Petiole Nitrate Concentration Research Articles

Comparative Efficacy of White Mustard (Sinapis alba L.) and Soybean (Glycine max L. Merr.) Seed Meals as Bioherbicides in Organic Broccoli (Brassica oleracea Var. Botrytis) and Spinach (Spinacea oleracea) Production

White mustard and soybean seed meals were compared for weed control and yield of organically grown broccoli and spinach. The meals were incorporated into the soil 2 weeks before crop planting at two rates (1.24 and 4.48 t ha−1). Weed densities and hand-weeding time were recorded twice during the growing seasons and weed biomass was measured at crop harvest. Compared to the 1.24 t ha−1 soybean treatment, weed densities were 52 to 95% and 41 to 45% less at 3 and 6 weeks after planting, respectively, in both crops with the 4.48 t ha−1 white mustard seed meal treatment. Time required for hand weeding at these times was also reduced by up to 82% and 48%, respectively. Broccoli yield was similar in all the treatments, but spinach yield was greatest in the 4.48 t ha−1 treatments for both seed meals. Petiole nitrate and nutrient concentrations in both crops were generally similar in all the treatments.

Efficacy and Cost of Cultivators, Steam, or an Organic Herbicide for Weed Control in Organic Vineyards in the San Joaquin Valley of California

Weed management is an important problem faced by organic grape (Vitis vinifera) growers as there are few effective and economic options available. However, new organically acceptable weed control products have become available in recent years. Several studies were conducted to compare the efficacy of two mechanical weed control methods (French plow and Bezzerides tree and vine cultivator) with steam, and an organic herbicide (d-limonene) in organic raisin and wine grape vineyards. The experiments were designed as split plots with the aforementioned treatments as main plots with additional weed control treatments (handhoeing and no handhoeing in the raisin grape vineyards; hoeing, no hoeing, steam, and d-limonene in the wine grape vineyard) one month after the main plot treatment as subplots. The plow provided the greatest level of weed control among the treatments followed by the cultivator. The time required to hoe mechanically cultivated plots was also generally lower than the other treatments. Steam and herbicide only suppressed weeds for 2–3 weeks, and the time needed to hoe plots in these treatments was generally similar to the untreated control at all sampling dates. The mechanical treatments also were two to four times more cost-effective than steam or herbicide. Therefore, mechanical treatments were the most effective and economical weed control methods, though none of the treatments affected vine growth, midday stem water potential, petiole nitrate concentration at bloom, grape yield, or quality.

Detection of Nitrogen Sufficiency in Potato Plants Using Gene Expression Markers

In-season chemical or optical measures of crop N status can be effective tools in optimizing potato fertilizer N management. The feasibility of using a gene expression as an alternative approach for early detection of potato nitrate deficiency was examined using three potato cultivars (Shepody, Russet Norkotah, and Red Pontiac) with abundant (7.5 mM NO3), limited (0.75 mM NO3) or deficient (0 mM NO3) nitrate supply in nutrient culture over a 7 d period. RNA was extracted from the last fully expanded leaf and quantified using realtime RT-qPCR. Reduced nitrate supply had no measurable effect on shoot dry weight or leaf chlorophyll concentration, but decreased petiole nitrate concentration. Under deficit nitrate supply, down-regulation of nitrate reductase and nitrite reductase was measured within 3 days for all cultivars, and down-regulation of asparagine synthetase was measured in two cultivars. Nitrate supply had no effect on expression of ammonium transporter. In this experimental system, plant gene expression markers detected a reduction of nitrate supply prior to measureable reductions in plant growth or in N status measured using common chemical or optical methods.

Effects of Various Preplant and In‐Season Nitrogen Management Practices for Potatoes on Plant and Soil Nitrogen Status

