Effects of Nitrogen Nutrition on Leaf Expansion and Photosynthesis of Trifolium subterraneum L. 1. Comparison between Different Levels of Nitrogen Supply

Growth analysis showed that reductions in the relative growth-rateof subterranean clover plants (cv. Mt. Barker), even those dueto moderate nitrogen deficiencies, were reflected in reductionsof the leaf-area ratio and particularly of the net assimilationrate. A decline in nitrogen supply in the culture solutions was foundto depress net rates of carbon dioxide uptake per unit leafarea and leaf expansion per plant to about the same extent,even at moderate levels of nitrogen stress. Four days aftertransfer of plants grown with adequate nitrogen to solutionswithout nitrogen, leaf area and net carbon dioxide uptake haddeclined to 84 per cent and 89 per cent of the values for thecontrol plants. After a further 4 days these values had decreasedto 71 per cent and 52 per cent respectively. When net carbon dioxide uptake was expressed per unit weightof chlorophyll, the effect of changes in nitrogen supply onnet photosynthesis largely disappeared, indicating a close relationshipwith the chlorophyll content of the leaves. However, anotherand perhaps more direct effect of nitrogen on photosynthesiswas suggested by the fact that, during the early stages of recoveryfrom a severe nitrogen stress, photosynthesis began to increasebefore the chlorophyll content of the leaves.     

PHYSIOLOGICAL EFFECTS OF MANGANESE DEFICIENCY RELATED TO AGE IN SOYBEANS (GLYCINE MAX)

Cooper , Eugene E., and Raymond E. Girton . (Purdue U., Lafayette, Ind.) Physiological effects of manganese deficiency related to age in soybeans (Glycine max). Amer. Jour. Bot. 50(2): 105–110. Illus. 1963.—Soybean plants when grown in manganese-deficient silica sand cultures developed typical manganese deficiency symptoms of interveinal chlorosis and necrosis. Physiological effects including depression of photosynthesis, respiration, growth, and relative chlorophyll contents were studied. The depression of photosynthesis was not always proportional to reduced chlorophyll content. This is taken to indicate the importance of manganese in reactions concerned in photosynthesis in addition to chlorophyll formation. Age of leaves related to position on the plant and actual aging of the plants with time sometimes produced different results when related to photosynthetic rates, which mainly decreased with age of plants. Chlorophyll content in young leaves increased with plant age, except for a consistent decrease after leaf maturity. Respiration rates generally decreased with age. For the most part, the effects of aging on photosynthesis, respiration, and chlorophyll contents were the same for soybeans as for other species reported in the literature.     

Leaf Photosynthesis and Respiration of High CO(2)-Grown Tobacco Plants Selected for Survival under CO(2) Compensation Point Conditions

Four self-pollinated, doubled-haploid tobacco, (Nicotiana tabacum L.) lines (SP422, SP432, SP435, and SP451), selected as haploids by survival in a low CO₂ atmosphere, and the parental cv Wisconsin-38 were grown from seed in a growth room kept at high CO₂ levels (600-700 parts per million). The selected plants were much larger (especially SP422, SP432, and SP451) than Wisconsin-38 nine weeks after planting. The specific leaf dry weight and the carbon (but not nitrogen and sulfur) content per unit area were also higher in the selected plants. However, the chlorophyll, carotenoid, and alkaloid contents and the chlorophyll a/b ratio varied little. The net CO₂ assimilation rate per unit area measured in the growth room at high CO₂ was not higher in the selected plants. The CO₂ assimilation rate versus intercellular CO₂ curve and the CO₂ compensation point showed no substantial differences among the different lines, even though these plants were selected for survival under CO₂ compensation point conditions. Adult leaf respiration rates were similar when expressed per unit area but were lower in the selected lines when expressed per unit dry weight. Leaf respiration rates were negatively correlated with specific leaf dry weight and with the carbon content per unit area and were positively correlated with nitrogen and sulfur content of the dry matter. The alternative pathway was not involved in respiration in the dark in these leaves. The better carbon economy of tobacco lines selected for low CO₂ survival was not apparently related to an improvement of photosynthesis rate but could be related, at least partially, to a significantly reduced respiration (mainly cytochrome pathway) rate per unit carbon.     

