Effects of magnesium deficiency on the photosynthesis and respiration of leaves of sugar beet

The effects of Mg deficiency on the photosynthesis and respiration of sugar beets (Beta vulgaris L. cv. F58-554H1) were studied by withholding Mg from the culture solution and by following changes in CO₂ and water vapor exchange of attached leaves. Leaf blade Mg concentration decreased from about 1200 to less than 200 meq kg⁻¹ dry matter without change in the rate of photosynthetic CO₂ uptake per unit leaf area, while from 200 to 50 meq kg⁻¹ the rate decreased to one-third. Rates of photorespiratory evolution of CO₂ into CO₂-free air responded to Mg like those of photosynthetic CO₂ uptake, the rates decreasing to one-half, below 200 meq kg⁻¹. Respiratory CO₂ evolution in the dark increased almost 2-fold in low Mg leaves. Magnesium deficiency had less effect on leaf (mainly stomatal) diffusion resistance (r₁) than on mesophyll resistance (r_m); in Mg-deficient plants r_m increased from 2.9 to 7.1 sec cm⁻¹, whereas r₁ became significantly greater than the control value only in the most severe instances of Mg deficiency.     

Effects of phosphorus deficiency on the photosynthesis and respiration of leaves of sugar beet

Phosphorus deficiency was induced in sugar beet plants (Beta vulgaris L. var. F5855441), cultured hydroponically under standardized environmental conditions, by removal of phosphorus from the nutrient supply at the ten leaf stage 28 days after germination. CO₂ and water vapor exchange rates of individual attached leaves were determined at intervals after P cutoff. Leaves grown with an adequate nutrient supply attained net rates of photosynthetic CO₂ fixation of 125 ng CO₂ cm⁻² sec⁻¹ at saturating irradiance, 25 C, and an ambient CO₂ concentration of about 250 μl l⁻¹. After P cutoff, leaf phosphorus concentrations decreased as did net rates of photosynthetic CO₂ uptake, photorespiratory evolution of CO₂ into CO₂-free air, and dark respiration, so that 30 days after cutoff these rates were about one-third of the control rates. The decrease in photosynthetic rates during the first 15 days after cutoff was associated with increased mesophyll resistance (r_m) which increased from 2.4 to 4.9 sec cm⁻¹, while from 15 to 30 days there was an increase in leaf (mainly stomatal) diffusion resistance (r_l′) from 0.3 to 0.9 sec cm⁻¹, as well as further increases in r_m to 8.5 sec cm⁻¹. Leaf diffusion resistance (r_l′) was increased greatly by low P at low but not at high irradiance, r_l′ for plants at low P reaching values as high as 9 sec cm⁻¹.     

Effects of potassium deficiency and replacement of potassium by sodium on sugar beet plants

Two sugar beet (Beta vulgaris L.) genotypes were cultivated at different K⁺/Na⁺ concentration in nutrient solutions (mM, 3/0 (control groups), 0.03/2.97 (K-Na replacement groups), and 0.03/0 (K deficiency groups)) to investigate the effects of potassium deficiency and replacement of potassium by sodium on plant growth and to explore how sodium can compensate for a lack of potassium. After 22 days of growth were determined: (i) dry weights of leaves, stems, and roots, (ii) the Na⁺ and K⁺ contents, (iii) MDA level, (iv) the activities of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPX), and (v) the level of free amino acids. Potassium deficit inhibited plant growth, decreased the K⁺ content in leaves and roots, activated GPX and SOD, suppressed CAT activity, and increased the content of most amino acids. In K-Na replacement groups, the effects of K⁺ deficiency, including changes in the MDA level, antioxidant enzyme activities, and the level of free amino acids, were alleviated, but the degree of recovery did not reach the values characteristic for the control groups. Based on these results, we concluded that low potassium could lead to the inhibition of seedling growth, oxidative damage, and amino acid accumulation. While sodium was able to substitute potassium to a large extent, it cannot fulfil potassium fundamental role as an essential nutrient in sugar beet.     

