The fate of15N labeled nitrogen applied to mature citrus trees

Summary The efficiency and balance of nitrogen from one year's application was studied in a long-term fertigation experiment. Enriched nitrogen fertilizer, K¹⁵NO₃, was applied to a 22-year-old Shamouti orange tree with a history of high N applications (N₃) and to an N-starved tree (N₁). The distribution of N in the different parts of the trees and in the soil was determined after the experimental trees were excavated. Similar total recovery of the labeled fertilizer N was found in the trees and soil in both treatments (N₁−61.7% N₃−56%). However, the distribution between tree and soil was different. The amount of recovered residual fertilizer in the soil was much larger in the N₃ treatment than in N₁. The highest percentage of fertilizer N was found in the new organs,i.e. fruits, twigs and leaves. The roots and branches took up only 6–14% from the labeled fertilizer. Only 20.9% of the leaf N and 23.4% of the fruit N in the N₃ tree originated in the labeled fertilizer, indicating translocation of N from older parts of the tree to new growth. Evidence was found of storage of N in the wooded branches, while the roots contained a surprisingly small part of labeled fertilizer. Contribution 1599E.     

Analytical studies on the responses of sunflower (Helianthus annuus) to various defoliation treatments

The effects of defoliation treatments on plant growth in sunflower (Helianthus annuus) were studied in the field. Four defoliation treatments, 0 (control), 37.4, 56.1 and 93.4% of the total leaf dry weight, were applied to plants that had small third leaves. Decreased leaf weight/whole plant weight (F/W) ratios in defoliated plants rapidly recovered to almost the same ratio as that observed in the control within 12 to 16 days after defoliation according to the degree of defoliation. The mechanism involved in the recovery of the F/W ratio in defoliated plants mainly consisted of three parameters: enhancement of (1) carbon distribution ratios in the leaves, (2) photosynthetic activity in the remaining leaves, and (3) retranslocation of carbon from the stem and/or roots to leaves. Inhibitive effects of defoliation on relative growth rate and net assimilation rate were seen at an early stage, but subsequently both rates became larger in defoliated plants than in controls. Defoliated plants tended to show rapid development and expansion of new leaves, and to show increased specific leaf area and protein synthesis in individual leaves. The sugar content of leaves in defoliated plants was higher than that in controls, while the content in both stem and roots was lower. These responses seem to be advantageous for development of the photosynthetic system. Heights of defoliated plants were clearly depressed according to the degree of defoliation, and this was attributed largely to differences in the elongation rates of the internodes resulting from defoliation.     

Determination of carbon-reduction-cycle intermediates in leaves of Arbutus unedo L. suffering depressions in photosynthesis after application of abscisic acid or exposure to dry air

Gas exchange and contents of photosynthetic intermediates of leaves of Arbutus unedo L. were determined with the aim of recognizing the mechanisms of inhibition that were responsible for the midday depression of photosynthesis following exposure to dry air, and the decline in photosynthetic capacity following application of abscisic acid (ABA). Rapidly killed (<0.1 s) leaf samples were taken when gas analysis showed reduced CO₂ assimilation. Determination of the contents of 3-phosphoglyceric acid (PGA), ribulose 1,5-bisphosphate (RuBP), triose phosphates, fructose 1,6-bisphosphate and hexose phosphates in the samples showed that significant variation occurred only in the level of PGA. As a result, the ratio PGA/RuBP decreased with increasing inhibition of photosynthesis, particularly when application of ABA had been the cause. A comparison of metabolite patterns did not bring out qualitative differences that would have indicated that effects of ABA and of dry air had been caused by separate mechanisms. Depression of photosynthesis occurred in the presence of sufficient RuBP which indicated that the carboxylation reaction of the carbon-reduction-cycle was inhibited after application of ABA or exposure to dry air.Abbreviations and symbols ABA abscisic acid - C _a partial pressure of CO₂ in the ambient air - C _i partial pressure of CO₂ in the intercellular spaces - I quantum flux - PGA 3-phosphoglyceric acid - RuBP ribulose 1,5-bisphosphate - I _L leaf temperature - w water-vapor pressure difference between leaf and air     

