Soil sciences

Sulfur (S) fertilization has increasingly become an important issue in crop management. In Argentina S deficiencies have been reported, but there has been no success in finding soil tests for predicting soybean response to S fertilization. The objectives of this study were to: (i) evaluate seed S and N:S ratio as a tool for identifying S deficient soybean crops in field conditions and (ii) determine shifts in stoichiometric relationships among nitrogen (N), phosphorus (P) and S in soybean seeds which can be used for identifying S responsive sites under field conditions. Seed samples from 20 field experiments designed to explore response to S fertilization in soybean were collected in a large area in the Argentinean Pampas region. Seed yield (SY) and P (%P), S (%S) and N (%N) concentration in seeds were determined. Different data analyses were tested in order to find an S deficiency index. Moderate SY responses to S fertilization were observed in the present study (i.e. relative SY was never below 75%). seed S concentration increased in response to S fertilization in most responsive sites, and N:S ratio was governed by variations in seed S concentration. Stoichiometric relationships were more accurate for identifying S responsive sites than using single variables (e.g. seed S concentration or N:S). The isometric variations among N, P and S suggest that the accumulation of these nutrients is proportional in seed tissue. A significant increase in the intercept of the stoichiometric relationship between N and S in S deficient crops was observed. An S deficient index (SDI) was calculated based on this difference that can be used as a diagnostic tool for identifying S responsive sites in soybean. The present study shows a novel approach for using stoichiometric concepts in fertilizer management in soybean.

The cellular and molecular events associated with cell death during compatible interaction between Fusarium oxysporum sp. linii and a susceptible flax (Linum usitatissimum) cell suspension are reported here. In order to determine the physiological and molecular sequence of cell death of inoculated cells, reactive oxygen species (ROS) production, mitochondrial potential, lipoxygenase, DNase, protease and caspase-3-like activities, lipid peroxidation and secondary metabolite production were monitored. We also used microscopy, in situ terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) and DNA fragmentation assay. Cell death was associated with specific morphological and biochemical changes that are generally noticed in hypersensitive (incompatible) reaction. An oxidative burst as well as a loss of mitochondrial potential of inoculated cells, an activation of lipoxygenase and lipid peroxidation were noted. Enzyme-mediated nuclear DNA degradation was detectable but oligonucleosomal fragmentation was not observed. Caspase-3-like activity was dramatically increased in inoculated cells. Phenylpropanoid metabolism was also affected as demonstrated by activation of PAL and PCBER gene expressions and reduced soluble lignan and neolignan contents. These results obtained in flax suggest that compatible interaction triggers a cell death sequence sharing a number of common features with the hypersensitive response observed in incompatible interaction and in animal apoptosis.

The potential fl)r extending the production of winter oilseed crops into the drier region of the cereal belt of eastern Australia was examined by comparing canola, Indian mustard and Linola in field experiments in the contrasting environments of Junee and Condobolin, NSW. Junee is in the region where canola production is concentrated, and Condodolin is in the drier part of the cereal belt currently considered marginal for canola. Different rates of nitrogen (N) fertilizer were applied in all experiments, and there were two times of sowing at Condobolin. Wheat was grown for comparison in all experiments. Crop performance was evaluated over two seasons by measuring dry-matter production, grain yield, water-use efficiency, and seed oil and protein contents. Maximum grain yields of all crops at Junee were higher than at Condobolin, and yield responses to N fertilizer in both environments were greater in the wetter 1992 season than in the drier 1991 season. Canola and wheat showed the largest yield responses to N fertilizer, but Linola generally had the lowest yield responses because of poor seedling emergence at high N rates, intolerance of drought during grain filling (1991) and lodging (1992). Based on the overall growth and grain yield responses to N fertilizer, and on N (protein) removal in grain, it is suggested that about 25% more N be applied to canola than to wheat, that Indian mustard receive about the same N fertilizer rate as wheat, and that Linola requires about 20% less N than wheat. Sowing late at Condobolin, when there was below-average rainfall during grain filling (1991) reduced the yield of canola and Linola, but not Indian mustard. There was no yield penalty for sowing the crops late in 1992 when conditions were favourable during grain filling. Grain harvest indices based either on dry matter or the biosynthetic cost of seed production were higher tk)r wheat and canola than for Indian mustard and Linola, and were largely unaffected by the N treatments. A potential transpiration efficiency value of 12.5 kg seed ha i mm ~ was derived for the oilseeds based on the biosynthetic costs of seed production. Oil concentrations were reduced by dry conditions during seed filling, by late sowing and by N fertilizer. Oil concentrations were higher at Junee than Condobolin, due mainly to differences in ambient temperature as there was a 2.7% decrease in oil concentration tor each I°C rise in mean temperature during seed filling. Oilseed meal and wheal grain protein levels were more responsive to N fertilizer when conditions were dry during grain filling. Protein levels in oilseed meals were higher but wheat grain protein was lower when conditions were favourable during grain filling. The study indicated that there arc prospects for extending the production of canola and Indian mustard into drier regions of the cereal belt because, when sown early, they • Corresponding author. have similar water-use efficiencies to wheat based on the biosynthetic costs of grain production. While the yield of Indian mustard is limited by a low harvest index, its yield stability under dry post-anthesis conditions indicates greater potential than current canola cultivars for more marginal areas. Linola showed the least potential as an oilseed for the drier parts of the cereal belt because of shallow rooting and susceptibility to drought.

