2017 ASHS Annual Conference

Supplemental lighting (SL) is necessary for optimal plant growth and yield in greenhouse tomato production, particularly during winter months in temperate climates. High-pressure sodium (HPS) lamps presently are the most common type of SL because of their high-intensity capabilities. Overhead HPS lamps can not only induce a radiant thermal load on tomato but also promote early senescence of lower leaves due to shading from the upper canopy. Light-emitting diode (LED) intracanopy towers provide a uniform lighting environment vertically along the plant canopy without emitting radiant heat and have a more efficient light spectrum to enhance photosynthesis. The objectives of this study were to compare physiological responses of high-wire greenhouse tomato to different SL sources [overhead HPS lamps vs. intracanopy LED towers] and light qualities along the plant canopy, and to determine how such responses affect fruit yield. ‘Merlice’ scions grafted onto ‘Maxifort’ rootstocks were supplemented with different combinations of blue (B), red (R), and/or far-red (FR) LED lighting: B + R (39%B∶61%R), R (100%R), R + low FR (82%R∶18%FR), or R + high FR (71%R∶29%FR), and compared with HPS (38%B:35%R∶27%FR) and unsupplemented controls. All the light treatments provided the same photosynthetic photon flux density (PPFD) at 230 µmol·s-1·m-2. Stomatal conductance (gs), transpiration rate (E), photosynthetic rate (Pn), and intercellular CO2 concentration (Ci) were examined at three canopy levels (upper, middle, and lower) after growing points were removed. Overall, the E, gs, and Pn were the highest in the upper canopy, while Ci was the lowest in the upper canopy. Interestingly, HPS treatment significantly increased E in the upper canopy by 2- to 3-fold, along with a tendency to increase gs, which was associated with higher leaf temperature (r2=0.7) . Moreover, tomato plants grown under HPS lamps had higher Pn in the upper canopy but significantly lower Pn in the lower canopy in comparison with other treatments. Compared with HPS, intracanopy LED treatments promoted Pn at low canopy by an average of 9-fold, and had 49% higher yield. It is concluded that tomato plants grown with intracanopy LED towers maintain higher photosynthetic capacity in the lower leaves, which appears to be associated with higher yield.