Effects of timing and supply of nitrogen on nitrogen remobilization from vegetative organs and redistribution to developing seeds of sunflower

A glasshouse study was made of the distribution of ¹⁵N among vegetative organs of sunflower and its later remobilization and redistribution to seeds, as influenced by the developmental stage at which ¹⁵N was provided, and by the N status of the plants. Plants of Hysun 30 sunflower were grown in sand culture and provided with K¹⁵NO₃ for a 3-day period at: (a) 3 days before the end of floret initiation; (b) 3 days before anthesis; (c) the start of anthesis; (d) full anthesis; and (e) 8 days after full anthesis. The plants were grown on a range of N supply rates, from severely deficient to more than adequate for maximum growth. Nitrogen-15 was distributed to all parts of the plant at the end of the ¹⁵N uptake periods. With the exception of the most N-stressed plants, subsequent remobilization of ¹⁵N from roots, stems and leaves occurred irrespective of the time the ¹⁵N was taken up. However, the percentage redistribution to seeds of ¹⁵N taken up at the end of floret initiation was less than for ¹⁵N taken up at anthesis. Remobilization of ¹⁵N from leaves and roots was higher (70%) for ¹⁵N taken up during and after anthesis than for ¹⁵N taken up at the end of floret initiation (45%), except for plants grown on the lowest N supply. By contrast, remobilization of ¹⁵N from the stem was lower for ¹⁵N taken up after full anthesis (40%) than before or during anthesis (>70%). The proportion of ¹⁵N remobilized from the top third of the stem was less than that from the bottom third, and decreased with increasing plant N status. Nitrogen-15 taken up over the 3-day supply periods during anthesis contributed from 2 to 11% of the total seed N at maturity; the contribution to seeds was greatest for plants grown on the highest N supply. Nitrogen taken up just before and during anthesis contributed most of the N accumulated in mature seeds of plants grown on an adequate N supply, but N taken up between the end of floret initiation and just before anthesis, or after full anthesis seemed to make an equally important contribution to mature seeds as N taken up during anthesis for plants grown on a very low N supply. It was concluded that the development of florets and seeds of sunflower is supported by N taken up by the plant between the end of floret initiation and anthesis, and by N redistributed from vegetative organs. Unless soil N is so low as to impair early growth, split applications of N fertilizer would be best made just before the end of floret initiation (‘star stage’) and just before anthesis.     

Action potentials and variation potentials in sunflower: An analysis of their relationships and distinguishing characteristics

Sunflower plants ( Helianthus annuus L.) were given an electrical stimulus to the stem or a heat (flame)‐wound to a single leaf or a cotyledon. The resulting electrical activity was monitored with extracellular electrodes. An electrical stimulus applied to the stem frequently evoked an action potential (AP), but never a variation potential (VP). In contrast, a heat‐wound applied to a leaf virtually always elicited a VP, which was often accompanied by one or more superimposed spikes (putative APs). The kinetic parameters of the AP and the VP were investigated. The AP appears to propagate without decrement in velocity or magnitude, whereas the VP parameters decrease significantly with distance. The heat stimulus triggered rapid alterations in stem elongation/contraction, which preceded changes in electrical potential, indicating the transmission of a hydraulic signal. Light‐off and light‐on stimuli evoked negative‐ and positive‐going changes in extracellular electrical potential, respectively, corresponding to de‐ and hyper‐polarization of the plasma membrane. Membrane depolarization (extracellularly manifested as a VP) evoked by both the light‐off and heat‐wounding stimuli was able to trigger one or more APs. We interpret these results to suggest that APs are "genuine" electrical signals involving voltage‐gated ion channels or pumps, which can be evoked directly by electrical stimulation or indirectly by changes in membrane potential occurring during the VP or after the light‐off stimulus. In contrast, VPs appear to be a local (non‐transmissible) electrical consequence of the passage of a rapidly transmitted hydraulic signal in the xylem, presumably acting on mechanosensitive ion channels or pumps in adjacent living cells.     

The effect of CO2 on ethylene evolution and elongation rate in roots of sunflower (Helianthus annuus) seedlings

