Effect of respiratory homeostasis on plant growth in cultivars of wheat and rice
Some plants have the ability to maintain similar respiratory rates (measured at the growth temperature), even when grown at different temperatures, a phenomenon referred to as respiratory homeostasis. The underlying mechanisms and ecological importance of this respiratory homeostasis are not understood. In order to understand this, root respiration and plant growth were investigated in two wheat cultivars ( Triticum aestivum L. cv. Stiletto and cv. Patterson) with a high degree of homeostasis, and in one wheat cultivar ( T. aestivum L. cv. Brookton) and one rice cultivar ( Oryza sativa L. cv. Amaroo) with a low degree of homeostasis. The degree of homeostasis (H) is defined as a quantitative value, which occurs between 0 (no acclimation) and 1 (full acclimation). These plants were grown hydroponically at constant 15 or 25 ∞ ∞ ∞ C. A good correlation was observed between the rate of root respiration and the relative growth rates (RGR) of whole plant, shoot or root. The plants with high H showed a tendency to maintain their RGR, irrespective of growth temperature, whereas the plants with low H grown at 15 ∞ C showed lower RGR than those grown at 25 ∞ C. Among several parameters of growth analysis, variation in net assimilation rate per shoot mass (NAR m ) appeared to be responsible for the variation in RGR and rates of root respiration in the four cultivars. The plants with high H maintained their NAR m at low growth temperature, but the plants with low H grown at 15 ∞ C showed lower NAR m than those grown at 25 ∞ C. It is concluded that respiratory homeostasis in roots would help to maintain growth rate at low temperature due to a smaller decrease in net carbon gain at low temperature. Alternatively, growth rate per se may control the demand of respiratory ATP, root respiration rates and sink demands of photosynthesis. The contribution of nitrogen uptake to total respiratory costs was also estimated, and the effects of a nitrogen leak out of the roots and the efficiency of respiration on those costs are discussed.