Vacuum microwave dehydration decreases volatile concentration and soluble protein content of pea (Pisum sativum L.) protein.

BACKGROUND: Peas are an inexpensive yet nutritious and sustainable source of protein. However, it is challenging to incorporate pea proteins into food formulations owing to their beany or green off-flavours and their limited water solubility. RESULTS: Vacuum microwave dehydration (VMD) of pea protein with an initial moisture content of 425% (dry basis, db) at 2 W g-1 specific microwave energy and 200 Torr vacuum level for 88 min led to an 83% reduction in total volatile compound concentration. VMD processing at high initial moisture contents facilitated the Maillard reaction, enhancing the extent of protein cross-linking, leading to a marked decrease in soluble protein content, to 11 g kg-1 . Reducing the initial moisture content to 56% db greatly retained protein solubility (112-113 g kg-1 ), but it only led to a minor reduction in total volatile compound concentration (2-11% reduction). A high microwave energy (20 W g-1 )-short time (2 min) treatment at 200 Torr vacuum level was found optimal, reducing both volatile levels and soluble protein content by ~50%. CONCLUSION: Evidently, it is difficult to employ VMD without reduction of pea protein solubility and corresponding changing in functionality. Yet, if optimized, VMD has the capability to decrease volatile concentrations while retaining protein solubility. Future sensory analysis should be conducted to determine whether the aforementioned reductions in total volatile compound concentration may have a notable effect on consumer palatability. © 2020 Society of Chemical Industry.

Analyzing both the fast and the slow phases of chlorophyll a fluorescence and P700 absorbance changes in dark-adapted and preilluminated pea leaves using a Thylakoid Membrane model.

The dark-to-light transitions enable energization of the thylakoid membrane (TM), which is reflected in fast and slow (OJIPSMT or OABCDE) stages of fluorescence induction (FI) and P700 oxidoreduction changes (ΔA810). A Thylakoid Membrane model (T-M model), in which special emphasis has been placed on ferredoxin-NADP+-oxidoreductase (FNR) activation and energy-dependent qE quenching, was applied for quantifying the kinetics of FI and ΔA810. Pea leaves were kept in darkness for 15 min and then the FI and ΔA810 signals were measured upon actinic illumination, applied either directly or after a 10-s light pulse coupled with a subsequent 10-s dark interval. On the time scale from 40 µs to 30 s, the parallel T-M model fittings to both FI and ΔA810 signals were obtained. The parameters of FNR activation and the buildup of qE quenching were found to differ for dark-adapted and preilluminated leaves. At the onset of actinic light, photosystem II (PSII) acceptors were oxidized (neutral) after dark adaptation, while the redox states with closed and/or semiquinone QA(-)QB(-) forms were supposedly generated after preillumination, and did not relax within the 10 s dark interval. In qE simulations, a pH-dependent Hill relationship was used. The rate constant of heat losses in PSII antenna kD(t) was found to increase from the basic value kDconst, at the onset of illumination, to its maximal level kDvar due to lumenal acidification. In dark-adapted leaves, a low value of kDconst of ∼ 2 × 106 s-1 was found. Simulations on the microsecond to 30 s time scale revealed that the slow P-S-M-T phases of the fluorescence induction were sensitive to light-induced FNR activation and high-energy qE quenching. Thus, the corresponding time-dependent rate constants kD(t) and kFNR(t) change substantially upon the release of electron transport on the acceptor side of PSI and during the NPQ development. The transitions between the cyclic and linear electron transport modes have also been quantified in this paper.

Distinct UV-A or UV-B irradiation induces protochlorophyllide photoreduction and bleaching in dark-grown pea (Pisum sativum L.) epicotyls.

The effects of distinct UV-A and UV-B radiations were studied on etiolated pea (Pisum sativum L.) epicotyls. Emission spectra of the native protochlorophyll and protochlorophyllide forms were measured when epicotyls were excited with 360 or 300 nm light. The UV-A (360 nm) excited mainly the non-enzyme-bound monomers of protochlorophyll and protochlorophyllide and the UV-B (300 nm) excited preferentially the flash-photoactive protochlorophyllide complexes. These latter complexes converted into short- and long-wavelength chlorophyllide forms at 10-s illumination with both wavelength irradiations. As the spectral changes were very small, the effects of longer illumination periods were studied. Room temperature fluorescence emission spectra were measured from the same epicotyl spots before and after irradiation with various wavelengths between 280 and 360 nm for 15 min and the "illuminated" minus "dark" difference spectra were calculated. Both the UV-A and the UV-B irradiations caused photoreduction of protochlorophyllide into chlorophyllide. At 10 µmol photons m-2 s-1, the photoreduction rates were similar, however, at 60 µmol photons m-2 s-1, the UV-B irradiation was more effective in inducing chlorophyllide formation than the UV-A. The action spectra of protochlorophyllide plus protochlorophyll loss and chlorophyllide production showed that the radiation around 290 nm was the most effective in provoking protochlorophyllide photoreduction and the UV light above 320 nm caused strong bleaching. These results show that the effect of the UV radiation should be considered when discussing the protochlorophyllide-chlorophyllide photoreduction during germination and as a part of the regeneration of the photosynthetic apparatus proceeding in the daily run of photosynthesis.

