Changes in the synthesis of rubisco in rice leaves in relation to senescence and N influx

BACKGROUND AND AIMS: The amount of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) synthesized in a leaf is closely correlated with N influx into the leaf throughout its lifetime. Rubisco synthesis and N influx are most active in the young leaf during expansion, but are very limited in the senescent leaf. However, it is not established whether Rubisco synthesis can be observed if N influx is increased, even in a very senescent leaf. This study first investigated changes in the relationships between rbcS and rbcL mRNA contents and Rubisco synthesis per unit of leaf mass with leaf senescence. Next, leaves were removed during late senescence, to examine whether Rubisco synthesis is re-stimulated in very senescent leaves by an increase in N influx. METHODS: Different N concentrations (1 and 4 mm) were supplied to Oryza sativa plants at the early (full expansion), middle and late stages (respectively 8 and 16 d after full expansion) of senescence of the eighth leaf. To enhance N influx into the eighth leaf 16 d after full expansion, all leaf blades on the main stem, except for the eighth leaf, and all tillers were removed and plants received 4 mm N (removal treatment). KEY RESULTS: Rubisco synthesis, rbcS and rbcL mRNAs and the translational efficiencies of rbcS and rbcL mRNAs decreased with leaf senescence irrespective of N treatments. However, in the removal treatment at the late stage, they increased more strongly with an increase in N influx than in intact plants. CONCLUSIONS: Although Rubisco synthesis and rbcS and rbcL mRNAs decrease with leaf senescence, leaves at the late stage of senescence have the potential actively to synthesize Rubisco with an increase in N influx.     

Changes in the activation state of rubisco in bean leaves in relation to recovery of photosynthetic activity after various treatments

Changes in the activation level of rubisco from bean leaves were studied by measuring carboxylation rates after either a prolonged period in the dark after photoinhibitory treatment. Plants kept for 11 h in darkness were subjected to three stages of strong irradiance: stage I, photosynthesis in air; stage II, exposure in1% O₂ and CO₂-free N₂; stage III, reintroduction of air. Decrease in the rate of CO₂ uptake at stage III are due to photoinhibition. From the end of stage I to stage III, initial carboxylase activity, measured in extracts, exhibited large variations whereas changes for total activity were much smaller. Initial activity was four times lower at stage II compared to stage I. It increased in several minutes to high values at stage III where rates of CO₂ uptake were attained more gradually. No large amount of ribulose 1–5 bisp phosphate (RuBP) appeared to be bound to inactive enzyme at stage II. When darkened leaves were illuminated (stage I) the increase of both initial and total activity was slow and parallel to disappeance of the inhibitor carboxyarabinitol-1-phosphate (CA1P). Conversely, increase of CO₂ uptake rate was more rapid.     

Changes in photosystem II function during senescence of wheat leaves

Analyses of chlorophyll fluorescence were undertaken to investigate the alterations in photosystem II (PSII) function during senescence of wheat ( Triticum aestivum L. cv. Shannong 229) leaves. Senescence resulted in a decrease in the apparent quantum yield of photosynthesis and the maximal CO₂ assimilation capacity. Analyses of fluorescence quenching under steady‐state photosynthesis showed that senescence also resulted in a significant decrease in the efficiency of excitation energy capture by open PSII reaction centers (F'_v/F'_m) but only a slight decrease in the maximum efficiency of PSII photochemistry (F'_v/F'_m). At the same time, a significant increase in non‐photochemical quenching (q_N) and a considerable decrease in photochemical quenching (q_P) were observed in senescing leaves. Rapid fluorescence induction kinetics indicated a decrease in the rate of Q_A reduction and an increase in the proportion of Q_B‐non‐reducing PSII reaction during senescence. The decrease in both F'_v/F'_m and q_P explained the decrease in the actual quantum yield of PSII electron transport ((φ_PSII). We suggest that the modifications in PSII function, which led to the down‐regulation of photosynthetic electron transport, would be in concert with the lower demand for ATP and NADPH in the Calvin cycle which is often inhibited in senescing leaves.     

