Root and shoot respiration of perennial ryegrass are supplied by the same substrate pools: assessment by dynamic 13C labeling and compartmental analysis of tracer kinetics

Abstract

The substrate supply system for respiration of the shoot and root of perennial ryegrass (Lolium perenne) was characterized in terms of component pools and the pools' functional properties: size, half-life, and contribution to respiration of the root and shoot. These investigations were performed with perennial ryegrass growing in constant conditions with continuous light. Plants were labeled with (13)CO(2)/(12)CO(2) for periods ranging from 1 to 600 h, followed by measurements of the rates and (13)C/(12)C ratios of CO(2) respired by shoots and roots in the dark. Label appearance in roots was delayed by approximately 1 h relative to shoots; otherwise, the tracer time course was very similar in both organs. Compartmental analysis of respiratory tracer kinetics indicated that, in both organs, three pools supplied 95% of all respired carbon (a very slow pool whose kinetics could not be characterized provided the remaining 5%). The pools' half-lives and relative sizes were also nearly identical in shoot and root (half-life < 15 min, approximately 3 h, and 33 h). An important role of short-term storage in supplying respiration was apparent in both organs: only 43% of respiration was supplied by current photosynthate (fixed carbon transferred directly to centers of respiration via the two fastest pools). The residence time of carbon in the respiratory supply system was practically the same in shoot and root. From this and other evidence, we argue that both organs were supplied by the same pools and that the residence time was controlled by the shoot via current photosynthate and storage deposition/mobilization fluxes.

Figure 1.

Specific respiration rates of shoots (black symbols) and roots (white symbols) of perennial ryegrass, labeled for different time intervals, and of nonlabeled controls (C; at left). Each value is the mean of four to 10 replicate plants (±sd). Dashed lines indicate average values (see “Results”). Note the logarithmic scaling of the x axis.

Figure 2.

Evolution of the fraction of unlabeled carbon (f_unlabeled) in CO₂ respired by shoots (A) and roots (B) of perennial ryegrass during labeling. Each value is the mean of four to six replicate plants (±se). Lines denote model predictions (Fig. 3). Insets expand the first 48 h.

Figure 3.

Three-pool model of the substrate supply system of dark respiration of the shoot of perennial ryegrass. Carbon fixed in photosynthesis enters the respiratory system via pool Q₁, where it is either respired (respiratory flux F₁₀) or transferred to pool Q₂. In Q₂, carbon is either respired directly (F₂₀) or first cycles through Q₃ before being respired via Q₂. Respiratory tracer release from Q₂ is associated with a delay. Functional characteristics of the pools (size, half-life, and contribution to shoot and root respiration; Table I) were estimated by translating the model into a set of differential equations and fitting the model to the tracer kinetics of shoot respiration. The same model also fitted the tracer kinetics of dark respiration of the root but included an additional delay of 0.8 h for tracer release in F₁₀ and F₂₀. Arrows and boxes are scaled to indicate the magnitude of fluxes and pool sizes.

Figure 4.

Sensitivity of the goodness of model fits for the shoot (A–C) and the root (D–F) to departures from optimized values of pool size (A and D), half-life (B and E), and contribution to respiration (C and F). Sensitivity is expressed by the RMSE of the fit (minimum RMSE indicates the optimum value of a model parameter). The solid lines represent Q₁, the dotted lines represent Q₂, and the dashed lines represent Q₃. Note the logarithmic scaling of the x axis for pool size and half-life.

Figure 5.

Time course of the fraction of unlabeled carbon in CO₂, respired by shoots (black symbols) and roots (white symbols) of perennial ryegrass plants during respiration measurements, for plants that were previously labeled for 1 h (triangles) and 24 h (circles). Error bars denote se (n = 4). The dashed line denotes the linear regression for shoots labeled for 1 h (y = 0.83 + 0.48 x, r² = 0.74; see “Materials and Methods”). The regression for the other labeling times was nonsignificant (data not shown).