Elevated air humidity increases UV mediated leaf and DNA damage in pea (Pisum sativum) due to reduced flavonoid content and antioxidant power.

Growth in high relative air humidity (RH, >85%) affects plant morphology and causes diminished response to stomatal closing signals. Many greenhouses are prone to high RH conditions, which may negatively affect production and post-harvest quality. UV radiation induces stomatal closure in several species, and facilitates disease control. We hypothesised that UV exposure may trigger stomatal closure in pea plants (Pisum sativum) grown in high RH, thereby restoring stomatal function. The effects of UV exposure were tested on plants grown in moderate (60%) or high (90%) RH. UV exposure occurred at night, according to a disease control protocol. Lower stomatal conductance rates were found in UV-exposed plants, though UV exposure did not improve the rate of response to closing stimuli or desiccation tolerance. UV-exposed plants showed leaf curling, chlorosis, necrosis, and DNA damage measured by the presence of cyclobutane pyrimidine dimers (CPD), all of which were significantly greater in high RH plants. These plants also had lower total flavonoid content than moderate RH plants, and UV-exposed plants had less than controls. Plants exposed to UV had a higher content of cuticular layer uronic compounds than control plants. However, high RH plants had a higher relative amount of cuticular waxes, but decreased proteins and uronic compounds. Plants grown in high RH had reduced foliar antioxidant power compared to moderate RH. These results indicate that high RH plants were more susceptible to UV-induced damage than moderate RH plants due to reduced flavonoid content and oxidative stress defence.

Is carbonic anhydrase activity of photosystem II required for its maximum electron transport rate?

It is known, that the multi-subunit complex of photosystem II (PSII) and some of its single proteins exhibit carbonic anhydrase activity. Previously, we have shown that PSII depletion of HCO3-/CO2 as well as the suppression of carbonic anhydrase activity of PSII by a known inhibitor of α­carbonic anhydrases, acetazolamide (AZM), was accompanied by a decrease of electron transport rate on the PSII donor side. It was concluded that carbonic anhydrase activity was required for maximum photosynthetic activity of PSII but it was not excluded that AZM may have two independent mechanisms of action on PSII: specific and nonspecific. To investigate directly the specific influence of carbonic anhydrase inhibition on the photosynthetic activity in PSII we used another known inhibitor of α­carbonic anhydrase, trifluoromethanesulfonamide (TFMSA), which molecular structure and physicochemical properties are quite different from those of AZM. In this work, we show for the first time that TFMSA inhibits PSII carbonic anhydrase activity and decreases rates of both the photo-induced changes of chlorophyll fluorescence yield and the photosynthetic oxygen evolution. The inhibitory effect of TFMSA on PSII photosynthetic activity was revealed only in the medium depleted of HCO3-/CO2. Addition of exogenous HCO3- or PSII electron donors led to disappearance of the TFMSA inhibitory effect on the electron transport in PSII, indicating that TFMSA inhibition site was located on the PSII donor side. These results show the specificity of TFMSA action on carbonic anhydrase and photosynthetic activities of PSII. In this work, we discuss the necessity of carbonic anhydrase activity for the maximum effectiveness of electron transport on the donor side of PSII.

Involvement of the chloroplast plastoquinone pool in the Mehler reaction.

Light-dependent oxygen reduction in the photosynthetic electron transfer chain, i.e. the Mehler reaction, has been studied using isolated pea thylakoids. The role of the plastoquinone pool in the Mehler reaction was investigated in the presence of dinitrophenyl ether of 2-iodo-4-nitrothymol (DNP-INT), the inhibitor of plastohydroquinone oxidation by cytochrome b6/f complex. Oxygen reduction rate in the presence of DNP-INT was higher than in the absence of the inhibitor in low light at pH 6.5 and 7.6, showing that the capacity of the plastoquinone pool to reduce molecular oxygen in this case exceeded that of the entire electron transfer chain. In the presence of DNP-INT, appearance of superoxide anion radicals outside thylakoid membrane represented approximately 60% of the total superoxide anion radicals produced. The remaining 40% of the produced superoxide anion radicals was suggested to be trapped by plastohydroquinone molecules within thylakoid membrane, leading to the formation of hydrogen peroxide (H2 O2 ). To validate the reaction of superoxide anion radical with plastohydroquinone, xanthine/xanthine oxidase system was integrated with thylakoid membrane in order to generate superoxide anion radical in close vicinity of plastohydroquinone. Addition of xanthine/xanthine oxidase to the thylakoid suspension resulted in a decrease in the reduction level of the plastoquinone pool in the light. The obtained data provide additional clarification of the aspects that the plastoquinone pool is involved in both reduction of oxygen to superoxide anion radicals and reduction of superoxide anion radicals to H2 O2 . Significance of the plastoquinone pool involvement in the Mehler reaction for the acclimation of plants to light conditions is discussed.

Sugar demand, not auxin, is the initial regulator of apical dominance.