Adequate availability of nitrogen (N) to satisfy potato crop N requirement is critical for production of optimal tuber yields with high processing qualities, which will contribute to maximum net returns. The agroclimatic conditions in the Columbia Basin region in the Pacific Northwest (PNW) supports high potato tuber yields with high processing qualities. Best management of N fertilization is important to increase N uptake efficiency and to minimize N losses. Monitoring soil and petiole N provides a basis to evaluate available N in the soil and N status in the plants. This study was conducted on a Quincy fine sand (mixed, mesic Xeric Torripsamments; >95% sand) using ‘Ranger Russet’ potato cultivar. Potato followed by 2 years of sweet corn rotation was adapted in this study. Treatments included the following preplant (PP) + in‐season (IS) N rates in kg ha−1: (i) 56 + 280, (ii) 112 + 224, or (iii) 112 + 336 in 2004 or 112 + 112 in 2005. The IS N was applied in five equally divided doses at 2‐week intervals, 3 weeks after seedling emergence. An additional treatment was included with treatment (ii) N rates, except that IS N was delivered in 10 weekly applications. Total tuber yield ranged from 54 to 64 and 78 to 90 Mg ha−1, respectively, in 2004 and 2005. In 2004, during the first 65 days after emergence (DAE), petiole nitrate (NO3) concentration was lower in the treatment (i) than that in the other treatments. This difference was not evident in 2005. The petiole NO3 concentrations were lower in 2005 than that in 2004, particularly during latter part of the growing period. Soil available N [nitrate NO3‐N + ammonium (NH4) N in the top 30 cm] concentrations were also greater in 2004 than those in 2005. Soil N appeared to increase toward the end of the growing season in 2004, which has negative effects on tuber yield and quality. Across all treatments, total tuber yield and that of >340‐g grade tubers were greater in 2005 than that in 2004.

Effect of Straw and Fertilizer Nitrogen Management for Spring Barley on Soil Nitrogen Supply to a Subsequent Potato Crop

The effect of straw management (straw removed or retained) and fertilizer N rate (0, 80 or 160 kg N ha-1) on a preceding barley rotation crop on N availability to the subsequent potato crop was evaluated in two years. Nitrogen fertilizer application increased barley grain and straw yield and N uptake, and generally decreased straw C:N ratio. Barley N rate increased residual soil nitrate at harvest in one year, but not in the subsequent spring. Barley management had no effect on assays of soil N supply or on total tuber yield, tuber specific gravity, plant dry matter accumulation or petiole nitrate concentration for an unfertilized potato crop. However, straw retention reduced potato plant N accumulation in both years, and reduced soil nitrate content in vegetation-free plots in one year, compared with straw removal. In this cool, humid environment, management of a barley rotation crop has limited short-term effects on N supply to a subsequent potato crop.

Efeito da adubação azotada, fosfatada e potássica na cultura da batata.: Produtividade e eficiência de uso dos nutrientes

Results from fertilisation field trials with nitrogen, phosphorus and potassium on potato crop are presented. The study was carried out in Sta Apolonia farm, NE Portugal, in 2002. Three independent experiments, one for each nutrient, were organized in completely randomized designs. Phosphorus treatments included 0, 50, 100 and 200 kg P2O5/ha. Potassium treatments were 0, 50, 100, 200 kg K2O/ha and N treatments 0, 50, 100, 200 and 400 kg N/ha. Calcium superphosphate (18 % P2O5), potassium chloride (60 % K2O) and urea (45 % N), in rates according to the above-mentioned treatments, were broadcast and incorporated in their respective plots with pre-planting tillage. The higher tuber yield recorded for all treatments reached 42 Mg/ha. Phosphorus rate did not significantly (a < 0,05) influence tuber yield, while with potassium and nitrogen fertilisation a significant increase of 8 Mg/ha above the control was recorded. Exponential asymptotic equations were used to fit the crop response to applied K and N. The response curves show an initial increase on tuber yield, followed by a progressive attenuation as the nutrient rate increase and stabilising in an extended plateau for the higher nutrient rates. None of the elements influenced the size and the dry matter percentage of the tubers. Petiole nitrate concentrations and chlorophyll SPAD readings, performed with the portable tools: RQflex reflectometer and SPAD-502 chlorophyll meter, showed correspondence with fertiliser-N treatments. Thus, they can be used to diagnose the N nutritional status of crop. The N, P and K use efficiency was very low, stressing the importance of sowing catch crops in the fall, following the harvest of the potato, in order to minimize environmental impacts mainly associated with residual N.