Effects of N-fertilizers,straw, and dry fallow on the nitrogen balance of a flooded soil planted with rice

Summary Nitrogen balance studies were made on rice (Oryza sativa) grown in flooded soil in pots. A low rate of fertilizer (5.64 mg N. kg⁻¹ soil) did not depress the N gain, but a high rate (99.72 mg N. kg⁻¹ soil) elminated the N gain. Soil N loss was negligible since¹⁵N applied as ammonium sulfate and thoroughly mixed with the soil was recovered from the soil-plant system after 3 crops. The observed N gain, therefore, was caused by N₂-fixation, not by a reduction of soil N loss. Straw enhanced N gain at the rate of 2–4 mg per g straw. However, this gain was not observed when soil N availability was high. Dry fallow between rice crops decreased the N gain.     

Elemental composition and food intake of Phialidium hydromedusae in the laboratory

Elemental composition and feeding rate of hydromedusae Phialidium sp. on copepods were studied in the laboratory. Regression equations for both mature and immature medusae allowed the estimation of their dry weight (DW), total C and N content as a function of their diameter. The mean C content as percentage of the DW varied from 13.13% ( ) for the immature to 19.38% (5.68) for the mature individuals. The mean N content is 4.03% (2.49) of DW of immatures and 5.85% (2.70) of the matures. Ingestion rate of Phialidium sp. fed on copepods (200–500 μm) increased with prey density but reached a maximum at high prey concentrations. A maximum ingestion rate of 8.55 (1.6) copepods · medusa ⁻¹ · h⁻¹ was reached for prey concentrations of > 140 copepods · 1 ⁻¹ for both immature and mature medusae. Maximum daily consumption of prey weight varied from 1.41 to 978% C body weight for mature medusae and from 2.90 to 975% for the immature individuals.     

A simple, accurate method for determining wet and dry weight concentrations of plant cell suspension cultures using microcentrifuge tubes

Summary A simple method using microcentrifuge tubes for determining fresh and dry weights, and collecting cell-free supernatant from plant suspension cultures is described. This method offers improvements in accuracy, precision, and time efficiency over traditional filtration methods. Using 4-day-old Nicotinia tabacum cultures, the centrifuge method was shown to remove 25% more of the interstitial water from cell aggregates compared to a suction filtration method, with significantly less variation in fresh weight data.     

Fine root dynamics in two tropical dry evergreen forests in southern India

Seasonality in fine root standing crop and production was studied in two tropical dry evergreen forests viz., Marakkanam reserve forest (MRF) and Puthupet sacred grove (PSG) in the Coromandel coast of India. The study extended from December 89 to December 91 in MRF and from August 90 to December 91 in PSG with sampling at every 2 months. Total fine interval. Mean fine root standing crop was 134 g m⁻² in MRF and 234 g m⁻² in PSG. root production was 104 g m⁻² yr⁻¹ in MRF and 117 g m⁻² yr⁻¹ in PSG. These estimates lie within the range for fine roots reported for various tropical forests. Rootmass showed a pronounced seasonal pattern with unimodal peaks obtained during December in the first year and from October–December in the second year in MRF. In PSG greater rootmass was noticed from June–October than other times of sampling. The total root mass in MRF ranged from 114 to 145 g m−2 at the 13 sampling dates in the three sites. The live biomass fraction of fine roots in MRF ranged from 46 to 203 g m⁻² and in PSG it ranged from 141 to 359 g mm⁻² during the study periods. The dead necromass fraction of fine roots ranged from 6 to 37 g m⁻² in MRF and from 12 to 66 g m⁻² in PSG. Fine root production peaked during December in both the forest sites. The necromass fraction of newly produced roots was negligible. Total N was slightly greater in PSG than in MRF. Whereas total P level was almost similar in both the sites. The study revealed that season and site characteristics influenced fine root system.