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.     

Growth promotion effect of Fusarium spp. PPF1 from bermudagrass (Cynodon dactylon) rhizosphere on Indian spinach (Basella alba) seedlings are linked to root colonisation

A rhizosphere fungus was isolated from roots of bermudagrass (Cynodon dactylon) and identified as Fusarium spp. PPF1. A pot experiment was conducted to test its ability to promote the vegetative growth of Indian spinach seedlings (Basella alba). Indian spinach seeds were sown in sterilised field soil amended with wheat grain inoculum of PPF1 at the rate of 0.5 and 1.0% w/w, and plants were grown for 21?days in a net house. Significantly, higher germination percentage and vigour index were observed due to application of PPF1 in the potting soil. Treatment with PPF1 also significantly increased shoot length, shoot fresh weight, shoot dry weight, root length, root fresh weight, root dry weight, leaf area and leaf chlorophyll content of cucumber plants compared to non-treated control. The growth promotion rate increased with the increasing concentration of inoculum of PPF1 applied to the soil. The fungus was re-isolated from the roots of cucumber plants at higher frequencies, while a positive co-relation was found between the root colonisation ability and the plant growth enhancement by the isolate. These results suggest that growth promotion effect of Fusarium spp. PPF1 on Indian spinach (B. alba) are linked to root colonisation ability of the fungus.     

Interactive effects of growth‐limiting N supply and elevated atmospheric CO2 concentration on growth and carbon balance of Plantago major

To assess the interactions between concentration of atmospheric CO₂ and N supply, the response of Plantago major ssp. pleiosperma Pilger to a doubling of the ambient CO₂ concentration of 350 µl l^?1 was investigated in a range of exponential rates of N addition. The relative growth rate (RGR) as a function of the internal plant nitrogen concentration (N_i), was increased by elevated CO₂ at optimal and intermediate N_i. The rate of photosynthesis, expressed per unit leaf area and plotted versus N_i. was increased by 20-30% at elevated CO₂ for N_i above 30 mg N g^?1 dry weight. However, the rate of photosynthesis, expressed on a leaf dry matter basis and plotted versus N_i, was not affected by the CO₂ concentration. The allocation of dry matter between shoot and root was not affected by the CO₂ concentration at any of the N addition rates. This is in good agreement with theoretical models. based on a balance between the rate of photosynthesis of the shoot and the acquisition of N by the roots. The concentration of total nonstructural carbohydrates (TNC) was increased at elevated CO₂ and at N limitation, resulting in a shift in the partitioning of photosynthates from structural to nonstructural and, in terms of carbon balance, unproductive dry matter. The increase in concentration of TNC led to a decrease in both specific leaf area (SLA) and N_i at all levels of nutrient supply, and was the cause of the increased rate of photosynthesis per unit leaf area. Correction of the relationship between RGR and Ni for the accumulation of TNC made the effect of elevated CO₂ on the relationship between RGR and N_i disappear. We conclude that the shift in the relationship between RGR and N_i was due to the accumulation of TNC and not due to differences in physiological variables such as photosynthesis and shoot and root respiration, changes in leaf morphology or allocation of dry matter.     

Chilli rhizosphere fungus Aspergillus spp. PPA1 promotes vegetative growth of cucumber (Cucumis sativus) plants upon root colonisation

A rhizosphere fungus was isolated from roots of chilli plants and identified as Aspergillus spp. PPA1. The fungus was tested for its ability to promote the growth of cucumber plants in a pot experiment. Cucumber seeds were sown in sterilised field soil amended with wheat grain inoculum (WGI) of PPA 1 at the rate of 0.5, 1 and 1.5% w/w, and plants were grown for 21 days in a net house. The treatment with PPA1 significantly increased shoot length, shoot fresh weight, shoot dry weight, root length, root fresh weight, root dry weight, plant length, leaf area and leaf chlorophyll content of cucumber plants compared to non-treated control. The growth promotion rate increased with the increasing concentration of inoculum of PPA1 applied to the soil. The fungus was re-isolated from the roots of cucumber plants at higher frequencies. These results suggest that Aspergillus spp. PPA1 is a root colonising plant-growth promoting fungus for cucumber.     

