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Donald J. Merhaut and Rebecca L. Darnell

Commercial blueberry production is limited primarily to soils where ammonium, rather than nitrate, is the predominant N form. However, Vaccinium arboreum, a species native to northern Florida, often is found growing in soils where nitrate is the major N form. This species may serve as a breeding source or rootstock for commercial blueberries, expanding the potential soil types that may be used for blueberry cultivation. In our study, in vivo nitrate reductase activity (NRA) was measured in roots and leaves of 2-year-old seedlings of V. arboreum and a commercial cultivar, V. corymbosum `Sharpblue'. Plants were grown hydroponically in sand culture and fertilized with a modified Hoagland's solution containing N as either ammonium, ammonium nitrate, or nitrate. Vaccinium arboreum averaged nitrite at 200, 60, and 20 nmol/g fresh weight per h for nitrate, ammonium nitrate, and ammonium fertilized plants, respectively. `Sharpblue' root NRA was significantly lower, averaging nitrite 50, 38, and 8 nmol/g fresh weight per h for nitrate, ammonium nitrate, and ammonium fertilized plants, respectively. NRA was much lower in leaves than roots of V. arboreum, averaging nitrite at ≈15 nmol nmol/g fresh weight per h across N treatments. No NRA was detected in the leaves of `Sharpblue', regardless of N treatment. These data suggest that V. arboreum may be used as a rootstock or breeding source to expand blueberry production into soil types that are higher in nitrate than the soils typically used for blueberry production.

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Jun Ying Zhao, Li Jun Wang, Pei Ge Fan, Zhan Wu Dai, and Shao Hua Li

Half or whole root systems of micropropagated `Gala' apple (Malus ×domestica Borkh.) plants were subjected to drought stress by regulating the osmotic potential of the nutrient solution using polyethylene glycol (20% w/v) to investigate the effect of root drying on NO3- content and metabolism in roots and leaves and on leaf photosynthesis. No significant difference in predawn leaf water potential was found between half root stress (HRS) and control (CK), while predawn leaf water potential from both was significantly higher than for the whole root stress (WRS) treatment. However, diurnal leaf water potential of HRS was lower than CK and higher than WRS during most of the daytime. Neither HRS nor WRS influenced foliar NO3- concentration, but both significantly reduced NO3- concentration in drought-stressed roots as early as 4 hours after stress treatment started. This reduced NO3- concentration was maintained in HRS and WRS roots to the end of the experiment. However, there were no significant differences in NO3- concerntation between CK roots and unstressed roots of HRS. Similar to the effect on root NO3- concentration, both HRS and WRS reduced nitrate reductase activity in drought-stressed roots. Moreover, leaf net photosynthesis, stomatal conductance and transpiration rate of HRS plants were reduced significantly throughout the experiment when compared with CK plants, but the values were higher than those of WRS plants in the first 7 days of stress treatment though not at later times. Net photosynthesis, stomatal conductance and transpiration rate were correlated to root NO3- concentration. This correlation may simply reflect the fact that water stress affected both NO3- concentration in roots and leaf gas exchange in the same direction.

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Zhihui Chang, Laiqiang Zhuo, Fangfang Yu, and Xunzhong Zhang

biosolids on root length density and root surface area of turfgrass under moisture stress conditions. It has been reported that N metabolites (such as proline) are closely related to drought tolerance ( Caravaca et al., 2004 ). Nitrate reductase is one of

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Yan Zhang, Cuiyue Liang, Yan Xu, Thomas Gianfagna, and Bingru Huang

by low available N or P levels in the soil solution for plant uptake and/or low nutrient acquisition and assimilation capacity, which leads to growth inhibition and leaf chlorosis. Nitrate reductase (NRase) catalyses the reduction of nitrate to

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Yiwei Jiang, Yaoshen Li, Gang Nie, and Huifen Liu

involves the reduction of nitrate to nitrite and then to ammonium catalyzed by nitrate reductase (NR) and nitrite reductase ( Meyer and Stitt, 2001 ), and ammonium produced by this process is then converted into organic molecules by the glutamine synthetase

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Douglas A. Cox

Six cultivars of poinsettia (Euphorbia pulcherrima Willd. ex Kl.) were grown in unlimed growth medium consisting of equal volumes of sphagnum peat and perlite. Plants received fertilizer solutions supplying Mo at either 0.0 mg·liter-1 (-Mo) or 1.0 mg·liter -1 (+Mo). In the –Mo treatment, moderate to severe symptoms of Mo deficiency (marginal and interveinal chlorosis, marginal necrosis, and downward curling of the margins) developed on the middle-aged and some recently matured leaves of `Annette Hegg Brilliant Diamond' and `Eckespoint Lilo', while some interveinal chlorosis occurred on the same leaves of `Gutbier V-17 Angelika'. No symptoms appeared on `Gross Supjibi', `Peace Regal Velvet', and `Peace Noel'. All cultivars were symptomless in the + Mo treatment. In the –Mo treatment, upper, recently matured leaves of the symptomatic cultivars and two of three symptomless cultivars had Mo concentrations at or near the critical level for deficiency; however, nitrate reductase enzyme activity was higher and NO3-N was lower in the leaves of symptomless cultivars than of symptomatic cultivars.

