Photosynthetic Responses of Canola to Exogenous Application or Endogenous Overproduction of 5-Aminolevulinic Acid (ALA) under Various Nitrogen Levels

Abstract

Limited data are available on the effects of 5-aminolevulinic acid (ALA) on plant photosynthesis in relation to the nitrogen (N) level. In this study, we investigate photosynthetic responses to ALA in canola plants (Brassica napus L.). We used wild-type plants without ALA addition (controls), wild-type plants with exogenous ALA application, and transgenic plants that endogenously overproduced ALA. The plants were grown hydroponically in nutrient solutions with low, middle, and high concentrations of N. Our results indicate that plants in both treatment groups had higher chlorophyll contents and net photosynthetic rates and lower intracellular CO₂ concentrations in the leaves, as compared to controls. Furthermore, simultaneous measurement of prompt chlorophyll fluorescence and modulated 820-nm reflections showed that the active photosystem II (PS II) reaction centers, electron transfer capacity, and photosystem I (PS I) activity were all higher in treated plants than controls at all N levels; however, the responses of some photochemical processes to ALA were significantly affected by the N level. For example, under low N conditions only, a negative ΔK peak appeared in the prompt chlorophyll fluorescence curve, indicating a protective effect of ALA on electron donation via activation of the oxygen-evolving complex. Taken together, our findings suggest that ALA contributes to the promotion of photosynthesis by regulating photosynthetic electron transport under various N levels. These findings may provide a new strategy for improving photosynthesis in crops grown in N-poor conditions or reduced N-fertilization requirements.

Keywords: 5-aminolevulinic acid (ALA); canola; nitrogen supply; photosynthetic responses.

Figure 1

Differences in chlorophyll content (represented as SPAD values) in exogenously 5-aminolevulinic acid (ALA)-treated and endogenous ALA-overproducing canola leaves grown under low-, middle-, and high-N conditions. Means ± standard errors are presented (n = 10). Means with the same letter indicate a nonsignificant difference (p > 0.05), according to Duncan’s test. Sources of variation: ALA, N-level, and ALA × N-level interaction; ** p < 0.01; ns: not significant. N = nitrogen.

Figure 2

Differences in (a) net photosynthetic rate (Pn), (b) intercellular CO₂ concentration (Ci), (c) stomatal conductance (Gs), and (d) transpiration rate (Tr) in exogenously-treated and endogenous ALA-overproducing canola leaves grown under low-, middle-, and high-N conditions. Values are means ± standard errors (n = 10). The same letter indicates no significant difference (p > 0.05) according to Duncan’s test. Sources of variation: ALA, N-level, and ALA × N-level interaction; * p < 0.05; ** p < 0.01; ns: not significant.

Figure 3

Changes in prompt fluorescence (PF) curves in exogenously-treated and endogenous ALA-overproducing canola leaves under low- (a,d), middle- (b,e), and high-N conditions (c,f). (a–c) PF curves plotted on a logarithmic time scale from 20 μs to 1 s (JIP time). The steps O (at 20 µs), J (at 2 ms), I (at 30 ms), and P (peak) are marked. Each curve is the average of 10 replicate measurements. (d–f) Variable fluorescence curves (ΔV = Δ[(F_t − F_o)/(F_m − F_o)]), which were constructed by subtracting the normalized (between the O step and P step) values of the PF recorded in wild-type (WT) canola. The feature peaks ΔK, ΔJ, and ΔI are marked.

Figure 4

Radar plots of changes in JIP test parameters in exogenously treated and endogenous ALA-overproducing canola leaves relative to controls under (a) low-, (b) middle-, and (c) high-N conditions.

Figure 5

Changes in 820-nm modulated reflection (MR) curves, expressed as normalized MR (represented as MR_t/MRo), in exogenously-treated or endogenous ALA-overproducing canola leaves grown under (a) low-, (b) middle-, and (c) high-N conditions.

Figure 6

Changes in the velocities of (a) P700 and plastocyanin (PC) oxidation (V_ox) and (b) subsequent rereduction (V_red) in exogenously-treated and endogenous ALA-overproducing canola leaves grown under low-, middle-, and high-N conditions. Values are means ± standard error (n = 10). The same letter indicates a nonsignificant difference (p > 0.05), according to Duncan’s test. Source of variation: ALA, N-level, and ALA × N-level interaction; ** p < 0.01; ns: not significant.