Application of trehalose ameliorates heat stress and promotes recovery of winter wheat seedlings

Trehalose was supplied to wheat (Triticum aestivum L.) seedlings just before a high temperature (40 °C) treatment and some physiological parameters were measured during the heat stress and recovery. The application of trehalose decreased the net photosynthetic rate (P_N) of wheat seedlings under the heat stress, but to a small extent increased the dry mass (DM) and leaf water content (LWC) after recovery from the heat stress. The trehalose-induced decrease in PN under the heat stress was not associated with a stomatal response. The heat stress slightly decreased the maximal efficiency of photosystem II (PS II) photochemistry (the variable to maximum chlorophyll a fluorescence ratio, F_v/F_m) similarly in the trehalose treated or non-treated plants. Under the heat stress, the actual efficiency of PS II photochemistry (Φ_PSII) and the efficiency of excitation energy capture by open reaction centers (F_v′/F_m′) were lower in the trehalose-pretreated seedlings, whereas they were higher after the recovery. The patterns of changes in nonphotochemical quenching (NPQ) were contrary to those of ?_{PS II} and F_v′/F_m′. The chlorophyll content was lower, whereas the β-carotene content and the degree of de-epoxidation (DEPS) of xanthophyll cycle pigments were higher in the trehalose-pretreated wheat seedlings under the heat stress. These results suggest that exogenous trehalose partially promotes recovery of wheat by the increase of NPQ, β-carotene content, and DEPS.     

The Regulation of Exogenous Jasmonic Acid on UV-B Stress Tolerance in Wheat

Enhanced ultraviolet-B radiation (UV-B, 280?C320?nm) is recognized as one of the environmental stress factors that cannot be neglected. Jasmonic acid (JA) is an important signaling molecule in a plant??s defense against biotic and abiotic stresses. To determine the role of exogenous JA in the resistance of wheat to stress from UV-B radiation, wheat seedlings were exposed to 0.9?kJ?m^?2?h^?1 UV-B radiation for 12?h after pretreatment with 1 and 2.5?mM JA, and the activity of antioxidant enzymes, the level of malondialdehyde (MDA), the content of UV-B absorbing compounds, photosynthetic pigments, and proline and chlorophyll fluorescence parameters were measured. The results of two-way ANOVA illustrated that the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), MDA level, anthocyanin and carotenoid (Car) content, and almost all chlorophyll fluorescence parameters were significantly affected by UV-B, JA, and UV-B?×?JA (P?<?0.05) [the maximal efficiency of photosystem II photochemistry (F _v/F _m) was not affected significantly by UV-B radiation]. Duncan??s multiple-range tests demonstrated that UV-B stress induced a significant reduction in plant photosystem II (PSII) function and SOD activity and an increased level of membrane lipid peroxidation, indicative of the deleterious effect of UV-B radiation on wheat. JA pretreatment obviously mitigated the detrimental effect of UV-B on PSII function by increasing F _v/F _m, reaction centers?? excitation energy capture efficiency (F _v??/F _m??), effective photosystem II quantum yield (??PSII), and photosynthetic electron transport rate (ETR), and by decreasing nonphotochemical quenching (NPQ) of wheat seedlings. Moreover, the activity of SOD and the content of proline and anthocyanin were provoked by exogenous JA. However, the MDA level was increased and Car content was decreased by exogenous JA with or without the presence of supplementary UV-B, whereas the contents of chlorophyll and flavonoids and related phenolics were not affected by exogenous JA. Meanwhile, exogenous JA resulted in a decrease of CAT and POD activities under UV-B radiation stress. These results partly confirm the hypothesis that exogenous JA could counteract the negative effects of UV-B stress on wheat seedlings to some extent.     

