Abstract. Water status plays an important role for fruit quality and quantity in tomato (Solanum lycopersicum L.).
However, determination of the plant water status via measurements of sap flow (FH2O) or stem diameter (D) cannot be done
unambiguously since these variables are influenced by other effectors than the water status. We performed a semi-seasonal
and a diurnal analysis of the simultaneous response of FH2O and D to environmental conditions, which allowed us to
distinguish different influences on DD such as plant age, fruit load and water status and to reveal close diurnal relationships
between FH2O and DD. In addition, an analysis of the diurnal mechanistic link between both variables was done by applying
a slightly modified version of a water flow and storage model for trees. Tomato stems, in contrast with trees, seemed to
maintain growth while transpiring because a large difference between turgor pressure (Yp) and the yield threshold (G) was
maintained. Finally, the simultaneous response of D and FH2O on irrigation events showed a possibility to detect water
shortages.
Introduction
Because of its economic importance, tomato (Solanum
lycopersicum L.) is widely studied, in particular, its traits for
improving fruit organoleptic and nutritional quality (Dorais et al.
2001). Many reports (Mitchell et al. 1991; Cuartero and
Fernandez-Munoz 1999; Veit-Köhler et al. 1999; Plaut et al.
2004) show that salinity and water deficit influence the tomato
fruit soluble solids content and thus quality. Therefore, plant
water status appears to influence fruit quality to a large extent.
However, because previous research has reported an inverse
relationship between fruit soluble solids content and fresh
yield, a compromise must be found between reduced fruit
production and enhanced fruit quality. This balance is as
difficult to achieve as it is to maintain because the growth and
the development of a fruit is part of the integrated processes in the
whole plant in which water economy and intra-plant competition
mechanisms play a substantial role. Therefore, an understanding
of the plant’s water status and the mechanisms between water
status and growth is a topic of great interest.
Currently, several methods are available for automated
monitoring of plant water status. First, measurements of stem
sap flow (FH2O) can give information on the plant’s water status
since high transpiration rates (and thus high FH2O) lead to more
negative tensions inside the stem xylem. Vermeulen et al. (2007)
described the possibility of using stem sap flow measurements as
an indicator of water deficit. Since FH2O is a direct response of the
plant to the environmental conditions such as the atmospheric
water demand andthe substrate water supply,its measurement has
potential in the perspective of estimating plant water status (Jones
2004).