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Studies relating diurnal changes in endogenous levels of certain plant hormones such as abscisic acid (ABA) diurnal patterns of plant water status are generally lacking. Since ABA has been implicated in the response of plants to stress conditions, the effects of plant water deficits on endogenous abscisic acid (ABA) levels in greenhouse and field‐grown cotton (Gossypium hirsutumL. ‘Deltapine 16’) were investigated. Greenhouse‐grown plants at the 7 to 8 leaf stage and leaves at node 18 of irrigated and nonirrigated field plants were used. Leaves were harvested at intervals throughout a 24‐hour period, frozen in liquid N and stored at −30 C. Samples wee then extracted in 80% methanol and assayed for ABA using newly developed, high.pressure, liquid chromatographic techniques. Plant water potential measurements were made at each sampling time. Stomatal diffusion resistance measurements were made on the leaves harvested from the field. A diurnal pattern in the ABA levels was observed in both field and greenhouse plants. Abscisic acid levels were low during the early morning hours when plant water potentials were high. During the day ABA levels increased as plant water potentials decreased, and reached a peak near sunset in both field and greenhouse. Levels of ABA then decreased as plant water potentials increased during the night and early morning hours. ABA levels were always higher in the stressed plants throughout the day in both field and greenhouse with the greater differences occurring in greenhousegrown tissue. The levels of ABA also increased as stomatal conductance increased in the field plants and continued to increase after conductance values began to decline in the afternoon in both stressed and well‐watered leaves which may reflect the measurement of ABA levels in whole leaves instead of guard cells alone. The larger differences between the conductance values in the stressed and nonstressed tissue late in the afternoon, however, may be associated with the differences in the ABA levels at that time. Since threshold potentials for initial ABA accumulation as well as diurnal patterns can be established, these assay techniques for ABA, developed specifically for cotton, may be useful in drought tolerance research as a tool for relating these parameters to variances in drought tolerance among genotypes.

Germinating and developing cotton (Gossypium hirsutumL.) seedlings were examined for proline accumulation under severe water stress and N deficiency to examine the relationship of N to proline accumulation. Seeds of ‘Coker 201’ were dark‐germinated for 3 days with one‐half desiccated during the last 12 hours. In another experiment, plants were light grown under well watered conditions with or without nutrient solution containing NO3‐N. On the 12th day they were subjected to severe water stress. Dry weight, free amino acids, and plant water potentials were determined after stress developed. Three‐day desiccated cotyledons accumulated 28% more proline than the unstressed controls. Axes of these stressed seedlings, however, did not accumulate free proline. The older water stressed cotyledons and leaves accumulated free proline, but only when the seedlings had an exogenous source of N. Thus under severe N deficiency, proline accumulation ability is imparied in cotton leaf tissue. Free histidine, on the other hand, accumulated in N deficient leaves, but not cotyledons. The results indicate that proline accumulation by water stressed tissue is related to the N status of the tissue as well.

The phosphate defoliant DEF (S,S,S‐Tributylphosphorothrithioate) was sprayed on presquaring greenhousegrown cotton (Gossypium hirsutumL.) plants. Plant water status and leaf area data were taken at 3‐hour intervals during the 1st day and daily thereafter. Stomatal diffusion resistance was measured during the 1st day only. Leaves were analyzed for total N, soluble N, and distribution of free amino acids. Little change in any of the parameters measured was noted the 1st day. On subsequent days the DEF‐treated leaves had lower leaf water potential recovery values (predawn measurement) and lower specific weights (fresh weight basis). DEFtreated leaves, however, had a higher specific leaf weight on a dry weight basis. Leaf N percent did not change as a result of the DEF treatment. Soluble N increased up to twice that of the controls by the 3rd day. The distribution of free amino acids in DEF treated leaves was altered. The concentrations of all amino acids except aspartic acid, glutamic acid + glutamine, proline and valine were significantly increased in the DEF treated leaves. The exceptions remained stable or tended to decrease. The results indicated that DEF‐treated leaves may be mobilizing the leaf protein prior to abscission, but that transport out of the leaf is impeded. Further experiments are needed to substantiate this notion.