Developmental changes in energy dissipation in etiolated wheat seedlings during the greening process

We studied the developmental changes in photosynthetic and respiration rates and thermal dissipation processes connected with chloroplasts and mitochondria activity in etiolated wheat (Triticum aestivum L., var. Irgina) seedlings during the greening process. Etioplasts gradually developed into mature chloroplasts under continuous light [190 μmol(photon) m^?2 s^?1] for 48 h in 5-day-dark-grown seedlings. The net photosynthetic rate of irradiated leaves became positive after 6 h of illumination and increased further. The first two hours of de-etiolation were characterized by low values of maximum (F_v/F_m) and actual photochemical efficiency of photosystem II (PSII) and by a coefficient of photochemical quenching in leaves. F_v/F_m reached 0.8 by the end of 24 h-light period. During greening, energy-dependent component of nonphotochemical quenching of chlorophyll fluorescence, violaxanthin cycle (VXC) operation, and lipoperoxidation activity changed in a similar way. Values of these parameters were the highest at the later phase of de-etiolation (4–12 h of illumination). The respiration rate increased significantly after 2 h of greening and it was the highest after 4–6 h of illumination. It was caused by an increase in alternative respiration (AP) capacity. The strong, positive linear correlation was revealed between AP capacity and heat production in greening tissues. These results indicated that VXC in chloroplasts and AP in mitochondria were intensified as energy-dissipating systems at the later stage of greening (after 4 h), when most of prolamellar bodies converted into thylakoids, and they showed the greatest activity until the photosynthetic machinery was almost completely developed.     

Hexanoic acid 2-(diethylamino)ethyl ester enhances chilling tolerance in strawberry seedlings by impact on photosynthesis and antioxidants

Strawberry (Fragaria ananassa Duch.) seedlings were pretreated with hexanoic acid 2-(diethylamino)ethyl ester (DA-6) in concentrations of 0, 10, 20 and 40 mg dm⁻³ and then subjected to chilling and rewarming. The effects of applied DA-6 on the generation of reactive oxygen species (O₂ ⁻, H₂O₂), lipid peroxidation, proline accumulation and photosynthesis were evaluated. Pretreatment with DA-6 alleviated the inhibition of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activities caused by chilling stress thus reducing O₂ ⁻ and H₂O₂ production and lipid peroxidation in pretreated plants. DA-6 pretreatment also accelerated accumulation of proline and reduce the decrease in proline content after rewarming. DA-6 pretreatment increases maximum quantum yield of photosystem 2 (F_v/F_m), actual photochemical efficiency of photosystem 2 (Φ_PS2), photochemical quenching coefficient (qP) and net photosynthetic rate (P_N) and decreases non-photochemical quenching coefficient (qNP) of the seedlings under chilling stress. DA-6 pretreatment also increased the recovery rate of photosynthesis after rewarming.     

Photosynthetic activity and chlorophyll pigment concentration in Medicago x varia T. Martyn leaves treated with the Tytanit growth regulator

The purpose of the research was to determine the effect of the foliar use of a growth regulator with the trade name of Tytanit, containing titanium ascorbate, on photosynthetic activity and chlorophyll content in Medicago × varia T. Martyn leaves. There were two kinds of plots: C – control series; Ti – plants treated with Tytanit, containing 8.5 g of titanium in 1 dm³. The following parameters were determined: maximum photosystem II efficiency (F_v/F_m) in a dark-adapted state, actual photosystem II efficiency (ΔF/F_m’) in a light-adapted state, photochemical quenching factor (QP), non-photochemical quenching factor (QN), and chlorophyll a and b content. The Fisher-Snedecor test was used to determine whether the impact of experimental factors was significant, and the HSD 0.05 value was calculated using Tukey’s test. Compared to control, the photosynthetic apparatus performance of alfalfa was positively affected by the regulator compared to control. Tytanit applied to plant leaves increased their photosynthetic activity as a result of an increase in the content of chlorophyll pigments. It was also found that periods of rainfall deficiency did not affect the beneficial effects of the regulator.     

