A major agronomic problem in the southeastern USA is low yield of late-planted soybean [Glycine max (L.) Merr.]. This problem is aggravated by the adverse effect of waterlogging on crop growth. Our objectives were to identify soybean growth stages sensitive to waterlogging; identify yield components and physiological parameters explaining yield losses induced by waterlogging; and determine the extent of yield losses induced by waterlogging under natural field conditions. Greenhouse and field studies were conducted during 1993 and 1994 near Baton Rouge, LA, (30 degrees N Lat) on a Commerce silt loam. Waterlogging tolerance was assessed in cultivar Centennial (Maturity Group VI) at three vegetative and five reproductive growth stages by maintaining the water level at the soil surface in a greenhouse study. Using the same cultivar, we evaluated the effect of drainage in the field for late-planted soybean. Rain episodes determined the timing of waterlogging; redox potential and oxygen concentration of the soil were used to quantify the intensity of waterlogging stress. Results of the greenhouse study indicated that the early vegetative period (V2) and the early reproductive stages (R1, R3, and R5) were most sensitive to waterlogging. Three to 5 cm of rain per day falling on poorly drained soil was sufficient to reduce crop growth rate, resulting in a yield decline from 2453 to 1550 kg ha-1. Yield loss in both field and greenhouse studies was induced primarily by decreased pod production resulting from fewer pods per reproductive node. In conclusion, waterlogging was determined to be an important stress for late-planted soybean in high rainfall areas such as the Gulf Coast Region.

The leakage of solutes from foliar tissue is utilized as a dynamic measure of apparent changes in membrane integrity in response to desiccation. It is found that rehydrating leaf discs of cowpea (Vigna sixensis IL.l Endi.) show increasing leakiness in proportion to the extent of prior desiccation, whereas SelgineUa kpidophyla Spring., a resurrection plant, does not. RESULTS Using the A at 280 nm as a measure of the leakage of solutes from the leaf tissues immersed in water, one can compare the apparent relative leakiness from tissues that have been desiccated to various levels. Such an experiment for leaf discs of cowpea (a plant which is relatively susceptible to desiccation) and for frond pieces ofSelaginella (a plant with notable resistance to desiccation) is shown in Figure 1. When cowpea leaf discs had been dried to various degrees (90, 76, or 52% of original fresh weight), the subsequent leakage characteristics obtained from the discs in water show a period of relatively rapid loss of solutes lasting for about 4 or 5 min, and after that a relatively steady rate of leakage (Fig. IA). Both the slope of that leakage curve and the total amounts of solutes leaked out over a 20-min period indicate that, after greater amounts of desiccation, there is an increased amount of leakage. On the other hand, Selaginella was found to suffer no perceptable increase in leakage even when desiccation had advanced to 40o of initial fresh weight (Fig. IB).The relation of the extent of leakage and the extent of desiccation damage is further illustrated in Figure 2. When the total amount of leakage after 20 min is plotted against the extent of prior desiccation, it is evident that the cowpea leaf discs lose increasing amounts of solutes with greater extents of desiccation, whereas the Selaginella tissue does not.In the course of these experiments, it was found that differences in leakage rates were obtained as leaves of differing ages were used. A comparison of leaf discs desiccated to 55% of fresh weight and undesiccated discs of three different leaf ages are shown in Figure 3. In this case, the leakage is expressed as a percentage of total content of A28o-absorbing materials, since the total A2ws content increases with the age of the leaf. It can be seen that the extent of leakage increases with leaf age, and that the desiccation experience results in substantially greater solute leakage from

Winter wheat (Triticum aestivum L. emend. Thell.) yields in Louisiana are consistently below the national average because of a combination of biotic and abiotic environmental factors prevailing in the Gulf Coast region. This study was undertaken to estimate the yield loss for wheat that is attributable to soil waterlogging and to compare physiological performance under waterlogging‐stressed conditions by cultivars grown in Louisiana. In a 3‐yr pot study conducted in a greenhouse, waterlogging stress was imposed by raising the water level to the soil surface. This treatment reduced the soil redox potential in the pots from an average of 409 to 149 mV, indicating an absence of free oxygen in the rootzone. Compared with a well‐drained control treatment, grain weight was depressed 37 to 45% by waterlogging in the eight cultivars tested. In a field experiment with ‘Coker 9877’, grain weight was depressed 51% in poorly drained plots compared with well‐drained plots. Yield depression was due to reduced kernel number and kernel weight rather than to an effect on stand establishment. In the greenhouse experiments, flag‐leaf photosynthesis correlated well with grain weight in the cultivars tested. Waterlogging caused only a small suppression of flag‐leaf photosynthesis and leaf conductance, and there were no significant interactions between treatment and cultivar. These commercially available cultivars showed an equally poor tolerance of waterlogging stress. The results emphasize the need for identification of waterlogging tolerance in wheat cultivars developed for the Gulf Coast states.

Growth of Arabidopsis thaliana (L.) Heynh. in decreasing oxygen partial pressures revealed a linear decrease in seed production below 15 kPa, with a complete absence of seed production at 2.5 kPa oxygen. This control of plant reproduction by oxygen had previously been attributed to an oxygen effect on the partitioning between vegetative and reproductive growth. However, plants grown in a series of decreasing oxygen concentrations produced progressively smaller embryos that had stopped developing at progressively younger stages, suggesting instead that their growth is limited by oxygen. Internal oxygen concentrations of buds, pistils, and developing siliques of Brassica rapa L. and siliques of Arabidopsis were measured using a small-diameter glass electrode that was moved into the structures using a micromanipulator. Oxygen partial pressures were found to be lowest in the developing perianth (11.1 kPa) and pistils (15.2 kPa) of the unopened buds. Pollination reduced oxygen concentration inside the pistils by 3 kPa after just 24 h. Inside Brassica silique locules, partial pressures of oxygen averaged 12.2 kPa in darkness, and increased linearly with increasing light levels to 16.2 kPa. Measurements inside Arabidopsis siliques averaged 6.1 kPa in the dark and rose to 12.2 kPa with light. Hypoxia in these microenvironments is postulated to be the point of control of plant reproduction by oxygen.

Ultrastructural changes of pollen cytoplasm during generative cell formation and pollen maturation in Arabidopsis thaliana were studied. The pollen cytoplasm develops a complicated ultrastructure and changes dramatically during these stages. Lipid droplets increase after generative cell formation and their organization and distribution change with the developmental stage. Starch grains in amyloplasts increase in number and size during generative and sperm cell formation and decrease at pollen maturity. The shape and membrane system of mitochondria change only slightly. Dictyosomes become very prominent, and numerous associated vesicles are observed during and after sperm cell formation. Endoplasmic reticulum appears extensively as stacks during sperm cell formation. Free and polyribosomes are abundant in the cytoplasm at all developmental stages although they appear denser at certain stages and in some areas. In mature pollen, all organelles are randomly distributed throughout the vegetative cytoplasm and numerous small particles appear. Organization and distribution of storage substances and appearance of these small particles during generative and sperm cell formation and pollen maturation are discussed.