The relationship between leaf area growth and biomass accumulation in Arabidopsis thaliana

Abstract

Leaf area growth determines the light interception capacity of a crop and is often used as a surrogate for plant growth in high-throughput phenotyping systems. The relationship between leaf area growth and growth in terms of mass will depend on how carbon is partitioned among new leaf area, leaf mass, root mass, reproduction, and respiration. A model of leaf area growth in terms of photosynthetic rate and carbon partitioning to different plant organs was developed and tested with Arabidopsis thaliana L. Heynh. ecotype Columbia (Col-0) and a mutant line, gigantea-2 (gi-2), which develops very large rosettes. Data obtained from growth analysis and gas exchange measurements was used to train a genetic programming algorithm to parameterize and test the above model. The relationship between leaf area and plant biomass was found to be non-linear and variable depending on carbon partitioning. The model output was sensitive to the rate of photosynthesis but more sensitive to the amount of carbon partitioned to growing thicker leaves. The large rosette size of gi-2 relative to that of Col-0 resulted from relatively small differences in partitioning to new leaf area vs. leaf thickness.

Keywords: carbon partitioning; growth; leaf area; leaf thickening; photosynthesis; specific leaf area.

Figure 1

The underlying scheme of C flow represented in the Arabidopsis Leaf Area Growth Model. Processes of C assimilation, consumption, partitioning and accumulation accounted for in the present model is highlighted. During the day, while a portion of assimilated carbon is directly used to support growth and maintenance processes in the plant a significant portion of assimilated C is partitioned to starch, which is later degraded and mobilized to support growth and maintenance processes during the night. The net assimilation rate or NAR is the net amount of assimilated C remaining for plant growth after consumption in maintenance respiration, exudation, and defense processes. Some of the C partitioned to leaf, inflorescence/stem/seeds, and roots is used in growth respiration to produce the energy to transform the remaining C to new biomass. Carbon allocated to leaf growth is partitioned to increase leaf area (s) and to increase leaf thickness (t). The symbols σ, ι, ρ, λ, ^sλ, and ^tλ represent the partition coefficients of the corresponding processes.

Figure 2

Workflow of the parameterization process of the Arabidopsis Leaf Area Growth Model. This schematic diagram illustrates key steps followed during parameterization of the Arabidopsis Leaf Area Growth Model. First, plant growth measurements, a literature survey, and expert opinion were used to generate parameters manually which allowed a reasonable match of modeled data to measured data and generation of parameter constraints (Supplementary Table 1) for the multi-objective optimization. Then the manual parameters were fine-tuned with genetic programming in R. Starting with random values, a number of qualified parameter settings were identified after 200,000 computer iterations of optimization steps based on four sets of objectives, such that the differences between modeled and measurements for each of leaf area and masses of leaf, inflorescence and root were less than or equal to 1. The large number of qualified parameter settings were subjected to a hierarchical clustering algorithm to categorize Col-0 and gi-2 parameters into groups (Supplementary Table 2). From each cluster, the most representative and biologically feasible parameter settings were selected manually followed by further selection based on their best fit to leaf area and leaf mass.

Figure 3

Comparison of modeled and measured leaf and plant growth over time in Col-0 and gi-2. Modeled data generated using the selected parameter settings (Table 2) is compared to measured data for total leaf area (A), leaf dry mass (B), specific leaf area (C), and dry masses of the inflorescence (D), root (E), and the entire plant (F). Modeled data for Col-0 from simulation 1 (solid black lines) and simulation 2 (solid gray lines) and for gi-2 from simulation 1 (dotted lines) and 2 (dashed lines) is given. Simulated data is from 1 to 90 DAS. Measured data for Col-0 (filled circles) and gi-2 (filled squares) was initially taken at 26 DAS for Col-0 and 25 DAS for gi-2 and at 44, 66, 86 DAS for both lines. Measured values represent the mean ± SE and n = 10 plants per line. Measurements of gi-2 which showed a statistically significant difference from Col-0 at α = 0.05 are marked with an asterisk (^*).

Figure 4

Comparison of leaf overlap and petiolar length overtime in Col-0 and gi-2. A comparison between projected to total leaf area ratio and total leaf area for Col-0 (solid line and filled circles) and gi-2 (dashed line and filled squares) (A) and the total length of petioles (B) is shown. In (A), the 1st measurement (lowest leaf area and highest projected to total leaf area ratio) was taken at 26 DAS for Col-0 and 25 DAS for gi-2 and the remaining data at 44, 66, 86 DAS for both lines, and values represent the average of 10 measurements from 10 plants per line. In (B), data was taken at 26 DAS for Col-0 and 25 DAS for gi-2 and values represent the mean ± SE and n = 10 plants per line. Measurements of gi-2 which showed a statistically significant difference from Col-0 at α = 0.05 are marked with an asterisk (^*).