Growth, water relations and photosynthesis of seedlings and resprouts after fire

Seasonal patterns of growth, water relations, photosynthesis and leaf characteristics were compared between obligate seeders (Cistus monspeliensis and Cistus ladanifer) and resprouters (Arbutus unedo and Pistacia lentiscus) from the first to the second year after fire. We hypothesized that seedlings would be more water-limited than resprouts due to their shallower root systems. Regarding water use strategies, Cistus species are drought semi-deciduous and A. unedo and P. lentiscus are evergreen sclerophylls, therefore, comparisons were based on the relative deviation from mature conspecific plants. Seedlings and resprouts had higher shoot elongation and leaf production than mature plants, and over an extended period. Differences from mature plants were larger in resprouts, with two-fold transpiration, leaf conductance and photosynthesis in late spring/early summer. Seedlings of C. monspeliensis exhibited higher transpiration and leaf conductance than mature plants, while those of C. ladanifer only exhibited higher water potential. Growth increments and ameliorated water relations and photosynthesis after fire were attributed to an increase in water and nutrient availability. The small differences in water relations and photosynthesis between seedlings and mature conspecifics are in accordance with the prediction of seedlings experiencing higher water limitation than resprouts. We attribute these results to differences in root systems: resprouters benefited from an increase in root/shoot ratios and the presence of deep roots whereas Cistus seedlings relied on very shallow roots, which cannot provide assess to deep water during summer. Nevertheless, seedlings did not show evidence of experiencing a more severe water limitation than mature conspecifics, which we attributed to the presence of efficient mechanisms of avoiding and tolerating water stress. The results are discussed in relation to post-fire demography of seeders and resprouters in Mediterranean communities.     

Growth and carbon economy of a fast-growing and a slow-growing grass species as dependent on nitrate supply

In previous experiments systematic differences have been found in the morphology, carbon economy and chemical composition of seedlings of inherently fast- and slow-growing plant species, grown at a non-limiting nutrient supply. In the present experiment it was investigated whether these differences persist when plants are grown at suboptimal nutrient supply rates. To this end, plants of the inherently fast-growing Holcus lanatus L. and the inherently slow-growing Deschampsia flexuosa (L.) Trin. were grown in sand at two levels of nitrate supply. Growth, photosynthesis, respiration and carbon and nitrogen content were studied over a period of 4 to 7 weeks. At low N-supply, the potentially fast-growing species still grew faster than the potentially slow-growing one. Similarly, differences in leaf area ratio (leaf area:total dry weight), specific leaf area (leaf area:leaf dry weight) and leaf weight ratio (leaf dry weight:total dry weight), as observed at high N-supply persisted at low N-availability. The only growth parameter for which a substantial Species × N-supply interaction was found was the net assimilation rate (increase in dry weight per unit leaf area and time). Rates of photosynthesis, shoot respiration and root respiration, expressed per unit leaf, shoot and root weight, respectively, were lower for the plants at low N-availability and higher for the fast-growing species. Species-specific variation in the daily carbon budget was mainly due to variation in carbon fixation. Lower values at low N were largely determined by both a lower C-gain of the leaves and a higher proportion of the daily gain spent in root respiration. Interspecific variation in C-content and dry weight:fresh weight ratio were similar at low and high N-supply. Total plant organic N decreased with decreasing N-supply, without differences between species. It is concluded that most of the parameters related to growth, C-economy and chemical composition differ between species and/or are affected by N-supply, but that differences between the two species at high N-availability persist at low N-supply.     

Root cooling strongly affects diel leaf growth dynamics,water and carbohydrate relations in Ricinus communis

In laboratory and greenhouse experiments with potted plants, shoots and roots are exposed to temperature regimes throughout a 24 h (diel) cycle that can differ strongly from the regime under which these plants have evolved. In the field, roots are often exposed to lower temperatures than shoots. When the root‐zone temperature in Ricinus communis was decreased below a threshold value, leaf growth occurred preferentially at night and was strongly inhibited during the day. Overall, leaf expansion, shoot biomass growth, root elongation and ramification decreased rapidly, carbon fluxes from shoot to root were diminished and carbohydrate contents of both root and shoot increased. Further, transpiration rate was not affected, yet hydrostatic tensions in shoot xylem increased. When root temperature was increased again, xylem tension reduced, leaf growth recovered rapidly, carbon fluxes from shoot to root increased, and carbohydrate pools were depleted. We hypothesize that the decreased uptake of water in cool roots diminishes the growth potential of the entire plant – especially diurnally, when the growing leaf loses water via transpiration. As a consequence, leaf growth and metabolite concentrations can vary enormously, depending on root‐zone temperature and its heterogeneity inside pots.     

