Rates of Processes of Essential Plant Nutrients
[chapter]
Thomas Crawford
2014
Books in Soils, Plants, and the Environment
Within a range of concentrations from 0.1 to 10,000 mmol m −3 , net rates of uptake of Ca 2+ , K + , N, P, S, and Zn 2+ were found to range from less than 0.001 to greater than 1.0 μmol g FW root −1 h −1 (Pitman, 1975) . In maize, net rates of accumulation and loss of nitrogen in various parts of the maize shoot during reproductive growth have been estimated by the first derivatives of regression equations based upon periodic sampling and chemical determination of N content of various
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... ts of the multicompartment system. Estimated maximum rates of accumulation of total N in the grain were estimated to be approximately 80 mg N day −1 plant −1 , based upon estimated rates of accumulation of exogenous N and endogenous N in the grain. N fluxes (net flux resulting from influx and efflux) for stem, leaves below the ear, leaves above the ear, shank, cob, and husk were also estimated, based on the first derivatives of the regression equations (Crawford et al., 1982) . Net rates of accumulation and loss of nitrogen in the China 17 sorghum (Sorghum bicolor (L.) Moench) genotype, which is an efficient user of nitrogen, were estimated, per plant, to have varied among the following ranges: lowest half of the stalk, 0 to −40 g N week −1 ; third highest of the four sections of the stalk, +0.04 to −0.04 g N week −1 ; leaves of the third highest of the four sections of the stalk, +0.08 to −0.03 g N week −1 ; fourth highest of the four sections of the stalk, +0.16 to −0.07 g N week −1 , leaves of the fourth highest of the four sections of the stalk, +0.11 to −0.04 g N week −1 ; and the grain, 0-0.12 N g week −1 . Comparable data for the sorghum genotype, TX623, show that fluxes differ during the same period of growth for a less efficient user of nitrogen (Crawford et al., 2009 ). Flux rates of N, P, K, Cu, Fe, Mn, and Zn varied in the roots and shoots of cucumber (Cucumis sativus L.) plants, as affected by deficiency, sufficiency, or toxicity of Mn. During the period of 43-58 days after germination, Mn deficiency caused the roots to change from sink to source of N and K on days 56 and 53, respectively, and caused the shoot to change from sink to source of P and Fe on days 57 and 58 respectively. During the same period, Mn toxicity caused the roots to change from sink to source of N, K, and Cu on days 46, 51, and 46, respectively, and caused the shoot to change from sink to source of Fe on day 55 (Crawford et al., 1990) . Mycorrhizal associations with plant roots have been shown to increase the uptake of phosphate. Sanders and Tinker (1973) showed that during 14 days, the rate of mycorrhizal phosphate uptake by the roots of onion plants was 0.17 pmol cm −1 s −1 , compared to nonmycorrhizal uptake, which was 0.050 pmol cm −1 s −1 . Similarly, during a 10-day experiment, they found that the rate of mycorrhizal phosphate uptake by the roots of onion plants was 0.13 pmol cm −1 s −1 , compared to nonmycorrhizal uptake, which was 0.042 pmol cm −1 s −1 . Eight-day-old maize (Zea mays L.) seedlings replete with nitrogen, when placed in N-deficient solutions, increased their rate of absorption of nitrate and ammonium uptake from 200 μM NH 4 NO 3 . Patterns of uptake changed during a 72-h period. During the 72 h of exposure, the rate of uptake of ammonium increased from about 5 to 9 μmol g −1 h −1 , whereas the rate of uptake of nitrate increased from 2 to 5 μmol g −1 h −1 , then declining to about 4 μmol g −1 h −1 . More detailed rate data for shorter periods of time during the 72 h are also included in the report by Jackson and Volk (1992) . Selenate and selenite are absorbed differently in the roots of wheat. Plants absorbed similar amounts of Se within 1 d when supplied with selenite or selenate. Uptake of selenate and selenite was enhanced in sulfur-starved and phosphorus-starved plants, respectively. During a 30-min period when selenite concentration was increased from 0 to 10 μM, Se uptake rate increased from 0 to about 7 mmol Se g −1 root h −1 when no phosphorus was present, but when 0.1 mM phosphorus was present, Se uptake rate increases from 0 to only about 2 mmol Se g −1 root h −1 ; the investigators surmised that selenite is absorbed by a mechanism similar, if not identical, to that of phosphate (Li et al., 2008) . Selenate and sulfate are oxyanions of selenium and sulfate, respectively, and are in Group 6A of the periodic table. These two elements both have oxidation states of +4, +6, and −2 and display similar chemical behavior. Downloaded by [Thomas Crawford] at 11:12 03 April 2014
doi:10.1201/b16675-21
fatcat:lfgf46hwy5hdzfabxp2ssbjyke