WOFOST_GT2 introduction

A source of possible inconsistencies in the way SUCROStype models reproduce the underlying system is represented by their peculiar representation of the canopy structure. They divide the canopy in a fixed number of layers, for which instantaneous gross assimilation rates are calculated. This number appears to be arbitrary, e.g., it is three for WOFOST (van Keulen and Wolf, 1986) and five for SUCROS (van Keulen et al., 1982), apparently with no justification in both cases. Moreover, this number is maintained constant from emergence to maturity, ignoring differences in canopy structure occurring during crop cycle. One of the most critical point in this representation is that the division of the canopy in different layers is explicitly considered only for some processes, e.g., gross photosynthesis. On the contrary, other processes, e.g., leaves death, do not take into account the position of leaf area index (LAI) units within the canopy, neither for ageing nor for selfshading. "Dead LAI” (representing leaves no more photosynthetically active) is thus evenly allocated along the canopy profile (i.e., to all the layers). This representation leads to situations where the last emitted LAI units, representing the youngest leaves, die exactly like the oldest ones, and where portions of dead leaves shade green ones.

The methodology adopted to improve the representation of senescence dynamics within the canopy was based on the use of phenological development (via DVS) for deriving indirect information on the main variables involved, i.e., LAI and plant height. DVS identifies critical phases closely related to (i) photosynthetic area evolution, e.g., tillering (0.3 < DVS < 0.6) and flag leaf emission (DVS = 0.9), and (ii) stem elongation (0.6 < DVS < 0.9). In the same way, DVS e via its role in modulating the patterns of assimilates partitioning to the different plant organs e is indirectly related to plant height, since this variable is strictly related to the fraction of photosynthates daily allocated to stems (Confalonieri et al., 2011).

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