Significance of evergreen vs. deciduous leaves on whole plant carbon balance of fruit trees:

interpretation through a modelling approach applied to grapevines (Vitis vinifera) and citrus (Citrus sp.) grown in the semi arid northern Chile.

The carbon (C) balance of a whole plant: reduction, assimilation and allocation of C to different plant organs and activities (growth, maintenance and reserve accumulation) is one of the main bases of plant performance, both in natural and agricultural systems. This balance is the result of the “demand” of C by sink organs and activities; and the “offer” of C assimilated by source organs (mainly leaves). The C use efficiency of a plant (CUE) decreases when the C source offer fails to match the demand of the C sink(s) or when C assimilation by the source organs is down-regulated by low sink demand. In either case, the resulting low CUE directly affects the plants annual outcome: total biomass accumulation, total fruit production and availability of C reserves and structure (leaves, roots and fruiting sites) to sustain the following production cycle. We hypothesize that the leaf habit of evergreen (EFT) and deciduous fruit trees (DFT) differently affects the C balance, especially due to different seasonal patterns of source: sink relationships. In the case of fruit trees, sink activity (i.e. C demand) is mainly related to reproductive growth (fruits) which shows a seasonal pattern in EFT and DFT species. The difference resides in the source activity (i.e. C offer by leaves); whereas this activity is seasonal in DFT it tends to be relatively constant along the year in EFT.

The study and comparison of the C balance of two economically important crops for northern Chile differing in their leaf habit: citrus (Citrus sp.; EFT) and grapevines (Vitis vinifera; DFT) through a modelling approach is proposed. The model will consist of different plant compartments: leaves; branches and trunk; roots; and fruits, containing structural and nonstructural C. The fluxes of C will depend on source: sink relationships where the C offer by the sources will be estimated with a photosynthesis model coupled to a light interception and photoinhibition model and the C demand by sink organs will be estimated from growth and respiration models. Light interception will depend on the size of the leaf compartment and it will incorporate a three dimensional architectural model. Source:sink feedback down-regulation of assimilation of C by leaves will also be dealt with. Fruit load and leaf area of the EFT and the DFT species will be manipulated in order to achieve different source:sink relationship treatments on which biometrical, chemical (total C, nitrogen [N], carbohydrates [CHO]), gas exchange and chlorophyll fluorescence measurements will be performed during two subsequent seasons and used to parameterize the different models. During the third season the model will be validated and the interaction of C balance with water and N balance will be explored. The significance of the models parameters will be analyzed and compared in terms of evergreen and deciduous leaf habit.

The objective of this study is to achieve a clear understanding of the different factors affecting C balance in fruit trees with deciduous or evergreen leaves based on the interpretation of the model parameters that best explain these differences. This information will be useful for agriculturists seeking to achieve stable productivity and quality among successive production cycles through thinning and pruning management, and ecologists studying the C cycle of natural and/or intervened ecosystems. Furthermore, the parameters defined in this study may be used to analyze different phenotypes in breeding programs. Because in the particular case of the semiarid regions of northern Chile the added production area of EFT and DFT species is the first consumer of the scarce irrigation water, the results of these study will also contribute to enhance water use efficiency (i.e. units C accumulation / units water evapo-transpired), hereby increasing C sequestration by these production systems.