Carbon Balance of Tall Fescue (Festuca arundinacea Schreb.): Effects of Nitrogen Fertilization and the Growing Season

The C balance of a tall fescue sward grown under different ratesof N fertilization in summer, autumn, and spring was calculatedusing models derived from measurements of shoot growth, canopygross photosynthesis, shoot respiration and of C partitioningto the roots. Under the diverse growing conditions associatedwith the seasons and the N fertilization, C utilization forabove- and below-ground biomass accumulation never exceeded39 and 14% of the canopy gross photosynthesis, respectively.Carbon losses attributed to root respiration and exudation,which were estimated by difference between canopy net photosynthesisand total growth, ranged between 3 and 30% of canopy gross photosynthesis.Seasonal differences in shoot growth could be attributed tothe amount of intercepted radiation, the radiation-use efficiencyand the C partitioning to the roots. The effect of N deficiencyon shoot growth can be attributed to its effects on canopy photosynthesis(principally resulting from changes in intercepted photosyntheticallyactive radiation) and C partitioning. In comparison with theeffect on shoot growth, the effect of the N deficiency on thecanopy gross photosynthesis per unit of light intercepted overthe regrowth cycle was limited. It is concluded that most ofthe effect of N fertilization on shoot growth is due to changesin C partitioning which result in faster leaf area developmentand greater light interception.Copyright 1994, 1999 AcademicPress Tall rescue, Festuca arundinacea Schreb., carbon balance, nitrogen, grass, fertilization     

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid,water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO ₃ ^- pools and in NO ₃ ^- flux to the xylem, particularly in tomato which had smaller tissue NO ₃ ^- reserves. Even in barley, tissue NO ₃ ^- reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO ₃ ^- flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.Abbreviation and symbols ABA abscisic acid - c_i leaf internal CO₂ concentration - L_p root hydraulic conductance     

Growth direction of nodal roots in rice: its variation and contribution to root system formation

The root system of a rice plant (Oryza sativa L.) consists of numerous nodal roots and their laterals. The growth direction of these nodal roots affects the spatial distribution of the root system in soil, which seems to relate to yield and lodging resistance. The growth angle of a nodal root varies with the type and timing of emergence of the nodal root. The body of a rice plant can be recognized as an integrated set of shoot units, each unit consisting of an internode with a leaf and several roots. Nodal roots formed at the apical part of a shoot unit often elongate horizontally, whereas those formed at the basal part of the shoot unit show various growth directions depending on both the growth stages of the plant and the environmental conditions. Moreover, nodal roots that emerge from the most basal shoot unit of a tiller are usually thick and grow downwards. External factors such as planting density and nitrogen application affect the growth direction of nodal roots, probably partly because of the changing tillering pattern of the shoot. In addition to the growth angle of nodal roots, size of nodal roots may be another important factor determining the spatial distribution of the root system in soil.     

Growth responses to waterlogging and drainage of woody Hakea (Proteaceae) seedlings,originating from contrasting habitats in south-western Australia

We investigated the responses of seven woody Hakea (Proteaceae) species (two populations of each), to two months of waterlogging and subsequent drainage, in a controlled glasshouse experiment. The species originated from contrasting environments (winter-wetland versus non-wetland habitats), and differed in abundance (endangered ironstone species versus common species). Waterlogging arrested growth of the main root system, and stimulated the formation of superficial adventitious roots just below the root/shoot junction in all species. Wetland species produced at least twice the amount of adventitious root dry mass of that of non-wetland species, due to differences in number, length or degree of branching. Their adventitious roots also tended to have higher porosities (7–10% versus 5–6% gas spaces). The relative amount of adventitious roots formed was strongly, positively correlated with the maintenance of shoot growth, and only the non-wetland species showed significant shoot growth reductions (19%) upon waterlogging. Dry mass percentage of stems and leaves, and leaf dry mass per area (LMA) increased considerably during waterlogging in all species (averages of 15, 29 and 27%, respectively), returning to the values of continuously drained control plants after drainage. Similarly, upon drainage, a suppression of shoot growth (average 35%) and a stimulation of root growth (average 50%) restored the root mass ratios to those of control plants. We found a negative correlation between the maintenance of growth during waterlogging versus that after waterlogging, suggesting a trade-off in functioning of the superficial adventitious roots between waterlogged and drained conditions. The rare winter-wet ironstone endemics resembled the common winter-wet species in most of their responses to waterlogging and drainage. Therefore, the results presented here cannot offer an explanation for their different distribution patterns. Our results suggest that non-wetland species may be disadvantaged in a wetland environment, due to their lower capacity to form adventitious roots resulting in stronger growth reductions.     

Microcutting leaf area,weight and position on the stock shoot influence root vigour,shoot growth and incidence of shoot tip necrosis in grape plantlets in vitro

The physiological parameters of microcuttings, namely leaf area, weight (wt) and position on the stock shoot, had significant effects on root vigour, microshoot growth and incidence of shoot tip necrosis (STN) in cultures of grape (Vitis vinifera L.) ‘Arka Neelamani’. Single-node leafy cuttings cultured in MS medium containing 1 μM IAA and 0.1 μM GA₃ generally rooted first and subsequently sprouted into a single shoot. Small leafed cuttings exhibited slow root emergence, poor quality roots, early sprouting and weak shoot growth. Large leafed cuttings on the other hand, showed early rooting, vigorous roots, delayed sprouting and healthier shoots. Significant correlations were observed between fresh root wt per plantlet at 1 month in culture and wt of lamina, total wt and leaf area of the cuttings in that order. A significant correlation also existed between wt of roots and height or wt of the sprout that developed. The study suggests that the weight or area of the leaf governed the root growth in a microcutting. STN was observed in some plantlets particularly those derived from large leafed cuttings. Such cuttings showed vigorous roots and delayed but fast sprout growth that ended in STN. Plantlets showing STN had less Ca⁺⁺ and Mg⁺⁺ in the shoot tissue than in the shoots of normal plantlets while the roots showed similar Ca⁺⁺ or higher Mg⁺⁺ contents. Both had comparable amounts of cytokinins in shoot and root tissues. This revised version was published online in June 2006 with corrections to the Cover Date.     

Growth and gas exchange responses of Hevea brasiliensis seedlings to inoculation with Glomus mosseae

 The beneficial effect of arbuscular mycorrhizal (AM) fungi on plant growth is well known, but the physiological processes involved are still discussed. The purpose of this study was to determine if Glomus mosseae affects the growth of Hevea brasiliensis seedlings and, if it is the case, if it could be associated with variations in leaf CO₂ and H₂O gas exchange. H. brasiliensis rubber trees were grown for 9 months in a medium containing either propagules of G. mosseae or sterilized inoculum. Plant size, root collar diameter and leaf area, as well as net CO₂ assimilation, stomatal conductance (g_s) and water-use efficiency of photosynthesis were evaluated during the first 5 stages of growth. At stage 2, a growth depression occurred in the mycorrhizal seedlings coincident with the first AM infections. Then, at stage 5, Glomus mosseae-inoculated plants had moderate colonization (47% of root length) and were taller than control plants with a larger root collar diameter and an enhanced leaf organogenesis. This enhanced growth was accompanied by increased photosynthesis, transpiration, and stomatal conductance. After 9 months, dry weights of shoots and roots of inoculated plants were greater than those of controls by 27 and 17%, respectively. Received: 10 May 1997 / Accepted: 9 September 1997     

Effects of Cadmium and Gibberellin on Growth and Photosynthesis of Glycine Max

The effects of 0, 2.5, 5.0, and 10.0 mg(Cd²⁺) m^-3 [Cd(NO₃)2×4 H₂O] and 0 and 10.0 mg m^-3 gibberellin on certain parameters of photosynthesis and growth in soybean (Glycine max L. cv. Pershing) plants were studied. With increasing Cd²⁺ concentration in the Hoagland nutrient solution, the contents of chlorophyll and CO₂ compensation concentration decreased. The addition of 10 mg m^-3 gibberellin reduced the negative effects of Cd²⁺ in shoot and root growth. With increasing of Cd²⁺ concentration in the culture medium, the dry matter production in both the roots and shoots decreased as shown by the decline in growth rate (PGR), net assimilation rate (NAR), and leaf area ratio. The addition of gibberellin caused a partial elimination of the Cd effects on the roots and shoots and the PGR and NAR and it increased leaf area and length of stem.     

