Effect of phloem loading mechanism and sink capacity on potential carbon assimilation and growth of six horticultural crops

Carbon (C) assimilation (A) by leaves (C sources) via photosynthesis is the main driver of crop growth and yield. Rising ambient CO2 concentrations (a primary substrate for A) due to global change provides an opportunity for improving crop growth and yield (C fertilization). Nonetheless, the relationship between leaf photosynthetic capacity and crop growth and yield is less straight-forward than could be expected. One of the reasons for this is that, besides relying on potential A, crop growth and yield are also determined by factors which limit A down-stream from the source leaf mesophyll: phloem loading (PL) and the capacity of C sink organs to use photoassimilates for respiration, growth and storage. PL of sugars from the mesophyll to the minor veins of the source leaves allows driving the long-distance transport of these sugars to sink organs by elevating hydrostatic pressure in sieve elements. PL mechanisms can be active, mainly in herbaceous plants, or passive, mainly in woody plants. Active PL consumes energy for increasing the leaf phloem sugar concentration hereby keeping low concentrations of non structural C (NSC) in the mesophyll cells. Passive PL, on the other hand, maintains high NSC concentrations in mesophyll cells in order to drive symplastic diffusion of sugars to the phloem via plasmodesmatal connections. Keeping low NSC content in the leaf mesophyll cells has been recently proposed as the main role of active PL. Low mesophyll NSC increases the “return on investment” by reducing the “excess C inventory cost” of storing high NSC in the mesophyll (i.e. C which is not used for assimilating yet more C). Moreover, high NSC concentrations in the leaf mesophyll have been related to down-regulation of A in several horticultural crops. Such NSC mediated down-regulation of A has also been related to low sink demand (SD) for C in such crops. Actually, SD has been shown to control A and growth of horticultural crops, especially in the case of fruit trees. Although several studies have dealt with the physiological and molecular mechanisms of PL and source:sink relationships, the quantification of the combined effect of PL and SD on crop A and growth has not yet been attempted.

Young plants of 6 horticultural crops relevant for Chilean agriculture and belonging to different families will be grown in a green house in the Las Cardas Experiment Station. The species are 3 herbaceous crops: tomato, bean (both with active apoplastic PL) and cucumber (active symplastic PL via polymer trap); and 3 woody crops: grapevine (passive sucrose [Suc] PL), apple (passive Suc + polyol PL) and blueberry (active Suc PL). During 3 seasons (new planting each season) the plants will be grown under similar conditions and will be submitted to different SD treatments which will be achieved either by manipulating source:sink ratios via eliminating reproductive sinks and heterotrophic leaves, girdling shoots (disconnecting their phloematic connection with other sinks), and partial defoliation (reduced C offer by sources). Source:sink relationships will also be manipulated via modifying the environmental conditions: shading for reducing source C offer or increasing temperature with polyethylene shelters for increasing sink temperature and hence respiratory C demand. Gas exchange measurement of different organs (leaf net photosynthesis, sink organ respiration) and of the whole plants (net C exchange) will be monitored along the day and season with an IRGA connected to the corresponding measurement chambers. Dry mass, NSC and nitrogen (N) content in organs and the whole plant will also be measured along the season along with leaf mesophyll NSC content. A C balance model will be constructed in which canopy light interception will be estimated with virtual 3D representations of the plant’s architecture and used as an input for the Farquhar photosynthesis model. This model will be coupled to a C allocation model based on source:sink relationships and account for different PL mechanisms. Feedback down-regulation of A will be accounted for via daily estimations of excess NSC inventories. The combined effect of PL mechanisms and SD on the A and growth of the horticultural crops will be analyzed and the correlation of these variables with excess NSC inventories will be explored based on the results of direct measurements. The model will allow quantifying the separate and combined effects of PL mechanism (i.e. excess leaf C inventory) and sink feedback inhibition on A and growth. Simulations in which PL mechanisms will be exchanged between crops will allow evaluating the potential of each mechanism to restrict or enhance crop A and growth.

The results of this study will be of great use for improving horticultural crop yield in the future, both via agronomic practices and breeding. Moreover, because rising CO2 concentration in the atmosphere is expected to alleviate the restrictions to growth imposed by C sources (leaf mesophyll), a better understanding of the factors limiting growth and yield downstream in the source:sink pathway are of key importance for predicting the effects of climate change on horticulture and for designing yield efficient horticultural systems for the future.

Equipo de Trabajo

Director Proyecto:
Nicolás Franck Berger
Profesor Asistente. Universidad de Chile
Ingeniero Agrónomo. M.S. Ph. D


Víctor García de Cortázar
Profesor Asociado. Universidad de Chile
Ingeniero Agrónomo. M.S. Ph. D

Claudio Pastenes
Profesor Asociado. Universidad de Chile
Ingeniero Agrónomo. Ph. D

Rafael Coopman
Profesor Auxiliar, Universidad Austral
Ingeniero Forestal, Mg. Sc., Dr.

Supervisor de terreno
Eduardo Navarro
Ingeniero Agropecuario

Colaborador Internacional:
Professor Robert Turgeon
University of Cornell (USA)