Carbon and nitrogen economy of 24 wild species differing in relative growth rate

The relation between interspecific variation in relative growth rate and carbon and nitrogen economy was investigated. Twentyfour wild species were grown in a growth chamber with a nonlimiting nutrient supply and growth, whole plant photosynthesis, shoot respiration, and root respiration were determined. No correlation was found between the relative growth rate of these species and their rate of photosynthesis expressed on a leaf area basis. There was a positive correlation, however, with the rate of photosynthesis expressed per unit leaf dry weight. Also the rates of shoot and root respiration per unit dry weight correlated positively with relative growth rate. Due to a higher ratio between leaf area and plant weight (leaf area ratio) fast growing species were able to fix relatively more carbon per unit plant weight and used proportionally less of the total amount of assimilates in respiration. Fast growing species had a higher total organic nitrogen concentration per unit plant weight, allocated more nitrogen to the leaves and had a higher photosynthetic nitrogen-use efficiency, i.e. a higher rate of photosynthesis per unit organic nitrogen in the leaves. Consequently, their nitrogen productivity, the growth rate per unit organic nitrogen in the plant and per day, was higher compared with that of slow growing species.     

Trophic status of <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis> influences the impact of iron deficiency on photosynthesis

To investigate the impact of iron deficiency on bioenergetic pathways in Chlamydomonas, we compared growth rates, iron content, and photosynthetic parameters systematically in acetate versus CO₂-grown cells. Acetate-grown cells have, predictably (2-fold) greater abundance of respiration components but also, counter-intuitively, more chlorophyll on a per cell basis. We found that phototrophic cells are less impacted by iron deficiency and this correlates with their higher iron content on a per cell basis, suggesting a greater capacity/ability for iron assimilation in this metabolic state. Phototrophic cells maintain both photosynthetic and respiratory function and their associated Fe-containing proteins in conditions where heterotrophic cells lose photosynthetic capacity and have reduced oxygen evolution activity. Maintenance of NPQ capacity might contribute to protection of the photosynthetic apparatus in iron-limited phototrophic cells. Acetate-grown iron-limited cells maintain high growth rates by suppressing photosynthesis but increasing instead respiration. These cells are also able to maintain a reduced plastoquinone pool.     

Growth and photosynthesis of pea plants under different iron supply

Soil conditions, leading to iron deficiency or toxicity, are widespread in nature. Our objective was to study the effect of Fe supply, ranging from complete deficiency to excess, on growth and on some photosynthetic indices of pea plants. Both iron deficiency and toxicity decreased shoot and root growth. Complete deficiency resulted in a lower shoot/root ratio and a higher content of dry biomass per unit of fresh biomass in roots, while iron excess led to higher content of dry biomass per unit of fresh biomass in shoot. Complete deficiency was also characterized by low chlorophyll and carotenoid content, elevated ratios of chlorophyll a/chlorophyll b and carotenoids/chlorophylls, a drop of photosynthetic rate per leaf area, and an increase of photosynthetic rate per chlorophyll. The stomatal resistance substantially increased, while the transpiration rate decreased. Smaller changes in stomatal resistance and transpiration rate, but not in photosynthetic rate per leaf area, were found under partial iron deficiency and under excess of iron. In the first case, the chlorophyll content decreased, while in the second it increased. The maximum efficiency of photosystem II was unaffected by iron supply. Even when no genetic or experimental differences existed, changes in growth, pigment content and photosynthesis due to variation of Fe supply depended on the type and severity of the imposed stress, as well as on the studied parameter. A combination of indices described better the effect of iron supply, especially when small differences were characterized.     

Interaction of Mineral Deficiency and Nitrogen Source on Acid Phosphatase Activity in Leaf Extracts

Plants were grown in sand cultures to observe the effects ofmineral deficiencies, with different sources of nitrogen supply,on leaf acid phosphatase activity using phenolphthalein phosphateas substrate. Large increases ranging from 7 to 18 times normal (on freshweight or protein basis) occurred with deficiency of phosphoruswith nitrate, nitrite or ammonia nitrogen; increases from 4to 8 times normal occurred with deficiencies of zinc with nitriteor ammonia; and of copper with nitrite. Less pronounced increases,from 2 to 3 times normal on a protein basis, occurred with deficiencyof zinc with nitrate; of boron with all nitrogen sources; ofmanganese and of iron with nitrite. Activities on both proteinand fresh-weight basis were often greater with nitrite or ammoniumsulphate than with nitrate. Effects of phosphorus, zinc, copper,boron, and nitrogen sources were significant. Molybdenum deficiency caused approximately 2 times normal activityon a protein basis in tomato, tobacco, sunflower, cauliflower,lucerne, mustard, spinach beet, and lettuce when nitrate wasgiven. Effects with ammonia nitrogen were not consistent inmolybdenum-deficient plants but 50 per cent. increases occurredin the first 5 species listed. Activities varied widely betweenspecies. It was concluded that the increases in activity were causedby increases in amounts of the enzyme produced during growth,and that different combinations of deficiencies and nitrogensupplies produce different specific responses in terms of enzymesynthesis.     