Figure 5

Comparison of leaf thickness in Col-0 and gi-2. A comparison between leaf thickness measured from leaf sections of rosette leaves harvested from 38-day old plants (A) and representative photographs of leaf cross sections (B) for Col-0 and gi-2 is given. In (A) values represent the mean ± SE and n = 3 plants per line.

Figure 6

Comparison of area-based and mass-based relative leaf growth rate over time in Col-0 and gi-2. Modeled data generated using partitioning coefficients in simulation 1 (Table 2) and measured data (also shown expanded in smaller panels) for area-based relative growth rate (RGR_S) or relative increase in leaf area (A) and mass-based relative growth rate (RGR_M) or relative increase in plant mass (B) for Col-0 and gi-2 is provided. Modeled data for Col-0 (open circles) and gi-2 (open squares) is simulated from 5 to 90 DAS. Relative growth rates measured for Col-0 (solid lines and filled circles) were calculated from 26 to 44 DAS, 44 to 66 DAS, and 66 to 86 DAS. Relative growth rates measured for gi-2 (dashed lines and filled squares) was calculated from 25 to 44 DAS, 44 to 66 DAS, and 66 to 86 DAS. For measured data, values represent the mean ± SE and n = 10 plants per line. Measurements of gi-2 which showed a statistically significant difference from Col-0 at α = 0.05 are marked with an asterisk (^*).

Figure 7

Comparison of photosynthesis and respiration overtime in Col-0 and gi-2. Area-based photosynthesis (A), area-based nighttime respiration (B), photosynthesis on a whole plant basis (C), and nighttime respiration on a whole plant basis (D) is shown for Col-0 (solid lines and filled circles) and gi-2 (dashed lines and filled squares). The 1st measurement was taken at 26 DAS for Col-0 and 25 DAS for gi-2 followed by 44, 66, 86 DAS for both lines. Values represent the mean ± SE and n = 10 plants per line. Measurements of gi-2 which showed a statistically significant difference from Col-0 at α = 0.05 are marked with an asterisk (^*).

Figure 8

The density distribution of learned parameters from the Arabidopsis Leaf Area Growth Model. Density distribution of all 16 parameters namely, partitioning coefficients of C partitioning to inflorescence, root, leaf thickening and leaf area growth for four growth phases: germination phase, early and late vegetative phases, and reproductive phase as learned by the Arabidopsis Leaf Area Growth Model is given. The 84 identified qualified parameter settings for Col-0 and the 95 identified parameter settings for gi-2 used to determine the densities are given in Supplementary Table 2.

Figure 9

Modeled changes in C partitioning to respiration and growth processes overtime in Arabidopsis. The amounts of C partitioned to drive maintenance and growth respiration, exudation, inflorescence, root and leaf area growth and leaf thickening is presented as percentages of the daily available C at 26, 44, 66, and 86 DAS, based on the outputs of the Arabidopsis Leaf Area Growth Model using learned parameters from simulation 1 and 2 for Col-0 and gi-2 given in Table 2.

Figure 10

The relationship between leaf area growth and plant growth over time. Modeled data generated using partitioning coefficients in simulation 1 (Table 2) and measured data were used to determine the response of plant dry mass to variations in projected leaf area (A), total leaf area (B), and leaf mass (C). Modeled data for Col-0 (solid lines) and gi-2 (dotted lines) represent 1–90 DAS. Measured data for Col-0 (solid lines and filled circles) and gi-2 (dashed lines and filled squares) was initially taken on 26 DAS for Col-0 and 25 DAS for gi-2 followed by 44, 66, 86 DAS for both lines. Measured values represent the average of 10 measurements from 10 plants per line.

Figure 11

The relationship between area-based relative growth rate and-mass based relative growth rate over time. Modeled data generated using partitioning coefficients in simulation 1 (Table 2) and measured data were used to determine the relationship between plant mass and specific leaf area (A), and to plot the relationship between mass-based relative growth rate and area-based relative growth rate (B). In (A) modeled data for Col-0 (solid line) and gi-2 (dotted line) represent 5–90 DAS and measured data for Col-0 (solid line and filled circles) and gi-2 (dashed line and filled squares) was initially taken on 26 DAS for Col-0 and 25 DAS for gi-2 followed by 44, 66, 86 DAS for both lines. Measured values represent the average of 10 measurements from 10 plants per line. In (B) modeled data for Col-0 (open circles) and gi-2 (open squares) represent 21–90 DAS.