Simulation of Individual Leaf Size and Canopy Development: Approaches to Carbon Allocation and Growth Strategies. Dennis Timlin 1, David Fleisher 1, Soo-Hyung Kim 2, Yang Yang 3, and Vangimalla Reddy 1 1 USDA-ARS Crop Systems and Global Change Laboratory, Beltsville, MD, 2 Univ. of Washington, 3 Univ. of Maryland, Wye Research Center

Why Focus on Growth of Individual Leaves? Long term growth responses to N are mainly a function of increased leaf area and light interception. Advantages include better simulation of: Assimilate partitioning, Light interception, and Nitrogen response and dynamics CO2 response

Other Endogenous Factors That Need to Be Accounted for in a Model CO 2 as it affects: carbon assimilation rate carbon partitioning N content in the plant Temperature

Sink Strength Definition of sink strength (carbon) from Chamont, 1993 The ability of a sink organ to import assimilates for its growth, development and maintenance. It is a product of sink capacity (growth not limited by sink supply) and sink activity

The correlation between carbon fixation (net)and (A) carbon export (Δ) or carbon accumulation (or depletion) in leaves (O) Ho, 1976 Note linear increase of export with assimilation Export is about % of net assimilation Mobilization of carbon resources

Carbon Fixation and Carbon Export These data were measured during the light period. Does export continue during the night? (maybe at the low rate) Is there water movement in the plant to carry the fluxes? Does continuing export at night result in export the same as assimilation?

Approach to Leaf Modeling Recently, Fleisher and Timlin (2006) presented a method to simulate growth of individual leaves in a potato canopy as a function of temperature and carbon availability.

Objectives The objective of this study was to explore strategies for carbon allocation during early canopy expansion Look at different approaches for the optimization of carbon usage.

Vertical Distribution of Leaf Area

Expansion Rate vs Leaf # and N from Vos and Biemond, 1992

Mature Leaf Area vs Leaf Number, Vos and Biemond, 1992

Leaf Expansion Rate and Final Leaf Size The slope of this relationship is in units of days and can be understood as the time it takes to reach full leaf size (depends on how the rate is determined). Constant slope implies a constant time to reach full leaf size independent of N rate.

Simple Crop Model Put together from simple components to test different strategies for allocating carbon to new leaves in potato. Leaf addition as a function of temperature Leaf growth using a model developed by us Hourly time step

Simple Crop Model Leaf level photosynthesis based on the model developed by van Caemmerer, Berry and Farquhar Photosynthesis scaled to canopy level by calculating the sunlit and shaded photosynthesis component of each leaf individually. A constant Specific Leaf Area was used.

Simple Crop Model The tuber provided about 4 gr of Carbon per plant. Carbon was partitioned to root but not to tuber.

Carbon Allocation Tested different approaches to carbon allocation Equal increments of carbon assigned to each leaf Leaf with most sunlit area assigned highest priority for carbon Leaf with highest demand gets priority for carbon

Calculation of Growth Duration and Maximum Growth Rate. A is the area of a single leaf (cm 2 ), A 0 is initial leaf area (0.05 cm 2 ), A f is final leaf area, D is decay in specific leaf expansion rate (day -1 ) and DAA is the days after appearance of the leaf (day). A Gompertz type equation was used to smooth measured leaf expansion data.

Leaf Area as a Function of Time and CO 2

Derived Calculations The first derivative of this equation was used to describe the daily rate of canopy growth (RD i ) at day i. Growth duration (the time it takes the leaf to reach 95% of it’s final size) Average growth rate over that period

Potential Leaf Size Calculation Potential Leaf size at time i (L i ) was calculated as: f(T) and f(N) are functions to adjust for temperature and nitrogen effects Sink demand for carbon is a function of temperature.

Temperature Function from Fleisher and Timlin, 2006

Relative (L Ri ) growth rate was calculated as:

Simulated Leaf Area

Simulated Leaf Area Along Stem Surplus carbon was 68g for the low temperature and 2 g for the high temperature plant

Leaf Area Distribution at Three Times

Measured Leaf Area on Stem Younger tubers 10 month old tubers

Measured Total Leaf Area

Other Considerations Sink supply is via the conductive organs of the plant and hence dependent on water flux in the plant. Here is where water stress can affect the ability to convey sugars to growing tissues.

Other Considerations The carbon content of new leaves is about 38 to 39%, for older leaves it drops to about 31%. This provides a means to quantify carbon export as a leaf senesces.

Conclusions Carbon allocation where priority is given to leaves with the greatest demand produces a leaf area distribution to the distribution measured on a potato plant. At low temperatures there is a carbon surplus that can be used for additional stem or tuber growth. It is still not clear how we can allocate carbon to new stems.

Future Work Nitrogen is key to this process and is often found to be related to light interception. Do plants optimize leaf area index and leaf distribution based on availability of resources and light?

Limitations These results were achieved by using mean expansion data from several mainstem leaves. We need to see if this can be extended to single leaves, and to apical and branch leaves. Carbon limitations also affect leaf expansion rates, especially for higher order leaves in potato. This also will be addressed in the model.