Two-factor analysis of variance.

Despite being outside of the traditionally defined photosynthetically active radiation (PAR) waveband (400–700 nm), far-red (FR; 700–799 nm) light can increase photosynthesis and induce shade-avoidance responses, which increases light interception and thus, whole-plant growth. However, it is unclear how the promotion of growth from FR light depends on PAR wavebands and specifically how the substitution of red light (600–699 nm) with green light (500–599 nm) influences the efficacy of FR light on increasing shoot biomass accumulation. To determine this, we grew red- and green-leaf lettuce (Lactuca sativa) at a fixed total photon flux density (PFD) with 12 different fractions of red, green, and FR light and the same PFD of blue (400–499 nm) light. We postulated that decreasing the red:FR by substituting FR light for green light, red light, or both would increase shoot fresh mass (FM) until a fraction beyond which growth (but not leaf area) would begin to decrease. Indeed, the substitution of red with FR light increased the leaf area of both cultivars, but FM was greatest under an FR fraction [FR/(R+FR)] of approximately 0.25. Under the greatest FR PFD, FM was similar to lettuce grown without FR light, despite having greater leaf surface area for light interception. Green light had less of an effect on leaf expansion and FM than FR light, and plant diameter and leaf area of red-leaf ‘Rouxai’ were the greatest when green light fully replaced red light at the highest FR PFD. We conclude that under a modest light intensity and blue PFD, a spectrum that includes up to 25% of far-red photons can increase leaf area and biomass accumulation. While leaf area may continue to increase at higher far-red fractions, fresh mass does not, and plant quality begins to deteriorate.

尽管远红光（FR；700–799 nm）处于传统定义的光合有效辐射（photosynthetically active radiation, PAR）波段（400–700 nm）之外，但它能够提升光合效率并诱导避荫响应，进而增加光捕获量，最终促进整株植物生长。然而目前仍不明确，远红光对生长的促进作用如何依赖于光合有效辐射波段，以及用绿光（500–599 nm）替代红光（600–699 nm）具体会如何影响远红光提升地上部生物量积累的效果。为解答这一问题，本研究在固定总光子通量密度（photon flux density, PFD）的条件下，培育了红叶与绿叶生菜（Lactuca sativa），设置12种红光、绿光与远红光的比例组合，并保持蓝光（400–499 nm）的光子通量密度一致。我们提出假设：通过用远红光替代绿光、红光，或同时替代二者来降低红光与远红光的比值，将使地上部鲜重（FM）提升，直至达到某一临界比例，超过该比例后，植株生长（但非叶面积）将开始下降。实验结果确证，用远红光替代红光可提升两个生菜品种的叶面积，但当远红光占比[FR/(R+FR)]约为0.25时，地上部鲜重达到峰值。在远红光光子通量密度最高的处理组中，尽管植株用于光捕获的叶面积更大，但其鲜重与未添加远红光的对照组生菜并无显著差异。绿光对叶扩展与鲜重的影响弱于远红光；在远红光光子通量密度最高的条件下，当绿光完全替代红光时，红叶生菜品种‘Rouxai’的株径与叶面积达到最大。本研究得出结论：在适度光强与蓝光光子通量密度的条件下，包含至多25%远红光光子的光谱可提升叶面积与生物量积累。尽管远红光占比更高时叶面积可能持续增加，但鲜重不会随之提升，且植株品质将开始下降。