APPLICATION OF A DEMOGRAPHIC CROP GROWTH MODEL: AN EXPLORATIVE STUDY ON THE INFLUENCE OF NITROGEN ON GRAPEVINE PERFORMANCE

Simulated proportional reductions of observed levels (1.0) of soil N predicted decreases of total dry matter assimilation. A stepwise reduction of soil N to zero yielded a sigmoid decrease in total vegetative dry matter down to 30% of the observed value, but fruit mass was reduced only below 0.5 soil N. At a hypothetical soil N content of zero (0.0) 40% of the standard yield was produced, but plant N reserves were completely depleted at the end of the growing season. The proportional allocation of the photoassimilate to the three sinks, maintenance respiration, reproductive growth and vegetative growth, depended on the level of soil N. With increasing N deficiency, higher proportions of assimilate were incorporated into the fruit than at the observed soil N level (1.0). The allocation to vegetative mass reacted conversely, whereas maintenance respiration remained at 35% of the total carbohydrate production at all levels of soil N.

In addition, the model permitted assessment of the influence of soil N on the age structure and N composition of leaves that are food for different phytophagous arthropod pests. Simulations of three levels (1.0, 0.5, 0.1) of soil N showed minor increases of the average leaf age at low soil N, due to reduced production of new leaves in summer. The average N content of leaves (i.e. a measure of food quality for herbivores) decreased to a late season plateau of 2.5% at high soil N compared to 0.5% at low N supply.

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