Synergistic enhancement of biomass allocation from leaves to stem by far-red light and warm temperature can lead to growth reductions

The application of far-red (FR; 700–800 nm) light can improve plant growth, especially in leafy green vegetables, through enhancing leaf expansion and photon capture. However, higher levels of FR light may induce excessive stem elongation at the expense of leaf expansion. The morphological responses to FR light are mediated by phytochrome photoreceptors, with their activity further dependent on temperature. We aimed to quantify if different plant species respond differently to FR light and temperature conditions. Six economically-important plant species, including three cool-season species (lettuce, kale, and petunia) and three warm-season species (tomato, African marigold, and zinnia) were grown under three FR fractions [FR/(Red+FR); 0, 0.13, and 0.25] and two temperature set points (22 and 28 ℃) to characterize their growth and morphological responses. Increasing the FR fraction from 0 to 0.25 led to a 26–47 % increase in leaf expansion in lettuce, kale, petunia, and zinnia at 22 ℃. However, the total leaf area of tomato and African marigold decreased by 14–26 % as the FR fraction increased to 0.25 at 22 ℃. At a warmer temperature of 28 ℃, unlike the response observed under cooler temperature, increasing the FR fraction resulted in excessive stem elongation (a 36–101 % increase) and a 16–49 % reduction in leaf expansion in lettuce, kale, and petunia. For tomato, African marigold, and zinnia, the total leaf area increased by 15–26 % as the FR fraction increased from 0 to 0.13 at 28 ℃; however, further increasing the FR fraction from 0.13 to 0.25 resulted in a reduction in total leaf area. Across all six species, a high FR fraction combined with warm temperature synergistically stimulated stem elongation at the expense of leaf expansion. Shoot biomass responded to FR light and warm temperature similarly to leaf expansion in all six species. We further characterized the physiological responses to FR light and temperature in lettuce and tomato. In both crops, FR light generally increased the quantum yield of photosystem II, while decreasing the net CO₂ assimilation rate per unit leaf area, chlorophyll and carotenoid contents, and chlorophyll a:b ratio. Additionally, FR light increased soluble sugar:starch ratio in leaves at 28 ℃, but not at 22 ℃, suggesting that the synergistic effect of FR light and warm temperature on stem elongation may be mediated by increased soluble sugar translocation from leaves to stem. We concluded that the enhanced stem growth under FR light and warm temperature can lead to reduced plant biomass. Our results further indicate that the interactive effects between FR light and temperature on plant growth and morphology were species-dependent, with distinct responses observed among species with different temperature preferences.