Effects of Nitrogen Nutrition on Leaf Expansion and Photosynthesis of Trifolium subterraneum L. II. Comparison between Nodulated Plants and Plants Supplied with Combined Nitrogen

Subterranean clover plants depending on symbiotic nitrogen fixationhad smaller leaf areas than control plants supplied with combinednitrogen in the nutrient solutions. There were no differencesin chlorophyll content per unit fresh weight of leaves or petioles,nor in net rates of carbon dioxide uptake per unit leaf areaat light intensities above 2000 fc and at carbon dioxide concentrationsabove 300 ppm. Dark respiration by the shoots of the nodulatedplants was considerably higher than for the controls. This couldhave been a direct result of nodule activity and is suggestedas a possible factor contributing to the slower growth of theseplants compared with the controls. A comparison of the nitrogen contents of shoots and roots showeda sub-optimal nitrogen status, particularly in the roots, ofthe nodulated plants. This is suggested as another factor contributingto the slower growth of the nodulated plants compared with thecontrols. The response patterns before and after the addition of combinednitrogen differed in a number of important respects from thosefound previously under conditions of a sub-optimal nitrogensupply in the nutrient solution outside the roots. These arebriefly discussed.     

Effects of sulfur on the photosynthesis of intact leaves and isolated chloroplasts of sugar beets

Effects of sulfur on photosynthesis in sugar beets (Beta vulgaris L. cv. F58-554H1) were studied by inducing sulfur deficiency and determining changes in the photosynthesis of whole attached leaves and of isolated chloroplasts. The rates of photosynthetic CO₂ uptake by intact leaves, photoreduction of ferricyanide, cyclic and noncyclic photophosphorylation of isolated chloroplasts, and the rate of CO₂ assimilation by ribulose diphosphate carboxylase, decreased with decrease in total leaf sulfur from 2500 to about 500 μg g⁻¹ dry weight. Sulfur deficiency reduced photosynthesis through an effect on chlorophyll content, which decreased linearly with leaf sulfur, and by decreasing the rate of photosynthesis per unit chlorophyll. There was only a small effect of sulfur deficiency on stomatal diffusion resistance to CO₂ until leaf sulfur decreased below 1000 μg g⁻¹ when stomatal resistance became a more significant proportion of the total diffusion resistance to CO₂. Light respiration rates were positively correlated with photosynthesis rates and dark respiration was unchanged as leaf sulfur concentrations declined.     

The relative proportions of the chloroplast pigments as influenced by different levels of iron and potassium supply

Summary 1. Potato plants were grown in pot sand cultures at different levels of iron and potassium supply. Plants grown at the lowest level of iron developed iron deficiency chlorosis and potassium deficiency symptoms when maintained at the lowest level of potassium but not at the highest level.2. Rapid procedures were developed for the extraction and estimation of chloroplast pigments from small samples of lamina—usually less than 1 g of fresh material.3. The expression of pigment content on an area basis gave results which were in better agreement with visual observations than those expressed on either fresh or dry weight basis.4. Both iron and potassium additions increased the chloroplast pigment content. Leaves exhibiting iron deficiency contained reduced quantities of all pigments per unit area of lamina.5. A linear relationship existed between chlorophyll and carotene, chlorophyll and xanthophyll, and carotene and xanthophyll contents.6. The results suggested that laminae completely deficient in chlorophyll would contain no carotene but might still contain xanthophyll.7. The relative proportions of chlorophyll, carotene and xanthophyll do not remain constant under varying conditions of iron status. When iron is deficient the proportion of xanthophyll increased in relation to the chlorophyll and carotene contents.     