Both carbon dioxide and ethylene can affect the rate of root elongation. Carbon dioxide can also promote ethylene biosynthesis by enhancing the activity of 1-aminocylopropane-1-carboxylic acid (ACC) oxidase. Since the amount of CO₂ in the soil air, and in the atmosphere surrounding roots held in enclosed containers, is known to vary widely, we investigated the effects of varying CO₂ concentrations on ethylene production by excised and intact sunflower roots (Helianthus annuus L. cv. Dahlgren 131). Seedlings were germinated in an aeroponic system in which the roots hung freely in a chamber and were misted with nutrient solution. This allowed for treatment, manipulation and harvest of undamaged and minimally disturbed roots. While exposure of excised roots to 0.5% CO₂ could produce a small increase in ethylene production (compared to roots in ambient CO₂), CO₂ concentrations of 2% and above always inhibited ethylene evolution. This inhibition of ethylene production by CO₂ was attributed to a reduction in the availability of ACC: however, elevated CO₂ had no effect on ACC oxidase activity. ACC levels in excised roots were depressed by CO₂ at a concentration of 2% (as compared to ambient CO₂), but n-malonyl-ACC (MACC) levels were not affected. Treating intact roots with 2% CO₂ inhibited elongation by over 50%. Maximum inhibition of elongation occurred 1 h after the CO₂ treatment began, but elongation rates returned to untreated values by 6 h. Supplying these same intact roots with 2% CO₂ did not alter ethylene evolution. Thus, in excised sunflower roots 2% CO₂ treatment reduces ethylene evolution by lowering the availability of ACC. Intact seedlings respond differently in that 2% CO₂ does not affect ethylene production in roots. These intact roots also temporarily exhibit a significantly reduced rate of elongation in response to 2% CO₂.     

Deoxyradicinin,a novel phytotoxin from Alternaria helianthi

A novel major metabolite which possesses phytotoxic and antifungal properties has been isolated from culture filtrates of Alternaria helianthi and its structure elucidated as deoxyradicinin.     

Fecundity, phenology, and seed dormancy of F1 wild-crop hybrids in Sunflower (Helianthus annuus, Asteraceae)

Crop-to-wild hybridization has the potential to introduce beneficial traits into wild populations. Gene flow from genetically engineered crops, in particular, can transfer genes coding for traits such as resistance to herbicides, insect herbivores, disease, and environmental stress into wild plants. Cultivated sunflower (Helianthus annuus) hybridizes spontaneously with wild/weedy populations (also H. annuus), but little is known about the relative fitness of F1 hybrids. In order to assess the ease with which crop-to-wild introgression can proceed, we compared characteristics of F1 wild-crop progeny with those of purely wild genotypes. Two nontransgenic, cultivated varieties were crossed with wild plants from three different regions-Texas, Kansas, and North Dakota. Seed burial experiments in the region of origin showed that wild-crop seeds had somewhat higher germination rates (less dormancy) than wild seeds from Kansas and North Dakota, while no differences were seen in seeds from Texas. Progeny from each type of cross were grown in outdoor pots in Ohio and in a weedy field in Kansas to quantify lifetime fecundity and flowering phenology. Flowering periods of hybrid and wild progeny overlapped considerably, especially in plants from North Dakota and Texas, suggesting that these hybrids are very likely to backcross with wild plants. In general, hybrid plants had fewer branches, flower heads, and seeds than wild plants, but in two crosses the fecundity of hybrids was not significantly different from that of purely wild plants. In Ohio, wild-crop hybrids from North Dakota appeared to be resistant to a rust that infected 53% of the purely wild progeny, indicating a possible benefit of "traditional" crop genes. In summary, our results suggest that F1 wild-crop hybrids had lower fitness than wild genotypes, especially when grown under favorable conditions, but the F1 barrier to the introgression of crop genes is quite permeable.     

Role of roots and shoots in the regulation of the Fe efficiency responses in sunflower and cucumber

Several experimental approaches were used to study the role of roots and shoots in the regulation of the Fe efficiency responses in the roots of Fe deficient plants of sunflower ( Helianthus annuus L., inbred line RHA 274) and cucumber ( Cucumis sativus L., cv. Burpee pickler). The presence of the shoot apex and the youngest leaves in sunflower and cucumber plants was not indispensable to the development of the responses. In split-root experiments with sunflower plants, with either one or two shoots obtained by grafting, different parts of the root system could receive different Fe additions. Roots growing in a medium without Fe developed Fe efficiency re-sponses even when the other part of the root system was growing with Fe. Also, the part of the root system that was growing in a medium with Fe, developed some Fe efficiency responses, although delayed and at a lower level as compared with the root without external Fe. The results are discussed in relation to the idea that the development of the responses in the roots is controlled not only by the Fe content in the root, but also by some information transmitted between parts of a root system that differ in their Fe nutritional status.     

