The relationship between phosphate status and photosynthesis in leaves

Spinach (Spinacia oleracea L.) and barley (Hordeum vulgare L.) were grown in hydroponic culture with varying levels of orthophosphate (Pi). When leaves were fed with 20 mmol·l^–1 Pi at low CO₂ concentrations, a temporary increase of CO₂ uptake was observed in Pi-deficient leaves but not in those from plants grown at 1 mmol·l^–1 Pi. At high concentrations of CO₂ (at 21% or 2% O₂) the Pi-induced stimulation of CO₂ uptake was pronounced in the Pi-deficient leaves. The contents of phosphorylated metabolites in the leaves decreased as a result of Pi deficiency but were restored by Pi feeding. These results demonstrate that there is an appreciable capacity for rapid Pi uptake by leaf mesophyll cells and show that the effects of long-term phosphate deficiency on photosynthesis may be reversed (at least temporarily) within minutes by feeding with Pi.Abbreviation Pi orthophosphate     

Interaction between photosynthesis and respiration in illuminated leaves

Plants are sessile organisms that often receive excessive amounts of light energy. This excess energy can be exported from the chloroplasts and dissipated by the mitochondrial respiratory chain. The inner membrane of plant mitochondria possesses unique non-phosphorylating pathways, involving alternative oxidase and type II NAD(P)H dehydrogenases. There are accumulating amounts of evidence showing that these energy-wasteful pathways are up-regulated under excess light conditions, suggesting that they play key roles in efficient photosynthesis. Based on recent advances in our understanding about the metabolic interaction between chloroplasts and mitochondria, we discuss the importance of the respiratory chain for stabilizing the photosynthetic system.     

杂交酸模叶片线粒体交替氧化酶呼吸途径在光破坏防御中的作用

以杂交酸模(Rumex K-1)为试材,研究了不同光强下线粒体交替氧化酶呼吸途径(AOX途径)对酸模叶片光破坏的防御作用.结果表明:在200 μmol·m-2·s-1弱光下,用水杨基羟肟酸抑制AOX途径后,Rumex K-1叶片的PSⅡ实际光化学效率、光合线性电子传递速率以及光合放氧速率均显著下降,非还原性QB反应中心显著升高,加重了叶片的光抑制,而活性氧清除机制上调,避免了活性氧的过量积累,部分缓解了Rumex K-1叶片的光抑制;在800 μmol·m-2·s-1强光下,AOX途径受抑,导致Rumex K-1叶片发生严重的光抑制,而此时活性氧清除机制的上调不足以缓解活性氧过量的积累.无论在强光还是弱光下,AOX途径在Rumex K-1叶片的光破坏防御过程中都起着重要作用,而且在强光下,AOX途径对叶片的光破坏防御作用是叶绿体内其他光破坏防御途径所不能代替的.     

The development of photophosphorylation and photosynthesis in greening bean leaves

Photophosphorylation and oxygen evolution were measured in 8-day-old dark-grown bean leaves (Phaseolus vulgaris) after various times of greening in far red light and in white light. The sequence of development was the same for both greening regimes, but the processes were much more rapid in white light. The capacity for photophosphorylation, as assayed by the firefly luciferase assay, appeared after 12 hours in far red light. At this stage and for times up to 24 hours, photophosphorylation was not inhibited by 10⁻⁵m 3-(3,4-dichlorophenyl)-1,1-dimethylurea. At 24 hours, the capacity for oxygen evolution appeared and photophosphorylation became partially inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea at concentrations which inhibited oxygen evolution. In white light photophosphorylation appeared after 15 minutes, and oxygen evolution at one hour. Photophosphorylation became partially sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea when oxygen evolution appeared. Carbonylcyanide m-chlorophenyl-hydrazone inhibited photophosphorylation and photosynthesis at low concentrations, 10⁻⁵m, with immature leaves, but the leaves developed resistance to carbonylcyanide m-chlorophenyl-hydrazone as they greened.     

Differences between maize and rice in N-use efficiency for photosynthesis and protein allocation

The N-use efficiency for photosynthesis was higher in a C(4) plant, maize, than in a C(3) plant, rice, including rbcS antisense rice with optimal ribulose-1,5-bisphosphate carboxylase (Rubisco) content for CO(2)-saturated photosynthesis, even when photosynthesis was measured under saturating CO(2) conditions. The N cost for the C(4) cycle enzymes in maize was not large, and the lower amount of Rubisco allowed a greater N investment in the thylakoid components. This greater content of the thylakoid components as well as the CO(2) concentrating mechanism may support higher N-use efficiency for photosynthesis in maize.     