Glyphosate inhibits photosynthesis and allocation of carbon to starch in sugar beet leaves

Application of glyphosate (N-[phosphonomethyl] glycine) to exporting leaves of sugar beet (Beta vulgaris, L.) during the day lowered stomatal conductance and carbon fixation. Allocation of newly fixed carbon to foliar starch accumulation was nearly completely inhibited, being decreased by the same amount as net carbon fixation. In contrast, decreasing net carbon fixation in untreated leaves by lowering CO₂ concentration caused starch accumulation to decrease, but only in the same proportion as net carbon fixation. Shikimate level increased 50-fold in treated leaves but the elevated rate of carbon accumulation in shikimate was only 4% of the decrease in the rate of starch accumulation. Application of steady state labeling with ¹⁴CO₂ to exporting leaves confirmed the above changes in carbon metabolism, but revealed no other major daytime differences in the ¹⁴C-content of amino acids or other compounds between treated and control leaves. Less ¹⁴C accumulated in treated leaves because of decreased fixation, not increased export. The proportion of newly fixed carbon allocated to sucrose increased, maintaining export at the level in control leaves. Returning net carbon exchange to the rate before treatment restored starch accumulation fully and prevented a decrease in export during the subsequent dark period.     

Early iron deficiency stress response in leaves of sugar beet.

Iron nutrient deficiency was investigated in leaves of hydroponically grown sugar beets (Beta vulgaris) to determine how ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) gene expression is affected when thylakoid components of photosynthesis are diminished. Rubisco polypeptide content was reduced by 60% in severely iron-stressed leaves, and the reduction was directly correlated to chlorophyll content. The concentration of Rubisco protein in iron-stressed leaves was found to be regulated by availability of mRNAs, and CO2 fixation by Rubisco was reduced from 45 mumol CO2 m-2 s-1 in extracts from iron-sufficient leaves to 20 mumol CO2 m-2 s-1 in extracts from severely stressed leaves. The rate of CO2 fixation was directly correlated to leaf chlorophyll content. Rubisco in iron-sufficient control leaves was 59% activated, whereas in severely stressed leaves grown under the same light, Rubisco was 43% activated. RNA synthesis was reduced by about 50% in iron-deficient leaves, but 16S and 25S rRNA and ctDNA were essentially unaffected by iron stress.     

Effects of light intensity and oxygen on photosynthesis and translocation in sugar beet

The mass transfer rate of ¹⁴C-sucrose translocation from sugar beet (Beta vulgaris, L.) leaves was measured over a range of net photosynthesis rates from 0 to 60 milligrams of CO₂ decimeters⁻² hour⁻¹ under varying conditions of light intensity, CO₂ concentration, and O₂ concentration. The resulting rate of translocation of labeled photosynthate into total sink tissue was a linear function (slope = 0.18) of the net photosynthesis rate of the source leaf regardless of light intensity (2000, 3700, or 7200 foot-candles), O₂ concentration (21% or 1% O₂), or CO₂ concentration (900 microliters/liter of CO₂ to compensation concentration). These data support the theory that the mass transfer rate of translocation under conditions of sufficient sink demand is limited by the net photosynthesis rate or more specifically by sucrose synthesis and this limitation is independent of light intensity per se. The rate of translocation was not saturated even at net photosynthesis rates four times greater than the rate occurring at 300 microliters/liter of CO₂, 21% O₂, and saturating light intensity.     

Translocation and the control of photosynthesis in sugar beet

Summary Evidence is presented that the size and activity of sinks for the products of photosynthesis exert a large measure of control over carbohydrate levels in the leaves of the sugar beet plant. Further data is presented to show that a correlation exists between the level carbohydrate in the leaf and the rate of photosynthesis. The rate of photosynthesis can thus be linked directly to the sinks of the plant by their ability to control rates of translocation from leaves.     

Volatile components of sugar beet leaves

Extracts of both young and old sugar beet plants were obtained using a modified Likens and Nickerson apparatus. Constituents were identified by GC/MS, and using selected ion monitoring it was shown that the previously determined phenylacetonitrile was probably not of glucosinolate origin. Some unsaturated aldehydes, alcohols and derivatives (enzymic lipid degradation products) were formed to greater extents by the younger leaves, but otherwise such quantitative differences were relatively few and generally random. An interesting range of chlorinated compounds was obtained only from the older plants; a pesticide origin is suggested.     

Isolation of protoplasts from sugar beet leaves

Isolated protoplasts were prepared from sugar beet leaves by means of the action of a non-specific enzyme xylonase; homologous fusions and total yield were examined.     