Seasonal responses of photosynthetic parameters in maize and sunflower and their relationship with leaf functional traits

Estimates of seasonal variation in photosynthetic capacity (P_c) are critical for modeling the time course of carbon fluxes. Given the time‐intensive nature of calculating P_c parameters via gas exchange, it is appealing to calculate parameter variation via changes in chlorophyll (Chl) and nitrogen (N) content by assuming that P_c scales with these variables. Although seasonal changes in P_c and the relationships between N and P_c have been evaluated in forest canopies, there is limited data on seasonal parameter values in crops, nor is it clear if seasonal changes in P_c can be estimated from leaf traits under the high N fertility of managed systems. We characterized the seasonal variability of the maximum rates of carboxylation (V_cmax) and electron transport (J_max) under well‐fertilized conditions for maize (Zea mays L.) and sunflower (Helianthus annuus L.) and coupled these data with measurements of Chl, N, and leaf mass per unit area (LMA). The seasonal Chl–N relationship was significant in maize, but not in sunflower. Area‐based N–V_cmax relationships were not significant for either crop. Mass‐based N–V_cmax relationships were weak in sunflower, but highly significant in maize. Our results suggest that P_c can be seasonally adjusted in maize with reliable estimates of changes in LMA.     

Susceptibility of sunflower inbred lines and putative resistance sources to Smicronyx fulvus LeConte (Coleoptera: Curculionidae)

The red sunflower seed weevil, Smicronyx fulvus LeConte (Coleoptera: Curculionidae), is a primary seed-feeding pest of cultivated sunflowers, Helianthus annuus L., in North America. Host plant resistance is one tool available to complement insecticide-based management of S. fulvus. Artificial infestations of 30 adult weevils per head were used to determine whether variation for susceptibility to S. fulvus exists in previously released inbred lines, and how a new weevil-resistant line, HA 488, compares with other putative sources of resistance. Correcting for the number of seeds per head, 13 older inbred lines showed variation in per cent seed damage from 20% to 38%, with two lines (HA 412 HO, HA 821) being more damaged than most of the tested lines. Among four putative resistance sources, HA 488 was significantly less damaged (5%) than two previously identified open-pollinated varieties (PI 170424, PI 253417, with 13%–14% seed damage), while the source of the resistance in HA 488, PI 431542, was statistically intermediate (12%). The resistance available in HA 488 is a marked improvement, potentially reducing damage per weevil by two thirds or more, but additional work on genetic markers for resistance, economic thresholds and basic weevil biology (e.g. degree-day models for adult emergence) is needed to support implementation of integrated pest management for this key sunflower pest.     

Solvent-free enzymatic hydrolysis of non-polar lipids in crude sunflower lecithin using phospholipase A1 (Lecitase® Ultra)

In this contribution, a new chromatographic method was applied to study the hydrolysis of non-polar lipids, i.e. triacylglicerols (TAG), diacylglycerols (DAG) and monoacylglycerols (MAG), when crude sunflower lecithin is treated with Lecitase^® Ultra, an enzymatic preparation with phospholipase A₁ (PLA₁) activity. Results not only proved the enzyme lipase activity toward non-polar lipids in selected reaction conditions (aqueous system, T?=?50?°C, pH?=?5) but also suggested the occurrence of acyl-migration phenomenon observed by other authors in similar systems. Results showed that 1?h of reaction was enough to decrease the content of TAG in 54%, while DAG and MAG concentration increased from 0.4 to 3.5 and from 1.9 to 6.5?g/100?g of crude lecithin, respectively. Along the reaction, different contents of glycerides could be achieved, obtaining products with different composition which, in combination with the presence of phospholipids (PL) and/or lysophospholipids (LPL), could present specific emulsifying/stabilizing properties with a wide range of applications in food and pharmaceutical industry.     

Trace elements bioavailability to winter wheat (Triticum aestivum L.) grown subsequent to high biomass plants in a greenhouse study

Multielement-contaminated agricultural land requires the adaptation of agronomic practices to meet legal requirements for safe biomass production. The incorporation of bioenergy plants with, at least, moderate phytoextraction capacity into crop rotations with cereals can affect trace elements (TE) phytoavailability and, simultaneously, constitute economic revenues for farmers outside the food or forage sector. Hence, in a crop rotation pot study sunflower (Helianthus annuus L.), modified for high biomass and TE accumulation by chemical mutagenesis, was compared to winter oilseed rape (Brassica napus L.) as pre-crop. On two agricultural soils with different TE loads, the crops´ potential for phytoextraction and for impacts on TE uptake by subsequent winter wheat (Triticum aestivum L.) was studied. The results showed that rape tolerated high-level mixed contamination with metals (Cd, Pb and Zn) and As more than sunflower. In both soils, labile metals concentration increased and soil acidity remained high following sunflower. Furthermore, enhanced grain As accumulation in subsequent wheat was observed. By contrast, soil acidity and Cd or Zn accumulation of subsequent wheat decreased following rape. In the short term, moderate phytoextraction was superimposed by nutrient use or rhizosphere effects of pre-crops, which should be carefully monitored when designing crop rotations for contaminated land.