Ian G. Burns

Apparent recovery of fertilizer n by vegetable crops

Apparent recoveries of fertilizer N by crops were studied as their treatment in simulation models is a serious problem. Multi-level N fertilizer experiments in which fertilizer was broadcast and incorporated in soil immediately before drilling were carried out on a range of vegetable crops on adjacent sites of the same field. At final harvest, apparent recovery always declined approximately linearly with the increase in fertilizer N even when less was applied than was needed for maximum uptake; this contrasts with the well-known constancy of apparent recovery of Gramineae (winter wheat and grass) over widely different rates. At the seedling stage (of vegetables), when N-uptakes were very small, N-uptake and invariably dry weight, unexpectedly increased with the increase in fertilizer N until very high levels were reached. A single simulation model, with species differences accounted for by variations in the value of only one coefficient, defined, at least qualitatively, all the various phenomena. It is ar...

A dynamic model for the combined effects of N, P and K fertilizers on yield and mineral composition; description and experimental test

This paper describes an integrated model for calculating the interactive effects of N, P and K fertilizers on crop response by combining routines from separate N, P and K models which used readily available inputs. The new model uses the principle of the ‘Law of the Minimum’ to calculate actual daily increments in plant weight and uptake of each nutrient based on the nutrient least able to meet the plant requirements, although account is also taken of soil factors such as the dependence of soil solution K on the level of mineral N. The validity of the model was tested against the results of 4 field experiments with different combinations of crop species, times of harvest, and levels of N, P and K fertilizers. The integrated model gave good overall predictions of the plant dry weight (excluding fibrous roots) and %N of the dry weight. However, predictions of its %P and %K in the dry weight were less satisfactory, especially in the luxury range. Simulation studies with low levels of nutrients showed that, while most interactive effects on final yield conformed to the Law of the Minimum type of response, the inter-dependence of K and nitrate concentrations in the soil solution resulted in responses to K at different levels of N that were better represented by the Mitscherlich equation or the Multiple Limitation Hypothesis. Thus the adaptability of the model allowed it to reproduce crop responses predicted by three quite different non-mechanistic static equations previously used in the literature to summarise nutrient interaction data. This suggests that the model has the potential to provide a mechanistic basis for interpreting factorial NPK fertilizer trials. Opportunities for improving the model were provided by the experimental findings that %P was strongly correlated with %N throughout the entire range of treatments; that K fertilizers failed to increase %K when %N was low; that fertilizer N increased plant %K when the level of K fertilizer was substantial but not otherwise; and that fertilizer-P depressed plant %K. The model validation also showed that there is a need to improve the parameterization for major crops.

Screening lettuce cultivars for low nitrate content during summer and winter production

Summary Concerns about high nitrate levels in vegetable produce have led the European Union to introduce limits on nitrate concentrations in some salad crops including lettuce (Lactuca sativa L.), with the aim of decreasing nitrate intake by consumers. These limits are likely to create problems for lettuce growers in northern European countries such as the UK, where leafy vegetables can accumulate high nitrate concentrations in leaf tissues due to low light levels, especially during the winter. One option to overcome this problem is the use of low nitrate-accumulating genotypes. The objective of this work was to screen a selection of soil-grown glasshouse lettuce cultivars for head weight and nitrate concentration during the summer and the winter seasons. Two pairs of trials were carried out using a Trojan square design, following commercial cropping practices. Eight long-day commercial lettuce cultivars were grown for both early- and late-summer harvest, and eight short-day commercial cultivars for both early- and late-winter harvest. Both pairs of experiments included examples of cultivars belonging to the following types: Butterhead, Cos, Batavia, Curly, Oakleaf, Lollo Rosso and Lollo Bionda. It was found that the commercial fresh weight of the heads depends on season (summer or winter) and cultivar, with significant interaction between cultivar and experiment (i.e. harvest date within season). By comparison, nitrate concentration showed not only great variability between cultivars in general, but also between the main lettuce types and between cultivars within the Butterhead type. The data for the summer crops suggest that ‘Vegus’ (in particular) tended to accumulate less nitrate than the other cultivars of the Butterhead type. For the winter crops, although no single cultivar was found to exhibit consistently lower nitrate concentrations at harvest, the means for the Butterhead group were generally lower than the average of the cultivars for the other types. Several of the long-day cultivars were found to exceed the EU maximum summer nitrate limit of 3500 mg kg–1 fresh weight, whereas relatively few of the short-day cultivars had nitrate concentrations greater than the corresponding 4500.mg kg–1 limit for winter harvested crops.

How do nutrients drive growth

The relationship between plant nutrient concentration and relative growth rate (RGR) was studied under non-steady state conditions using data from a new N interruption experiment with young lettuce plants grown hydroponically in the glasshouse. RGRs estimated from the fit of a versatile growth model were shown to decline curvilinearly with plant N concentration as N deficiency increased. Similar curvilinear relationships were also derived when the same model was used to reanalyse data for N, P and K interruption treatments from other experiments previously published in the literature. These results clearly indicate that the rate of remobilisation of nutrient reserves varies with the nutrient status of the plant. This contrasts with the linear relationships observed where the changes in plant N concentration occurred solely as a response to increasing plant age, or when plants were grown under steady state conditions with constant relative nutrient addition rates. These differences in the pattern of response provide strong evidence to support the hypothesis that the form of the relationship between RGR and plant nutrient concentration can vary depending upon whether a plants external supplies or internal reserves of a particular nutrient are more limiting.

