Project

Influence of the water balance and the functioning of the vascular tissue on the fruit quality of tomato and grape

Duration
01 January 2013 → 31 December 2016
Funding
Regional and community funding: IWT/VLAIO
Research disciplines
  • Natural sciences
    • Ecology
    • Environmental science and management
    • Other environmental sciences
Keywords
fruit quality fruit development plant fruit interaction
 
Project description

Water availability, or the lack of it, is one of the most influential factors in plant
and fruit growth. Typically, well-watered plants will thrive and thus fruit yields are
highest under well-watered conditions. However, moderate water deficits can
have beneficial effects on the quality of the fruit by increasing the concentration of
sugars and a multitude of other quality-related compounds. As such, by changing
the soil water availability, a balance between yield and quality can be sought. The
plant plays a crucial role in this interplay between water, sugars and fruit, as it is
responsible for the supply of water and nearly all assimilates to the fruit. Possible
stress effects of diminished water availability on the fruit are therefore expressed
via the plant.
In this doctoral thesis, the effect of plant water status on fruit development as well
as on plant-fruit interaction was studied. Insight was sought, not only in fruit
quality aspects, but also in the underlying mechanisms. The research was
conducted on tomato and grape because of both their scientific and economic
relevance. Economically, tomato (botanically a fruit, yet a culinary vegetable) is
the most important vegetable crop worldwide, whereas grape is the most
important fruit crop. Scientifically, tomato and grape are the model fruits for two
distinctively different fruit growth strategies: tomatoes grow according to a
sigmoid pattern, whereas grape berries exhibit a double sigmoid growth pattern.
A similar research strategy was followed both for tomato and grape:
measurements of fruit growth and quality were combined with an array of plant
measurements (sap flow, stem diameter variations, stem water potential) under
different levels of water availability. This reduction in water availability for the
plant can not only be achieved by reducing the amount of irrigation (i.e. drought)
but also by increasing the salinity of the nutrient solution. The latter way is only
relevant for soilless crops and was, hence, only studied in tomato. We found that
increased salinity had a larger beneficial effect on fruit quality than drought, and
our plant measurements taught us that this was due to the fact that fruits became
the most important carbon sink earlier on in the season under increased salinity,
and because the phloem (which conducts sugars into the fruit) had a relatively
larger contribution to fruit growth as well. Because of this, higher quality tomatoes
could be achieved, without a reduction in yield on the short term.
In grapevine, we found that slight decreases in water availability, that are typically
not even considered to be drought stress in the field, had a large impact on plant
and fruit water and carbon status. Positive drought effects on fruit quality were
now coupled with a reduction in yield, as well as a reduction in plant growth.
However, yield is only of secondary importance for wine grape growers, and
berry quality is their main concern as it largely influences wine quality.
In addition to the research on fruit quality and plant-fruit interaction, we also
investigated to two pressing matters that needed to be solved to allow progress
both in our and future research. First, the validity of heat girdling as a means to
determine the contributions of xylem and phloem flow to fruit growth was
demonstrated. This technique has been used since the late 80s, yet was never
thoroughly validated. With a combination of histology and in vivo MRI (magnetic
resonance imaging), we finally provided this validation. As such we were able to
use this technique to study the effect of increased salinity on the relative
contributions of xylem and phloem flow to the fruit, and we found that increased
salinity increased the contribution of phloem to fruit growth from 20 to 60 %. The
second issue that was investigated was the occurrence of an irreversible
shrinkage in grapevine stems after veraison (i.e. the onset of ripening in grape
berries). This post-veraison shrinkage is unrelated to drought stress, renders
stem diameter variations useless during this period, and the reason behind it was
unresolved up until now. We found that this shrinkage was caused by the die-off
of the outer bark layers, a yearly occurring event that is unrelated to
environmental drivers.
Finally, our experimentally obtained knowledge was integrated with concepts
from literature in a mechanistic coupled plant-fruit model simulating plant and fruit
growth. This model was applied on both tomato and grape to study the driving
forces for transport of water and carbon from plant to fruit, and to look at the
differences in growth patterns and underlying mechanisms between tomato and
grape.
This research highlighted the strength of the combination of plant measurements
and mechanistic modelling in further understanding the mechanisms that drive
the development of fruit. This approach could, hence, be applied in commercial
cultivation to help growers with their irrigation management decisions, as such
allowing them to optimise the balance between yield and quality according to
their or the customer's needs.