For almost a century the plant hormone auxin has been central to theories on apical dominance, whereby the growing shoot tip suppresses the growth of the axillary buds below. According to the classic model, the auxin indole-3-acetic acid is produced in the shoot tip and transported down the stem, where it inhibits bud growth. We report here that the initiation of bud growth after shoot tip loss cannot be dependent on apical auxin supply because we observe bud release up to 24 h before changes in auxin content in the adjacent stem. After the loss of the shoot tip, sugars are rapidly redistributed over large distances and accumulate in axillary buds within a timeframe that correlates with bud release. Moreover, artificially increasing sucrose levels in plants represses the expression of BRANCHED1 (BRC1), the key transcriptional regulator responsible for maintaining bud dormancy, and results in rapid bud release. An enhancement in sugar supply is both necessary and sufficient for suppressed buds to be released from apical dominance. Our data support a theory of apical dominance whereby the shoot tip's strong demand for sugars inhibits axillary bud outgrowth by limiting the amount of sugar translocated to those buds.

Ethylene Signaling Influences Light-Regulated Development in Pea.

Plant responses to light involve a complex network of interactions among multiple plant hormones. In a screen for mutants showing altered photomorphogenesis under red light, we identified a mutant with dramatically enhanced leaf expansion and delayed petal senescence. We show that this mutant exhibits reduced sensitivity to ethylene and carries a nonsense mutation in the single pea (Pisum sativum) ortholog of the ethylene signaling gene ETHYLENE INSENSITIVE2 (EIN2). Consistent with this observation, the ein2 mutation rescues the previously described effects of ethylene overproduction in mature phytochrome-deficient plants. In seedlings, ein2 confers a marked increase in leaf expansion under monochromatic red, far-red, or blue light, and interaction with phytochromeA, phytochromeB, and long1 mutants confirms that ein2 enhances both phytochrome- and cryptochrome-dependent responses in a LONG1-dependent manner. In contrast, minimal effects of ein2 on seedling development in darkness or high-irradiance white light show that ethylene is not limiting for development under these conditions. These results indicate that ethylene signaling constrains leaf expansion during deetiolation in pea and provide further evidence that down-regulation of ethylene production may be an important component mechanism in the broader control of photomorphogenic development by phytochrome and cryptochrome.

Epidermal UV-A absorbance and whole-leaf flavonoid composition in pea respond more to solar blue light than to solar UV radiation.

Plants synthesize phenolic compounds in response to certain environmental signals or stresses. One large group of phenolics, flavonoids, is considered particularly responsive to ultraviolet (UV) radiation. However, here we demonstrate that solar blue light stimulates flavonoid biosynthesis in the absence of UV-A and UV-B radiation. We grew pea plants (Pisum sativum cv. Meteor) outdoors, in Finland during the summer, under five types of filters differing in their spectral transmittance. These filters were used to (1) attenuate UV-B; (2) attenuate UV-B and UV-A < 370 nm; (3) attenuate UV-B and UV-A; (4) attenuate UV-B, UV-A and blue light; and (5) as a control not attenuating these wavebands. Attenuation of blue light significantly reduced the flavonoid content in leaf adaxial epidermis and reduced the whole-leaf concentrations of quercetin derivatives relative to kaempferol derivatives. In contrast, UV-B responses were not significant. These results show that pea plants regulate epidermal UV-A absorbance and accumulation of individual flavonoids by perceiving complex radiation signals that extend into the visible region of the solar spectrum. Furthermore, solar blue light instead of solar UV-B radiation can be the main regulator of phenolic compound accumulation in plants that germinate and develop outdoors.

The role of strigolactones in photomorphogenesis of pea is limited to adventitious rooting.

The recently discovered group of plant hormones, the strigolactones, have been implicated in regulating photomorphogenesis. We examined this extensively in our strigolactone synthesis and response mutants and could find no evidence to support a major role for strigolactone signaling in classic seedling photomorphogenesis (e.g. elongation and leaf expansion) in pea (Pisum sativum), consistent with two recent independent reports in Arabidopsis. However, we did find a novel effect of strigolactones on adventitious rooting in darkness. Strigolactone-deficient mutants, Psccd8 and Psccd7, produced significantly fewer adventitious roots than comparable wild-type seedlings when grown in the dark, but not when grown in the light. This observation in dark-grown plants did not appear to be due to indirect effects of other factors (e.g. humidity) as the constitutively de-etiolated mutant, lip1, also displayed reduced rooting in the dark. This role for strigolactones did not involve the MAX2 F-Box strigolactone response pathway as Psmax2 f-box mutants did not show a reduction in adventitious rooting in the dark compared with wild-type plants. The auxin-deficient mutant bushy also reduced adventitious rooting in the dark, as did decapitation of wild-type plants. Rooting was restored by the application of indole-3-acetic acid (IAA) to decapitated plants, suggesting a role for auxin in the rooting response. However, auxin measurements showed no accumulation of IAA in the epicotyls of wild-type plants compared with the strigolactone synthesis mutant Psccd8, suggesting that changes in the gross auxin level in the epicotyl are not mediating this response to strigolactone deficiency.