mg CO2•g−1 compared with 849 and 664 mg CO2•g−1•for leaves from the two elevated CO2 concentrations, respectively. The maintenance coefficient was similarly reduced from a control rate of 114 mg CO2•g−1•d−1 to below 65 mg CO2•g−1•d−1 for leaves exposed to CO2 enrichment. Our results quantitatively describe the magnitude by which growth and maintenance respiration are affected by CO2 enrichment and as such should provide useful information for the future modeling of this phenomenon.

Estimating the true cost of dinitrogen fixation by nodulated plants in undisturbed conditions

Either symbiotic or nonsymbiotic N2 fixation in association with higher plants requires an appreciable amount of energy. This is mostly supplied by current photosynthesis, but an exact estimation of the true cost of the process is rather difficult. Simultaneous exposure of the association to 14CO2 and 15N2 for 1 day, analysis of labelled CO2 produced by soil respiration, and determination of N2 fixation make it possible to quantify the true respiratory cost associated with fixation. Preliminary trials with soybean suggest an amount of 2.5–7 mg C/mg of N2 fixed.