Study of photosynthetic characteristics of the Pyropia yezoensis thallus during the cultivation process

The photosynthetic characteristics of thalli of cultured Pyropia yezoensis strains collected in January, February, and March in seaweed cultivation area of South China Yellow Sea were studied. Results showed that the maximum quantum efficiency (F _v/F _m) of all P. yezoensis thallus collected at different times was 0.65. The actual quantum efficiency (ΔF/F _m′) of samples in January was the lowest of all samples, while the ΔF/F _m′ of samples in March was significantly higher than those in January and February. The increase of temperature and photosynthetic pigments ratios of phycoerythrin and chlorophyll a (PE/Chla) and phycocyanin and chlorophyll a (PC/Chla) from January to March may be the important reasons for the increase in light use efficiency of thallus; although the thallus in March was significantly thicker than in January which may have reduced the light energy absorbed by photosynthetic pigments, the increase of relative high energy use efficiency also helped to maintain the photosynthetic oxygen evolution rate in March. The thicker thallus also reduced photodamage, and the thallus area was increased obviously in March, so the growth rate of thallus in March was over 35 % higher than that in February. Our research indicates that the photosynthetic characteristics of P. yezoensis strains thalli have a close relationship with their growth stage and environmental factors especially temperature, and those photosynthetic characteristics are also reflected in the growth rate of the thalli.     

Metabolic recovery of the Antarctic liverwort <Emphasis Type="Italic">Cephaloziella varians</Emphasis> during spring snowmelt

We measured the responses of pigments and chlorophyll a fluorescence parameters of the Antarctic leafy liverwort Cephaloziella varians to snowmelt during austral spring 2005 at Rothera Point on the western Antarctic Peninsula. Although no changes to the concentrations of UV-B photoprotective pigments were detected during snowmelt, chlorophyll and carotenoid concentrations and maximum photosystem (PS)II yield (F _v /F _m) were respectively 88, 60 and 144% higher in the tissues of the liverwort that had recently emerged from snow than in those under a 10 cm depth of snow. A laboratory experiment similarly showed that effective PSII yield increased rapidly within the first 45 min after plants sampled from under snow were removed to an illuminated growth cabinet. The pigmentation and PSII yields of plants during snowmelt were also compared with those of plants in January, during the middle of the growing season at Rothera Point. During snowmelt, plants had lower F _v /F _m values, chlorophyll a/b ratios and concentrations of UV-B photoprotective pigments and carotenoids than during mid-season, suggesting that although there is some recovery of PSII activity and increases in concentrations of photosynthetic pigments during snowmelt, the metabolism of C. varians is restricted during this period.     

S-methylmethionine alleviates the cold stress by protection of the photosynthetic apparatus and stimulation of the phenylpropanoid pathway

Maize (Zea mays L.) seedlings were pretreated with 0.001 g dm^?3 S-methylmethionine (SMM) in the nutrient solution for 24 h and then subjected to chilling (6 °C for 2, 4, 6, 10, and 24 h). Cold stress significantly decreased the maximum quantum yield of photosystem II (variable to maximum chlorophyll fluorescence ratio, F_v/F_m) during the whole experiment but SMM pretreatment significantly reduced this decline. Content of phenolics and anthocyanins increased in response to low temperature, and SMM pretreatment further intensified the synthesis of these protective agents. These findings were supported by increased expression of genes coding enzymes of the phenylpropanoid pathway leading to synthesis of cinnamate-4-hydroxylase (C4H) and chalcone-synthase (CHS). Our results indicate that SMM pretreatment alleviates the low temperature stress by reducing the damage of the photosynthetic apparatus and stimulating the phenylpropanoid pathway.     

Development of Photosystem I and Photosystem II Activities in Leaves of Light-grown Maize (Zea mays)

To compare chloroplast development in a normally grown plant with etiochloroplast development, green maize plants (Zea mays), grown under a diurnal light regime (16-hour day) were harvested 7 days after sowing and chloroplast biogenesis within the leaf tissue was examined. Determination of total chlorophyll content, ratio of chlorophyll a to chlorophyll b, and O₂-evolving capacity were made for intact leaf tissue. Plastids at different stages of development were isolated and the electron-transporting capacities of photosystem I and photosystem II measured. Light saturation curves were produced for O₂-evolving capacity of intact leaf tissue and for photosystem I and photosystem II activities of isolated plastids. Structural studies were also made on the developing plastids. The results indicate that the light-harvesting apparatus becomes increasingly efficient during plastid development due to an increase in the photosynthetic unit size. Photosystem I development is completed before that of photosystem II. Increases in O₂-evolving capacity during plastid development can be correlated with increased thylakoid fusion. The pattern of photosynthetic membrane development in the light-grown maize plastids is similar to that found in greening etiochloroplasts.     

