C. David Raper

Interdependence of Root and Shoot Activities in Determining Nitrogen Uptake Rate of Roots

Tobacco plants (Nicotiana tabacum L.) were grown under PAR with accumulated daily totals of 10.5, 18.0, 25.5, 33.0, and 40.5 Einsteins m-2day-1 and day/night temperatures of 26/22 and 22/18 C. Plants were sampled at 2-day intervals during exponential growth. Total dry matter and nitrogen in the whole plant and dry matter in the roots increased with PAR and temperature; however, their proportionalities were not constant. The RGRR and RARNP were nearly equal to each other at each PAR and temperature, and both were approximately equal to the RARSR. Depending on treatment conditions, soluble carbohydrates in roots remained constant during exponential growth at 5%-9%. These results support a model for dependence of the nitrogen-supplying function of roots on the concurrent transport of soluble carbohydrate from the shoot.

Photosynthetic Rate During Steady-State Growth as Influenced by Carbon- Dioxide Concentration

Cultivars of burley and flue-cured tobacco (Nicotiana tabacum L.) were allowed to attain steady-state exponential growth at two nitrate levels and in 400 or 1,000 μl/liter CO2 in the ambient atmosphere. The unit leaf area rates of net CO2 exchange were measured at the same CO2 concentrations at which the plants were grown. The NCERs were greater at 400 than at 1,000 μl/liter CO2 in both cultivars and at both nitrate levels. The NCERs were also greater in the flue-cured than in the burley cultivar and at the high nitrate level.

Relationship between Growth and Nitrogen Accumulation for Vegetative Cotton and Soybean Plants

Experiments were conducted to determine whether the nitrogen absorption function of the roots is directly related to the photosynthate supplying function of the leaves under steady-state conditions during vegetative growth phase of cotton (Gossypium hirsutum L) and soybean (Glycine max [L.] Merrill) plants. A direct relationship between these two functions would imply a balanced interdependence and contradict the views that growth of all plant parts is dependent on carbohydrate supplied by the leaves and that nitrogen uptake is dependent only on existing root and soil characteristics. Plants were grown under both artificial light in CERs and natural light in air-conditioned greenhouses, at three thermoperiods, and with three levels of nutrient supply. The data show that RGRR and RARNP were not significantly different within thermoperiod or nutrient supply and agree with those obtained in a previous experiment with tobacco (Nicotiana tabacum L.) The results suggest that plant growth models should be based on the balanced interdependence of the nitrogen absorption and photosynthate supplying functions

Growth and Nitrogen Assimilation of Soybeans in Response to Ammonium and Nitrate Nutrition

Plants supplied with moderate concentrations of NH4 + in solution generally grow poorly compared with plants supplied with other sources of nitrogen. Experiments were conducted with a flowing solution culture system to determine whether growth restrictions could be avoided over an extended period in the presence of NH4 + if root-zone pH were strictly controlled and if plants were exposed to NH4 + during exponential growth when carbohydrate fluxes to the root are coordinated with the rate of nitrogen acquisition. Vegetative soybeans (Glycine max [L.] Merrill) initially were exposed to complete nutrient solutions containing NO3 - until the exponential growth stage and then were exposed for 4 wk to solutions in which nitrogen was supplied as either 1.0 mM NH4 +, 1 0 mM NO3 -, or 0.5 mM NH4 + plus 0.5 mM NO3 -. Acidity of the solutions was constantly maintained at pH 5 8 ± 0 1 by automated control In separate experiments, irradiance (photosynthetic photon flux density [PPFD] of 700 and 325 μE m-2 s-1) levels were controlled to produce distinct steady-state rates of leaf, root, and whole-plant growth The source of nitrogen did not alter growth or nitrogen accumulation within either environment Growth of whole plants and plant parts and accumulation of nitrogen remained exponential The results support the conclusion that plants can effectively utilize NH4 + as a nitrogen source as long as root-zone pH is strictly controlled and a balance is maintained between carbohydrate availability and acquisition of NH4 +

NITRATE UPTAKE, ROOT AND SHOOT GROWTH, AND ION BALANCE OF SOYBEAN PLANTS DURING ACCLIMATION TO ROOT-ZONE ACIDITY'

The effects of acidity on NO3 - absorption by nonnodulated, vegetative soybean (Glycine max [L.] Merrill) plants were examined during a 30-day growth period in flowing solution culture. The acidities of nutrient solutions were maintained at pH 6.1, 5.1, or 4.1 ± 0.1. Root growth rates were reduced for about 20 days by increasing acidity but then recovered to rates which exceeded those at low acidity so that root mass in all treatments was similar after 30 days. In contrast, the uptake rate of NO3 - per unit of root mass was enhanced by increased acidity within the first day of exposure to rates that remained constant throughout the treatment period. Total nitrate uptake and shoot growth rates were restricted at higher acidities until about day 20 and thereafter recovered to rates which exceeded those at low acidity in parallel with the recovery in root growth rates. Alterations in total NO3 - uptake in response to acidity also were evaluated in relation to changes in uptake of other ions and ionic balance in the plant tissue. As acidity increased, anion uptake was increased relative to cation uptake. Though greater reduction of NO3 -, and thus greater generation of internal OH- ions, occurred in plants at higher acidities, tissues contained smaller amounts of organic anions. Acidity of the root-zone, therefore, influenced partitioning of internal OH- generated during NO3 - reduction between synthesis of organic anions in the tissue and efflux to the external solution in association with excess anion uptake.