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Rebecca L. Darnell and Gary W. Stutte

Strawberries (Fragaria xananassa Duch. .Osogrande.) were grown hydroponically with three NO3-N concentrations (3.75, 7.5, or 15.0 mM) to determine effects of varying concentration on NO3-N uptake and reduction rates, and to relate these processes to growth and fruit yield. Plants were grown for 32 weeks, and NO3-N uptake and nitrate reductase (NR) activities in roots and shoots were measured during vegetative and reproductive growth. In general, NO3-N uptake rates increased as NO3-N concentration in the hydroponics system increased. Tissue NO3-. concentration also increased as external NO3-N concentration increased, reflecting the differences in uptake rates. There was no effect of external NO3-N concentration on NR activities in leaves or roots during either stage of development. Leaf NR activity averaged ~360 nmol NO2 formed/g fresh weight (FW)/h over both developmental stages, while NR activity in roots was much lower, averaging ~115 nmol NO2 formed/g FW/h. Vegetative organ FW, dry weight (DW), and total fruit yield were unaffected by NO3-N concentration. These data suggest that the inability of strawberry to increase growth and fruit yield in response to increasing NO3-N concentrations is not due to limitations in NO3-N uptake rates, but rather to limitations in NO3 - reduction and/or assimilation in both roots and leaves.

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Ruth Lavon, Raphael Salomon, and Eliezer E. Goldschmidt

The chlorotic appearance of mineral-deficient citrus leaves presumably reflects degradative changes in chloroplast components, most of which have nitrogen as a principal constituent. To examine this assumption the size of some major nitrogen pools, the SDS-PAGE pattern of soluble and chloroplast membranal proteins, and the activities of nitrate reductase and ribulose bisphosphate carboxylase (RuBPcase) were determined in leaves of rough-lemon (Citrus volkameriana Ten. & Pasq) plants grown hydroponically for 3 to 10 months under K, Mg, and Ca deficiencies. Plants grown under minerally deficient conditions produced less biomass. Leaves developing under K, Mg, and Ca-deficient conditions had significantly reduced concentrations of the respective elements. Chlorophyll levels of the chlorotic Mg and Ca-deficient leaves were lower than those of control leaves but chlorophyll a/b ratios were not markedly different. Calcium deficiency caused significant decreases in total nitrogen, nitrates, and the free amino acid pool. Proline which is the major component of the free amino acid pool decreased by 82.5%. Calcium-deficient leaves had significantly lower nitrate reductase and RuBPcase activities. The level of RuBPcase holoenzyme and its subunits were also reduced. Protein levels of K, Mg, and Ca-deficient leaves were not significantly altered. The SDS-PAGE patterns of soluble and chloroplast membranal proteins did not reveal major qualitative changes. In conclusion, the data do not demonstrate a general close link between chlorosis of minerally deficient citrus leaves and nitrogen metabolism. Calcium deficiency seems to specifically interfere with early stages of nitrogen assimilation and free amino acid accumulation but the metabolic integrity of the leaf is apparently maintained even under severe nutritional stress conditions.

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Rebecca L. Darnell and Steven A. Hiss

Most Vaccinium species have narrow soil adaptation and are limited to soils that have low pH, high available iron (Fe), and nitrogen (N) primarily in the ammonium (NH4+) form. Vaccinium arboreum Marsh. is a wild species that can tolerate a wider range of soil conditions, including higher pH and nitrate (NO3-) as the predominant N form. This wider soil adaptation may be related to the ability of V. arboreum to acquire Fe and NO3- more efficiently than cultivated Vaccinium species, such as V. corymbosum L. interspecific hybrid (southern highbush). Nitrate and Fe uptake, and nitrate reductase (NR) and ferric chelate reductase (FCR) activities were compared in these two species grown hydroponically in either 1.0 or 5.0 mm NO3-. Nitrate uptake rate (on a whole-plant and FW basis) and root NR activity were significantly greater in V. arboreum compared with V. corymbosum. Iron uptake on a FW basis was also greater in V. arboreum, and was correlated with higher root FCR activity than was found in V. corymbosum. Increased Fe and NO3- uptake/assimilation in V. arboreum were reflected in increased organ and whole-plant dry weights compared with V. corymbosum. Vaccinium arboreum appears to be more efficient in acquiring and assimilating NO3- and Fe than is the cultivated species, V. corymbosum. This may partially explain the wider soil adaptation of V. arboreum.

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Giedrė Samuolienė, Akvilė Urbonavičiūtė, Pavelas Duchovskis, Zenonas Bliznikas, Pranciškus Vitta, and Artūras Žukauskas

initiated by nitrate reductase (NR) ( Beevers and Hagemann, 1969 ; Campbell, 1999 ; Hoff et al., 1994 ). Nitrate reductase is subjected to regulation by several factors. The triggering signal for the NR expression is the presence of nitrate ( Crawford