酸雨胁迫对苦槠幼苗气体交换与叶绿素荧光的影响

常绿阔叶树苦槠(Castanopsis sclerophylla)是亚热带地带性顶极群落的建群种之一, 在区域森林资源保护和可持续利用方面具有重要的地位。在该区域日益严重的酸雨胁迫下, 研究其对于胁迫的生理生态响应具有重要的理论价值和实践意义。该文以苦槠幼苗为研究材料, 研究了酸雨胁迫对苦槠幼苗光合生理的影响。结果表明: (1)短时间内, pH 2.5处理下的幼苗叶绿素相对含量最低, 且与pH 5.6处理下的存在显著性差异(p < 0.05); 经过一段时间处理后, pH 4.0处理下的叶绿素相对含量最高, 表明低浓度的酸雨会促进叶绿素相对含量的增加。(2) 2007年4月, 光合速率(P_n)、PSⅡ最大光化学效率(F_v/F_m)和PSⅡ的潜在活性(F_v/F₀)在不同酸雨浓度处理下基本无变化。随着酸雨处理时间的延长, pH 2.5处理下的P_n、光饱和点、光补偿点和暗呼吸速率有显著降低, 且与pH 5.6处理下的存在显著性差异(p < 0.05)。pH 2.5处理组与pH 5.6处理组之间的F_v/F_m和F_v/F₀差异性逐渐减小。表观量子效率和气孔导度变化规律不明显。综合来说, 酸雨处理前期, 高浓度的酸雨胁迫对苦槠幼苗叶绿素相对含量、光合生理参数有显著影响, 但随着酸雨处理时间的增加, 酸雨胁迫对苦槠幼苗的影响逐渐减小, 表明其对外界不良环境具有一定的抵御能力和适应能力。     

盐雾胁迫对极小种群植物日本荚蒾光合生理的影响

为评价日本荚蒾(Viburnum japonicum)的耐盐雾能力,对4 a生实生苗用不同盐雾浓度处理(盐雾NaCl质量浓度分别为0%、1%、2%、3%),测定叶片净光合速率、最大光化学效率(F_v/F_m)和叶绿素含量(Chl)等指标的变化。结果表明,1%盐雾处理的日本荚蒾植株能够存活,但生长不良,大于2%的盐雾处理的植株全部死亡。随着浓度的升高,日本荚蒾叶片的最大光合速率、F_v/F_m及Chl含量下降,而光饱和点及光补偿点总体呈上升趋势。这说明盐雾胁迫通过伤害光系统Ⅱ反应中心、改变植物可利用光能范围及降低叶绿素含量影响植物的光合作用。     

Responses of two field-grown coffee species to drought and re-hydration

The gas exchange, parameters of chlorophyll fluorescence, contents of pigments, and activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), as well as lipid peroxidation were investigated in two field-grown coffee species, Coffea arabica and C. liberica, exposed to drought and re-hydration. Drought caused a more pronounced inhibition of net photosynthetic rate in C. liberica compared to C. arabica. The de-epoxidation of xanthophyll cycle pigments at midday estimated by leaf reflectance was much higher in C. arabica than in C. liberica, but no significant change was found in response to drought. Under moderate drought, the activities of SOD and APX increased significantly only in C. arabica. The maximum photochemical efficiency of photosystem 2, PS2 (F_v/F_m) at predawn did not change and there was no lipid peroxidation during this time. Under severe drought F_v/F_m decreased and initial fluorescence (F₀) increased for both species, and SOD activity increased, APX activity remained relatively high, and malondialdehyde (MDA) accumulated in C. arabica, while APX decreased in C. liberica. The photosynthetic apparatus of C. arabica was completely recovered after 5 d of re-irrigation as indicated by the restoration of F_v/F_m to the control values. A lack of recovery upon rewatering of C. liberica indicated irreversible damage to PS2. Hence compared to C. liberica, C. arabica possesses a higher desiccation-induced antioxidative protection and higher portion of the total pigment pool used in photoprotection, which might aid alleviating photoinhibitory damage during desiccation and photosynthesis recovery when favourable conditions are restored.The research was financially supported by the project of the Chinese Academy of Sciences (KSCX2-SW-104).     