Traditional plant breeding approaches have not been very effective in the development of higher yielding cotton (Gossypium hirsutumL.) cultivars for growth under water‐deficient field conditions. The purpose of this research was to evaluate the ability of a laboratory technique to identify cotton genotypes with higher growth rates under water stress conditions in the field. The technique consisted of detaching cotton leaves from the plant at sunset and hydrating those leaves overnight. The following day, the leaves were placed under a light‐bank in the laboratory and weighed every 2 min as they dried. From that data, transpiration decline curves were plotted. The curves exhibited stomatal and cuticular transpiration phases. The point where those two linear phases intersected was termed “mean stomatal closure” (MSC), and both time and relative water content (RWC) at that point were estimated. Sixteen entries were evaluated for transpiration decline curve components and growth rates in irrigated and dryland field plots in 1978 at Big Spring, Tex. In 1980, 12 different entries were evaluated for the same traits under dryland conditions at the same location. In 1978, entry differences occurred for all transpiration decline curve components in the irrigated test and for all components, except stomatal transpiration rates, in the dryland test. The interaction between entries and water levels was significant for all components. In 1980, only RWC at MSC and cuticular transpiration rates were different among entries. Growth rates among entries could not be separated statistically within any experiment. However, large negative correlation coefficients were measured between mean RWC at MSC and mean growth rates in the dryland tests in both 1978 and 1980. This relationship indicated that growth rates tended to be higher when the stomates remained open to a lower RWC before closure. The relationship was not significant in the 1978 irrigated test. Transpiration decline curves appear to be useful in screening cotton genotypes for differences in growth rates under conditions of water stress.

An understanding of the growth and development of diverse nonflowering photoperiodic (long‐night) exotic cottons (Gossypium hirsutumL.) differing in traits associated with drought tolerance is needed before this material can be fully utilized in genetic selection programs. Therefore, glasshouse studies were conducted in 1980 to relate previously observed differences in dry matter production to differences in leaf morphology and development. Ten exotic and two commercial cotton lines were investigated at Lubbock; three of the exotic cottons were also grown in another study at Lubbock and at Temple, TX. Plants were grown in a commercial growing medium in the Lubbock experiments and in an Amarillo fine sandy loam soil (fine‐loamy, mixed, thermic, Aridic Paleustalfs) in the Temple experiment. The number of mainstem nodes was counted in the first experiment at Lubbock; in the second experiment the number of mainstem leaves was counted and harvested for dry weight, and total leaf number, total number of nodes, and total leaf area were recorded at 30,44,58,72, and 86 days after planting. Similar measurements were made at 44 and 56 days after planting at Temple. Leaf initiation rates varied significantly, from 0.35 to 0.29 leaves per day, among the exotic cottons in the first experiment, and significant differences occurred between the two locations. The development of leaf area, total dry matter accumulation, rate of dry matter production per unit leaf area, leaf size and total leaf number differed significantly among strains that had similar leaf initiation rates. Observed differences between greenhouse and field‐grown plants at Lubbock indicated a strong environmental influence on the relationships between the growth parameters. Leaf area was directly correlated with dry matter production for all strains. The development of and increase in leaf area, however, was strain‐specific and depended on either increased leaf numbers or increased leaf size.

A mathematical model, based on probabilistic assumptions concerning root growth directions, was applied to observations from minirhizotrons, in order to characterize the dominant root system orientation. The model uses the average number of roots that intersect the wall of several tubes buried in the soil, and not the numbers of root on individual observation tubes. By determining the ratio of the number of roots that intersect the top of an angled minirhizotron to the number that intersect the bottom, the model predicts whether the root system is more horizontally or more vertically oriented than random. Application of the model to measurements of root intersections with minirhizotrons indicated that the cotton root system was more horizontally oriented than random. This orientation was confirmed by examining the cotton root system on a vertical trench wall. Application of the model also suggested the presence of upward root growth to a greater extent than was observed in the trench. An analysis of the procedures used in the trench profile indicated, however, that the magnitude of upward growth possibly was underestimated. The modeled and observed ratios had the same absolute value when the probability was considered random that a given root was oriented upward or downward. This result indicates that ratios calculated from minirhizotron observations can be used to adequately predict the dominant plant root system orientation.

Greenhouse‐grown cotton (Gossypium hirsutumL.) plants were subjected to a drying cycle when the plants had four true leaves and the proline content of the leaf tissue was determined. Free proline accumulated to about 100 times the concentration of the well‐watered controls as the stress increased. The threshold of the predawn leaf water potential for proline accumulation was about —15 to —17 bars. Total proline content also increased, which suggested that there was some proline synthesis. Free glutamic acid concomitantly decreased. Because proline did not accumulate until the water deficits were quite severe concentration of free proline is not a good indicator of the onset of plant water stress.

