Control of leaf expansion in sunflower (Helianthus annuus L.) by nitrogen nutrition

Seedlings of Helianthus annuus L. were grown at an initiallyhigh relative nitrate supply rate (0.27 mol N mol N^–1d^–1). The supply was subsequently reduced to a low rate(0.04 mol N mol N^–1 d^–1). The response of leaf areadevelopment to this abrupt decrease in nitrate availabilitywas characterized by following the expansion of the primaryand secondary leaf pairs. The timing of the drop in nitratesupply was when cell division in the epidermis of the primaryleaf pair was largely complete. Reducing the availability ofnitrate had a strong effect on leaf area expansion. The finalleaf size of the primary leaf pair was affected indicating aneffect of nitrate availability on cell expansion. By the endof the experiment the secondary leaf pair was only one-thirdthe area of that on control seedlings. The role of epidermalcell turgor pressure in this growth response was assessed bydirect measurements with a miniature cell pressure probe. Noreduction in cell turgor pressure following the decrease innitrate availability was detected. It is concluded that a reductionin turgor pressure was not responsible for the reduction inleaf area expansion and it is suggested that reduced cell expansionwas due to changes in cell wall properties. Concentrations ofleaf and root abscisic acid increased following the reductionin nitrate availability. Key words: Abscisic acid, cell size, cell turgor pressure, nitrate, nitrogen, relative rate of nitrate supply     

Growth under elevated atmospheric CO(2) concentration accelerates leaf senescence in sunflower (Helianthus annuus L.) plants

Some morphogenetic and metabolic processes were sensitive to a high atmospheric CO(2) concentration during sunflower primary leaf ontogeny. Young leaves of sunflower plants growing under elevated CO(2) concentration exhibited increased growth, as reflected by the high specific leaf mass referred to as dry weight in young leaves (16days). The content of photosynthetic pigments decreased with leaf development, especially in plants grown under elevated CO(2) concentrations, suggesting that high CO(2) accelerates chlorophyll degradation, and also possibly leaf senescence. Elevated CO(2) concentration increased the oxidative stress in sunflower plants by increasing H(2)O(2) levels and decreasing activity of antioxidant enzymes such as catalase and ascorbate peroxidase. The loss of plant defenses probably increases the concentration of reactive oxygen species in the chloroplast, decreasing the photosynthetic pigment content as a result. Elevated CO(2) concentration was found to boost photosynthetic CO(2) fixation, especially in young leaves. High CO(2) also increased the starch and soluble sugar contents (glucose and fructose) and the C/N ratio during sunflower primary leaf development. At the beginning of senescence, we observed a strong increase in the hexoses to sucrose ratio that was especially marked at high CO(2) concentration. These results indicate that elevated CO(2) concentration could promote leaf senescence in sunflower plants by affecting the soluble sugar levels, the C/N ratio and the oxidative status during leaf ontogeny. It is likely that systemic signals produced in plants grown with elevated CO(2), lead to early senescence and a higher oxidation state of the cells of these plant leaves.     

Basal leaf senescence in a sunflower (Helianthus annuus) canopy: responses to increased R/FR ratio

Senescence of lower leaves (LS) begins before anthesis in sunflower crop canopies. Using isolated field-grown sunflower plants, it has previously been shown that pre-anthesis LS is dependent on photosynthetic photon flux density (PPFD) and is hastened by increases in far-red light. We tested the hypothesis that increasing the red/far-red ratio (R/FR) perceived by basal leaves within canopies delays LS. To do this, light impinging on the lower surface of north-oriented 8th leaves (cotyledons=0) of crops with maximum leaf area indexes of 3.3 (Experiment 1) and 2.4 (Experiment 2) was enriched (+8.33 μmol m⁻² s⁻¹) with red light using light emitting diode (LED) panels. LED panels constructed with unlit LED or with green LED (PPFD slightly greater than the red LED panels, to compensate for lower efficiency) were used as controls. Compared with controls, additional R significantly ( P <0.05) increased R/FR perceived by the lower surface and significantly ( P <0.01) delayed LS. On average, leaf duration, as time between full expansion and a 70% diminution of chlorophyll content, was 5 days greater for leaves receiving extra red light (maximum observed LD=27 days). We conclude that an increase in the R/FR ratio can delay LS in crop canopies.     