The Influence of Dark Adaptation Temperature on the Reappearance of Variable Fluorescence following Illumination

The effect of chilling temperatures (5°C) on chlorophyll fluorescence transients was used to study chilling-induced inhibition of photosynthesis in plant species with differing chilling sensitivities. A Brancker SF-20 fluorometer was used to measure induced fluorescence transients from both attached and detached leaves of chilling-sensitive cucumber (Cucumis sativus L. cv Ashley) and chilling-resistant pea (Pisum sativum L. cv Alaska). The rate of reappearance of the variable component of fluorescence (F_v), following a period of illumination at 25°C, was dependent on the temperature at which the leaf was allowed to dark adapt in chilling-sensitive cucumber, but not in chilling-resistant pea. In cucumber, dark adaptation at 25°C following illumination resulted in a much faster return of F_v than dark adaptation at 5°C following illumination. However, F_v reappearance during the dark adaptation period in chilling-resistant pea was temperature independent. The difference in the temperature response of F_v following illumination correlated with temperature sensitivity of these two species. The process responsible for the difference in F_v may represent a site of chilling sensitivity in the photosynthetic apparatus.     

Effects of chilling and high temperatures on photosynthesis and chlorophyll fluorescence in leaves of watermelon seedlings

The effects of chilling (CT, day/night temperatures of 12/10 °C, an irradiance of 250 μmol m^?2 s^?1), chilling combined with a low irradiance (CL, 12/10 °C, 80 μmol m^?2 s^?1), and a high temperature (HT, 42/40 °C, 250 μmol m^?2 s^?1) on chlorophyll content, chlorophyll fluorescence, and gas exchange were studied in two watermelon cultivars, ZJ8424 and YS01, differing in their resistance. The chlorophyll content, net photosynthetic rate (P_N), stomatal conductance (g_s), and transpiration rate (E) decreased substantially, whereas the intercellular CO₂ concentration (c_i) increased when the two watermelon cultivars were grown under these stresses. The photosynthetic parameters showed greater changes at chilling than at the high temperature, and the CL caused a more pronounced inhibition in PN compared with the CT. After 2 d exposure to the CT, YS01 had higher P_N, g_s, and E, but a lower ci compared with ZJ8424. The maximum efficiency of photosystem (PS) II photochemistry (F_v/F_m), effective quantum yield of PS II photochemistry (Φ_PSII), photochemical quenching (qP), and electron transport rate (ETR) decreased under the CT and CL but showed only a slight drop under the HT. All these stresses significantly increased non-photochemical quenching (NPQ). The CT brought more damage to the photosynthetic apparatus of leaves compared with the CL. In addition, after returning to normal conditions (25/15 °C, 250 μmol m^?2 s^?1) for 3 d, the photosynthetic parameters recovered to pre-stress levels in HT treated seedlings but not in CT treated seedlings. In conclusion, the low irradiance could help to alleviate the extent of photoinhibition of PS II photochemistry caused by chilling and cv. ZJ8424 was more sensitive to the extreme temperatures than cv. YS01.     