Nitrogen efficiency is a complex trait. Identification of secondary plant traits correlating with N efficiency would facilitate the breeding for N-efficient cultivars. Sixteen tropical maize cultivars differing in grain yield at low N supply (N efficiency) under field conditions in Zimbabwe exhibited a significant negative correlation between N efficiency and leaf senescence during grain filling. The same cultivars were studied for leaf senescence under N deficiency in a short-term nutrientsolution experiment. Leaf chlorophyll contents as estimated by SPAD values and photosynthesis rates were used as measures for leaf senescence. Cultivars differed both in SPAD values and photosynthesis rates of the older leaves during N deprivation. Significant negative correlations were found between SPAD values, photosynthesis rates in the nutrient-solution experiment, and leaf-senescence scores in the field experiments, and positive correlations were found between photosynthesis rates and grain yield under low-N conditions in the field. Relationships between physiological root parameters, which were also investigated in the nutrientsolution experiment, and N uptake or grain yield of the cultivars in the field could not be established. It is concluded, that the assessment of the capacity of a genotype to maintain a higher photosynthetic capacity of older leaves during N deficiency-induced senescence at the seedling stage is a suitable selection parameter for the N efficiency of tropical maize cultivars.

Despite the considerable efforts directed over the past four decades to investigate, via correlative physiology and genetic engineering, the contribution of osmolytes to osmotolerance of higher plants, their physiological function(s) still remain a matter of debate. Here, we summarize the functions of proline and glycine betaine, recently documented in our group by using canola (Brassica napus L. var. oleifera), leaf discs treated in vitro with medium of low or high osmolarities. These discs accumulate proline when subjected to osmotic upshifts, whereas proline is rapidly consumed when the discs are further treated under non-stressing conditions. The period of time needed to osmoregulate the amount of proline is assumed to be too long to assign to the proline response a relevant function in turgor regulation which can be achieved with carbohydrates. In addition, proline accumulation is suppressed by supplying the osmotically stressing medium with polyamines, phyto-oxylipins and glycine betaine with no detrimental effect on the capacity to cope with osmotic stress. In opposition to proline, glycine betaine does not occur in canola. We take advantage of this situation to test the compatibility of exogenously supplied betaine in this plant material. Comparisons are done with spinach (Spinacia oleracea L.) leaf discs, which contain an endogenous amount of betaine and also exhibit the capacity to absorb a large amount of it. The betaine absorbed is not degraded and thus cannot be considered in this situation as a true osmoregulator. In turgid tissues of plants that do not produce betaine, this compound induces an elevated level of glutamine, a decreased level of protein, and photo-dependent glycine accumulation. This can rely on senescent-like effects and on a blockage in the glycolate cycle. Such damages to nitrogen metabolism are not observed in leaf explants treated simultaneously with betaine and the stressing agent. We also find that the treatments of turgid canola leaf discs with proline or betaine induce important changes in polyamine contents, mimicking, but with a higher order of magnitude, the effects of osmotic stress. Interestingly, this inducing activity is not observed when osmolytes are supplied under osmotic upshift conditions. Antagonistic effects between osmolytes and osmotic stress are described for the first time in higher plants. They suggest that proline and betaine can act as signal molecules in counteracting destabilizing effects to polyamine metabolism and lead us to decipher other counter-stress effects of nitrogenous osmolytes which, obviously, do not behave just as inert osmobalancers.

The aim of our research was to obtain information on the isotopic fingerprint of nitrous oxide (N 2 O) associated with its production and consumption during denitrification. An arable soil was preincubated at high moisture content and subsequently amended with glucose (400 kg C ha −1 ) and KNO 3 (80 kg N ha −1 ) and kept at 85% water-filled pore space. Twelve replicate samples of the soil were incubated for 13 days under a helium-oxygen atmosphere, simultaneously measuring gas fluxes (N 2 O, N 2 and CO 2 ) and isotope signatures (δ 18 O-N 2 O, δ 15 N bulk -N 2 O, δ 15 N α , δ 15 N β and 15 N site preference) of emitted N 2 O. The maximum N 2 O flux (6.9 ± 1.8 kg N ha −1 day −1 ) occurred 3 days after amendment application, followed by the maximum N 2 flux on day 4 (6.6 ± 3.0 kg N ha −1 day −1 ). The δ 15 N bulk was initially −34.4‰ and increased to +4.5‰ during the periods of maximum N 2 flux, demonstrating fractionation during N 2 O reduction, and then decreased. The δ 18 O-N 2 O also increased, peaking with the maximum N 2 flux and remaining stable afterwards. The site preference (SP) decreased from the initial +7.5 to −2.1‰ when the N 2 O flux peaked, and then simultaneously increased with the appearance of the N 2 peak to +8.6‰ and remained stable thereafter, even when the O 2 supply was removed. We suggest that this results from a non-homogenous distribution of NO − 3 in the soil, possibly linked to the KNO 3 amendments to the soil, causing the creation of several NO − 3 pools, which affected differently the isotopic signature of N 2 O and the N 2 O and N 2 fluxes during the various stages of the process. The N 2 O isotopologue values reflected the temporal patterns observed in N 2 O and N 2 fluxes. A concurrent increase in 15 N site preference and δ 18 O of N 2 O was found to be indicative of N 2 O reduction to N 2 .