Phenotypic plasticity in oilseed rape in response to resource availability

Abstract

Over the last 50 years, seed production of oilseed rape (Brassica napus L.), the source of one of the healthiest edible oils for human consumption, has increased globally, reaching 71 million tonnes in 2017. However, food security will be challenged by the growth of the world's population, reaching 9.7 billion people in the next 50 years, increasing the demand for food, vegetable oils for human consumption, feed, and biofuel. Therefore, new strategies are urgently needed to satisfy the growing demand for oilseed rape, and one such strategies revolves around making use of plasticity in oilseed rape. The overall objective of this thesis was to evaluate the plastic response of oilseed rape to environmental factors and agronomic practices by identifying the main drivers and mechanisms underlying plant C and N distribution, with emphasis in the interaction between plant C status with plant architectural traits and physiological traits associated with seed yield. In this study, N fertilizer supply determined N content per unit of leaf mass (Nmass), while it played a subordinate role in altering N per unit of area (Narea), which was the main determinant of C assimilation. The rate of photosynthesis per unit of leaf area was strongly correlated with N content per leaf area (Narea) through an asymptotical relationship.Narea was more closely associated with leaf mass per unit of leaf area (LMA) than with Nmass.LMA was affected by a significant interaction between N supply and plant population density.Such an interaction plays an important role in the discrepancies observed in literature. Leaf area was affected by N supply, with important effects on light distribution and C assimilation at crop level. Indeed, the interaction between N supply and plant population density influences the leaf area index, which determines the proportion of light intercepted by the plant canopy.The role of N supply as determinant of plant source-sink status (which is, in turn, the main determinant of branch outgrowth and silique production) was evaluated by combining experimental and Functional-Structural Plant (FSP) modelling approaches. To evaluate the role of N supply on branch outgrowth, an FSP model was adapted for oilseed rape. Branch and silique outgrowth were implemented as dependent on the plant source/sink ratio status, mainly driven by light interception. Since the numbers of branches and siliques per plant simulated by the FSP modelling approach were similar to the measured numbers, the hypothesized mechanism that the number of siliques is determined by the source/sink ratio, which in itself is affected by the number of branches, was confirmed. Additionally, the hypothetical limits of oilseed rape plants to produce siliques under a simulated branch inhibition were also explored.The results suggest that the main raceme of an oilseed rape plant has the potential to develop a similar number of siliques as a plant with several branches. However, to achieve this full compensation, the model assumed several phenomena that are not very real...