Effects of salt stress on photosynthesis,PSII photochemistry and thermal energy dissipation in leaves of two corn (<Emphasis Type="Italic">Zea mays</Emphasis> L.) varieties

The effect of four different NaCl concentrations (from 0 to 102 mM NaCl) on seedlings leaves of two corn (Zea mays L.) varieties (Aristo and Arper) was investigated through chlorophyll (Chl) a fluorescence parameters, photosynthesis, stomatal conductance, photosynthetic pigments concentration, tissue hydration and ionic accumulation. Salinity treatments showed a decrease in maximal efficiency of PSII photochemistry (F_v/F_m) in dark-adapted leaves. Moreover, the actual PSII efficiency (ϕ_PSII), photochemical quenching coefficient (q_p), proportion of PSII centers effectively reoxidized, and the fraction of light used in PSII photochemistry (%P) were also dropped with increasing salinity in light-adapted leaves. Reductions in these parameters were greater in Aristo than in Arper. The tissue hydration decreased in salt-treated leaves as did the photosynthesis, stomatal conductance (g _s) and photosynthetic pigments concentration essentially at 68 and 102 mM NaCl. In both varieties the reduction of photosynthesis was mainly due to stomatal closure and partially to PSII photoinhibition. The differences between the two varieties indicate that Aristo was more susceptible to salt-stress damage than Arper which revealed a moderate regulation of the leaf ionic accumulation.     

Analysis of Light-Induced Depressions of Photosynthesis in Leaves of a Wheat Crop during the Winter

The photosynthetic performances of individual leaves of a wheat (Triticum aestivum cv Bezostaya) crop were assessed daily and throughout individual days during the winter when temperature and light levels were fluctuating. Measurements of chlorophyll fluorescence induction and the maximum quantum yield of O₂ evolution were made on individual leaves. Depressions in the ratio of variable to maximal fluorescence (F_v/F_m) were correlated with low temperatures and high light levels throughout the winter and during the course of individual days. Depressions in F_v/F_m observed in the field during the day were not accompanied by any significant change in the ability of photosystem II complexes to bind 3-(3,4-dichlorophenyl)-1-dimethyl urea, indicating that the depressions in F_v/F_m were not attributable to photodamage to the D1 protein of the photosystem II reaction center. Decreases in F_v/F_m were associated with increases in the rate of dissipation of excitation energy by radiationless decay processes and decreases in the quantum efficiency of CO₂ assimilation, indicative of a rapidly reversible light-induced “downregulation” of photosynthesis. No major changes were observed in the maximum quantum efficiency of O₂ evolution of leaves throughout periods of fluctuating temperature and light, because light-induced depressions in photosynthetic efficiency recovered within the time required to make these measurements.     

Comparative analysis of photosynthetic characteristics and productivity in F2 hybrids of winter rye

Diversity of photosynthetic characteristics determined by plant genotypes provides the grounds for the increase in potential crop productivity by means of producing plant forms whose photosynthetic apparatus (PSA) has optimal size and functional efficiency. Parental forms of winter rye (Secale cereale L.) and their interline and intravarietal reciprocal F₂ hybrids were compared in terms of photosynthetic pigment (PSP) content, characteristics of chlorophyll a (Chl) fluorescence, some morphological traits, and grain productivity. At the booting and anthesis stages, significant divergence in chlorophyllous pigment content and Chl a fluorescence parameters was observed for winter rye inbred lines, rye varieties, and hybrids. The hybrids were revealed whose elevated grain productivity correlated with high PSP content and the highest photosystem II (PSII) activity. Analysis of correlations in reciprocal F₂ hybrids at booting and anthesis stages of rye development showed that accumulation of PSII is related to stem-forming capacity and to the flag leaf surface area. In reciprocal hybrids, the correlations between morphological traits, grain productivity, plastid pigments, and photochemical activity were subject to variations. The relationships between PSA parameters and grain productivity in winter rye F₂ hybrids varied depending on the developmental stage as well as on crossing combination; this relation was largely determined by the choice of pairs for hybridization, by direction of crosses, and by genetic features of parental forms.     

