Effect of red to far-red ratio on tomato (Solanum lycopersicum) stem hydraulic properties

MSc-thesis abstract (submitted 23 November 2017): Plants grown at high plant density or understory are prone to excessive stem elongation due to the competition of light. This excessive elongation might inhibit the efficiency of the water transport system by decreasing the stem hydraulic conductance. The young tomato seedlings are especially sensitive to this shading signal.

This study focuses on investigating the effects of increasing doses of far red light (FR) at similar PAR radiation
(i.e. decreasing PSS) on the hydraulic properties (stem hydraulic conductance, stem hydraulic conductivity and underlying
xylem anatomy) and the ability of the stems to supply water to the leaves (leaf specific conductance) of young tomato
plants. The role of phytochrome A and the phytochrome B in tomato (PHYA and PHYB1B2, respectively) in moderating these
responses was examined in mutants (phyA and phyB1B2) as well at the Moneymaker background.

The results showed that the main reason for the decrease in stem hydraulic conductance, induced by FR, was stem elongation. When Moneymaker plants were grown at a high dose of FR, the stem hydraulic conductivity increased due to wider xylem vessels, but this was not enough to offset the negative effects of stem elongation on hydraulic conductance.
Remarkably, the phytochrome-deficient plants lacked this compensatory mechanism displayed by Moneymaker plants.It was also observed that increasing the FR light fraction led to a decrease in LSC, which means that the FR light caused a reduction of the capacity of the stem’s vascular tissue to supply water to the leaves.

A reduction of LSC has implications for plant survival. Literature shows that plants in which the water transport capacity is uncoupled from the transpirational area are at a greater risk of a embolism event occurring under conditions of high evaporative demand.

Keywords: stem hydraulic properties ; stem elongation ; LED light ; Red ; Far-Red ;