Ammonium in relation to proline accumulation in detached rice leaves

Ammonium accumulation in relation to prolineaccumulation in detached rice leaves under stressconditions was investigated. Ammonium accumulation indark-treated detached rice leaves preceded prolineaccumulation. Ammonium accumulation caused by waterstress coincided closely with proline accumulation indetached rice leaves. Exogenous NH₄Cl andmethionine sulfoximine (MSO), which caused anaccumulation of ammonium in detached rice leaves,increased proline content. It was found that prolinein NH₄Cl- or MSO-treated rice leaves is lessutilized than in water-treated rice leaves (controls). These results are in agreement with the observationthat a decrease in proline utilization contributes tothe accumulation of proline in dark-treated and waterstressed rice leaves. Although ammonium contentincreased in Cd- and Cu-treated rice leaves, theincrease in ammonium content was only observed afterthe increase in proline content.     

Changes in 1-(malonylamino)cyclopropane-1-carboxylic acid content in wilted wheat leaves in relation to their ethylene production rates and 1-aminocyclopropane-1-carboxylic acid content

In excised wheat (Triticum aestivum L.) leaves, water-deficit stress resulted in a rapid increase, followed by a decrease, in ethylene production rates and in the levels of 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene. However, the level of N-malonyl-ACC (MACC), the major metabolite of ACC, increased gradually, then leveled off. This increase in MACC was much greater than the decrease in ACC level. The MACC levels were positively correlated with severity of water stress. Once established, the MACC levels did not decrease even after the stressed tissues were rehydrated. Administration of labeled ACC and MACC showed that the conjugation of ACC to MACC was essentially irreversible. Repeated wilting treatments following the first wilting and rehydration cycle resulted in no further increase in ethylene production and in the levels of ACC and MACC. However, when benzyladenine was supplied during the preceding rehydration process, subsequent wilting treatment resulted in a rise in MACC level and a rapid rise followed by a decline in ethylene production rates and in the level of ACC. The magnitude of these increases was, however, smaller in these rewilted tissues than that observed in the first wilting treatment. Since MACC accumulates with water stress and is not appreciably metabolized, the MACC level is a good indicator of the stress history in the detached leaves used.     

Changes in activity of energy dissipating mechanisms in wheat flag leaves during senescence

Abstract: Excitation energy dissipation, including the xanthophyll cycle, during senescence in wheat flag leaves grown in the field was investigated at midday and in the morning. With progress of senescence, photosynthesis (Pn) and actual PSII photochemical efficiency (ΦPSII) decreased markedly at midday. The decrease in extent of Pn was greater than that of ΦPSII. However, there was no significant decline in Pn and ΦPSII observed in the morning, except in leaves 60 days after anthesis. The kinetics of xanthophyll cycle activity, thermal dissipation (NPQ), and q_f observed at midday during senescence exhibited two distinct phases. The first phase was characterized by an increase of xanthophyll cycle activity, NPQ, and q_f during the first 45 days after anthesis. The second phase took place 45 days after anthesis, characterized by a dramatic decline in the above parameters. However, the qI, observed both at midday and in the morning, always increased along with senescence. A larger proportion of NPQ insensitive to DTT (an inhibitor of the de-epoxidation of V to Z) was also observed in severely senescent leaves. In the morning, only severely senescent leaves showed higher xanthophyll cycle activity, NPQ, q_f, and qI. It was demonstrated that, at the beginning of senescence or under low light, wheat leaves were able to dissipate excess light energy via NPQ, depending on the xanthophyll cycle. However, the xanthophyll cycle was insufficient to protect leaves against photodamage under high light, when leaves became severely senescent. The ratio of (Fj - Fo)/(Fp - Fo) increased gradually during the first 45 days after anthesis, but dramatically increased 45 days after anthesis. We propose that another photoprotection mechanism might exist around reaction centres, activated in severely senescent leaves to protect leaves from photodamage.     

