High temperatures adversely affect crop productivity of several plant species including bell pepper ( Capsicum annuum L. var. annuum ). The objectives of this study were: (1) to determine whether flower ontogeny is adversely affected by high temperature during different phases of development, including pre-and post-pollination events; (2) to determine the duration of high temperature exposure necessary to cause reduction in fruit set; and (3) to determine whether injury to the pistil or stamen during development is responsible for reduced fruit set during high temperature. We determined that flower buds at <2·5 mm in length, corresponding to microspore mother cell meiosis to tetrad dissolution, and flowers that reached anthesis during the high temperature exposure had reduced fruit set when exposed to 33 °°°° C for 48 or 120 h. Flower buds at <2·5 mm in length also had reduced pollen viability when exposed to 33 °°°° C for 120 h. Morphological examination demonstrated that meiocytes initiated tetrad formation, but after dissolution the microspores remained small and clumped without a thick exine. High temperature exposure at a latedevelopment, pre-anthesis stage did not affect pistil or stamen viability, but high post-pollination temperatures inhibited fruit set, suggesting that fertilization is sensitive to high temperature stress.

Steady-state flow rates and exudate osmotic potentials were measured from complete root systems from warm-(28/23 C) or cold-(17/11 C) grown soybean or broccoli (Brassica oleracea) plants at various pressures or different temperatures.In warm-grown soybean roots systems, a break occurred at 14.7 C in the Arrhenius plot of total flow at constant pressure. When plants were grown at lower temperatures, the break point shifted to 8 C. Broccoli, a chileing-resistant species, showed no break for the temperature range used, but cooler growth temperatures decreased the activation energy for water flow through the root system from 18 kilocalories per mole to 9 kilocalories per mole. In both broccoli and soybean, cold-grown plants had lower exudate potentials and greater flow rates at low hydrostatic pressures than the warm-grown plants.These observations indicate that the rate-limiting site for passive water transport is a membrane which may be modified as the plant acclimates to varying growth temperatures. An additional part of the acclimation process is an increase in activity of root ion pumps.Since the time of Hales (1727) it has been known that cold soil reduces the absorption of water by plants. Differences among species in reaction to water absorption at low temperatures were reviewed by Kramer (1 1). Kuiper (13) was among the first to note that at a "critical" temperature, which varied with the temperature at which roots were grown, a sharp drop occurred in water absorption. Some investigators attributed anomalies such as this to discontinuities in properties of surface water at 15, 30, 45, and 60 C (7, 15, 18).A more likely hypothesis attributes sharp breaks in Arrhenius plots of biological phenomena to phase transitions in membrane lipids in which the hydrophobic core changes from a liquid to a solid or crystalline state at some specific temperature. Such changes alter membrane permeability and enzyme activity (8,10,19). Arrhenius plots were used by Clarkson (2) to study exudation rates and ion concentration of the xylem exudate from excised barley and rye roots preconditioned at various temperatures. With rye roots grown at 20 C he found a break in the Arrhenius plot of exudation rate versus temperature at 10 C, but for roots preconditioned at 8 C for 3 days the break occurred at 5 C.

ABSTRACILeaf area expansion, dry weight, and water relations of Phaseolus Pulgaris L. and P. acutifolius Gray were compared during a drying cycle in the greenhouse to understand the characteristics which contribute to the superior drought tolerance ofP. acutifolius. Stomates ofP. acutifolius closed at a much higher water potential than those of P. vulgaris, delaying dehydration of leaf tissue. P. acutifolius had a more deeply penetrating root system, which also contributes to its drought tolerance. Root-shoot ratios did not differ between the two species either under well watered or water stressed conditions. Leaf osmotic potential was also similar in the two species, with no apparent osmotic adjustment during water stress. These results indicate that P. acutifolius postpones dehydration and suggest that sensitive stomates and a deeply penetrating root system are characteristics which, if incorporated into cultivated beans, might increase their drought tolerance.Phaseolus vulgaris L., the common bean, provides an important source of protein in many developing countries. However, it is extremely sensitive to drought, and significant yield reductions due to mild drought are common (4,12). A related species, Phaseolus acutifolius Gray, tepary bean, has been reported to produce extremely well with limited moisture (2,3,13,15,20,21 Leaf area during the drying cycle was determined nondestructively by measuring the length and width of the center leaflet of each trifoliolate leaf. Area of the entire leaf was calculated from regression lines determined from at least 50 leaves of each cultivar grown under similar conditions. Adaxial and abaxial leaf diffusive resistances were measured with a diffusion porometer (Delta Instruments) on the center leaflet ofthe youngest fully expanded leaf. For the common bean this was actually the first trifoliolate leaf. Because of the bushy habit of the tepary bean, a fully expanded, fully exposed leaf was selected. In each case, the leaf was immediately covered with a plastic bag, the petiole excised from the stem, and the water potential measured in a Scholander-type (17) pressure bomb (Soil Moisture Equipment). The leaf was then removed from the bomb and transferred to another zip lock plastic bag and stored on ice for measurement of osmotic potential. Preliminary experiments showed that the measurement of the water potential had no effect on subsequent osmotic potential measurements.Leaves for osmotic potential determination were returned to the laboratory and stored at -20°C until measurement. The leaf 2Abbreviations: WHR, white half runner; PPFD, photosynthetic photon flux density; DAP, days after planting.3 Turface is a product

Reports of the effects of abscisic acid (ABA) on ion and water fluxes have been contradictory. Some of the confusion seems due to the interaction of ion and water transport across membranes. In these experiments root systems were subjected to hydrostatic pressures up to 5.0 bars to enable measurement of root conductance that was independent of measurement of osmotic potentials or ion fluxes.ABA between 5 x 10-I molar and 2 x 10' molar resulted in a decrease in the conductance of the soybean root systems as compared with the controls. ABA treatment also eliminated the discontinuity in the Arrhenius plot of total flow versus reciprocal temperature at constant pressure. The results suggest that ABA acts at the membrane that is rate-limiting to water flow directly, or by altering metabolism that in turn affects the membrane.The dramatic increase in ABA content in stressed plants (20,23) (12) found that ABA stimulated ion uptake in Phaseolus roots, whereas Erlandsson and co-workers (4) observed a decrease in "Rb uptake in sunflower roots.Cram and Pitman (3) reported that ABA did not change the hydraulic conductivity of maize or barley roots, but Glinka and Reinhold (9, 10) reported that ABA increased both the diffusional permeability and hydraulic conductivity of roots to water flow under a pressure of 0.8 bar. Most recently, ABA was reported to have no effect on osmotically driven water flow in excised sunflower roots (4). Pitman and Wellfare (18) found dramatic decreases in barley root exudation rates treated with ABA, but concluded that this was due to an inhibition of active salt transport and not hydraulic conductivity. ' Research supported by National Science Foundation Grants PCM76-11 142-AO1-2 to Dr. P. J. Kramer and DEB77-15845 These contradictions could be due to the difficulty in evaluating a complex transport system as a root. The standing osmotic gradient (I) and the removal of ions from the ascending xylem water (13) make determination of the osmotic driving force for water flow difficult. At low hydrostatic pressures the interaction between osmotic and hydrostatic driving forces makes it impossible to assess whether a change in flow rate is due to a change in hydraulic conductivity or ion movement (6). The analytical technique of Fiscus (6) provides a method of measuring the hydraulic conductance (L)5 of a root system independent of ion transport.The experiments reported here use applied hydrostatic pressures to determine the effect of ABA on water flux through root systems, to determine if changes in flux are due to changes in L and to determine the nature of the changes ABA induces in the ratelimiting barrier to water flux. MATERIALS AND METHODSCulture conditions and basic experimental procedures have been previously described (15). Soybean plants were grown in a half-strength Hoagland nutrient solution under a 14-h photoperiod and a thermoperiod of 28/23 C. Root systems of decapitated plants were sealed in a 10.3-liter pressure chamber filled with fresh nutrient solution. Hydrostatic pres...

ABSTRACrThe fluorescent compound trisodium, 3-hydroxy-5,8,10-pyrenetrisulfonate (PTS) was used to quantify the apoplastic flux through red pine (Pings resinosa Ait.) root systems-that portion of the total water flux reaching the xylem without ever crossing a semipermeable membrane. Flow was induced by pressure through detopped root systems, and by transpiration through intact seedlings. Apoplastic flux was determined by multiplying total flux by the ratio of PT'S concentration in the xylem exudate to PTS concentration in the bathing medium.Under aeration, apoplastic flux was less than 1% of total flux. Under anaerobic conditions, up to 50% of total flux was apoplastic suggesting that anaerobic conditions change the pathway of water flow into root xylem. The change under anaerobic conditions was reversible. Detopped root systems under pressure and intact seedlings under transpiration gave similar results. In detopped root systems, the magnitude of the pressure gradient may alter the apoplastic contribution to total flux. component of water flux through root systems.The existence of an exclusively apoplastic pathway is demonstrated by the appearance in the xylem of materials totally excluded by membranes (15). In roots without secondary cambium, breaks in the endodermis in the emergence zone around lateral roots (15, 16), wounds, and the root tips where the casparian strip is incompletely developed (4) all provide potential apoplastic pathways. The pathway across the endodermis has been traced with electron opaque elements, radioactively labeled substances, and apoplastic dyes. PTS was used as the marker of apoplastic water flux in our study. PTS is fluorescent, highly water soluble, nontoxic, not adsorbed onto cell walls, and is totally excluded from the symplast (14, 15, 19). The proportion of uptake which was exclusively apoplastic was calculated using an equation modified from one used by Mees and Weatherley ( 11) to calculate leaks in their root system under pressure:Water flow through root systems may be expressed by the following relationship: Q4= Qa+Qs (1) where Q, is the total water flux; Qa is the exclusively apoplastic flux, involving water which reaches the xylem without ever crossing a semipermeable membrane; and Qa is the symplastic flux, involving water that travels most of its pathway either in the symplast or apoplast but which must cross a membrane at least once during its passage to the xylem. Apoplastic flux is considered relatively unimportant since Q, is severely inhibited by metabolic inhibitors, and ion movement into roots appears to function as if it were crossing a membrane (5, 13). However, quantitative estimates of the apoplastic contribution to total flux were not found. A need to quantify the components in flux models is frequently indicated (1, 2, 10). A major objective of this study was to quantify the apoplastic