Response of respiration of soybean leaves grown at ambient and elevated carbon dioxide concentrations to day-to-day variation in light and temperature under field conditions

BACKGROUND AND AIMS: Respiration is an important component of plant carbon balance, but it remains uncertain how respiration will respond to increases in atmospheric carbon dioxide concentration, and there are few measurements of respiration for crop plants grown at elevated [CO(2)] under field conditions. The hypothesis that respiration of leaves of soybeans grown at elevated [CO(2)] is increased is tested; and the effects of photosynthesis and acclimation to temperature examined. METHODS: Net rates of carbon dioxide exchange were recorded every 10 min, 24 h per day for mature upper canopy leaves of soybeans grown in field plots at the current ambient [CO(2)] and at ambient plus 350 micromol mol(-1) [CO(2)] in open top chambers. Measurements were made on pairs of leaves from both [CO(2)] treatments on a total of 16 d during the middle of the growing seasons of two years. KEY RESULTS: Elevated [CO(2)] increased daytime net carbon dioxide fixation rates per unit of leaf area by an average of 48 %, but had no effect on night-time respiration expressed per unit of area, which averaged 53 mmol m(-2) d(-1) (1.4 micromol m(-2) s(-1)) for both the ambient and elevated [CO(2)] treatments. Leaf dry mass per unit of area was increased on average by 23 % by elevated [CO(2)], and respiration per unit of mass was significantly lower at elevated [CO(2)]. Respiration increased by a factor of 2.5 between 18 and 26 degrees C average night temperature, for both [CO(2)] treatments. CONCLUSIONS: These results do not support predictions that elevated [CO(2)] would increase respiration per unit of area by increasing photosynthesis or by increasing leaf mass per unit of area, nor the idea that acclimation of respiration to temperature would be rapid enough to make dark respiration insensitive to variation in temperature between nights.     

The effects of simazine and temperature on photosynthesis in rye

Temperature during growth had a strong influence on light-saturated rates of photosynthesis and respiration in rye (Secale cereale). Carbon dioxide uptake was depressed at the low temperatures and reached maximal values higher than 30 milligrams of carbon dioxide per square decimeter per hour at the intermediate temperatures. Respiration rates increased substantially while growth temperature decreased. Simazine treatment (0.00, 0.03, 0.08, 0.12 milligrams per liter) resulted in no significant stimulatory or inhibitory effects at low temperatures, but progressively inhibited growth and photosynthesis at the higher temperatures. Respiration rates were not significantly influenced. Thus, the effect of simazine is strongly temperature-dependent and is more pronounced on photosynthesis than respiration.     

The Physiological Mechanisms Underlying N₂-Fixing Common Bean (<i>Phaseolus vulgaris</i> L.) Tolerance to Iron Deficiency

Iron is an essential element for plants as well as all living organisms, functioning in various physiological and biochemical processes such as photosynthesis, respiration, DNA synthesis, and N₂ fixation. In the soil, Fe bioavailability is extremely low, especially under aerobic conditions and at high pH ranges. In contrast, plants with nodules on their roots that fix atmospheric nitrogen need much more iron. To highlight the physiological traits underlying the tolerance of N₂-fixing common bean to iron deficiency, two genotypes were hydroponically cultivated in a greenhouse: Coco nain (CN) and Coco blanc (CB). Plants were inoculated with an efficient strain of Rhizobium tropici, CIAT899, and received a nutrient solution added with 0 µM Fe (severe Fe deficiency, SFeD), 5 µM Fe (moderate Fe deficiency, MFeD) or 45 µM Fe (control, C). Several physiological parameters related to photosynthesis and symbiotic nitrogen fixation were then analyzed. Iron deficiency significantly reduced whole plant and nodule growth, chlorophyll biosynthesis, photosynthesis, leghemoglobin (LgHb), nitrogenase (N₂ase) activity, nitrogen, and Fe nutrition, with some genotypic differences. As compared to CB, CN maintained better Fe allocation to shoots and nodules, allowing it to preserve the integrity of its photosynthetic and symbiotic apparatus, thus maintaining the key functional traits of the plant metabolism (chlorophyll biosynthesis and photosynthesis in shoots, leghemoglobin accumulation, and nitrogenase activity in root nodules). Plant growth depends on photosynthesis, which needs to be supplied with sufficient iron and nitrogen. Fe deficiency stress index (FeD-SI) and Fe use efficiency (FeUE) are two physiological traits of tolerance that discriminated the studied genotypes.     

