Effects of foliar applied gibberellic acid and benzyladenine upon yield components in sunflower (Helianthus annuus L.)

Sunflower yield is determined by seed number/m^–2 and by achene weight. Frequently, a high percentage of empty achenes in the inner portion of the capitulum, probably due to a reduced vascularization of that section of the flower head, decreases final yield. The objective of the present research is to determine if foliarly applied gibberellic acid (GA) and benzyladenine (BA) can enhance the vascularization in the inner portion of the capitulum, improving photoassimilate translocation. Field experiments were conducted during 1989/90 with hybrid SPS 894 and during 1990/91 with hybrid ACA 882. GA (150 mg/l^–1), BA (150 and 250 mg/l^–1) and GA 150+BA 150 mg/l^–1 each were foliarly applied 20, 40, or 60 days after emergence. For both seasons and hybrids plant growth regulator (PGRs) applications significantly reduced the percentage of empty achenes, increased achene weight, achene weight (× 1000) and achene number in the inner portion of the capitulum and in the middle and outer portion during 1990/91. A 25% increase in seed yield was achieved due to PGR application and the capitulum partition index (achene weight/receptacle weight^–1, CPI) was significantly increased due to an improvement in photoassimilate distribution. A distribution model was derived showing that preferential allocation of photoassimilates in the outer portion of the capitulum can be modified by PGR application, demonstrating that photoassimilate distribution is under hormonal control.Consejo Nacional de Investigaciones Científicas y Tecnológicas (CONICET)     

Induction of leaf senescence by low nitrogen nutrition in sunflower (Helianthus annuus) plants

Different parameters which vary during the leaf development in sunflower plants grown with nitrate (2 or 20 mM) for a 42‐day period have been determined. The plants grown with 20 mM nitrate (N+) showed greater leaf area and specific leaf mass than the plants grown with 2 mM nitrate (N?). The total chlorophyll content decreased with leaf senescence, like the photosynthetic rate. This decline of photosynthetic activity was greater in plants grown with low nitrogen level (N?), showing more pronounced senescence symptoms than with high nitrogen (N+). In both treatments, soluble sugars increased with aging, while starch content decreased. A significant increase of hexose to sucrose ratio was observed at the beginning of senescence, and this raise was higher in N? plants than in N+ plants. These results show that sugar senescence regulation is dependent on nitrogen, supporting the hypothesis that leaf senescence is regulated by the C/N balance. In N+ and N? plants, ammonium and free amino acid concentrations were high in young leaves and decreased progressively in the senescent leaves. In both treatments, asparagine, and in a lower extent glutamine, increased after senescence start. The drop in the (Glu+Asp)/(Gln+Asn) ratio associated with the leaf development level suggests a greater nitrogen mobilization. Besides, the decline in this ratio occurred earlier and more rapidly in N? plants than in N+ plants, suggesting that the N? remobilization rate correlates with leaf senescence severity. In both N+ and N? plants, an important oxidative stress was generated in vivo during sunflower leaf senescence, as revealed by lipid peroxidation and hydrogen peroxide accumulation. In senescent leaves, the increase in hydrogen peroxide levels occurred in parallel with a decline in the activity of antioxidant enzymes. In N+ plants, the activities of catalase and ascorbate peroxidase (APX) increased to reach their highest values at 28 days, and later decreased during senescence, whereas in N? plants these activities started to decrease earlier, APX after 16 days and catalase after 22 days, suggesting that senescence is accelerated in N‐leaves. It is probable that systemic signals, such as a deficit in amino acids or other metabolites associated with the nitrogen metabolism produced in plants grown with low nitrogen, lead to an early senescence and a higher oxidation state of the cells of these plant leaves.     

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     

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.     

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.     

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.     

Response of sunflower (Helianthus annuus L) genotypes to PEG-mediated water stress

Drought tolerance of two sunflower (Helianthus annuus L.) genotypes, cultivated cultivar 1114 and interspecific line H. annuus × H. mollis, was studied under laboratory conditions using PEG-6000. Four levels of osmotic stress (?0.4, ?0.6, ?0.8 and ?1.0 MPa) were created and performances were monitored against a control. Physiological and biochemical stress determining parameters such as malondialdechyde (MDA), proline content, and hydrogen peroxide (H₂O₂) were compared between seedlings of both genotypes. The results indicated that both genotypes have similar responses at four osmotic potentials for all traits studied. All seedling growth parameters such as germination percentage, root length, shoot length, root and shoot dry weight decreased with increasing osmotic stress. MDA, proline, and H₂O₂ were found to be increased at different osmotic gradients in comparison to control. Cultivar 1114 was less affected than the interspecific line under these stress conditions. The data observed in the experiments revealed that perennial wild H. mollis can hardly be considered to be an excellent candidate of drought tolerance genes.     

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.     

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.     

