Correlation between dynamic tomato fruit-set and source-sink ratio: a common relationship for different plant densities and seasons?

Abstract

Background and aims: It is widely accepted that fruit-set in plants is related to source-sink ratio. Despite its critical importance to yield, prediction of fruit-set remains an ongoing problem in crop models. Functional-structural plant models are potentially able to simulate organ-level plasticity of plants. To predict fruit-set, the quantitative link between source-sink ratio and fruit-set probability is analysed here via a functional-structural plant model, GreenLab.

Methods: Two experiments, each with four plant densities, were carried out in a solar greenhouse during two growth seasons (started in spring and autumn). Dynamic fruit-set probability was estimated by frequent observation on inflorescences. Source and sink parameter values were obtained by fitting GreenLab outputs for the biomass of plant parts (lamina, petiole, internode, fruit), at both organ and plant level, to corresponding destructive measurements at six dates from real plants. The dynamic source-sink ratio was calculated as the ratio between biomass production and plant demand (sum of all organ sink strength) per growth cycle, both being outputs of the model.

Key results and conclusions: Most sink parameters were stable over multiple planting densities and seasons. From planting, source-sink ratio increased in the vegetative stage and reached a peak after fruit-set commenced, followed by a decrease of leaf appearance rate. Fruit-set probability was correlated with the source-sink ratio after the appearance of flower buds. The relationship between fruit-set probability and the most correlated source-sink ratio could be quantified by a single regression line for both experiments. The current work paves the way to predicting dynamic fruit-set using a functional structure model.

Fig. 1.

Evolution of the leaf appearance rate (LAR) during plant development. LAR decreased at the transition from vegetative stage (regression lines, dotted: y = 0·0323x + 4·53 for A, y = 0·0219x + 3·59 for B) to reproductive stage (regression lines, dashed). D1, 1 plant m⁻²; D2, 3 plants m⁻²; D3, 6 plants m⁻²; D4, 11 plants m⁻². Thermal time was expressed in growing degree-days, with a temperature base of 10 °C.

Fig. 2.

Environmental conditions in the greenhouse: light level (A), temperature (B) and daily potential evapotranspiration (PET, C).

Fig. 3.

Evolution of fruit-set probability. Fruit-set probability at any given time is the percentage of flower buds appearing at that stage that give rise to a fruit. D1, 1 plant m⁻²; D2, 3 plants m⁻²; D3, 6 plants m⁻²; D4, 11 plants m⁻². Thermal time was expressed in growing degree-days, with a temperature base of 10 °C.

Fig. 4.

Multi-fitting on plant data from six samplings. All data were fitted simultaneously, including total dry weight of each type of organ (data not shown) and the dry weight of individual laminae (A), petioles (B), internodes (C) and trusses (D), shown with D3 (6 plants m⁻²) of the autumn experiment.

Fig. 5.

Computed evolution of plant biomass production per cycle (Q, cf. eqn 4) in both spring (A) and autumn (B) experiments. D1, 1 plant m⁻²; D2, 3 plants m⁻²; D3, 6 plants m⁻²; D4, 11 plants m⁻². Thermal time was expressed in growing degree-days, with a temperature base of 10 °C.

Fig. 6.

Computed evolution of plant demand per cycle (D, cf. eqn 3). D1, 1 plant m⁻²; D2, 3 plants m⁻²; D3, 6 plants m⁻²; D4, 11 plants m⁻². Thermal time was expressed in growing degree-days, with a temperature base of 10 °C.

Fig. 7.

Evolution of the plant source–sink ratio (Q/D), computed as the ratio between plant biomass production and demand. It increased in the vegetative stage and then decreased when fruit-set began, in both spring (A) and autumn (B) experiments. D1, 1 plant m⁻²; D2, 3 plants m⁻²; D3, 6 plants m⁻²; D4, 11 plants m⁻². Thermal time was expressed in growing degree-days, with a temperature base of 10 °C.

Fig. 8.

Quantitative relationship between fruit-set probability and source–sink ratio for four plant densities, from spring and autumn experiments, with data (symbols) and regression line: fruit-set probability = 1−e^{−2·39 (Q/D−0·12)}, R² = 0·77.