Effects of potassium deficiency on water relations and photosynthesis of the tomato plant

Potassium deficient (−K) and potassium sufficient (+K) plants were exposed to four days of water stress. Well watered −K and +K plants had comparable rates of transpiration. But +K plants had a larger leaf area and depleted the soil moisture to a greater extent on day 1 of stress. For days 2 and 3 their transpiration rate, leaf water potential and relative water content fell below those of −K plants. Well watered −K plants had a significantly lower rate of photosynthesis than +K plants. Photosynthesis of −K plants was more sensitive to reduction in plant water potential than that of +K plants. Reduction of photosythesis in −K leaves was due to impairment of photosynthetic capacity and not to stomatal closure. Growth was significantly reduced in −K plants.     

The influence of phosphate deficiency on photosynthesis, respiration and adenine nucleotide pool in bean leaves

The decrease in inorganic phosphate (Pi) content of 10-d-old Phaseolus vulgaris L. plants did not affect rates of photosynthesis (PN) and respiration (RD), leaf growth, and adenylate concentration. Two weeks of phosphate starvation influenced the ATP content and leaf growth more than PN and RD. The ATP concentration in the leaves of 15- and 18-d-old phosphate deficient (-P) plants after a light or dark period was at least half of that in phosphate sufficient (+P, control) plants. Similar differences were found in fresh and dry matter of leaves. However, PN declined to 50 % of control in 18-d-old plants only. Though the RD of -P plants (determined as both CO2 evolution and O2 uptake) did not change, an increased resistance of respiration to KCN and higher inhibition by SHAM (salicylhydroxamic acid) suggested a higher engagement of alternative pathway in respiration and a lower ATP production. The lower demand for ATP connected with inhibition of leaf growth may influence the ATP producing processes and ATP concentration. Thus, the ATP concentration in the leaves depends stronger on Pi content than on PN and RD.     

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

Effects of calcium on photosynthesis in sugar beets (Beta vulgaris L. cv. F58-554H1) were studied by inducing calcium deficiency and determining changes in CO₂ uptake by attached leaves, electron transport, and photophosphorylation by isolated chloroplasts, and CO₂ assimilation by ribulose diphosphate carboxylase extracts. Calcium deficiency had no significant effect on leaf CO₂ uptake, photoreduction of ferricyanide, cyclic or noncyclic ATP formation of isolated chloroplasts, or on ribulose diphosphate carboxylase CO₂ assimilation, when the rates were expressed per unit chlorophyll. When expressed per unit leaf area CO₂ uptake increased by about 15% in low calcium leaves. The most noticeable effect of calcium deficiency was reduction in leaf area: low calcium had no effect on dark respiratory CO₂ evolution, on leaf diffusion resistance, or on mesophyll resistance to CO₂. We concluded that only small amounts of calcium are required for normal photosynthetic activity of sugar beet leaves.     

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.     

Effects of moisture stress on the multiplication and expansion of cells in leaves of sugar beet

Summary Sugar beets were subjected to moisture stress by decreasing the water potential of the culture solution osmotically with polyethylene glycol by a known amount, , and, alternatively by applying matric potential, , at the plant roots. Lowering the water potential at the root surface less than 200 millibars by either method resulted in significant decreases in the rate of cell multiplication. The final number of cells per leaf at = -372 mb the final was 165% of that at = -473 mb ( = –101 mb); similarly at = –15 mb the final cell number was 198% of that at = –196 mb ( = –181 mb). The mean cell volume of leaves was not significantly affected by these levels of moisture stress.     

Effects of decreased net carbon exchange on carbohydrate metabolism in sugar beet source leaves

The relationship between CO₂ concentration and starch synthesis and degradation was studied by measuring leaf starch content and disappearance of ¹⁴C-starch. At a concentration of 340 microliters CO₂ per liter, starch accumulated without degradation of previously synthesized starch. Degradation of starch began when CO₂ concentration was lowered, but its synthesis continued. At 120 microliters CO₂ per liter rates of synthesis and degradation were equal. Even at the CO₂ compensation point, synthesis of starch continued. Concomitant starch synthesis and mobilization supported export from the leaf. Changes in starch metabolism that occur when photosynthesis is CO₂-limited provide a means to study regulation of starch metabolism and carbon allocation in translocating leaves.     

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.