The role of ethylene in the regeneration of Helianthus annuus (sunflower) plants from callus

Hypocotyl-derived callus from the Helianthus annuus L. inbred line SS415B regenerated significantly more plants if the seedlings were grown in the light. The difference between light- and dark-grown seedlings was not correlated with differences in seedling ethylene production, but seemed to be due to a difference in sensitivity to ethylene at a specific time during seedling growth. Treating 3-day-old dark-grown seedlings with 10 μ M aminoethoxyvinylglycine (AVG) effectively inhibited ethylene production for at least 7 days. Hypocotyl callus derived from AVG-treated seedlings gave the same amount of regeneration as callus from light-grown seedlings. Promotion of regeneration by AVG was not seen unless the 3-day-old seedlings were grown for 4 additional days prior to culturing hypocotyl explants. The effects of AVG could be reversed by treatment with 1-aminocyclopropane-1-carboxylic acid (ACC) during these 4 days. After the 4 days, ACC was no longer effective.     

The domestication of Helianthus annuus L. (sunflower)

All modern domesticated sunflowers can be traced to a single center of domestication in the interior mid-latitudes of eastern North America. The sunflower achenes and kernels recovered from six eastern North American sites predating 3000 b.p. that document the early history of this important crop plant are reanalyzed, and two major difficulties in the interpretation of archaeological sunflower specimens are addressed. First, achenes and kernels obtained from a modern wild sunflower population included in a prior genetic study because of its minimal likelihood for crop-wild gene flow, and its close genetic relationship to domesticated sunflowers, provide a new and more tightly drawn basis of comparison for distinguishing between wild and domesticated achene and kernel specimens recovered from archaeological contexts. Second, achenes and kernels from this modern wild baseline population were carbonized, allowing a direct comparison between carbonized archaeological specimens and a carbonized modern wild reference class, thereby avoiding the need for the various problematic shrinkage correction conversion formulas that have been employed over the past half century. The need for further research on museum collections is underscored, and new research directions are identified.     

Composition of lipids from sunflower pollen (Helianthus annuus)

The contents of the pollen lipids of the sunflower Helianthus annuus are described. The major component is the seco-triterpene helianyl octanoate, followed by new beta-diketones as second major group of compounds. They exhibit a shorter chain length and often other positions of the functional group compared to already known beta-diketones. Of particular note are the 1-phenyl-beta-diketones, not previously reported from nature. Further lipid classes present are related hydroxyketones and diols. Interestingly, new beta-dioxoalkanoic acids are present in the extracts, which most likely are biogenetic precursors of the diketones. Additionally, we investigated the composition of the pollen coat which resembles the total extract, but lacks the dioxoalkanoic acids and certain estolides.     

Regeneration of fertile plants from protoplasts of sunflower (Helianthus annuus L.)

Summary Sunflower hypocotyl protoplasts (Helianthus annuus L.) from 5 PIONEER genotypes (PT024, SMF3, EMIL, HA300*PT024, VK5F) and 1 public line (RHa 274) formed colonies at frequencies of up to 60% when plated in 0.25ml agarose beads in a modified L4 medium (Lenée and Chupeau 1986) containing 3mg/l NAA, 1mg/l BA and 0.1mg/l 2,4-D, and 1000mg/l casamino acids. Protoplast-derived colonies grew slowly into calli. Organogenesis was obtained from callus of PT024 on a MS medium containing NAA and BA at 1mg/l and GA at 0.1mg/l. Freshly excised shoots were induced to root by an IAA treatment. Regenerated plants were transferred to the greenhouse and seed was harvested within 7 months of the initial protoplast isolation.Abbreviations BA 6-benzylaminopurine - NAA -naphtaleneacetic acid - GA gibberellic acid - IAA indole-3-acetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - MS Murashige and Skoog mineral elements - B5 Gamborg mineral elements     

Adventitious rooting in hypocotyls of sunflower (Helianthus annuus) seedlings

Removal of the main root system of sunflower ( Helianthus annuus ) initiates adventitious root development on the lower portion of the hypocotyl. The first cytological changes (enlarged nuclei in the interfascicular parenchymatous cells adjacent to the phloem and some cell divisions) are observed 24 h after root excision. On the basis of experiments in which (a) roots, apical buds and various amounts of cotyledonary tissue were removed, (b) cuttings were subjected to various light regimes, (c) benzyladenine oas applied to cotyledons to create an artificial sink, it was concluded that the roots normally produce factors inhibiting to adventitious rooting and might be a sink for stimulatory substances produced in the shoots. The cotyledons seem to be the major source of these stimulators. Application of aqueous and ethanolic extracts of cotyledons and hypocotyls to cuttings promoted adventitious rooting.     