Screening for genotype and environment effects on nitrate accumulation in 24 species of young lettuce

BACKGROUND Nitrate accumulates in plants in response to N supply, aerial environment (predominantly light), and genotype. This paper characterises the effects of genotype, environment, and their interactions on nitrate accumulation by 24 cultivated and wild lettuce accessions grown hydroponically in winter and summer. The results will inform future strategies for selecting for low-nitrate varieties. RESULTS A preliminary study in which two accessions were sampled for nitrate over time showed largest differences between cultivars in the early-middle period of growth. Sampling the whole population of lettuce at this stage revealed significant effects of genotype, environment (with nitrate concentrations generally higher in winter), and genotype × environment interactions (largely due to a wider range of concentrations in summer). Changes in the ranking of accessions for nitrate accumulation between the two growing seasons were generally small for cultivated morphotypes. Shoot nitrate concentrations and water contents were positively associated, particularly in summer when separate relationships for different cultivated morphotypes (butterhead, cos/Romaine, crisp, leaf, and stem lettuce) were detected. Expressing nitrate concentration on either a shoot fresh or dry matter basis had relatively little effect on the ranking of most cultivated accessions, but not for the wild types. CONCLUSION There is a well-defined sampling window when differences in nitrate accumulation between lettuce genotypes are at a maximum. Delaying sampling may allow morphological changes in head form to mask earlier genotypic differences. Genotype × environment interactions are predominantly of the non-crossover type and have only a small effect on changes in the ranking of accessions between seasons, allowing selections to be made at any time of year. At least part of the genotypic variation in nitrate accumulation is associated with differences in shoot water content.

The sulphate-S/total S ratio in plants as an index of their sulphur status

SummarySeveral authors are advocating the use of the SO4−S/total S ratio in the plant as the best index of S status. We have traced the arguments put forward in support of this index, and we show that they are based either on unfair comparisons with other indices, such as SO4−S or total S alone, or inappropriate statistical treatment.The SO4−S/total S index has two fundamental disadvantages compared with SO4−S or total S alone: (1) the numerator (SO4−S) is the major variable in the denominator, so the ratio is likely to be less sensitive than either of the measurements alone; (2) its determination involves twice as much analytical work as either measurement alone.Examination of some of the source references indicates that SO4−S by itself is the most satisfactory S index. Whenever whole plants are analysed, any index which includes organic S is subject to variation due to tissue age.

ISO-OSMOTIC REGULATION OF NITRATE ACCUMULATION IN LETTUCE

Concerns about possible health hazards arising from human consumption of lettuce and other edible vegetable crops with high concentrations of nitrate have generated demands for a greater understanding of processes involved in its uptake and accumulation in order to devise more sustainable strategies for its control. This paper evaluates a proposed iso-osmotic mechanism for the regulation of nitrate accumulation in lettuce (Lactuca sativa L.) heads. This mechanism assumes that changes in the concentrations of nitrate and all other endogenous osmotica (including anions, cations and neutral solutes) are continually adjusted in tandem to minimize differences in osmotic potential of the shoot sap during growth, with these changes occurring independently of any variations in external water potential. The hypothesis was tested using data from six new experiments, each with a single unique treatment comprising a separate combination of light intensity, nitrogen (N) source (nitrate with or without ammonium) and nitrate concentration carried out hydroponically in a glasshouse using a butterhead lettuce variety. Repeat measurements of plant weights and estimates of all of the main soluble constituents (nitrate, potassium, calcium, magnesium, organic anions, chloride, phosphate, sulphate and soluble carbohydrates) in the shoot sap were made at intervals from about 2 weeks after transplanting until commercial maturity, and the data used to calculate changes in average osmotic potential in the shoot. Results showed that nitrate concentrations in the sap increased when average light levels were reduced by between 30 and 49% and (to a lesser extent) when nitrate was supplied at a supra-optimal concentration, and declined with partial replacement of nitrate by ammonium in the external nutrient supply. The associated changes in the proportions of other endogenous osmotica, in combination with the adjustment of shoot water content, maintained the total solute concentrations in shoot sap approximately constant and minimized differences in osmotic potential between treatments at each sampling date. There was, however, a gradual increase in osmotic potential (i.e., a decline in total solute concentration) over time largely caused by increases in shoot water content associated with the physiological and morphological development of the plants. Regression analysis using normalized data (to correct for these time trends) showed that the results were consistent with a 1:1 exchange between the concentrations of nitrate and the sum of all other endogenous osmotica throughout growth, providing evidence that an iso-osmotic mechanism (incorporating both concentration and volume regulation) was involved in controlling nitrate concentrations in the shoot.