Growth response to step-decrease in nutrient availability in small birch (Betula pendula Roth)

Abstract Changes in the uptake and allocation of carbon and nitrogen, after a step-decrease in nutrient availability, were investigated in small birch (Betula pendula Roth). By demonstrating stable nutrition, before and after the decrease in nutrient supply, it was possible to eliminate the effects of plant size and age. Immediately following the step-decrease in nutrient availability, net nitrogen uptake to leaves and the relative rate of increase in shoot area tended to zero. Although photosynthetic rate per shoot area decreased, carbon uptake remained in excess of that used in structural growth and respiration. More of the excess carbon was accumulated as starch in leaves than in roots. After a lag phase, the relative rates of increase in plant dry matter, starch amount, net nitrogen uptake to leaves and shoot area development equalled that of the reduced rate of nutrient supply. It is concluded that the reduction in plant relative growth rate was much more attributable to the reduced allocation of photosynthate to leaf area growth than to the reduction in photosynthesis per shoot area.     

Root Confinement and its Effects on the Water Relations, Growth and Assimilate Partitioning of Tomato (Lycopersicon esculentum Mill)

Although it is well established that the root growth in manyspecies is very sensitive to mechanical impedance or to confinementin small volumes, little is known about the consequent effectson growth of the whole plant and the mechanisms involved. Thiswork investigated the effects of root confinement on the waterrelations, growth and assimilate partitioning of tomato (Lycopersiconesculentum Mill) grown in solution culture. Six-week old plants were transferred to either 4500 ml or 75ml containers filled with nutrient solution, and allowed togrow for 14 d. Transpiration, leaf-air temperature differences,and leaf diffusive resistances were measured frequently. Leaf,stem and shoot dry masses, leaf area and root length, were estimatedwhen the treatments were imposed and at the end of the experiment.After 14 d growth the root and shoot hydraulic resistances wereestimated from measurements of leaf water potential and transpirationrate, using a steady-state technique. Confining root growth to the small containers substantiallyreduced shoot and root growth and increased the proportion oftotal dry matter present in the stems. These effects were dueto drought stress. The hydraulic resistance of the root systemwas greatest in the confined plants. This led to more negativeleaf water potentials, increased leaf diffusive resistance,and reduced the net assimilation rate by a factor of 2.5. Transpirationper unit leaf area was less affected. However, cumulative transpirationwas also reduced by a factor of 2.5. mostly because of the smallerleaf area on the confined plants. Root hydraulic resistivitywas measured at 3.1 x 10¹²s m^–1 in the control treatment,but increased to 3.9 x 10¹² s m^–1 for roots in the smallcontainer. The mechanisms by which root confinement caused drought stressand disrupted the pattern of assimilate partitioning are discussedin detail. Assimilate partitioning, Lycopersicon esculentum, root confinement, plant growth, root growth, root resistance, shoot resistance, tomato, transpiration, water-use efficiency     

Influence of aqueous leaf extract of common lambsquarters and NaCl salinity on the germination, growth, and nutrient content of wheat

Aqueous leaf extract of common lambsquarters (Chenopodium album L.) was evaluated alone or in combination with NaCl salinity for its influence on germination, seedling growth and contents of Na, K, and Ca by shoots and roots of wheat. The leaf extract and NaCl alone or in combination did not have any significant effect on germination, but the shoot and root lengths of seedlings and their dry weight decreased significantly by the treatments. Root growth was affected more than the shoot. The combination of leaf extract and NaCl drastically reduced the growth more than the separate effects of these stress treatments. The incorporation of leaf extract in the growth media decreased the content of Na by shoot, whereas the contents of K and Ca increased. NaCl treatments in combination with leaf extract increased the content of Na. Similar increases were observed for K and Ca in shoot, while these nutrients were increased in roots compared to control.     