Nitrogen reallocation and photosynthetic acclimation in response to partial shading in soybean plants

The first trifoliate of soybean was shaded when fully expanded, while the plant remained in high light; a situation representative for plants growing in a closed crop. Leaf mass and respiration rate per unit area declined sharply in the first few days upon shading and remained rather constant during the further 12 days of the shading treatment. Leaf nitrogen per unit area decreased gradually until the leaves were shed. Leaf senescence was enhanced by the shading treatment in contrast to control plants growing in low light. Shaded leaves on plants grown at low nutrient availability senesced earlier than shaded leaves on plants grown at high nutrient availability. The light saturated rate of photosynthesis decreased also gradually during the shading treatment, but somewhat faster than leaf N, whereas chlorophyll contents declined somewhat slower than leaf N.
Partitioning of N in the leaf over main photosynthetic functions was estimated from parameters derived from the response of photosynthesis to CO₂. It appeared that the N exported from the leaf was more at the expense of compounds that make up photosynthetic capacity than of those involved in photon absorption, resulting in a change in partitioning of N within the photosynthetic apparatus. Photosynthetic nitrogen use efficiency increased during the shading treatment, which was for the largest part due to the decrease in leaf N content, to some extent to the decrease in respiration rate and only for a small part to change in partitioning of N within the photosynthetic apparatus.     

The influence of water stress preconditioning on dark respiration

The respiration rate of individual leaves of cotton (Gossypium hirsutum L. cv. Stoneville 213), beans (Phaseolus vulgaris L. cv. Bush Blue Lake), and sorghum (Sorghum vulgare Pers.) which had been fully expanded prior to a series of severe water stresses was compared with those of unstressed leaves of similar age. Measurements were made over a range of leaf temperatures. The respiration rate per unit area of the leaves of all rewatered plants were significantly lower than those of the plants which had not undergone water stress. During the stress periods, the leaves of all species suffered dry matter loss. The respiration rates per unit dry matter for cotton and beans were the same for the plants which had undergone stress as they were for the plants which had not undergone stress, thus for these two C3 plants the decrease in dark respiration due to water stress may be explained entirely by the loss of dry matter. Respiration rates of the water stressed sorghum leaves expressed on a per unit weight basis were significantly lower than those which had not undergone water stress preconditioning. The lower respiration rates of the stressed leaves when expressed on both a per unit area basis and a per unit weight basis for the C4 species indicate that water stress adaptation of C4 plants may include alterations in the respiratory mechanism or on the amount of residual respirable substrate. The light compensation points of all the species were not altered by the water stress preconditioning. This indicates that the mechanisms controlling the net photosynthetic exchange per unit leaf area underwent adaptations as a result of repeated water stress which decreased its efficiency.     

Effect of sulphate on photosynthesis in greenhouse–grown tomato plants

Sulphate accumulates in the rhizosphere of plants grown in hydroponic systems. To avoid such sulphate accumulation and promote the use of environmentally sound hydroponic systems, we examined the effects of four sulphate concentrations (0.1, 5,2, 10.4 and 20.8 m M ) on photosynthesis, ribulose-l,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) activities and related physiological processes in greenhouse–grown tomato plants ( Lycopersicon esculentum Mill. cv. Trust). The lowest sulphate concentration (0.1 m M ) significantly decreased photosynthetic capacity (P_c) and Rubisco activities on a leaf area basis. This result was supported by our data for dry matter per plant, which was low for plants in the 0.1 m M treatment. The photosynthesis-related variables such as leaf conductance, chlorophyll and soluble protein were lowest for the 0.1 m M treatment. Both total Rubisco activity and the activated ratio were reduced with this treatment. However, Rubisco activities expressed per g of protein or per g of chlorophyll were not significantly affected. These results suggest that sulphur deficiency depressed P_c– by reducing the amount of both Rubisco and chlorophyll and by causing an inactivation of Rubisco. The ratio of organic sulphur vs organic nitrogen (S/N) in plants of the 0.1 m M treatment was far below the normal values. This low S/N ratio might be accountable for the negative effect of low sulphate on P_c and plant growth. P_c and dry matter were not affected until sulphate concentration in the nutrient solution reached a high level of 20.8 m M .     

Photosystem II efficiency in low chlorophyll,iron-deficient leaves

Iron deficiency (iron chlorosis) is the major nutritional stress affecting fruit tree crops in calcareous soils in the Mediterranean area. This work reviews the changes in PS II efficiency in iron-deficient leaves. The iron deficiency-induced leaf yellowing is due to decreases in the leaf concentrations of photosynthetic pigments, chlorophylls and carotenoids. However, carotenoids, and more specifically lutein and the xanthophylls of the V+A+Z (Violaxanthin+ Antheraxanthin+Zeaxanthin) cycle are less affected than chlorophylls. Therefore, iron-chlorotic leaves grown in either growth chambers or field conditions have increases in the molar ratios lutein/chlorophyll a and (V+A+Z)/chlorophyll a. These pigment changes are associated to changes in leaf absorptance and reflectance. In the chlorotic leaves the amount of light absorbed per unit chlorophyll increases. The low chlorophyll, iron-deficient leaves showed no sustained decreases in PS II efficiency, measured after dark adaptation, except when the deficiency was very severe. This occurred when plants were grown in growth chambers or in field conditions. However, iron-deficient leaves showed decreases in the actual PS II efficiency at steady-state photosynthesis, due to decreases in photochemical quenching and intrinsic PS II efficiency. Iron-chlorotic leaves were protected not only by the decrease in leaf absorptance, but also by down-regulation mechanisms enhancing non-photochemical quenching and thermal dissipation of the light absorbed by PS II within the antenna pigment bed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Physiological and Biochemical Characteristics of Sugar Beet Plants Grown at an Increased Carbon Dioxide Concentration and at Various Nitrate Doses

