Jochen Hanssens

Non-destructive estimation of root pressure using sap flow, stem diameter measurements and mechanistic modelling

BACKGROUND Upward water movement in plants via the xylem is generally attributed to the cohesion-tension theory, as a response to transpiration. Under certain environmental conditions, root pressure can also contribute to upward xylem water flow. Although the occurrence of root pressure is widely recognized, ambiguity exists about the exact mechanism behind root pressure, the main influencing factors and the consequences of root pressure. In horticultural crops, such as tomato (Solanum lycopersicum), root pressure is thought to cause cells to burst, and to have an important impact on the marketable yield. Despite the challenges of root pressure research, progress in this area is limited, probably because of difficulties with direct measurement of root pressure, prompting the need for indirect and non-destructive measurement techniques. METHODS A new approach to allow non-destructive and non-invasive estimation of root pressure is presented, using continuous measurements of sap flow and stem diameter variation in tomato combined with a mechanistic flow and storage model, based on cohesion-tension principles. KEY RESULTS Transpiration-driven sap flow rates are typically inversely related to stem diameter changes; however, this inverse relationship was no longer valid under conditions of low transpiration. This decoupling between sap flow rates and stem diameter variations was mathematically related to root pressure. CONCLUSIONS Root pressure can be estimated in a non-destructive, repeatable manner, using only external plant sensors and a mechanistic model.

High light decreases xylem contribution to fruit growth in tomato.

Recently, contradicting evidence has been reported on the contribution of xylem and phloem influx into tomato fruits, urging the need for a better understanding of the mechanisms involved in fruit growth. So far, little research has been performed on quantifying the effect of light intensity on the different contributors to the fruit water balance. However, as light intensity affects both transpiration and photosynthesis, it might be expected to induce important changes in the fruit water balance. In this study, tomato plants (Solanum lycopersicum L.) were grown in light and shade conditions and the fruit water balance was studied by measuring fruit growth of girdled and intact fruits with linear variable displacement transducers combined with a model-based approach. Results indicated that the relative xylem contribution significantly increased when shading lowered light intensity. This resulted from both a higher xylem influx and a lower phloem influx during the daytime. Plants from the shade treatment were able to maintain a stronger gradient in total water potential between stem and fruits during daytime, thereby promoting xylem influx. It appeared that the xylem pathway was still functional at 35 days after anthesis and that relative xylem contribution was strongly affected by environmental conditions.

Metabolic Fingerprinting to Assess the Impact of Salinity on Carotenoid Content in Developing Tomato Fruits

As the presence of health-promoting substances has become a significant aspect of tomato fruit appreciation, this study investigated nutrient solution salinity as a tool to enhance carotenoid accumulation in cherry tomato fruit (Solanum lycopersicum L. cv. Juanita). Hereby, a key objective was to uncover the underlying mechanisms of carotenoid metabolism, moving away from typical black box research strategies. To this end, a greenhouse experiment with five salinity treatments (ranging from 2.0 to 5.0 decisiemens (dS) m−1) was carried out and a metabolomic fingerprinting approach was applied to obtain valuable insights on the complicated interactions between salinity treatments, environmental conditions, and the plant’s genetic background. Hereby, several hundreds of metabolites were attributed a role in the plant’s salinity response (at the fruit level), whereby the overall impact turned out to be highly depending on the developmental stage. In addition, 46 of these metabolites embraced a dual significance as they were ascribed a prominent role in carotenoid metabolism as well. Based on the specific mediating actions of the retained metabolites, it could be determined that altered salinity had only marginal potential to enhance carotenoid accumulation in the concerned tomato fruit cultivar. This study invigorates the usefulness of metabolomics in modern agriculture, for instance in modeling tomato fruit quality. Moreover, the metabolome changes that were caused by the different salinity levels may enclose valuable information towards other salinity-related plant processes as well.

Model-assisted analysis of elevated temperature and vapour pressure deficit effects on tomato stem and fruit water balance

Maintaining good plant water status is crucial for optimal production and quality of tomato in greenhouses. Various new climate control technologies have been introduced to make greenhouse cultivation more energy-efficient, resulting in a modified greenhouse climate. Recently, there has been growing interest in the use of plant-based methods to steer the climate. Monitoring stem diameter variations (SDV) has been extensively studied in tree species, but is also very promising for herbaceous crops. Stem and fruit diameter variations provide crucial information about plant water status, though unambiguous interpretation of these dynamics is often difficult. Mechanistic modelling can help to elucidate the mechanisms driving plant behaviour and is therefore an important tool for interpreting the dynamic response of the plants to changes in microclimate. In the present study, tomato plants (Solanum lycopersicum L.) were subjected to elevated air temperature (Ta) and vapour pressure deficit (VPD), while SDV, sap flow and fruit growth were continuously monitored. Results indicated that stem shrinkage became more pronounced and fruits shrank during periods of high Ta and VPD. Simulation results showed that reduced fruit growth resulted from both increased fruit transpiration and decreased phloem inflow. Moreover, xylem backflow appeared when Ta and VPD reached maximum values. It was demonstrated that the reduced fruit growth resulted mainly from changes in stem water potential, rather than fruit water potential.