Rubisco content and photosynthesis of leaves at different positions in transgenic rice with an overexpression of RBCS

As ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) activity limits light-saturated photosynthesis under present atmospheric condition, the effects of an overexpression of RBCS on Rubisco content and photosynthesis were examined in the leaves at different positions in rice ( Oryza sativa L.). Rubisco content in the transformant was significantly greater in the uppermost, fully expanded leaves but decreased to levels similar to those in wild-type plants in the lower leaves. The mRNA levels of total RBCS and rbcL in these leaves were much less than those in the expanding leaves, where Rubisco synthesis is active, suggesting commensurately low level of synthesis. Although the activation state of Rubisco was lower in the uppermost, fully expanded leaves of the transformant, it recovered to its full level in the lower leaves. As a result, the photosynthetic rate did not differ in leaves at the same position between the transformant and the wild type. Similarly, whole plant biomass did not differ between these genotypes. Thus, we conclude that although the overexpression of RBCS led to an enhancement of Rubisco protein content in the uppermost, fully expanded leaves, it does not result in increased photosynthetic rates or plant biomass, because of an apparent down-regulation in its activation state.     

Longevity and factors influencing photosynthesis in tea leaves

Quadratic relationship between the age of a tea leaf and the net photosynthetic rate (PN) has been found. A progressive increase in PN was recorded for four months. Then the PN slowly declined, yet even seven-month-old tea leaves sustained a low PN. In a tea shoot, the PN increased from the first leaf onwards. Besides the physiological maturity and proximity, photon flux density (PFD) played an important role in reducing the PN. The tea leaf PN was influenced by cultivation procedures which in turn disrupted the quantum of PFD transmitted through the canopy.     

The photosynthesis of ryegrass leaves grown in a simulated sward

Plants were taken from simulated swards of perennial ryegrass (Lolium perenne) grown in a controlled environment and the rates of photosynthesis of the youngest fully expanded leaves, and the second and third youngest leaves on the same tillers were measured. The youngest leaves had the highest rates and the third the lowest, with the second leaves intermediate. The rate of photosynthesis in bright light of successive youngest expanded leaves decreased as the swards increased in leaf area, but did not when plants were grown so that the main stem was not shaded. When plants were grown at different densities and the photosynthetic rates of leaves of a particular ontogenetic rank were measured, it was found that leaves on plants from higher densities had lower rates of photosynthesis. Also leaves on plants grown in bright light had higher photosynthetic rates than those on plants grown in dim light. It is concluded that the decline in the photosynthetic capacity of successive leaves in a rapidly growing simulated sward is due to the intense shading to which they are subjected during their expansion.     

Different drought-stress responses in photosynthesis and reactive oxygen metabolism between autotetraploid and diploid rice

Photosynthetic light curve, chlorophyll (Chl) content, Chl fluorescence parameters, malondialdehyde (MDA) content, phosphoenolpyruvate carboxylase (PEPC) activity and reactive oxygen metabolism were studied under drought stress in two autotetraploid rice lines and corresponding diploid rice lines. Net photosynthetic rate decreased dramatically, especially under severe drought stress and under high photosynthetic active radiation in diploid rice, while it declined less under the same conditions in autotetraploid lines. Compared with the corresponding diploid lines, the Chl content, maximum photochemical efficiency of photosystem (PS) II, and actual photochemical efficiency of PSII were reduced less in autotetraploid lines. PEPC activities were higher in autotetraploid rice lines. PEPC could alleviate inhibition of photosynthesis caused by drought stress. The chromosome-doubling enhanced rice photoinhibition tolerance under drought stress. The lower MDA content and superoxide anion production rate was found in the autotetraploid rice indicating low peroxidation level of cell membranes. At the same time, the superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities were higher in autotetraploid rice lines. SOD, POD, and CAT could effectively diminish the reactive oxygen species and reduced the membrane lipid peroxidation.     

Metabolic regulation in diseased leaves. I. The respiratory rise in barley leaves infected with powdery mildew

Photosynthetic and respiratory activities have been measured in leaves of Hordeum vulgare L. var. Manchuria (barley) after infection with Erysiphe graminis var. hordei (powdery mildew). Two isogenic lines, one resistant to infection and the other highly susceptible, were examined.

These isogenic lines showed very different physiological responses following infection. Photosynthesis and the chlorophyll content of resistant leaves was unaffected by infection. Respiration increased slightly and this was accompanied by small increases in activities of enzymes of glycolysis, the pentose-P pathway and the tricarboxylic acid cycle.