不同蛋白质含量小麦品种叶片NRA与氮素积累关系的研究

以鲁麦５号和昌乐５号两种不同蛋白质含量小麦品种为材料,研究了各生育期叶片ＮＲＡ与氮素积累的关系。其结果是,在不同施氮量下,各生育期叶片ＮＲＡ、ＮＯ３＾－－Ｎ、ＮＨ２－Ｎ、还原Ｎ含量皆随施氮量增加而增加,但生育前期昌乐５号的毕大于鲁麦５号,而生育后期则相反,鲁麦５号的皆大于昌乐５号,籽粒蛋白质含量亦为鲁麦５号高于昌乐５号。表明生育后期（开花期后）叶片ＮＲＡ是反映籽粒蛋白质含量高低的一项重要指标。     

Changes in root size and distribution in relation to nitrogen accumulation during maize breeding in China

Background and aims

Modern maize breeding has increased maize yields worldwide. The changes in above-ground traits accompanying yield improvement are well-known, but less information is available as to the effect of modern plant breeding on changes in maize root traits.

Methods

Root growth, nitrogen uptake, dry matter accumulation and yield formation of six maize hybrids released from 1973 to 2000 in China were compared. Experiments were conducted under low and high nitrogen supply in a black soil in Northeast China in 2010 and 2011.

Results

While nitrogen accumulation, dry matter production and yield formation have been increased, modern maize breeding in China since 1990 has reduced root length density in the topsoil without much effect on root growth in the deeper soil. The efficiency of roots in acquiring N has increased so as to match the requirement of N accumulation for plant growth and yield formation. The responses of root growth, nitrogen and dry matter accumulation, and grain yield to low-N stress were similar in the more modern hybrids as in the older ones.

Conclusions

Modern maize breeding has constitutively changed root and shoot growth and plant productivity without producing any specific enhancement in root responsiveness to soil N availability.     

Heterogeneity of the content of endogenous gibberellins in the leaves of winter wheat in relation to their insertion and ontogeny

Free and bound gibberellins were estimated in individual leaves of winter wheat using TLC a lettuce hypocotyl bioaasay. Bound gibberellins were extracted from the hydrolyzate of the water remnant after the removal of free gibberellins. Each leaf exhibits its own dynamics of free and bound gibberellins during its individual ontogeny, the gradual changes tending to be identical in every leaf. In young developing leaves the level of free gibberellins is high and that of bound is low. Further, inhibitors are sometimes present. The free gibberellin level rises with the activisation of growth and vital activity of a given leaf. Towards the end of the life cycle the content of free gibberellins is reduced to its minimum and inhibitory substances appear. The dynamics of bound gibberellins is opposite to that of free ones during ontogeny.     

Hardiness dependent accumulation of phospholipids in leaves of wheat cultivars

Phospholipid content and composition in the leaves of thirteen cultivars of wheat differing in frost hardiness, were compared before and after hardening in order to see whether phospholipids play a role in surviving at low temperatures. The content of phospholipids in the leaves at the end of the hardening could be related to the aquired hardiness, and the relationship between the two phenomena could be described by the regression equation y = (0.174 ± 0.0016) x + 1311. Accumulation of phospholipids represents a hardiness dependent augmentation of membranes. Phosphatidylcholine appeared to be preferentially accumulated in hardy cultivars during the hardening process. As there are no significant differences in phospholipid fatty acid compositions investigated earlier, these results suggest the importance of the polar head group composition of membranes in avoiding frost injury.     