Effects of simulated root herbivory and fertilizer application on growth and biomass allocation in the clonal perennialSolidago canadensis

Summary Compensatory growth in response to simulated belowground herbivory was studied in the old-field clonal perennialSolidago canadensis. We grew rootpruned plants and plants with intact root systems in soil with or without fertilizer. For individual current shoots (aerial shoot with rhizome and roots) and for whole clones the following predictions were tested: a) root removal is compensated by increased root growth, b) fertilizer application leads to increased allocation to aboveground plant organs and increased leaf turnover, c) effects of fertilizer application are reduced in rootpruned plants. When most roots (90%) were removed current shoots quickly restored equilibrium between above-and belowground parts by compensatory belowground growth whereas the whole clone responded with reduced aboveground growth. This suggests that parts of a clone which are shared by actively growing shoots act as a buffer that can be used as source of material for compensatory growth in response to herbivory. Current shoots increased aboveground mass and whole clones reduced belowground mass in response to fertilizer application, both leading to increased allocation to aboverground parts. Also with fertilizer application both root-pruned and not root-pruned plants increased leaf and shoot turnover. Unfertilized plants, whether rootpruned or not, showed practically no aboveground growth and very little leaf and shoot turnover. Effects of root removal were as severe or more severe under conditions of high as under conditions of low nutrients, suggesting that negative effects of belowground herbivory are not ameliorated by abundant nutrients. Root removal may negate some effects of fertilizer application on the growth of current shoots and whole clones.     

The impact of decreased Rubisco on photosynthesis, growth, allocation and storage in tobacco plants which have been transformed with antisense rbcS

Transgenic tobacco plants tranformed with antisense to rbcS to decrease expression of ribulose-1,5–bisphosphate carboxylase-oxygenase (Rubisco) have been used to investigate (a) whether Rubisco is limiting for photosynthesis and plant growth and (b) whether biomass allocation and storage of carbohydrate and nitrogen are regulated in response to decreased rate of photosynthesis. The rate of photosynthesis (measured in growth conditions) and plant growth were not strongly inhibited until almost half of the Rubisco was removed. When Rubisco was decreased further there was a large decrease of photosynthesis and plant growth. When photosynthesis decreased in the ‘antisense’ plants there was an increase in the shoot/root ratio and the specific leaf area. As a result, the leaf area ratio (leaf area per g plant dry weight) increased 3–4–fold. This shows that tobacco compensates for decreased photosynthesis by maximizing leaf area. The decrease of photosynthesis also resulted in lower starch and free hexose in the leaf, but the volume of the diurnal starch turnover was largely maintained. This indicates that partitioning to starch is regulated to decrease non-productive accumulation of starch, but still maintain a pool of transient starch for export during the night. The decrease of photosynthesis was also accompanied by a large increase of the nitrogen/ carbon balance, due to a large accumulation of nitrate in the leaf. This shows that assimilation of nitrate is inhibited in response to low availability of photo-synthate.     

Exaggerated root respiration accounts for growth retardation in a starchless mutant of Arabidopsis thaliana

The knock‐out mutation of plastidial phosphoglucomutase (pgm) causes a starchless phenotype in Arabidopsis thaliana, and results in a severe growth reduction of plants cultivated under diurnal conditions. It has been speculated that high soluble sugar levels accumulating during the light phase in leaf mesophyll might cause a reduction of photosynthetic activity or that shortage of reduced carbon during the night is the reason for the slow biomass gain of pgm. Separate simultaneous measurements of leaf net photosynthesis and root respiration demonstrate that photosynthetic activity per unit fresh weight is not reduced in pgm, whereas root respiration is strongly elevated. Comparison with a mutant defective in the dominating vacuolar invertase (AtβFruct4) revealed that high sucrose concentration in the cytosol, but not in the vacuole, of leaf cells is responsible for elevated assimilate transport to the root. Increased sugar supply to the root, as observed in pgm mutants, forces substantial respiratory losses. Because root respiration accounts for 80% of total plant respiration under long‐day conditions, this gives rise to retarded biomass formation. In contrast, reduced vacuolar invertase activity leads to reduced net photosynthesis in the shoot and lowered root respiration, and affords an increased root/shoot ratio. The results demonstrate that roots have very limited capacity for carbon storage but exert rigid control of supply for their maintenance metabolism.