A Basis for Selecting Light Spectral Distribution for Evaluating Leaf Photosynthetic Rates of Plants Grown under Different Light Spectral Distributions

The photosynthetic rate is one of the most important and fundamental aspects for plant growth. In many studies this rate is measured, evaluated, and compared among the leaves of plants cultivated under different conditions. The measured rates are also used to calculate other photosynthesis-related indices, such as photosynthetic light-, water-, and nitrogen-use efficiencies. In agricultural and horticultural researches, the effectiveness of treatments is sometimes discussed based on the measured photosynthetic rates and calculated indices. A number of researches have reported that the relative spectral photon flux density (PFD) distribution (i.e. the spectral distribution normalized to the peak or mean value) of light used for measurement (i.e. measuring light or actinic light) affects leaf net photosynthetic rates (Pn) (e.g. McCree, 1972; Inada, 1976). To eliminate this direct effect from the comparison, Pn is usually measured under a common spectral distribution of measuring light irrespective of growth conditions in agricultural and horticultural studies. One of the most widely-used measuring lights is a mixture of blue and red light (BR-light) provided by light-emitting diodes (LEDs) installed in commercial photosynthesis analysis systems (e.g. LI-6400, LI-COR Inc., Lincoln, NE, USA; GFS-3000, Heinz Walz GmbH, Effeltrich, Germany). The use of artificial light sources enables precise control of the spectral distribution of measuring light on the leaf, and therefore, ensures reproducibility and reliability among experiments. Walters (2005) noted that photosynthetic rates measured with a relative spectral distribution of light different from that of the growth light do not necessarily reflect the functioning of photosynthesis under the actual growth conditions. Indeed, we have demonstrated this problem in Pn measurements in our recent experiment (Murakami et al., 2016). In that experiment, cucumber seedlings were grown under white LED (300 mol m 2 s ) without and with supplemental far-red (FR) LED light (70 mol m 2 s ) (W and WFR, respectively), and the Pn of the leaves was subsequently compared under BR-light and under light with a relative spectral distribution approximating to that of sunlight (‘artificial’ sunlight) at a photosynthetic PFD (PPFD) of 300 mol m 2 s . The mean Pn of W-grown-leaves (mean ± SE: 12.2 ± 0.5 mol m 2 s ) was 36% greater than that of WFR-grown-leaves (8.9 ± 0.7 mol m 2 s ) under BR-light (95% confident interval: +0.6 to +5.9, P = 0.027), while the mean value of W-grown-leaves (10.1 ± 0.5 mol m 2 s 1 ) were comparable to or 3% smaller than that of WFR-grown-leaves (10.4 ± 0.3 mol m 2 s ) under the artificial sunlight (95% confident interval: –1.9 to +1.3, P = 0.65) (Murakami et al., 2016). Based on the results obtained from measurement under BR-light, the prospective leaf photosynthetic rate (i.e. leaf photosynthetic rates after the measurements) of WFR-grown-plants may