Study of the senescence process in primary leaves of sunflower (Helianthus annuus L.) plants under two different light intensities

Different parameters that vary during leaf development may be affected by light intensity. To study the influence of different light intensities on primary leaf senescence, sunflower (Helianthus annuus L.) plants were grown for 50 days under two photon flux density (PFD) conditions, namely high irradiance (HI) at 350 μmol(photon) m^?2 s^?1 and low irradiance (LI) at 125 μmol(photon) m^?2 s^?1. Plants grown under HI exhibited greater specific leaf mass referred to dry mass, leaf area and soluble protein at the beginning of the leaf development. This might have resulted from the increased CO₂ fixation rate observed in HI plants, during early development of primary leaves. Chlorophyll a and b contents in HI plants were lower than in LI plants in young leaves. By contrast, the carotenoid content was significantly higher in HI plants. Glucose concentration increased with the leaf age in both treatments (HI and LI), while the starch content decreased sharply in HI plants, but only slightly in LI plants. Glucose contents were higher in HI plants than in LI plants; the differences were statistically significant (p<0.05) mainly at the beginning of the leaf senescence. On the other hand, starch contents were higher in HI plants than in LI plants, throughout the whole leaf development period. Nitrate reductase (NR) activity decreased with leaf ageing in both treatments. However, the NR activation state was higher during early leaf development and decreased more markedly in senescent leaves in plants grown under HI. GS activity also decreased during sunflower leaf ageing under both PFD conditions, but HI plants showed higher GS activities than LI plants. Aminating and deaminating activities of glutamate dehydrogenase (GDH) peaked at 50 days (senescent leaves). GDH deaminating activity increased 5-fold during the leaf development in HI plants, but only 2-fold in LI plants. The plants grown under HI exhibited considerable oxidative stress in vivo during the leaf senescence, as revealed by the substantial H₂O₂ accumulation and the sharply decrease in the antioxidant enzymes, catalase and ascorbate peroxidase, in comparison with LI plants. Probably, systemic signals triggered by a high PFD caused early senescence and diminished oxidative protection in primary leaves of sunflower plants as a result.     

Photosynthesis and chlorophyll fluorescence in sunflower (Helianthus annuus L.) leaves as affected by phosphorus nutrition

The effects of phosphate concentration on plant growth and photosyntheticprocesses in primary leaves of young sunflower (Helianthus annuusL.) plants were examined. Plants were grown for 3 weeks on half-strengthHoagland's solution containing 0, 0.1, 0.5, 1.0, and 3.0 molm^–3 orthophosphate (Pi). It was shown that optimal photosynthesisand the highest light utilization capacity were achieved at0.5 mol m^–3 Pi in the growth medium, which was in goodagreement with the maximum content of organic phosphorus inthe leaves. Low phosphate in the medium inhibited plant growthrate. Phosphate deficiency appreciably decreased photosyntheticoxygen evolution by leaves, the efficiency of photosystem two(PSII) photochemistry and quantum efficiency of PSII electrontransport. High oxidation state of PSII primary electron acceptorQA, at 0.1 mol m^–3 Pi, however, indicates that photosyntheticelectron transport through PSII did not limit photosynthesisin Pi-deficient leaves. The results indicate that diminishedphotosynthesis under sub- and supra-optimal Pi was caused mainlyby a reduced efficiency of ribulose 1, 5-bisphosphate (RuBP)regeneration at high light intensities. These results suggestthat, under non-limiting C0₂ and irradiance, photosynthesisof the first pair of leaves could be diminished by both sub-and supra-optimal phosphorus nutrition of sunflower plants. Key words: Helianthus annuus L, phosphate nutrition, photosynthesis, photochemical efficiency     

Genetic control of organogenesis in cotyledons of sunflower (Helianthus annuus)

Genetic variability for regeneration ability was evaluated by studying direct organogenesis from cotyledons of thirteen genotypes including three cytoplasmic male sterile, three maintenor, three restorer inbred lines, and four F1 hybrids obtained by crosses between some of these inbred lines. The experimental design was a complete randomized block with three replications. A high genetic variability for organogenesis parameters between genotypes was observed in this study. Evidence of cytoplasmic effect and nucleo-cytoplasmic interaction for some of regeneration parameters was observed. The data also showed the importance of additive genetic control for organogenesis parameters in most genotypes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Circadian regulation of leaf hydraulic conductance in sunflower (Helianthus annuus L. cv Margot)