Genotype and environment effects on nitrate accumulation in a diversity set of lettuce accessions at commercial maturity: the influence of nitrate uptake and assimilation, osmotic interactions and shoot weight and development

BACKGROUND The causes of the natural variation in nitrate accumulation and associated traits are studied using a diverse population of 48 mature lettuce accessions grown hydroponically in winter and summer seasons. Information on the effects of genotype, environment and their interactions will inform future selection strategies for the production of low-nitrate varieties more suited to meeting EU requirements for harvested produce. RESULTS The effects of genotype (G), environment (E) and G × E interactions were all significant, with nitrate concentrations lower but covering a wider range in summer. Concentrations of nitrate-N were positively correlated with those of water and total-N and negatively with assimilated-C in the shoot in both seasons, with all relationships partitioned according to morphotype and/or seasonal type. Corresponding relationships between nitrate-N and assimilated-N or with shoot fresh or dry weight were generally weak or inconsistent. Nitrate concentrations at an early growth stage were strongly related to those at maturity in winter, but not in summer when light levels were less variable. CONCLUSION The effects of genotype and environment on nitrate accumulation in lettuce are strongly influenced by morphotype, with most G × E interactions between accessions within the same morphotype predominantly of the non-crossover type. All low-nitrate-accumulating genotypes have increased concentrations of organic solutes (concentration regulation) and reduced water (volume regulation) to help stabilise osmotic potential within the shoots. Variability in nitrate accumulation arises more from differences in uptake than in efficiency of its chemical reduction. Genotypic differences in nitrate accumulation can be masked by changes in head morphology during maturation, provided that they are not confounded by substantial changes in intercepted light. Recent selection strategies do not appear to have produced lower-nitrate-accumulating cultivars.

The influence of genetic variation and nitrogen source on nitrate accumulation and iso-osmotic regulation by lettuce

Background and aimsCharacterisation of genetic variation in nitrate accumulation by lettuce will inform strategies for selecting low-nitrate varieties more capable of meeting EU legislation on harvested produce. This study uses a population of recombinant inbred lines (RILs) of lettuce to determine how genotypic differences influence N uptake, N assimilation and iso-osmotic regulation, and to identify key related traits prior to future genetic analysis.MethodsMeasurements were made on plants grown to maturity in soil fertilised with ammonium nitrate, and in a complete nutrient solution containing only nitrate-N. A simple osmotic balance model was developed to estimate variations in shoot osmotic concentration between RILs.ResultsThere were significant genotypic variations in nitrate accumulation when plants were grown either with nitrate alone or in combination with ammonium. Ammonium-N significantly reduced nitrate in the shoot but had no effect on its relative variability, or on the ranking of genotypes. Shoot nitrate-N was correlated positively with total-N and tissue water, and negatively with assimilated-C in both experiments. Corresponding relationships with assimilated-N and shoot weight were weaker. Estimated concentrations of total osmotica in shoot sap were statistically identical in all RILs, despite variations in nitrate concentration across the population.ConclusionsApproximately 73% of the genotypic variability in nitrate accumulation within the population of RILs arose from differences in nitrate uptake and only 27% from differences in nitrate assimilated, irrespective of whether or not part of the N was recovered as ammonium, or whether the plants were grown in soil or solution culture. Genotypic variability in nitrate accumulation was associated with changes in concentrations of other endogenous solutes (especially carboxylates and soluble carbohydrates) and of tissue water, which minimised differences in osmotic potential of shoot sap between RILs. This offers the opportunity of using the regulation of these solutes as additional traits to manipulate nitrate accumulation.

Effect of humidity and nutrient feed K/Ca ratio on physiological responses and the accumulation of dry matter, Ca and K in tomato.

Summary Two levels of humidity, high, 0.1 kPa vapour pressure deficit (vpd) and control 0.5 kPa vpd, and four nutrient feed K/Ca mM ratios (4/7, 10/7, 4/2 and 10/2) were applied in all factorial combinations to a nine-week old tomato (Lycopersicon esculentum Mill.) crop for 63 d. The effect on gas exchange, water relations, vegetative growth, yield and accumulation of Ca and K in the shoot was examined. High humidity had a deleterious effect on leaf expansion, delayed truss and fruit maturity and reduced fruit yield. Water uptake was reduced and the Ca concentration of leaf and fruit tissue was increased under high humidity compared with plants grown under control humidity; K accumulation was unaffected. The accumulation of K and Ca in the shoot appeared to be poorly related to the rate of transpiration. The high (10/2 mM) K/Ca ratio nutrient feed had little effect on vegetative growth and yield compared with the low (4/7 mM), but restricted Ca uptake to the fruits at both the high and the control humidity. During the measurement period, 0900–1300 hours, stomatal conductance and leaf water status remained high at elevated humidity, compared with a progressive reduction in leaf water status and low stomatal conductance in the control humidity. A/ci gas exchange analysis where A is the net CO2 assimilation rate and ci is the intercellular partial pressure of CO2 suggested that, at high humidity, the photosynthetic capacity of the leaves was reduced because of a lower in vivo carboxylation efficiency. However, the mechanism(s) responsible for reduced leaf expansion remains unclear. The complex interrelations between physiological responses, leaf expansion and the uptake and distribution of K and Ca to the shoot, are discussed.