Three-week-old sugar beet (Beta vulgaris L.) seedlings were grown for an additional four weeks under controlled conditions: in river sand watered with a modified Knop mixture containing one half-fold (0.5N), standard (1N), and or threefold (3N) nitrate amount, at the irradiance of 90 W/m² PAR, and at the carbon dioxide concentrations of 0.035% (1C treatment) or 0.07% (2C treatment). The increase in the carbon dioxide concentration and in the nitrogen dose resulted in an increase in the leaf area and the leaf and root dry weight per plant. With the increase in the nitrogen dose, morphological indices characterizing leaf growth increased more noticeably in 1C plants than in 2C plants. And vice versa, the effects of increased CO₂ concentration were reduced with the increase in the nitrogen dose. Roots responded to the changes in the CO₂ and nitrate concentrations otherwise than leaves. At a standard nitrate dose (1N), the contents of proteins and nonstructural carbohydrates (sucrose and starch) in leaves depended little on the CO₂ concentration. At a double CO₂ concentration, the content of chlorophyll somewhat decreased, and the net photosynthesis rate (P _n) calculated per leaf area unit increased. An increase in the nitrogen dose did not affect the leaf carbohydrate content of the 1C and 2C plants except the leaves of the 2C-3N plants, where the carbohydrate content decreased. In 1C and 2C plants, an increase in the nitrogen dose caused an increase in the protein and chlorophyll content. Specific P _n values somewhat decreased in 1C-0.5N plants and had hardly any dependence on the nitrate dose in the 2C plants. The carbohydrate content in roots did not depend on the CO₂ concentration, and the content was the highest at 0.5N. Characteristic nitrogen dose-independent acclimation of photosynthesis to an increased carbon dioxide concentration, which was postulated previously [1], was not observed in our experiments with sugar beet grown at doubled carbon dioxide concentration.     

Dependence of photosynthetic rates on leaf density thickness in deciduous woody plants grown in sun and shade

Comparisons of photosynthetic rates were made on leaves of ten species of woody dicotyledons grown in the field under full sun or under a canopy which transmitted approximately 18% of full light. Photosynthesis and dark respiration were measured and compared on various bases: area, chlorophyll, fresh weight of lamina, density thickness (fresh weight per unit area), and protein.

Light-saturated photosynthesis per unit area or unit chlorophyll was about 1.5 times greater in the sun leaves than in the shade leaves and essentially equal per unit fresh weight or unit protein. Sun leaves were thicker but the enzymes per unit fresh weight remained constant as thickness varied. Chlorophyll per unit area remained about constant; chlorophyll per unit fresh weight varied inversely with changes in leaf thickness. Thus, density thickness variation is important in photosynthetic adaptation to sun and shade. This is also shown by the relationship between light-saturated photosynthesis per unit area and density thickness.

     

缺铁对大豆叶片光合作用和光系统Ⅱ功能的影响

通过气体交换和叶绿素荧光测定研究了缺铁对大豆叶片碳同化和光系统Ⅱ的影响。缺铁条件下大豆光合速率(Pn)大幅下降;最大光化学效率(po)下降幅度较小;荧光诱导动力学曲线发生明显的变化,其中电子传递活性明显下降,K相(VK)相对荧光产量提高。缺铁大豆的天线转化效率(Fv'/Fm')、光化学猝灭系数(qP)和光系统Ⅱ实际光化学效率(ΦPSⅡ)降低,而非光化学猝灭(NPQ)则明显增加。此外,缺铁大豆的光后荧光上升增强。据此,认为铁缺乏伤害了光系统Ⅱ复合物供体侧和受体侧的电子传递;缺铁条件下光系统I环式电子传递的增强可能在维持激发能耗散和ATP供给方面起一定作用。