On the effect of additional doses of far-red light on the stem hydraulic conductivity of tomato.

MSc-thesis abstract (submitted 20 June 2016): We investigated the effect of increasing doses of far-red light (FR) under a stable PAR background (decreasing PSS) on morphological development and stem hydraulic conductance (Kstem), conductivity (Ks), and xylem anatomy of tomato (Solanum lycopersicum ‘Moneymaker’, M).
The role of phytochrome A and phytochrome B1B2 (phyA and phyB, respectively) in moderating these responses was examined, via the inclusion of the tomato phytochrome A (phyA) and phytochrome B1B2 (phyB) mutants.

After germination under white light, plants were grown under four different light treatments, each composed of a different dose of FR (0, 30, 65, 150µmol m-2s-1) and a stable dose of 100µmol m-2s-1 of PAR, delivered via a combination of red (R, 640 nm) and blue (B, 420 nm) LEDs (R:B, 90:10). Hence, the treatments exhibited a gradient in calculated phytochrome photoequilibria (PSS) ranging from PSS 0.89 (no FR), to PSS 0.80, PSS 0.72, and PSS 0.58 (150µmol m-2s-1 FR).

The addition of FR resulted in a significant increase in the height of the three genotypes examined, suggesting that in tomato in addition to phyB, other Type II phytochromes (phyE and phyF) are also playing a role in this response. The addition of FR resulted in a significant reduction in the root: shoot ratio of both M and phyA plants, suggesting that the FR-mediated inactivation of phyB is influencing this process. It was argued that other Type II phytochromes may be acting redundantly with phyB under low levels of FR (PSS 0.80) to control the reduction in root: shoot ratio, but that the FR-mediated activation of phyA under high levels of FR may be sufficient to significantly reverse this response in the absence of phyB.

All three genotypes displayed a significant reduction in Kstem with decreasing PSS, consistent with internode length changes. Only phyB plants presented a significant decrease in hydraulic conductivity (Ks) and xylem number with decreasing PSS. A decreasing PSS was not found to significantly influence the Ks and xylem number of M and phyA plants, suggesting that the presence of functional phyB is important to prevent a reduction in stem hydraulic conductivity (Ks) under increasing FR, and that this could be mediated through its influence on the number of xylem vessels. The loss of functional phyA and phyB was found to lead to a significant reduction in Ks under nearly all treatments, suggesting that the presence of active phyA and phyB is important to promote the conductivity of tomato stems.

An analysis of the diameter distribution of the xylem vessels in the measured internodes revealed that both a lack of functional phyA or phyB resulted in a decreased formation of larger-diameter vessels (80-160 µm), which are of relative high importance for Ks.

The stem hydraulic conductivity, xylem number, and xylem diameter distribution of M and phyB plants were not significantly different in the absence of FR (PSS 0.89), suggesting that the activation state of phyB is not directly related to this process, or that other Type II phytochromes are able to support this role in the absence of functional phyB. It is concluded that the role of the tomato phyA and phyB genes in the control of stem elongation and root: shoot ratio is consistent with the established Arabidopsis th. phytochrome paradigm.

Moreover, it is concluded that the addition of FR results in a decreased stem hydraulic conductance of M, phyA, and phyB plants. For M and phyA plants, this decrease seems to be mediated primarily through the increased length of the internodes, since the conductivity of stem segments (Ks) was not significantly affected by the light treatments.

The presence of functional phyB was found to be important to prevent the FR-mediated reduction in conductivity, which appears to be controlled through its promotion of xylem vessel formation under high levels of FR.