Far-red radiation stimulates dry mass partitioning to fruits by increasing fruit sink strength in tomato

Abstract

Far-red (FR) light promotes fruit growth by increasing dry mass partitioning to fruits, but the mechanism behind this is unknown. We hypothesise that it is due to an increased fruit sink strength as FR radiation enhances sugar transportation and metabolism. Tomato plants were grown with or without 50-80 μmol m^-2 s^-1 of FR radiation added to a common background 150-170 μmol m^-2 s^-1 red + blue light-emitting diode lighting. Potential fruit growth, achieved by pruning each truss to one remaining fruit, was measured to quantify fruit sink strength. Model simulation was conducted to test whether the measured fruit sink strength quantitatively explained the FR effect on dry mass partitioning. Starch, sucrose, fructose and glucose content were measured. Expression levels of key genes involved in sugar transportation and metabolism were determined. FR radiation increased fruit sink strength by 38%, which, in model simulation, led to an increased dry mass partitioned to fruits that quantitatively agreed very well with measured partitioning. FR radiation increased fruit sugar concentration and upregulated the expression of genes associated with both sugar transportation and metabolism. This is the first study to demonstrate that FR radiation stimulates dry mass partitioning to fruits mainly by increasing fruit sink strength via simultaneous upregulation of sugar transportation and metabolism.

Keywords: LED lighting; Solanum lycopersicum (tomato); dry mass partitioning; far red; sink strength; sugar metabolism; sugar transportation.

Fig. 1

Spectral composition of red + blue (RB) and RB + far‐red (FR) light treatment provided by the LEDs measured at the top of the canopy.

Fig. 2

Effects of adding far‐red (FR) to red + blue (RB) light on the fraction of dry mass partitioned to fruits, stem and leaves in tomato (Solanum lycopersicum) plants where fruits were pruned to five fruits per truss or not pruned (7–8 fruits per truss). Data were based on cumulative dry mass of plants 90 d after transplanting. Different letters denote significant differences between treatments according to Fisher’s protected least significant difference (LSD) test conducted independently for fruit, stem, and leaf (n = 4, α = 0.05).

Fig. 3

Effects of adding far‐red (FR) radiation to red + blue (RB) light on potential fruit growth (a) and potential fruit growth rate (b) in tomato (Solanum lycopersicum). Curves represent Gompertz function (a) and its derivative (b) fitted for RB + FR (dashed lines) and RB (solid lines) light conditions. Symbols represent measured fruit dry mass for RB + FR (open symbols) and RB (closed symbols) and error bar represents standard error of means (n = 4).

Fig. 4

Simulated total fruit sink strength per plant for plants with five fruits per truss (a) and dry mass fraction of fruits and vegetative parts (b) of tomato (Solanum lycopersicum) plants grown with or without additional far‐red radiation. Curves represent fruit sink strength simulated for a fruit load of five fruits per truss. Both simulations were conducted for a period of 90 d after transplanting. RB, red + blue; FR, far red.

Fig. 5

Effects of adding far‐red (FR) radiation to red + blue (RB) light on concentration of starch (a), sucrose (b), fructose (c) and glucose (d) in tomato (Solanum lycopersicum) fruits measured every 10 d after anthesis until fully ripe. Starch concentration is expressed as equivalent glucose concentration. Error bar represents standard error of means (n = 4). Asterisks denote statistically significant effects of FR radiation as tested with Student’s t‐test (n = 4; *, P < 0.05; **, P < 0.01; ***, P < 0.001).

Fig. 6

Effects of adding far‐red (FR) radiation to red + blue (RB) light on relative expression of genes related to sugar transportation, sucrose synthase, invertase, starch synthesis and starch catabolism in tomato (Solanum lycopersicum) flowers/fruits at 0, 10–20 and 30 d after anthesis (DAA). Different colours represent expression levels of each gene under RB + FR relative to that under RB. Asterisks denote statistically significant effects of FR radiation as tested with Student’s t‐test (n = 4; *, P < 0.05; **, P < 0.01).