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Georgios Psarras and Ian A. Merwin

One-year-old potted `Mutsu' apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] trees on scion invigorating Malling-Merton 111 (MM.111) and scion dwarfing Malling 9 (M.9) rootstocks were grown outdoors in containers under three levels of water availability (irrigated at -20, -80, and -200 kPa) to investigate the effects of soil water availability on combined soil/root (rhizosphere) respiration rates, and developmental morphology of root systems. Rhizosphere respiration was measured with a portable infrared gas analyzer, and root biomass was estimated by electrical capacitance. These nondestructive measurements were compared with final root dry weights of harvested trees, to determine their reliability for estimating relative differences in root biomass. Water stress reduced final biomass similarly for both rootstocks, but the relative reduction in shoot growth was greater for MM.111. Root to shoot ratios were higher and average specific root respiration was lower for M.9 rootstock compared with MM.111. M.9 appeared to be more tolerant of water stress then MM.111, due to reduced canopy transpiration relative to root system mass. Water stress increased root to shoot ratios, specific root length, and the carbohydrate costs of root maintenance as indicated by specific respiration rates. Root dry weight (DW) was better correlated to rhizosphere respiration than to root electric capacitance. The observed r 2 values between root capacitance and root DW were as high as 0.73, but capacitance measurements were also influenced by soil water content and rootstock type. Electrical capacitance estimated total root biomass more accurately for M.9 than for MM.111.

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David M. Eissenstat, Javier F. Espeleta, and Eric L. Whaley

Despite the frequent occurrence of hot, dry surface soil, little is known about the fate of roots in this soil layer during and following periods of surface drought. Phosphorus uptake kinetics were examined in surface roots of citrus seedlings exposed to different periods of drought. Exposure to dry soil for up to 43 days had no effect on phosphate uptake of excised roots measured at solution concentrations of 50, 750, and 1500 μm phosphate. Effects of surface drought on fine root behavior of seedlings and bearing trees were examined in the field using a split-pot arrangement. At the end of 5 weeks of drought, only about 1% of the roots in the top pot of either the seedling or tree had died. By 8 to 9 weeks of drought, about 26% of the fine laterals of the adult citrus had died, but only 6% had died in the seedling. After 15 weeks of drought, 33% of the mature tree fine roots had died but only 8% had died in the seedling. Root-soil respiration per gram root at this time was at least twice as high in the seedling as in the mature tree. Fine roots of citrus are remarkably tolerant to dry surface soils of about 5 weeks duration, both in terms of root survival and resumption of physiological activity after rewetting.

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G. Psarras, I. Merwin, A. Lakso, and R. Zobel

We are evaluating techniques for measuring intact apple rootstock (Malus domestica cv. M.9 and MM.111) responses to low, medium, and high soil-water potential, and low, medium, and high concentrations of N, K, and Ca, in sterile sand culture. Root respiration and functional surface area were estimated with an IRGA chamber and electric capacitance meter, respectively. Root length and surface area were determined by digital image analysis of extracted root systems. Low N supply reduced root respiration, while low K levels increased respiration relative to well-nourished controls. Calcium effects were inconsistent among the rootstocks. Total root length and respiration rates of MM.111 were higher than M.9, but M.9 had higher root:shoot ratios. Root capacitance was correlated with total root length (P < 0.001); and M.9 root systems had greater capacitance than MM.111. In a related field experiment, root growth and respiration of 4-year-old `Mutsu' apple trees on M.9 rootstock were measured in soil under low and moderate drought stress established by rain exclusion shelters, using capacitance and IRGA meters, and a minirhizotron video camera inserted into Plexiglas tubes transecting the rhizosphere. Root growth rates peaked in July (coinciding with maximal shoot growth), then declined gradually during late summer; but variability among trees was greater than among water stress treatments. Root/soil respiration maxima of 4.5 μmol CO2/m2 per s occurred in mid June, late July (when new root counts peaked), and the end of August (when root turnover was maximal).

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John Jifon*

Use of plastic mulch to increase rhizosphere temperatures is a common practice in spring production of vegetable crops. However, supraoptimal soil temperatures during the fruit maturation period in early summer can impair root function and reduce produce quality. The effects of colored plastic mulch on rhizosphere temperature and `Primo' muskmelon root respiration were investigated in the field during Fall (Aug.-Nov. 2002) and Spring (Mar.-May 2003) seasons. Rhizosphere temperatures (measured at 0.1 m below the soil surface with thermo-couples) and respiration under four plastic mulches (black, silver, white, and clear), and a bare ground control were studied. The soil warming properties of the different mulches differed between Spring and Fall. Bare ground rhizosphere temperatures declined from ≈33 to 21°C in the Fall and increased from 14 to 26 °C in Spring. In both studies, black and clear plastic mulches had the highest rhizosphere warming effects (3-8 °C) compared to bare ground. In the Fall, average midday soil temperatures under the white and silver mulches were 2-3 °C cooler than the bare ground treatment. Canopy establishment was accelerated by plastic mulches in Spring but not in Fall. Root + soil respiration was positively correlated with measured rhizosphere temperatures (r = 0.69), with the highest respiration rates recorded under the clear and black plastic mulches. More than 80% of fruits from the clear plastic treatment were deformed and unmarketable. The number of marketable fruit was similar among the black, white and silver mulch treatments and significantly greater (32% in Spring & 12% in Fall) than in the bare ground treatments.