Petiole and Soil Nitrogen Concentrations during the Growing Season of Two Potato Cultivars as Influenced by Different Nitrogen‐Management Practices

Fine‐tuning potato nitrogen (N) management for irrigated sandy soils is a desirable goal to optimize the production and minimize negative impacts on the environment. Effects of different rates of preplant N (PP‐N) applications and different rates and frequencies of in‐season N (IS‐N) for two potato cultivars were evaluated for 2 years (2001 and 2002) in a Quincy fine sand (mixed, mesic, Xeric Torripsamments) in the Pacific Northwest (PNW). In the 2001 experiment, the petiole nitrate (NO3‐N) concentrations in both the cultivars were in the optimum range in the samples taken 22 days after emergence (DAE). In the subsequent sampling, the petiole NO3‐N concentrations showed a wide range proportional to the rates of IS‐N. In the 2002 experiment, the petiole NO3‐N concentrations were in the excess range in most treatments until about 51 DAE in both the cultivars, followed by a decline in concentrations to the low range. The petiole phosphorus (P) and potassium (K) concentrations were mostly above the excess range in both the cultivars over the entire sampling period. The soil N data showed that the transformation of urea N was quite rapid and that the extractable ammonium (NH4‐N) and NO3‐N concentrations in the soil returned to the background levels within 90 days after the PP‐N application. The results of this study demonstrate a rapid transformation of preplant applied urea N into NH4‐N and NO3‐N and depletion of the available N in the top 120 cm of soil. This, in turn, supports the need for IS‐N applications to meet the crop N requirement during the later growing period.

Comparison of petiole nitrate concentrations, SPAD chlorophyll readings, and QuickBird satellite imagery in detecting nitrogen status of potato canopies

Nitrogen (N) management is critical in optimizing potato yield and quality and reducing environmental pollution. Six N rates from 34 to 270 kg ha −1, and different timing of N application were used in a 3-year field experiment to contrast SPAD-502 chlorophyll meter and QuickBird satellite imagery data against the conventional petiole sampling technique for assessing canopy N status. Overall treatment variations in SPAD readings were consistent with those in petiole nitrate-nitrogen (NO 3-N) concentrations. However, the ability of the SPAD meter to detect treatment differences varied with growth stage and growing season. Severe N deficiency was detected about 1 month after emergence with SPAD readings, but as early as 2 weeks after emergence with petiole NO 3-N concentrations. Petiole NO 3-N concentrations tended to differentiate more treatment variations than SPAD readings at all growth stages except at hilling. N deficiency was detected with QuickBird image-derived vegetation indices (VIs) at the hilling stage in 2002, but not in 2003. At the post-hilling stage, treatment differences in VI values were minimal and insignificant except very late in the growing season. SPAD meters could be used as an indirect method for detecting N deficiency at the hilling stage when making supplemental N applications, but they are not as sensitive as the petiole sampling method. The sensitivity of QuickBird imagery to canopy N variations needs to be further tested with more pixel data. However, cloud interference and high cost of images could limit the use of QuickBird data in making timely management decisions.

Establishment of Continuous Critical Levels for Indices of Plant and Presidedress Soil Nitrogen Status in the Potato Crop

Critical levels for six plant and presidedress soil nitrogen (N) indices were established for the growing season, and their relative accuracy to diagnose the need for supplemental sidedress N was also compared. Field trials were conducted from 1996 to 1998 with irrigated potato in northeastern Portugal. Fertilizer treatments included several preplant and sidedress N rates. Petiole nitrate concentrations (determined by a standard laboratory method and with the portable RQflex reflectometer), leaf N content, leaf greenness (SPAD-502 chlorophyll meter), presidedress soil NO3-N and presidedress soil inorganic-N (NO3-N + NH4-N) were selected as N indicators. The Cate-Nelson graphical method and an analytical procedure using Mitscherlich type curves were used to determine critical levels. In both cases, a yield reduction of 10% was accepted. The accuracy of the diagnostics was estimated from the Cate-Nelson graphical method, quantifying the point percentage that appears in negative quadrants (the error rate). The graphical method yielded lower critical levels, appearing as the most conservative sidedress N recommendation basis. The critical levels decreased linearly between 15 and 45 days after emergence (DAE). The linear equations achieved provide continuous critical levels for the growing season and are shown below: Petiole NO3-N (g kg−1, dry wt basis) = − 0.737 DAE + 36.879(r 2 = 0.92); Petiole NO3 (g kg−1, from fresh tissue) = − 0.182 DAE + 9.417 (r 2 = 0.69); Leaf N (g kg−1, dry wt basis) = − 0.453 DAE + 61.028 (r 2 = 0.91); Chlorophyll-SPAD (SPAD units) = − 0.463 DAE + 64.400 (r 2 = 0.93); Soil NO3-N (mg kg−1) = − 1.096 DAE + 49.279 (r 2 = 0.92); and Soil inorganic-N (mg kg−1) = − 1.245 DAE + 56.599(r 2 = 0.92). The N indicators with lower error rate were the presidedress soil NO3-N and presidedress soil inorganic-N (both showing an error rate of 8.3%), followed by petiole nitrate concentration [determined in laboratory (12.0%) and with RQflex reflectometer (12.5%), leaf N content (13.0%), and leaf greenness (14.6%)]. Error rates were similar throughout the growing season, meaning that it is possible to get information about the need for supplementary N in the very early growth stages.