The infection of susceptible leaves resulted in a slight increase in photosynthesis 48 hours after inoculation, but subsequently there was a progressive decrease in the photosynthesis of these leaves compared with that of noninfected leaves. The capacity of infected leaves for partial reactions of photosynthesis such as the Hill reaction and the photoreduction of nicotinamide adenine dinucleotide phosphate (NADP¹) decreased during the later stages of infection. The levels of chlorophyll, NADPH-diaphorase and aldolase also declined. There was no detectable difference in the respiration of infected and noninfected leaves until 48 hours after inoculation. After this time, the infected leaves showed a higher respiration, the maximum difference occurring about 144 hours after inoculation. The respiratory increase was not accompanied by significant changes in the levels of enzymes of glycolysis and the tricarboxylic acid cycle with the exception of malate dehydrogenase which was lower in infected leaves. In contrast, the activities of glucose-6-P dehydrogenase and 6-P-gluconate dehydrogenase showed changes similar to that observed for respiration.

The respiration and the activities of glucose-6-P dehydrogenase and 6-P-gluconate dehydrogenase did not increase in infected leaves of etiolated plants, even when excellent growth of the fungus was established by growing the plants in White's basal medium supplemented with sucrose. The respiration of a susceptible mutant barley (the yellow-green virescent mutant of the variety Himalaya) when grown in the light at 11° was not changed by infection although the characteristic respiratory rise occurred in plants grown at 15°. At the lower temperature chloroplasts fail to develop in this mutant, although development is normal at 15°.

It is suggested that the pathogen is not directly responsible for the increase in respiration in green leaves, rather that this is a response in the host cells to a loss of photosynthetic capacity.

     

探究非绿叶植物的光合作用

光合作用是高中生物学中重要的一部分,现行各个版本的高中教材均将绿叶中色素的提取作为一个重要的实验。自然界中非绿叶植物虽然只占很小的比例,但是由于叶片的颜色与常见的绿色叶片不同,使其光合作用很容易引起学生的注意,作为学生课外探究实验非常具有可行性。本实验通过提取、分离非绿叶植物叶片中的色素及测定其色素的作用光谱来探究非绿叶植物的光合作用,并以此为契机引入探究实验的一般模式．引导学生掌握科学探究的过程。     

Limitations to photosynthesis in coffee leaves from different canopy positions.

Limitations to photosynthesis were explored in leaves from four canopy positions of field-grown, unshaded coffee (Coffea arabica L.), a tropical tree species classified as shade-obligatory. Overall, compared to shade (lower) leaves, sun (upper) leaves had higher net carbon assimilation rate (A) (4.5 against 2.0mumolm(-2)s(-1) at most) associated with higher electron transport rate (due to a greater irradiance availability) but unrelated to stomatal and mesophyll conductances, which were similar regardless of leaf position. Neither physiological variable directly involved with photosynthetic carbon gain nor those involved with light capture were able to adjust themselves to match the capacity of the photosynthetic machinery to the light supply. We concluded that: (i) there was no major difference in photosynthetic capacity between sun and shade leaves; (ii) the intrinsic low A in coffee was greatly associated with remarkable low diffusive limitations rather than with biochemical or photochemical constraints; and (iii) morphological (e.g., variations in specific leaf area and leaf inclination) or anatomical plasticity should be of greater acclimative value than physiological plasticity as a mean of coffee leaves to respond to changing irradiance.     

Induced volatiles in elicitor-treated and rice blast fungus-inoculated rice leaves

The volatiles released from elicitor (copper chloride, jasmonic acid, UV, L-methionine and chitosan oligomer)-treated and rice blast fungus-inoculated rice leaves were collected by the solid-phase microextraction technique and analyzed by GC-MS. (Z)-3-Hexen-1-ol, monoterpenes, methyl salicylate, and sesquiterpenes were identified as elicitor-induced volatiles by a comparison of their GC retention times and mass spectra with those of authentic compounds. The different elicitors resulted in some qualitative and quantitative differences in the production of volatiles. Monoterpenes and sesquiterpenes were identified as the rice blast fungus-induced volatiles.     

Water relations and photosynthesis in Cucumis sativus L. leaves under salt stress

Hydroponically grown cucumber plants were exposed to 14-d period of salinity (0, 50, 100 mM NaCl). NaCl caused reduction in the relative water content in the leaves. The Na⁺ content increased and the K⁺ content decreased. The net photosynthetic rate, stomatal conductance and transpiration rate were markedly decreased by all of the salt treatments. Salinity decreased also the maximum quantum efficiency of photosystem 2 (PS 2) determined as the variable to maximum fluorescence ratio, the photochemical quantum yield of PS 2 and the photochemical fluorescence quenching, while the non-photochemical quenching increased. Above results indicate that NaCl affects photosynthesis through both stomata closure and non-stomatal factors.