Variability in leaf turgidity has been observed among cotton (Gossypium hirsutumL.) strains grown under water deficit field conditions. Two photoperiodic (long‐night) strains were identiffed as having extreme phenotypic expressions of leaf turgidity. Under water deficit conditions T25 retained turgid leaves, while T169 wilted severely. The objective of this research was to determine the potential utility of using the leaf turgidity trait to select germplasm with enhanced drought tolerance. Three types of studies were conducted on these two strains: (i) growth evaluations in wet and dry field conditions and in glass house pots, (ii) evaluations of physiological responses in wet and dry field conditions, and (iii) seed cotton yield evaluations in dry field conditions on germplasm that was introgressed with genes for the day‐neutral flowering habit. Under irrigated field conditions, shoot biomass did not differ between T25 and T169, but the interaction between tests and entries was significant. Compared under water‐deficit field conditions, the nonwilting T25 strain produced significantly more shoot biomass than did T169. In glass house studies, T25 produced more root and shoot biomass with significantly higher water‐use efficiency than did T169. Physiological evaluations showed that under irrigated field conditions, T25 maintained higher leaf water potential, lower leaf conductance, and equal gross photosynthetic rates compared to T169. Under dry (rainout shelter) conditions, T25 always had higher water potential than did T169. At 70 to 80 days after planting (DAP), T25 had higher leaf conductance and photosynthetic rates than did T169. Seed cotton yield was higher for the T25 germplasm than for the T169 germplasm in either population or selected line comparisons. Observed variability in leaf turgidity of cotton under water‐deficit field conditions may be useful in selecting germplasm with enhanced drought‐tolerance.

Seventeen photoperiodic sensitive cotton (Gossypium hirsutumL.) stocks were grown for 2 years (1980 and 1981) at Big Spring, Tex. to evaluate the relationship between turgor maintenance and shoot dry matter accumulation (growth). During each year, leaf‐cutter thermocouple psychrometers were used to measure predawn and midday leaf water and osmotic potentials and to estimate turgor pressure at 50 and 80 days after planting (DAP). Shoot dry weights were measured also at 50 and 80 DAP. The 1st year was hot and dry while the 2nd year had about average temperatures and rainfall. Linear regressions between predawn and midday water and turgor potentials were used to estimate turgor maintenance as i) the change in turgor pressure per unit change of water potential (slope) and, ii) the osmotic potential where zero turgor pressure occurred (X intercept). Significant differences did not occur among the stocks for changes in turgor pressure per unit change in water potential (slope), but they did occur among stocks for osmotic potential at zero turgor (πo) all four sampling dates. Analysis over‐all of the sampling dates, identified a consistently low (T178) and high (T292) stock for πo. Linear correlation and regression coefficients between shoot dry weights and πowere negative and statistically significant during the sampling period (80 DAP, 1980) when the plants were water‐stressed. The linear correlation and regression coefficients were negative, but not statistically significant during the other sampling periods (50 DAP, 1980, and 50 and 80 DAP, 1981). Linear correlation coefficients between the change in shoot dry weights between 50 and 80 DAP in 1980 and the change in πowere negative (r = ‐0.82) and statistically significant. These relationships suggesthat if selection pressure is directed towards enhancing osmotic adjustment under water stress, a reduced growth potential may result.

It has been shown previously that in order to explain the electronic processes in a whole class of aromatic solids starting from the aromatic molecular systems and running through condensed aromatics, baked carbons, and polycrystalline graphite to perfect graphite crystals, the existence of an energy gap between the filled π band and the conduction band has to be assumed, the energy gap decreasing gradually with the increase of molecular dimensions and disappearing in the limit for large graphite crystals. This process of the gradual decrease of the energy gap was later investigated for pyrolyzed cellophane films heat treated up to temperatures of 800°C by study of the position of the long-wavelength limit of the absorption. It was found that the position of the absorption limit does in fact move from the visible into the infrared with increase in molecular size (increase of maximum heat-treatment temperature). The possibility of an exact control in the position of the infrared limit and furthermore the possibility of shifting it as far into the infrared as necessary should make such substances of great value as photodetectors. In this paper, preliminary results of experiments performed with thin films of condensed aromatic substances are reported. The presence of a photoresponse with a sensitivity limit roughly corresponding to the absorption limit has been established using square-wave illumination with frequencies up to somewhat more than 2000 cycles per second. The current shows saturation within about one-twentieth of a second. The amplitude of the fundamental is given as a function of pulse frequency in Fig. 2. The signal-tonoise ratio is found to be considerably inferior to that shown by commercial PbS photocells. The simplicity of such cells is remarkable, however, and might be a great asset for some special applications. Cells with infrared limits of photoresponse up to somewhat more than 5µ were obtained, the extension into the farther infrared being prevented by the increase in the conductivity of the films. The photoresponse seems to be a complicated process involving a combination of a photoelectric excitation with an effect of essentially bolometric nature.