Quantify the response of purslane plant growth,photosynthesis pigments and photosystem II photochemistry to cadmium concentration gradients in the soil

The cadmium (Cd), being a widespread soils pollutant and one of the most toxic heavy metals in the environment, adversely affects sustainable crop production and food safety. Pot experiment was conducted to quantify and simulate the response of purslane (Portulaca oleracea L.) plants to Cd toxicity. The purslane germinated seeds were cultivated in twelve Cd concentrations (from 0 to 300 mg/kg of Cd in soil) for six weeks and then some growth characteristics, photosynthesis pigments, and chlorophyll a fluorescence parameters were measured. The influence of Cd gradients in the soil on all growth parameters, photosynthesis pigments and chlorophyll a fluorescence parameters (except F_m and carotenoid content) were described by a segmented model. Furthermore, F_m and carotenoid contents were fitted to a linear model. The growth characteristics, chlorophyll content, photosynthetic pigments and some parameters of chlorophyll a fluorescence such as F_v, F_v/F_m, Y(II) and ETR decreased when Cd concentration increased. In contrast, F₀, Y(NPQ) and Y(NO) increased and F_m was not significantly affected. In general, most variations in the studied parameters were recorded with low concentrations of cadmium, which ranged from 0 to 125 mg/kg. Also, the growth characteristics (especially stem, leaf, and shoot dry weights) were more sensitive to Cd contamination than other parameters. Moreover, among chlorophyll fluorescence parameters, Y(NPQ) was the most sensitive to Cd concentration gradients in the soil that can be due to disturbances of antennae complex of PSII.     

Photocontrol of the Accumulation of Plastid Polypeptides during Greening of Tomato Cotyledons : Potentiation by a Pulse of Red Light

A pulse of red light acting through phytochrome accelerates the formation of chlorophyll upon subsequent transfer of dark-grown seedlings to continuous white light. Specific antibodies were used to follow the accumulation of representative subunits of the major photosynthetic complexes during greening of seedlings of tomato (Lycopersicon esculentum). The time course for accumulation of the various subunits was compared in seedlings that received a red light pulse 4 h prior to transfer to continuous white light and parallel controls that did not receive a red light pulse. The light-harvesting chlorophyll-binding proteins of photosystem II (LHC II), the 33-kD extrinsic polypeptide of the oxygen-evolving complex (OEC1), and subunit II of photosystem I (psaD gene product) all increased in the light, and did so much faster in seedlings that received the inductive red light pulse. The red light pulse had no significant effect on the abundance of the small subunit of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), nor on several plastid-encoded polypeptides: the large subunit of Rubisco, the β subunit of the CF₁ complex of plastid ATPase, and the 43- and 47-kD subunits of photosystem II (CP43, CP47). Subunits I (cytochrome b₆f) and III (Rieske Fe-S protein) of the cytochrome b₆f complex showed a small or no increase as a result of the red pulse. The potentiation of greening by a pulse of red light, therefore, is not expressed uniformly in the abundance of all the photosynthetic complexes and their subunits.     

Closure of Stomata in Water-Stressed Pine Needles Results from the Decreased Water Potential of the Mesophyll Apoplast in the Substomatal Cavity

Pine (Pinus sylvestris L.) seedlings grown under controlled conditions were subjected to water deficit (external water potentials ranging from–0.15 to–1.5 MPa) by adding polyethylene glycol 6000 (PEG) to the nutrient solution. Following this treatment, the dry weights of plant shoots and roots, as well as the ratio of variable to maximum chlorophyll fluorescence (F_v/F_m), nonphotochemical quenching (NPQ) of chlorophyll excitations, photosynthetic CO₂/H₂O exchange, dark respiration of needles, and water potential of mesophyll apoplast in the substomatal cavity of pine needles, were measured. The imposed water deficit was followed by the inhibition of seedling growth, suppression of photosynthesis and transpiration, and by the decreased content of photosynthetic pigments. It is shown for the first time that the closure of stomata in the needles of water-stressed pine seedlings falls into the physiological reaction norm and is caused by the reduction of water potential in the mesophyll apoplast of the substomatal cavity.