Response of Ultra–Narrow and Conventional Spaced Cotton to Variable Nitrogen Rates

Insufficient nitrogen (N) uptake in cotton (Gossypium hirsutum L.) has been shown to delay maturity and decrease lint yields, while excessive amounts will cause a host of disadvantages, ranging from increased insect damage to defoliation difficulties. Proper nitrogen fertilization rates are therefore essential to maximizing lint production while minimizing input costs. This test was designed to determine if ultra–narrow row cotton (UNRC) has similar nitrogen requirements as conventional row cotton (CRC). Two sites, one in peanuts and the other fallow the previous growing season, and four nitrogen rates (45, 67, 90, and 112 kg N ha−1) were used to compare soil fertility and nitrogen effects on petiole nitrate concentrations, lint yields, and ginning percentages. Although petiole nitrate concentrations between early and peak bloom were lower in ultra–narrow row cotton with 45 and 67 kg N ha−1, there was no correlation between petiole nitrate concentrations and lint yield. Conventional spacing receiving 67 kg N ha−1 had the highest lint production at both locations, while application of 90 kg N ha−1 produced the highest yields in UNRC at both locations. Ultra–narrow row cotton produced numerically higher yields than conventional row at all nitrogen rates at the location without a previous legume crop, but no significant trend was observed. Ultra–narrow row cotton lint percentages were significantly lower than conventional spacing, independent of nitrogen application rate.

Quantitative sidedress nitrogen recommendations for potatoes based upon crop nutritional indices

The quantification of sidedress N rate from in-season information on crop N nutritional status is extremely important in order to use only the necessary amount of nutrient to achieve maximum yields without significant N losses from soil. During three consecutive years (1996, 1997 and 1998) field N fertilizer experiments with irrigated potato ( Solanum tuberosum L., cv. Désirée) were conducted in Bragança (NE Portugal). Eight preplant treatments (0, 50, 100, 200 and 300 kg urea-N/ha and poultry manure, farmyard manure and municipal solid waste in rates equivalent to 100 kg organic-N/ha) were arranged as main plots and five sidedress N rates (0, 25, 50, 100 and 200 kg urea-N/ha) included as subplots. Petiole nitrate concentration, determined by a laboratory method (PNLab, g NO 3-N kg −1, dry weight basis) and by the portable RQflex reflectometer (PNRQflex, mg NO 3 kg −1, from fresh tissues), and leaf N content (LeafN, g kg −1, dry weight basis) were used as N nutritional indices. From the five sidedress N rates applied over each of the preplant treatments, critical sidedress N rates were estimated for several different crop N nutritional status. This was achieved by establishing exponential asymptotic curves between sidedress N rates and tuber yields and solving the equations for 95% of maximum tuber yield. In a second step, multiple regression equations were established between the estimated critical sidedress N rates (SNrate, kg N/ha), as dependent variable, and each crop N nutritional indices (PNLab, PNRQflex or LeafN) and days after emergence (DAE). The equations obtained are: SNrate = 182.7 − 4.146 × PNLab −1.87 × DAE SNrate = 161.0 − 0.013 × PNRQflex −1.34 × DAE SNrate = 562.2 − 8.416 × LeafN −3.59 × DAE These equations provide quantitative sidedress N rates for any level of crop N nutritional indices and sampling dates from 10 to 48 days after emergence.