Morphological response of soybeans as governed by photoperiod temperature and age at treatment

The morphological response of soybeans to photoperiod and temperature was analyzed. The plants were exposed to seven combinations of short- and long-day photoperiods at two stages of development and grown under four day/night temperature regimes. Development of axillary branches at lower nodes of the main stem varied with temperature and with stage of development at induction. The number of potential sites for floral initiation was related to the total number of (1) main stem nodes, (2) axillary branches, and (3) nodes on the axillary branches. Development of floral primordia was influenced by the interaction between photoperiod and temperature. Two- to threefold differences between the mean flower:node and pod:node ratios were obtained.

Changes in Ratio of Soluble Sugars and Free Amino Nitrogen in the Apical Meristem During Floral Transition of Tobacco

Under a modification of the nutrient diversion hypothesis, we propose that an inequality in carbohydrate and nitrogen translocation to the apical meristem may be a controlling factor in floral transition. Experiments were conducted in controlled-environment chambers to determine the associations between microscopic characteristics of the transition from vegetative to floral stages of the apical meristem of fluecured tobacco and to assimilate concentrations in the plant and apical meristem. Low temperature, nitrogen withdrawal, and restriction of nitrogen uptake were used as treatment variables. In all of these stress treatments, flowering occurred at a lesser number of leaves than in control treatments. Low temperature stress accelerated the time of transition to the floral stage as compared with a high temperature control; however, nitrogen stress did not accelerate the time of transition. All stress treatments affected the levels of nitrogen and carbohydrate in whole plants. Most notable was an increase in the percentage of starch and a decrease in the percentage of total soluble carbohydrate induced by the stress treatments. These data indicate that tobacco plants under stress accumulate excess carbohydrate in the form of starch. An apparent inequality in the relative concentrations of carbohydrate and nitrogen in the apical meristem was observed in all treatments at the time of floral transition and is in support of the nutrient diversion hypothesis.

Relative growth and nutrient accumulation rates for tobacco

SummaryTobacco plants (Nicotiana tabacum L.) were grown from transplanting until floral expression in the phytotron units of Southeastern Plant Environment Laboratories to evaluate the relationship between relative growth rate (RGR) and relative accumulation rates (RAR) of N, P, K, Ca, and Mg. RAR is calculated to be analogous to RGR. Plants were grown in both controlled-environment rooms with artificial light and air-conditioned greenhouses with natural light at three temperature conditions and three application rates of N-P-K. RGR and RAR were calculated only for the period of grand growth which occurred within the interval from 7 to 32 days after transplanting.In general, neither RGR nor RAR were affected by temperature or nutrient level. However, both temperature and nutrient level affected dry matter accumulation of the plants apparently by an influence on the rapidity with which plants adjusted to their new environment during the initial 7-day interval after transplanting. RAR for P and K were coequal with RGR of the whole plant; thus, the concentrations of P and K within the plant tended to remain constant during growth. RAR for N, Ca, and Mg were less than RGR for the whole plant; thus, internal concentrations of these nutrients declined during growth. RAR of N, Ca, and Mg for the whole plant were equivalent to RGR of the roots. As a rationale for the association of RGR of roots and RAR of N, it is proposed that the soluble carbohydrate pool in the roots concurrently influences both N absorption, as NO3-, and growth of new roots of immature plants.

GENOTYPIC DIFFERENCES IN ROOT ANATOMY AFFECTING WATER MOVEMENT THROUGH ROOTS OF SOYBEAN

The ability of root systems to absorb water was determined as the root hydraulic conductance for five exotic genotypes (PI 416937, H2L16, N95‐SH‐259, PI 407859‐2, and PI 471938) and the commercial cultivar Young of soybean (Glycine max [L.] Merrill). The genotypes were grown for 28 d in flowing hydroponic culture to minimize possible variations in physical or chemical constraints on root development and functioning. Root hydraulic conductance was determined in response to applied hydrostatic pressure to the solution inside a pressure vessel to induce solution flow through the root system to the nonpressurized cut‐stem surface. Almost twofold differences in hydraulic conductance of from 0.43 to \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape