Degree-day modeling applications in turfgrass management have recently seen increased interest. The predictive capacity of any degreeday model is dependent on an accurate determination of the basal growth temperatures for the species under consideration. The objective of this study was to determine basal growth temperatures and growth rate constants for eight warm season turfgrasses (five species). Sprigs from bermudagrass [Cynodon dactylon (L.) Pers. cv. Arizona Common and C. dactylon Ic C. transvaalensis Burtt Davey cv. Midiron], buffaiograss [Buchlo# dactyloides (Nutt.) Engeim. cv. Kansas Common and Texoka], zoysiagrass (Zoysia japonica Steudel cv. Meyer), St. Augustinegrass [Stenotaphrum secundatum (Walter) Kuntze cv. Raleigh and Floratam], and centipedegrass [Ereraochloa ophiuroides (Munro) Hackel, cv. Common] were grown at temperatures ranging from 5 to 30°C in a controlled environment chamber under 14-h photoperiods. Chamber temperature was decreased in a step-wise fashion to the next temperature after two leaves were fully expanded. Leaf growth rates at each temperature were calculated and expressed as millimeters per day. Base temperature and growth rate constants for each turfgrass were calculated with segmented nonlinear regression analysis. Base temperatures for the eight tested cnltivars ranged from 0 to 13°C. Interspecific and intraspecific differences for basal growth temperature were found, indicating that degree-day model application accuracy is dependent on proper determination of target species and cultivar basal growth temperature.
Information is lacking on the precision of atmometers and empirical models used to estimate turfgrass evapotranspiration (ET). Experiments were conducted to evaluate the precision of black Bellani plates, a class A evaporation pan, and the Penman‐Monteith empirical model for estimating ET of ‘Mustang’ tall rescue (Festuca arundinacea Schreb.), ‘Meyer’ zoysiagrass (Zoysia japonica Steud.), ‘Prairie’ buffalograss [Buchloe dactyloides (Nutt.) Engelm.], and ‘Midlawn’ bermudagrass [Cynodon dactylon (L.) Pers. × transvaalensis Burtt‐Davy] under well‐watered conditions. Tall rescue was mowed once weekly at 6.5 cm, and warm‐season grasses were mowed at 4.5 cm twice weekly. Diurnal ET was measured between June and September in 1993 and 1994 using weighing lysimeters and the water balance method. Evaporation from atmometers was measured during the hour that turfgrass ET was determined, and Penman‐Monteith‐estimated ET was calculated for the same time period. Black Bellani plate evaporation was correlated most closely with measured turf ET (R2 = 0.73), followed by class A pan evaporation (R2 = 0.67), and Penman‐Monteith‐estimated ET (R2 = 0.60). Ranking of grasses for mean daily ET was tall fescue (6.8 mm d−1) > zoysiagrass (5.6 mm d−1) > buffalograss (5.1 mm d−1) = bermudagrass (5.0 mm d−1). The black Bellani plate provides the most precise estimate of turfgrass ET under well‐watered conditions.
Possibilities to account for the mechanism of freeze-thaw injury to isolated protoplasts of Spinacia oleracea L. cv. Winter Bloomsdale were investigated. A freeze-thaw cycle to-3.9 C resulted in 80% lysis of the protoplasts. At-3.9 C, protoplasts are exposed to the equivalent of a 2.1 osmolal solution. Isolated protoplasts behave as ideal osmometers in the range of concentrations tested (0.35 to 2.75 osmolal), arguing against a minimum critical volume as a mechanism of injury. Average protoplast volume after a freeze-thaw cycle was not greatly different than the volume before freezing, arguing against an irreversible influx of solutes while frozen. A wide variety of sugars and sugar alcohols, none of which was freely permeant, were capable of protecting against injury which occurred when protoplasts were frozen in salt solutions. The extent of injury was also dependent upon the type of monovalent ions present, with Li = Na > K = Rb = Cs and Cl-Br > I, in order of decreasing protoplast survival. Osmotic conditions encountered during a freeze-thaw cycle were established at room temperature by exposing protoplasts to high salt concentrations and then diluting the osmoticum. Injury occurred only after dilution of the osmoticum and was correlated with the expansion of the plasma membrane. Injury observed in frozen-thawed protoplasts was correlated with the increase in surface area the plasma membrane should have undergone during thawing, supporting the contention that contraction of the plasma membrane during freezing and its expansion during thawing are two interacting lesions which cause protoplast lysis during a freeze-thaw cycle. Research concerned with freezing injuryin plants and work on microorganisms and animal tissues have essentially been two distinct fields of study and little effort has been made to ".... incorporate the results into more effective concepts of freezing injury" (12). Direct comparisons between the two systems have not and should not have been made since the freezing processes of the two systems are quite disparate. With the possible exception of algae, the environment of plant cells frozen in vivo is considerably different from that of single mammalian cells frozen in vitro. Although the latter are suspended in isotonic aqueous solutions, the former are osmotically stable in H20 due, in large part, to pressure developed by constraints on the cell membrane by a relatively rigid cell wall. Ice, which surrounds single cells when suspensions are frozen (16), forms only in isolated regions of some plant tissues (6), presumably due to the sparsity of extracellular water available for nucleation and crystallization and/or the location offavorable nucleation sites (7). These disparities between in vivo and in vitro studies preclude a comparison between the two systems. 'Department of Agronomy Series Paper 1214. 2Present address:
Cyanide-resistant 02 consumption can be stimulated by either treating whole white potato tubers (Norchip) with ethylene, in the presence of 100% 02, or aging slices obtained from untreated potato tubers. A comparison of alternative pathway activity elicited by either treatment was undertaken. The proportion of electrons flowing through the alternative path in the presence of intermediate concentrations of KCN and at various concentrations of salicylhydroxamic acid was identical in both cases. However, the respiration of slices from ethylene-treated tubers was in every case stimulated by KCN, whereas the aged slices never exhibited this phenomenon.Furthermore, the metabolism of D-IU-'4Clglucose was several hundred times greater in aged slices than in fresh slices from C2H4-treated tubers.These results, along with the respiratory kinetics of aged slices from ethylene-treated tubers, suggest that aged slices and fresh slices from ethylene-treated tubers are biochemically dissimilar.was monitored using the Warburg manometric technique (8).The oxidation of exogenously applied glucose was measured as described elsewhere (7). Two g of slices were placed in 4 ml 100 mm K-phosphate, pH 5.5, with 2 ACi (0.75 x 10-'0 mol) D-[U-'4CJglucose. 14CO2 was collected by suspending plastic cups containing 10% NaOH in the flasks. These cups were removed after 15, 30, 60, 90, and 120 min and placed into Hydromix scintillation cocktail (Yorktown Research, South Hackensack, N.J.), and new cups containing NaOH were placed in the sample chambers. Rates of "CO2 evolution were determined by regression analysis of the resulting time course.Total glucose was measured as described by Bergmeyer et al. (2) based on the reduction of NADP by glucose-6-P dehydrogenase. Five g of potato slices were homogenized in 25 ml hot 80% ethanol. The homogenate was centrifuged at 20,000g for 30 min and the supernatant was reduced to dryness. The residue was redissolved in H20 and passed through Dowex H+ and Cl-to remove ionic materials. The pH of the solution of unbound material was adjusted to 7. An aliquot of this neutral fraction was used for the analysis of glucose (2). Experiments were repeated at least two times.Slices obtained from white potato tubers exhibit a respiratory rise above that of the whole tuber (11). However, this type of respiration is not cyanide-resistant (9). Upon aging, these slices develop an additional respiratory increment and also show the development of an alternative, cyanide-resistant pathway (5). Recently (3,8), it was demonstrated that fresh slices obtained from potato tubers previously treated with C2H4 in air or 100%0 02 would possess the alternate CN--insensitive respiration prior to any aging.To gain a more complete understanding of the development of CN-resistance under these two conditions (i.e. C2H4 treatment in 100%0 02 or aging), this study was undertaken. It is apparent that, although alternative pathway activity is the result of both treatments (3,5,8)
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