An in situ incubation technique to measure the contribution of organic nitrogen to potatoes

Field experiments which consisted of the incubation of soil within polyvinylchloride (PVC) tubes inserted in glass jars previously buried in potato crop rows were conducted in order to measure the contribution of soil organic matter and organic amendments to the N nutrition of the crop. The experiments were carried out in Braganca, NE Portugal, in the summer seasons of 1996-1998. Five treatments including farmyard manure, poultry manure, municipal solid wastes, urea and the control were used. Manures and urea were applied in rates which correspond to 100 kg N/ha. Nitrogen released from soil organic matter and manures was checked through crop N recovered and petiole nitrate concentrations. In the plots of urea treatments 80.1, 68.4 and 98.8 mg NO 3 -N kg -1 were released during the entire growing seasons of 1996, 1997 and 1998, respectively. In the control treatments 64.1, 41.5 and 55.4 mg NO 3 -N kg -1 were recorded. The mean values of NO 3 -N yielded from amended plots were not statistically different than control, excluding the plots of poultry manure on the initial sampling dates at the start of incubation. The measured amounts of inorganic N mineralized in the field from native soil organic matter were five to eight times higher than previous laboratory estimates from soil analysis. These results demonstrated that soils with low levels of organic matter could release significant amounts of inorganic N during a cropping season. Soil N availability estimated from the incubation technique was confirmed by petiole nitrate concentrations and closely related to the crop N recovered. However, in the plots of urea treatments some inconsistencies were found, probably originating from the leaching of urea as a molecular form to layers below the limit of soil coring. Soil N balance showed that more than 82% of inorganic N released from organic matter came from the 14-cm soil surface layer.

Critical petiole nitrate concentration of two processing potato cultivars in Eastern Canada

Plant-based diagnostic methods of nitrogen (N) nutrition such as petiole nitrate (NO3-N) concentration can be used to improve the efficiency of N utilization, and hence decrease the risks of N losses to the environment. Our first objective was to determine the effect of N fertilization and supplemental irrigation on the petiole NO3-N concentration during tuber growth of two potato cultivars, Russet Burbank and Shepody, widely grown for processing in Eastern Canada. Our second objective was to establish the critical petiole NO3-N concentration using the relationship between petiole NO3-N concentration and the N nutrition index (NNI), an index based on the N concentration of shoots and tubers. This on-farm study was conducted at two sites in each of three years, 1995 to 1997. The N fertilization rates ranged from 0 to 250 kg N ha−1 with three rates in 1995, six rates in 1996, and four rates in 1997. The NO3-N concentration of petioles from the most recently mature leaves was measured on three sampling dates in 1995 and four sampling dates in 1996 and 1997. The petiole NO3-N concentration generally decreased with time. At all sites and on all sampling dates, the petiole NO3-N concentration increased with increasing N fertilization and was significantly greater for Shepody than for Russet Burbank. Irrigation had no consistent effect on petiole NO3-N concentration. Petiole NO3-N concentration was related to NNI (0.29<R2<0.62). Critical petiole NO3-N concentrations required to reach a NNI of 1.0, indicating a situation where N is not limiting growth, were greater for Shepody than for Russet Burbank, and they decreased with time. Critical petiole NO3-N concentrations (Y) expressed as a function of the number of days after planting (X) are Y = 4.80 - 0.055X for Russet Burbank and Y = 5.03 - 0.054X for Shepody.

Preliminary Estimates of Nitrogen Availability from Liquid Dairy Manure to Potato

Within Wisconsin, there is a distinct movement toward dairy herd expansion and consolidation of small farms. These large dairies are considering various land and manure management arrangements with non-livestock farmers, such as potato (Solanum tuberosum) producers to increase their manure management options. This study used a fertilizer equivalence approach to evaluate the availability of nitrogen from dairy manure to potatoes. Nitrogen (N) availability was evaluated in field experiments in 2000 and 2001, conducted in northeast Wisconsin using a moderate and a high liquid dairy manure rate [10,000 and 20,000 gal/acre (93,536 and 187,072 L·ha-1)] compared with results obtained from N fertilizer applied at five rates [0 to 240 lb/acre (269.0 kg·ha-1)]. Availability estimates using the fertilizer equivalence method based on tuber yield, harvested tuber N concentration and uptake, petiole nitrate concentration, and soil nitrate levels resulted in apparent availability of manurial N from 10% to 40%, with an overall average across both years of 29.2%. This level is only slightly less than values typically measured where corn has been used as the test crop. In spite of being more shallow-rooted and perhaps somewhat less efficient in N use it does not appear that adjustments to manurial N availability estimates are warranted when potatoes are grown.

