Near-isogenic lines of maize varying in their genes for flavonoid biosynthesis were utilized to examine the effects of foliar flavonoids and nutrient deficiency on maximum net photosynthetic rate (PN) and chlorophyll (Chl) fluorescence (Fv/Fm) in response to ultraviolet-B (UV-B) radiation. Plants with deficient (30 to 70 % lower N, K, Mn, Fe, and Zn) and sufficient nutrients were exposed to four irradiation regimes: (1) no UV-B with solar photosynthetically active radiation (PAR), (2) two day shift to ambient artificial UV-B, 8.2-9.5 kJ m-2 d-1 (21-25 mmol m-2 d-1); (3) continuous ambient artificial UV-B; (4) continuous solar UV-B in Hawaii 12-18 kJ m-2 d-1 (32-47 mmol m-2 d-1). The natural ratio of UVB: PAR (0.25-0.40) was maintained in the UV-B treatments. In the adequately fertilized plants, lines b and lc had higher contents of flavonoids and anthocyanins than did lines hi27 and dta. UV-B induced the accumulation of foliar flavonoids in lines hi27 and b, but not in the low flavonoid line dta or in the high flavonoid line lc. In plants grown on deficient relative to adequate nutrients, flavonoid and anthocyanin contents decreased by 30-40 and 40-50 %, respectively, and Chl a and Chl b contents decreased by 30 and 70 %, respectively. The UV-B treatments did not significantly affect PN and Fv/Fm in plants grown on sufficient nutrients, except in the low flavonoid lines dta and hi27 in which PN and Fv/Fm decreased by ∼15 %. PN, Fv/Fm, and stomatal conductance decreased markedly (20-30 %) in all lines exposed to UV-B when grown on low nutrients. The decrease in Fv/Fm was 10 % less in higher flavonoid lines b and lc. The photosynthetic apparatus of maize readily tolerated ambient UV-B in the tropics when plants were adequately fertilized. In contrast, ambient UV-B combined with nutrient deficiency significantly reduced photosynthesis in this C4 plant. Nutrient deficiency increased the susceptibility of maize to UV-B-induced photoinhibition in part by decreasing the contents of photoprotective compounds. and T. S. L. Lau ... [et al.].
The effects of Mn-deficiency on CO2 assimilation and excitation energy distribution were studied using Mn-starved maize leaves. Mn-deficiency caused about 70 % loss in the photon-saturated net photosynthetic rate (PN) compared to control leaves. The loss of PN was associated with a strong decrease in the activity of oxygen evolution complex (OEC) and the linear electron transport driven by photosystem 2 (PS2) in Mn-deficienct leaves. The photochemical quenching of PS2 (qP) and the maximum efficiency of PS2 photochemistry (Fv/Fm) decreased significantly in Mn-starved leaves under high irradiance, implicating that serious photoinhibition took place. However, the 'high-energy' fluorescence quenching (qE) decreased, which was associated with xanthophyll cycle. The results showed that the pool of de-epoxidation components of the xanthophyll cycle was lowered markedly owing to Mn deficiency. Linear electron transport driven by PS2 de-creased significantly and was approximately 70 % lower in Mn-deficient leaves than that in control, indicating less trans-thylakoid pH gradient was built in Mn deficient leaves. We suggest that the decrease of non-radiative dissipation depending on xanthophyll cycle in Mn-starved leaves is a result of the deficiency of trans-thylakoid pH gradient. and C. D. Jiang, H. Y. Gao, Q. Zou.
Maize (Zea mays) seedlings were exposed for 6 h to strong irradiance (1 000 μmol m-1 s-1 of PPFD) at 5, 12, 17, or 25 °C, followed by an exposure to the darkness for 6 h at 22 °C. Leaf chlorophyll fluorescence, net photosynthetic rate (PN), and the amount of superoxide radicals (O2-⋅) in relation to chilling-induced photoinhibition were investigated. During the photophase, a good correlation (r=-0.879) was observed between ΦPS2 (relative quantum efficiency of PS2 electron transport) and the amount of O2-⋅. Treatment with exogenous O2-⋅ reduced the PN and ΦPS2 as the chilling stress did, that was inhibited by specific scavenger of O2-⋅. Hence chilling-induced photoinhibition might be due to the production of O2-⋅. In contrast, in the dark period, PN and ΦPS2 of the seedlings treated with the exogenous O2-⋅ were enhanced, but they were inhibited by the specific scavenger of O2-⋅, showing the photoprotective role of O2-⋅ in the recovery phase. Furthermore, in terms of the effect of exogenous O2-⋅ on the xanthophyll cycle, the O2-⋅ production suggested a promotion effect for the de-epoxidation of violaxanthin during the photophase, the epoxidation of zeaxanthin at the dark stage, and the increase of the xanthophyll pool both in the photophase and dark phase, resulting in an enhancement of the ability of non-photochemical quenching to avoid or alleviate the damage to photosynthetic apparatus. and D. Ke, G. Sun, Y. Jiang.
The influence of chilling (8 °C, 5 d) at two photon flux densities [PFD, L = 200 and H = 500 μmol(photon) m-2 s-1] on the gas exchange and chlorophyll fluorescence was investigated in chilling-tolerant and chilling-sensitive maize hybrids (Zea mays L., K383×K130, K185×K217) and one cultivar of field bean (Vicia faba L. minor, cv. Nadwiślański). The net photosynthetic rate (PN) for the both studied plant species was inhibited at 8 °C. PN of both maize hybrids additionally decreased during chilling. Changes in the quantum efficiency of PS2 electron transport (ΦPS2) as a response to chilling and PFD were similar to PN. Measurements of ΦPS2/ΦCO2 ratio showed that in field bean seedlings strong alternative photochemical sinks of energy did not appear during chilling. However, the high increment in ΦPS2/ΦCO2 for maize hybrids can indicate reactions associated with chill damage generation. At 8 °C the non-photochemical quenching (NPQ) increased in all plants with chilling duration and PFD. The appearance of protective (qI,p) and damage (qI,d) components of qI and a decrease in qE (energy dependent quenching) took place. NPQ components of field bean and maize hybrids differed from each other. The amount of protective NPQ (qE + qI,p) components as part of total NPQ was higher in field bean than in maize hybrids at both PFD. On 5th day of chilling, the sum of qE and qI,p was 26.7 % of NPQ in tolerant maize hybrids and 17.6 % of NPQ in the sensitive one (averages for both PFD). The increased PFD inhibited the ability of all plants to perform protective dissipation of absorbed energy. The understanding of the genotypic variation of NPQ components in maize may have implications for the future selection of plants with a high chilling tolerance. and J. Kościelniak, J. Biesaga-Kościelniak.