Influence of temperature and soil drying on respiration of individual roots in citrus: integrating greenhouse observations into a predictive model for the field

In citrus, the majority of fine roots are distributed near the soil surface - a region where conditions are frequently dry and temperatures fluctuate considerably. To develop a better understanding of the relationship between changes in soil conditions and a plant's below-ground respiratory costs, t he effects of temperature and soil drying on citrus root respiration were q uantified in controlled greenhouse experiments. Chambers designed for measu ring the respiration of individual roots were used. Under moist soil condit ions, root respiration in citrus increased exponentially with changes in so il temperature (Q(10) = 1.8-2.0), provided that the changes in temperature were short-term. However, when temperatures were held constant, root respir ation did not increase exponentially with increasing temperatures. Instead, the roots acclimated to controlled temperatures above 23 degreesC, thereby reducing their metabolism in warmer soils. Under drying soil conditions, r oot respiration decreased gradually beginning at 6% soil water content and reached a minimum at <2% soil water content in sandy soil. A model was cons tructed from greenhouse data to predict diurnal patterns of fine root respi ration based on temperature and soil water content. The model was then vali dated in the field using data obtained by CO2 trapping on root systems of m ature citrus trees. The trees were grown at a site where the soil temperatu re and water content were manipulated. Respiration predicted by the model w as in general agreement with observed rates, which indicates the model may be used to estimate entire root system respiration for citrus.