Effects of nitrogen fertiliser on potato (Solanum tuberosum L., cv. Russet Burbank) in Tasmania. 2. Petiole and soil analysis

Thirteen field trials were conducted on ferrosols and tenosols in Tasmania to assess the response of potatoes (Solanum tuberosum L., cv. Russet Burbank) to basal and topdressed nitrogen fertiliser. The objectives of this study were to assess whether petiole nitrate concentrations could be used to indicate potato nitrogen status and crop response to nitrogen topdressing, and whether various soil nitrogen measures could be used to determine the responsiveness of potatoes to nitrogen fertilisation. Petiole nitrate-nitrogen concentrations in the youngest fully expanded leaves were found to be an indicator of nitrogen status. Critical nitrate concentrations varied in a linear fashion from about 25 000 mg/kg at tuber initiation to less than 5000 mg/kg when tubers were 100 mm long (about 100 days after planting). These concentrations are consistent with those determined for Russet Burbank in other studies. In contrast to petiole measures, various pre-plant and early season soil nitrogen measures (mineral, mineralisable and total nitrogen) did not adequately (adjusted R2&lt;0.25) explain variation in crop nitrogen responsiveness and could not be recommended as diagnostic or predictive tests. Soil nitrogen mineralisation rates in spring varied from 0.4 to 5 kg N/ha.day and were higher at sites previously in pasture than at continuously cropped sites. Although petiole nitrate concentrations responded to nitrogen topdressing, most nitrogen-deficient crops in this study did not show yield responses to topdressed nitrogen. This limited the usefulness of petiole nitrate-nitrogen tests.

COMPARISON OF SPAD CHLOROPHYLL METER READINGS vs. PETIOLE NITRATE CONCENTRATION IN SUGARBEET

ABSTRACT Proper nitrogen (N) management of sugarbeet is important as excessive N increases the potential for leaching of nitrate into groundwater and also results in decreased sugar concentration in the beet, thus decreasing profitability of the crop. Petiole nitrate analysis is a standard practice in sugar beet production to guide in-season application of N. The SPAD chlorophyll meter has been found to be useful in guiding N application in corn with the advantage of providing rapid, in-field measurement of crop N status. The objective of this study was to provide a comparison of SPAD meter readings vs. petiole nitrate concentration in sugarbeet. Towards this end, an experiment was conducted with three cultivars of sugarbeet grown under six levels of N fertilization using a split-plot design (N levels as main plots and cultivar as subplot). SPAD meter readings and petiole nitrate concentration were determined from samples of all subplots taken at 47 and 61 DAP, and from a subset of a single variety at all N levels on 54 and 74 DAP. The SPAD meter reading lagged about two weeks behind the petiole nitrate assay in being able to discriminate significant differences between N treatments. The cultivars tested did not significantly differ in their SPAD readings. SPAD meter readings were well correlated with petiole nitrate concentration at concentrations less than 10,000 mg per kg. Above this level, SPAD meter readings were not correlated with petiole nitrate concentration. From this study, it appears that the SPAD reading is a less sensitive measurement of plant N status than is petiole nitrate concentration.

Evaluation of petiole nitrate measurement as an aid for N fertilization of potatoes (Solanum tuberosum L.) on sandy soils

Potatoes are produced in the Sandveld of the Western Cape on soils with extremely low clay and organic material contents. A low nitrogen mineralisation potential and high leaching hazard complicate the management of the nitrogen supply to the plant. This study investigated the viability of a rapid field measurement of plant nitrate content as a decision support tool in three field trials. In the first two trials the standard practice of weekly N and K fertigation was used at four different N-levels on two cultivars. The nitrate levels were monitored in petioles during two seasons, using two different techniques. The procedure in which dried petioles were used for nitrate extraction and laboratory readings taken with an auto-analyser was less accurate than that where petiole sap was squeezed from petioles and indicator paper strips used for nitrate determination. The highest petiole nitrate levels in these trials corresponded with the highest tuber yields for both cultivars and were found at an application of 400 kg N ha−1. Increasing N levels (100 - 400 kg N ha−1) significantly reduced the specific gravity (SG) of Hertha but had no effect on the SG of Up-to-Date. With three irrigation treatments and three fertigation frequencies in another trial, the petiole nitrate concentrations were the highest when no leaching occured. The highest petiole nitrate concentration was obtained with two fertigations per week. The petiole nitrate levels differed between cultivars and peaked at five to six weeks after planting. These factors should be kept in mind when petiole nitrate readings are used as an aid for fertiliser programs. The Merck reflectoquant proved to be a viable decision support tool in the N management of potatoes under these conditions.

