Alexandra Jullien

Pre-harvest growth and development, measured as accumulated degree days, determine the post-harvest green life of banana fruit

Summary We aimed to define a more robust indicator for banana harvest date that ensures an optimal fruit green life (GL). Our hypothesis was that development rather than growth would account for GL more accurately. To this end, five indicators were compared: one related to fruit size (i.e., growth, expressed as the diameter of fruit); two related to fruit age [i.e., development, expressed as the age of fruit measured in the number of days or in degree days (°Cd) from inflorescence emergence]; and two related to metabolism during maturation (i.e., the concentrations of malate and citrate in the pulp). Different treatments (e.g., fruit removal, leaf shading, bunch bagging, defoliation, water deficit, and flooding) were applied to modify the fruit growth rate. On different dates between the emergence of the inflorescence and harvest, fruit GL and the five indicators were measured. The results showed that there was a decreasing exponential relationship between GL and accumulated °Cd from inflorescence emergence (r2 = 0.77). This was more reliable than the relationships between GL and fruit diameter (r2 = 0.39), or between GL and fruit age, expressed in days (r2 = 0.39). Relationships were also established between GL and malate or citrate concentrations, but they were not sufficiently reliable to estimate GL. The results illustrate that GL is related to fruit development, and that °Cd is a more reliable criterion for harvest date than the number of days, or fruit diameter, because it is less sensitive to different fruit growth rates. Banana growers in the French West Indies usually use fruit diameter and age in days to determine harvest date. However, they face problems of fruit ripening during transportation. The use of °Cd as an indicator may help to determine the optimum harvest date more accurately.

Modelling of Branch and Flower Expansion in GreenLab Model to Account for the Whole Crop Cycle of Winter Oilseed Rape (Brassica napus L.)

Functional-structural Plant Models are interesting tools to study interactions between architecture and environmental conditions. In the case of Winter Oilseed Rape (WOSR), we need a plant model that accounts for the role of source:sink relationships in the architectural development. GreenLab model is a good candidate because it was already used to evidence interactions between source:sink relationships and architecture for other species. However, its adaptation to WOSR is a challenge because of the complexity of its developmental scheme especially during reproductive phase. Indeed, we need to take into account the different timings of branch expansion and pod setting. Therefore two equations were added in GreenLab model to compute expansion delays for respectively branching and flowering of each axis.Experimental field data were used to estimate morphological parameters such as phyllochron, podochron,(equivalent to phyllochron but for pods), leaf expansion duration, and leaf life span. These data were also used to calibrate the source:sink module of the model. First results indicated that the model simulates properly the dynamics of plant growth and development during both vegetative and reproductive phases.

Floral bud damage compensation by branching and biomass allocation in genotypes of Brassica napus with different architecture and branching potential

Plant branching is a key process in the yield elaboration of winter oilseed rape (WOSR). It is also involved in plant tolerance to flower damage because it allows the setting of new fertile inflorescences. Here we characterize the changes in the branching and distribution of the number of pods between primary and secondary inflorescences in response to floral bud clippings. Then we investigate the impacts of the modifications in branching on the biomass allocation and its consequence on the crop productivity (harvest index). These issues were addressed on plants with contrasted architecture and branching potential, using three genotypes (Exocet, Pollen, and Gamin) grown under two levels of nitrogen fertilization. Clipping treatments of increasing intensities were applied to either inflorescences or flower buds. We were able to show that restoration of the number of pods after clipping is the main lever for the compensation. Genotypes presented different behaviors in branching and biomass allocation as a function of clipping treatments. The number of fertile ramifications increased for the high intensities of clipping. In particular, the growth of secondary ramifications carried by branches developed before clipping has been observed. The proportions of yield and of number of pods carried by these secondary axes increased and became almost equivalent to the proportion carried by primary inflorescences. In terms of biomass allocation, variations have also been evidenced in the relationship between pod dry mass on a given axis and the number of pods set, while the shoot/root ratio was not modified. The harvest index presented different responses: it decreased after flower buds clipping, while it was maintained after the clipping of the whole inflorescences. The results are discussed relative to their implications regarding the identification of interesting traits to be target in breeding programs in order to improve WOSR tolerance.

Stochastic Models in Floral Biology and its Application to the Study of Oilseed Rape (Brassica napus L.) Fertility

The number of seeds per pod is an important determinant of yield. New clues of yield and seed quality improvement can be provided by studying the relation between the developmental patterns of floral organs and seed production. In this article, a probabilistic model of plant inflorescence fertility is presented. From a biological point of view, seed development can be viewed as the combination of several physiological processes that can be modeled with stochastic laws. Experiments were made on oilseed rape in Grignon (France) in 2008 to calibrate the model. A Generalized Least Square method was implemented to estimate the model parameters. The variations of parameters were analyzed according to the position of flowers. Furthermore, we discussed the causes that lead to the variation of seed production within the inflorescence and related them to our model. The model reproduces well the distribution of the number of ovules per flower as well as the number of final seeds per pod. We deduced a law to describe the distribution of pollen grains on the stigma that is quite difficult to be observed experimentally. This model is the first step towards a dynamic model taking into account the complexity of the oilseed rape architecture, which is aimed to quantify the influence of pollination or trophic competition on seed production.

Are Yield and Biomass Distribution Affected by Sink Organ Clipping During Reproductive Phase of Winter Oilseed Rape (Brassica napus L.)

As many crops, Winter Oilseed Rape plants are sensitive to biotic or abiotic stresses, but, due to its plasticity reproductive organ losses can be compensated. In this case, biomass is allocated to remaining organs changing yield distribution within the plant. However, compensation remains variable and causes of this variability are still not completely understood. Due to sequential development, pod yield is distributed among axes unevenly. Indeed biomass of axis and biomass allocation to pods varies according to axis position. We suppose that efficiency of compensation at plant scale would depend on the position of axis implied. In the following study axes were clipped. Yield and biomass distribution within plant as well as efficiency of biomass allocation to reproductive organs were characterized. Our data assume that basal axes were mainly involved in compensation and that increase in pod yield on these axes was related to increase in dry mass with no modification of the efficiency allocation of biomass.

Use of a structure-function plant model to assess the morphogenetic plasticity. How does variation in phyllochron modify plant growth and development of Brassica napus in the GreenLab model?

A functional-structural model of winter oilseed rape (WOSR) has been developed to study plant morphogenetic plasticity, i.e. how processes of morphogenesis are adapted in response to environmental constraints. The phyllochron (time between emergence of two successive leaves) is one of the variables sensitive to environment. The aim of this article is to use model sensitivity analysis to quantify the impact of an increase or a reduction in phyllochron on plant growth and source/sink functioning.

Assessment of the Role of initial conditions in the setting of heterogeneity of functional Traits in a population of oilseed rape plants

Individual plant models have been developed in recent years to satisfy different objectives. However most of them focus on average plant and do not integrate the variability observed in cultivated fields. Population scale models are often based on very simplified representation of the plant, and most of them remain theoretical. The objective of this work is to use an experimental design to select the main variables driving plant growth in order to use them as key factors in a plant population scale model. Destructive and non-destructive measurements were carried out from February to June. Measured variables are commonly used in such models. In our experimental conditions, local density has little impact on model outputs. On the contrary, the plant initial size is highly correlated to final height, dry mass and number of ramifications. This result confirms that variability within the field is very dependent on plant development at the first stages.