Physiological, Biochemical and Molecular Assessment of UV-A and UV-B Supplementation in Solanum lycopersicum

Abstract

Daily UV-supplementation during the plant fruiting stage of tomato (Solanum lycopersicum L.) growing indoors may produce fruits with higher nutraceutical value and better acceptance by consumers. However, it is important to ensure that the plant's performance during this stage is not compromised by the UV supplement. We studied the impact of UV-A (1 and 4 h) and UV-B (2 and 5 min) on the photosynthesis of greenhouse-grown tomato plants during the fruiting/ripening stage. After 30 d of daily irradiation, UV-B and UV-A differently interfered with the photosynthesis. UV-B induced few leaf-necrotic spots, and effects are more evidenced in the stimulation of photosynthetic/protective pigments, meaning a structural effect at the Light-Harvesting Complex. UV-A stimulated flowering/fruiting, paralleled with no visible leaf damages, and the impact on photosynthesis was mostly related to functional changes, in a dose-dependent manner. Both UV-A doses decreased the maximum quantum efficiency of photosystem II (Fv/Fm), the effective efficiency of photosystem II (ΦPSII), and gas exchange processes, including net carbon assimilation (P_N). Transcripts related to Photosystem II (PSII) and RuBisCO were highly stimulated by UV supplementation (mostly UV-A), but the maintenance of the RuBisCO protein levels indicates that some protein is also degraded. Our data suggest that plants supplemented with UV-A activate adaptative mechanisms (including increased transcription of PSII peptides and RuBisCO), and any negative impacts on photosynthesis do not compromise the final carbohydrate balances and plant yield, thus becoming a profitable tool to improve precision agriculture.

Keywords: fruiting stage; indoor growth; photosynthesis; tomato; ultraviolet supplementation.

Figure 1

Chlorophyll a fluorescence after 30 d of exposure to different UV-A/B treatments. Minimal fluorescence yield of dark-adapted leaves with all PSII centers closed, F₀ (a); maximum fluorescence of dark-adapted leaves with all PSII centers closed, Fm (b); maximum quantum efficiency of PSII, Fv/Fm (c); PSII maximum efficiency in saturating light if all reaction centers are open, Fv’/Fm’ (d); photochemical quenching, qP (e); effective efficiency of PSII, ΦPSII (f) and non-photochemical quenching, NPQ (g). Within each parameter, *, *** and **** mean significant differences for p < 0.05, 0.01, 0.001 and 0.0001, respectively. Values are expressed as the mean and standard deviation (n = 6).

Figure 2

Leaf gas-exchange after 30 d of exposure to different UV-A/B conditions. Net photosynthetic rate, P_N (a); intercellular CO₂ concentration, Ci (b); stomatal conductance, gs (c); transpiration rate, E (d); intrinsic water-use efficiency iWUE, (P_N/gs) (e). Within each parameter, *, **, *** and **** represent significant differences for p ≤ 0.05, 0.01, 0.001 and 0.0001, respectively. Values are expressed as the mean and standard deviation (n = 6).

Figure 3

Carbon fixation in plants exposed 30 d to different UV-A/B conditions. Starch (a), Total Soluble Sugars (b), and Relative RuBisCO content (c) values. FM (Fresh Matter). Within each parameter, * represent significant differences for p < 0.05. Values are expressed as the mean and standard deviation (n = 10).

Figure 4

Regulation of photosynthesis pathway genes by different UV-A/B conditions. All parameters were measured in control, UV-A 4 h and UV-B 5 min (the last two are the higher exposure times for each radiation). The relative expression of the photosynthetic components was determined for psbA (a) and psbB (b) which encodes the D1 protein and CP47, respectively. At the same time, the relative expression of the genes encoding for the two subunits of RuBisCO were also assessed: rbcS (c) and rbcL (d) for small and large subunits, respectively. Within each parameter, *, **, *** and **** represent significant differences for p < 0.05, 0.01, 0.001 and 0.0001, respectively. Values are expressed as the mean and standard deviation. (n = 10).

Figure 5

PCA analysis of functional responses of tomato fruit plants exposed to UV-A (1 and 4 h) and to UV-B (2 and 5 min) for 30 d.

Figure 6

Major photosynthetic impacts and changes induced by moderate UV-A 4 h/day, for 30 d, in tomato flowering plants. Overall, the PSII fluorescence is affected by the decreased efficiency of ΦPSII, although LHC-pigment (Chl/car = chlorophyll/carotenoids) levels are not affected. This leads to fewer electrons being transported and thus a decrease in NADPH and ATP production and availability for the Calvin cycle. This reduction is related with the decrease in the net photosynthetic rate (P_N), meaning that internal CO₂ concentration (Ci) is not so depleted, and the stomatal conductance (gs) may decrease, therefore decreasing transpiration rate (E). Simultaneously, a degradation of RuBisCO may occur, but it can be replaced by new protein due to the stimulated accumulation of its transcripts (and increase its transcription), which overall may reset the negative impacts on the Calvin cycle, thus not having negative impacts on total amounts of soluble sugars and starch. Solid red arrows mean a decrease and solid blue arrows an increase. Dashed red and blue arrows mean a putative decrease and increase, respectively.