The circadian regulation of leaf hydraulic conductance (K_leaf) was investigated in Helianthus annuus L. (sunflower). K_leaf was measured with an high pressure flow meter during the light and dark period from plants growing at a photoperiod of 12 h. K_leaf was 4.0 e−4 kg s⁻¹ m⁻² MPa⁻¹ during the light period (LL) and 30–40% less during the dark period (DL). When photoperiod was inverted and leaves were measured for K_leaf at their subjective light or dark periods, K_leaf adjusted to the new conditions requiring 48 h for increasing from dark to light values and 4 d for the opposite transition. Plants put in continuous dark showed K_leaf oscillating from light to dark values in phase with their subjective photoperiod indicating that K_leaf changes were induced by the circadian clock. Several cuts through the minor veins reduced leaf hydraulic resistance (R_leaf) of both LL and DL to the same value (1.0 e + 3 MPa m² s kg⁻¹) that equalled the vascular resistance (R_v). The contribution of the non-vascular leaf resistance (R_nv) to R_leaf was of 71.9% in DL and of 58.4% in LL. The dominant R_nv was shown to be reversibly modulated by mercurials, suggesting that aquaporins play a role in diurnal changes of K_leaf.     

Callus formation from isolated sunflower (Helianthus annuus) mesophyll protoplasts

Mesophyll protoplasts of the cultivated sunflower,Helianthus annuus, have been consistently found not to divide or regenerate calli, despite the efforts of several groups. In the present report, we describe the conditions for donor plant culture, protoplast isolation, and their culture that were suitable for repeated regeneration of green, nodular, vigorously growing calli from isolated sunflower mesophyll protoplasts. The best conditions for protoplast isolation employed the use of both CAYLA cellulase and CAYLA pectinase. Culture conditions were not much different from those established earlier for sunflower hypocotyl protoplasts. The most startling observation was the great variability of division frequencies between experiments even under strictly controlled, identical experimental conditions. This finding points to an important influence of a variable in the physiological state of the donor plant which is difficult to control.     

Reevaluation of biosecurity of Ophraella communa against sunflower (Helianthus annuus)

Ophraella communa (Coleoptera: Chrysomelidae), originally from North America, has been used for biological control of common ragweed, Ambrosia artemisiifolia, in China since 2007. However, there is still a debate on whether O. communa can attack sunflowers under field conditions. To re-evaluate the biosecurity of O. communa against sunflower (Helianthus annuus), we investigated the population density of O. communa on three sunflower varieties that were intercropped with or planted in circumambience of A. artemisiifolia under field conditions. Our results showed that only very few O. communa eggs (<0.5 eggs/plant) were found on sunflower plants at the last two surveys when sunflowers were planted in circumambience of common ragweed. O. communa eggs were not found on sunflower plants at each survey when sunflowers were intercropped with common ragweed. The first–second instar larvae, third instar larvae, pupae and adults of O. communa were occasionally found on sunflower plants, but their densities were very low under either case of planting patterns. Based on these results, we conclude that sunflower is not a potential host plant for O. communa and the beetle is an effective host-specific biological control agent of common ragweed.     

Effects of leaf position and nitrogen supply on the expansion of leaves of field grown sunflower (Helianthus annuus L.)

Leaf growth responses to N supply and leaf position were studied using widely-spaced sunflower plants growing under field conditions. Both N supply (range 0.25 to 11.25 g added N per plant) and leaf position significantly (p=0.001) affected maximum leaf area (LA_max) of target leaves through variations in leaf expansion rate (LER); effects on duration of expansion were small. Specific leaf nitrogen (SLN, g N m^-2) fell quite rapidly during the initial leaf expansion phase (LA < 35% LA_max) but leveled off during the final 65% increase of leaf area. This pattern held across leaf positions and N supply levels. Leaf nitrogen accumulation after 35% LA_max continued up to achievement of LA_max; reductions in the higher SLN characteristic of the initial phase were insufficient to cover the nitrogen requirements for expansion during the final phase. LER in the quasi-linear expansion phase (35 to 100% of LA_max) was strongly associated with SLN above a threshold that varied with leaf position (mean 1.79±0.225 g N m^-2). This contrasts with the response of photosynthesis at high irradiance to SLN, which has previously been shown to have a threshold of 0.3 g N m^-2; in the present work saturation of photosynthetic rate was evident when SLN reached 1.97 g N m^-2. Thus, once the area of a leaf exceeds 35% of LA_max, expansion proceeds provided SLN values are close to the levels required for maximum photosynthesis. However, growth of leaves during the initial expansion phase ensures a minimum production of leaf area even at low N supply levels.