Changes in nicotinamide nucleotide content and glucose metabolism in wheat embryos during vernalization

Contents of nicotinamide nucleotides, NAD, NADP, NADH₂ and NADPH₂,in wheat embryos were determined during normal germination andcold treatment. They increased markedly in the early stage ofgermination then decreased later. During cold treatment, thesenucleotides increased initially as in normal germination, butdid not decrease in the later stages. The C₆/C₁ ratios during germination decreased gradually withaging, but after about the fourth day increased. This suggeststhat the pentose phosphate pathway operates actively. Duringcold treatment, the C₆/C₁ ratio decreased slightly with aging.This suggests that the EMP pathway operates predominantly inthe glucose metabolism. Based on these results, the correlationbetween NADPH₂ and glucose metabolism during cold treatmentwas discussed. Embryos which had absorbed labelled glucose were fractionatedinto several chemical components. The radioisotope accumulatedmainly in the sugar fraction, with small amounts in the organicacid, amino acid and nucleic acid fractions. Changes in thecontent of each component showed that sugar, organic acid andamino acid increased during cold treatment. ¹ Present address: Department of Biochemistry, Faculty of PharmaceuticalScience, Higashi Nippon Gaguen University, Ishikari-Tobetsu,Hokkaido 061-02, Japan. ² Present address: Hokkaido Forest Products Research Institute,Asahikawa 070, Japan. ³ Present address: Kakuto 534, Komae City, Tokyo 182, Japan. (Received November 1, 1976; )     

Ascorbic acid content in relation to frost hardiness injury in Citrus limon leaves

Leaves of the lemon tree have a variable content of ascorbic acid throughout the year. When they are kept under low, damaging temperatures the original content of ascorbic acid decreases sharply, so that when it loses about 70% the damage of the tissue is irreversible, as judged from the results of a test for survival in appropriate tissue culture. Losses of ascorbic acid are due to oxidation because they do not occur in the absence of oxygen. Analysis of the content of ascorbic acid can serve as a rapid and simple test for survival of the leaves to freezing temperatures. By this method critical temperatures and times are determined for two very different rates of thawing. It is also found that hardening of leaves, that takes place in fall, provides an extra resistance of about 3 °C over the leaves in spring-summer. Natural content of ascorbic acid, in contrast to that found in other plants, is lower in hardened (winter) than in dehardened leaves.     

Cell size and chloroplast size in relation to chloroplast replication in light-grown wheat leaves

As part of an investigation into the control of chloroplast replication the number and size of chloroplasts in mesophyll cells was examined in relation to the size of the cells. In first leaves of Triticum aestivum L. and T. monococcum L. the number of chloroplasts in fully expanded mesophyll cells is positively correlated with the plan area of the cells. The linear relationship between chloroplast number per cell and cell plan area is also consistent over a fivefold range of cell size in isogenic diploid and tetraploid T. monococcum. In T. aestivum the chloroplast number per unit cell plan area varies among cells in relation to the size of the chloroplasts. Those cells containing chloroplasts with a relatively small face area have a correspondingly higher density of chloroplasts, and consequently, the total chloroplast area per unit cell plan area is very similar in all the cells. The results indicate that the proportion of the cell surface area covered by chloroplasts is precisely regulated, and that this is achieved during cell development by growth and replication of the chloroplasts.     

Changes in amino Acid content of excised leaves during incubation I. The effect of water content of leaves and atmospheric oxygen level

Excised leaves were incubated at various water contents to determine the effect of water status on amino acid composition. Considerable proteolysis took place during incubation with a resultant increase in each amino acid in the non-protein fraction. However, serine, proline, γ-aminobutyric acid and methyleysteine sulfoxide were the only amino acids in which there was an accumulation (i.e., net synthesis). Serine showed a small but consistent accumulation lasting for 6 days. Proline showed a greater accumulation but this ceased after 2 days.     

Manganese content of mustard leaves in relation to iron and major nutrient supply

Summary In a study of manganese uptake by mustard plants in nutrient culture, the Mn/Fe, P/Fe, and K/Ca ratios in the leaves have been shown to vary in the same way with change in iron concentration of the medium. Copper shows similar inverse changes with iron. The metabolic significance of these ratios is discussed.