YIELD RESPONSE OF COTTON TO FOLIAR NITROGEN AS INFLUENCED BY SINK STRENGTH, PETIOLE, AND SOIL NITROGEN

Foliar-nitrogen (N) fertilization based on petiole nitrate concentration during boll development is a widely used production practice in cotton. The objectives of this study were: (1) to examine the response of cotton plants to foliar-N fertilization based on petiole N, two boll loads (low and high), and two soil-N levels (low: 55 kg Nha−1; high: 110 kg N ha−1), and (2) study the effect of soil-N levels on petiole characteristics of the leaf positioned fourth from the top of the canopy. High boll load plants in both soil-N levels had significantly greater yield than low boll load plants. Also, the foliar-N sprays in high boll load plants out yielded the nonfoliar-sprayed high boll load plants grown in high soil-N, as well as, low and high boll load plants growing in low soil-N. Petiole lengths of big leaves in low and high soil-N levels were significantly greater than that of small leaves. The same was true for petiole diameter. Petiole diameter, leaf area, and chlorophyll content of small and big leaves from high soil-N levels were significantly greater than in the low soil-N level. Petiole dry weight was similar between low and high soil-N levels, whereas the boll dry weight was significantly greater in the high soil-N level than in the low soil-N level. High soil-N resulted in greater petiole NO3 concentration than the low soil-N. These findings suggested that although petiole characteristics varied between low and high soil-N levels, the size of the sink determined the plants need for additional N, and therefore, governed plant response to foliar-applied N.

A petiole sap test for nitrate and potassium in Sultana grapevines

A sap test has been developed to measure petiole nitrate and potassium concentrations in Sultana grapevines. Sap extraction was carried out after freezing the petioles to cause cell rupture. The thawed samples were macerated along with water in a blender. Merck RQflex(r) test strips were used to measure nutrient levels and the test strip colour intensity was measured using the RQflex reflectometer. The accuracy of the sap test was checked against conventional analysis and found to be satisfactory. Considerable variation in nitrate and potassium concentrations were observed in petiole samples collected in a vineyard. More studies are needed to determine the number of petioles that must be collected for the sap test. In the meantime, it is tentatively suggested that at least 200 petioles be collected for the sap test. Nitrate and potassium concentrations showed slight non‐uniform diurnal variations between 0800 h and 1600 h (Eastern Summer Time). The effect of diurnal nutrient variations on the test results may be reduced by collecting petioles between 0800 h and 0930 h. The petiole dry weight percent increased from about 6 to 12% during the season. This change should be taken into account when sap test results are converted from fresh to dry weight basis.

Effect of Nitrogen Fertilization on Growth, Canopy Density, and Yield ofVitis viniferaL. cv. Cabernet Sauvignon

The effect of nitrogen supply on the vegetative and reproductive capacity of vines of low nitrogen status was investigated in a field trial conducted in Western Australia. Five rates of nitrogen fertilizer (0, 50, 100, 200, and 400 g N/vine) were applied to irrigated, 12-year-old Cabernet Sauvignon vines over three seasons. Two-thirds of the nitrogen was applied late budbreak and the rest at two weeks after flowering. Moderate rates of nitrogen fertilization stimulated vine growth and vigor (shoot extension rate) resulting in an increase in canopy density. Prior to flowering, maximum vine vigor was observed upon addition of 100 g N/vine. This effect was no longer evident after flowering. Those vines supplemented with 100 g of nitrogen also achieved maximum petiole nitrate concentrations at flowering, growth (shoot length, pruning weight, and leaf area), and canopy density (leaf layer number). Vines receiving 400 g N/vine had shorter shoots and less pruning weight than vines receiving 100 g N/vine, as vigor did not respond to nitrogen fertilizer prior to flowering. However, the petiole nitrate concentration, total leaf area, and canopy density of vines supplied with 200 to 400 g N/vine were no different to those vines supplemented with 100 g N/vine. Nitrogen had no effect on the total vine yield in the first and last season. However, the highest yield came from vines receiving 100 g N/vine in the second season. Additional applications of 200 g and 400 g N/vine increased the yield no further. Higher berry numbers per bunch were associated with the increase in total vine yield. It appeared that moderate rates of nitrogen fertilization can have a beneficial effect on vine productivity in situations where vine nitrogen status is low. In contrast, excessive nitrogen fertilization was an unprofitable exercise as it provided no further benefits in terms of vine productivity.