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.     

Systemic acquired resistance in sunflower (Helianthus annuus L.)

Systemic acquired resistance (SAR) to infection by Botrytis cinerea in the leaves of sunflower (Helianthus annuus L.) plants was induced following cotyledon inoculation with B. cinerea or treatment with abiotic inducers. Salicylic acid (SA), benzo-(1,2,3)-thiadiazole-7-carbothioic S-methyl ester (BTH), 2,6-dichloroisonicotinic acid (INA) or EDTA protected sunflower plants against Botrytis infection, that was revealed by a reduction in the number and area of the necrotic lesions in upper leaves after challenge inoculation with the pathogen. SA and BTH were more potent inducers than INA, EDTA or pre-inoculation with the fungus. In addition to resistance to B. cinerea, the upper leaves have also developed resistance to maceration by a mixture of cell wall-degrading enzymes. Calcium nitrate inhibited both the protective effect and the resistance of leaf discs to cell-wall degrading enzymes. All the tested chemicals increased the synthesis and excretion of sunflower phytoalexins--coumarins scopoletin and ayapin and induced the PR-proteins chitinase and 1,3-beta-glucanase, being the inducer effect of each activator correlated with the level of protection against B. cinerea (BTH > SA > INA > EDTA). Thus, SAR induction is mediated by general increase of plant defence responses. This is the first report on SAR in sunflower.     

Resistance to broomrape (Orobanche spp.) in sunflower (Helianthus annuus L.) is temperature dependent

The effects of various temperature regimes in the range 29-17/21-9 degrees C day/night on each stage of the parasitism process of Orobanche cumana and O. aegyptiaca on sunflower were studied under controlled conditions in polyethylene bags. The response of the resistant sunflower variety 'Ambar' was expressed as the degeneration of the parasite tissues after its establishment in the plant roots, and this stage was found to be temperature dependent. The degeneration rate of Orobanche tubercles in the resistant sunflower variety was also found to be temperature dependent and was about five times as great as that in the sensitive variety in the highest temperature regime tested of 29/21 degrees C day/night. The ability to reject the parasite by causing its degeneration and death is the main factor that determines the resistance. As the temperature rises, more tubercles degenerate and die, that is the sunflower plant expresses higher levels of resistance.     

Changes in mass and dimensions of sunflower (Helianthus annuus L.) Achenes and seeds due to carbonization

When analyzing sunflower (Helianthus annuus L.) remains, which are often carbonized, archaeobotanists commonly differentiate between wild and domesticated achenes and seeds based on the measured length (L) and width (W) or the calculated index L*W. Carbonization reduces the dimensions. To compensate for these reductions, archaeobotanists use a single correction factor proposed by Richard Yarnell (1978) for all cases. The use of a single correction factor can bias the reconstructed dimensions as carbonization is a highly variable process. The current study determines the relationship between carbonization and the dimensions of length and width. Measurements established that a decrease of 2.5-22.5% in achene length and 10-29% in achene width can occur, depending on temperature, heating rate, and variety. For seeds, temperature is of most importance, and shrinkage ranges from 0-27% for the length and from 0-20% for the width. These ranges make the use of a single correction factor problematic. A method is developed in which reflectance (an optical property applied in coal technology to determine coal rank) is used to measure the carbonization temperature, and in turn the shrinkage can be calculated. Subsequently, correction factors are calculated to reconstruct the original length and width. When applied to an assemblage of carbonized sunflower achenes, the newly developed method shows that the Yarnell single correction factor may bias the dimensions towards classifications of “wild” or “ruderal” forms of sunflower     

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.     

Utilization of leaf temperature for the selection of leaf gas-exchange traits to induce heat resistance in sunflower (Helianthus annuus L.)

Heat stress is a major production constraint of sunflower worldwide. Therefore, various populations (parental, F₁, F₂, F₃, and plant progenies) of sunflower were screened for leaf gas-exchange traits with the objectives to formulate selection criteria of heat resistance and development of heat-resistant lines. Initial screening and F₂ seeds exposed to heat stress (45°C) resulted in the development of an adapted F₂ population that showed leaf gas-exchange and morphological traits better than the unadapted population. Correlation coefficients of traits were partitioned into direct and indirect effects via a path analysis technique to determine the cause of their relationship with a basic parameter such as a reproductive head mass (HM). Path analysis showed a positive direct effect of leaf temperature (T_leaf) (0.32) on HM and also an indirect effect (0.77) of the transpiration rate (E) on HM. Moreover, T_leaf showed high heritability estimates. T_leaf was used to select superior plants within the F₂ population. This selection brought about an improvement in the net photosynthetic rate (P _N) and E as it was indicated from progeny performance and realized heritability. Progenies selected on the basis of T_leaf also showed an increase in achene yield and heat resistance over unselected F₃ progenies and a commercial hybrid.