Respiratory Q10 of Lettuce Increases with Increasing Plant Size

Literature reports on the Q10 for respiration vary widely, both within and among species. Plant size and metabolic activity may be responsible for some of this variation. To test this, respiration of whole lettuce plants was measured at temperatures ranging from 6 to 31 °C during a 24-h period. Subsequently, plant growth rate (in moles of carbon per day) was determined by measuring the CO2 exchange rate of the same plants during a 24-h period. Environmental conditions during this 24-h period resembled those that the plants were exposed to in the greenhouse. The measured growth rate was then used to estimate the relative growth rate (RGR) of the plants. The respiratory Q10 ranged from 1.4 for small plants to 1.75 for large plants. The increase in Q10 with increasing plant size was highly significant, as was the decrease in Q10 with increasing RGR. However, growth rate had little or no effect on the respiratory Q10. One possible explanation for these findings is that the Q10 depends on the ratio of growth to maintenance respiration (which is directly related to RGR). The growth respiration coefficient generally is considered to be temperature-insensitive, while the maintenance respiration coefficient normally increases with increasing temperature. Based on this concept, the Q10 for the maintenance respiration coefficient can be estimated as the estimated Q10 at a RGR of zero (i.e. no growth and thus no growth respiration), which was 1.65 in this experiment. Although the concept of dividing respiration into growth and maintenance fractions remains controversial, it is useful for explaining changes in respiratory Q10 during plant development.

Postanthesis biomass accumulation and respiration in shaded and unshaded wheat (Triticum aestivum L.)

Respiration significantly influences the carbon balance of a crop. In wheat (Triticum aestivum L.), biomass equivalent to between 40 and 75% final grain mass can be lost through shoot respiration during grain fill. This study examines the relationship between changes in biomass and respiration of the aboveground plant parts of shaded and unshaded wheat during grain fill. Two spring wheat cultivars, Max and Katepwa, were grown indoors with and without shade, and various biomass components and aboveground CO2 efflux rates were determined from anthesis to maturity. Maximum leaf biomass in Max was attained prior to anthesis while in unshaded Katepwa plants leaf biomass increased up to 35 d after anthesis. The stem changed from functioning as a source and became a sink 28 and 35 d after anthesis in the control plants of Max and Katepwa, respectively. The effect of shading on spike growth became apparent two weeks after anthesis. The CO2 efflux rate for unshaded Max and Katepwa plants declined significantly from 279 to 122 and from 210 to 141 mg CO2 plant−1 d−1, respectively, from anthesis through to maturity. Imposition of shade resulted in significantly lower CO2 efflux rates compared to the unshaded plants. Shade, however, exerted no influence on the estimated maintenance respiration coefficient (m) of a two component respiration model, although this coefficient declined 88% in Max throughout grain fill and declined up to 14 d after anthesis and remained stable thereafter in Katepwa. It was concluded, therefore, that shading affects total respiration through its impact on growth, but exerts no direct effect on the basic pattern of change in maintenance respiration during grainfill. Key words: Biomass accumulation, respiration rate, maintenance respiration coefficient, Triticum aestivum L.

Low levels of ozone increase bean leaf maintenance respiration

Pinto bean (Phaseolus vulgaris) plants were exposed to charcoal-filtered air with or without added low levels of ozone (90 nL∙L−1). Dark respiration (CO2 efflux) by expanding primary leaves of the plants was measured and mathematically partitioned into growth and maintenance components. The growth respiration coefficient was unaffected by ozone, whereas the maintenance respiration coefficient increased 15%. Such a relative increase in maintenance respiration results in a diversion of energy and metabolic intermediates from growth processes.