John W. Radin

Photosynthesis of cotton plants exposed to elevated levels of carbon dioxide in the field

The cotton (Gossypium hirsutum L.) plant responds to a doubling of atmospheric CO2 with almost doubled yield. Gas exchange of leaves was monitored to discover the photosynthetic basis of this large response. Plants were grown in the field in open-top chambers with ambient (nominally 350 μl/l) or enriched (nominally either 500 or 650 μl/l) concentrations of atmospheric CO2. During most of the season, in fully-irrigated plants the relationship between assimilation (A) and intercellular CO2 concentration (ci) was almost linear over an extremely wide range of ci. CO2 enrichment did not alter this relationship or diminish photosynthetic capacity (despite accumulation of starch to very high levels) until very late in the season, when temperature was somewhat lower than at midseason. Stomatal conductance at midseason was very high and insensitive to CO2, leading to estimates of ci above 85% of atmospheric CO2 concentration in both ambient and enriched chambers. Water stress caused A to show a saturation response with respect to ci, and it increased stomatal closure in response to CO2 enrichment. In fully-irrigated plants CO2 enrichment to 650 μl/l increased A more than 70%, but in water-stressed plants enrichment increased A only about 52%. The non-saturating response of A to ci, the failure of CO2 enrichment to decrease photosynthetic capacity for most of the season, and the ability of the leaves to maintain very high ci, form in part the basis for the very large response to CO2 enrichment.

A physiological basis for the division of nitrate assimilation between roots and leaves

Abstract In vivo activities of nitrate reductase (NR) were determined in soybean ( Glycine max ) and cotton ( Gossypium hirsutum ) roots and leaves, with either exogenous nitrate or endogenous nitrate as substrate. Differences between endogenous-nitrate and exogenous-nitrate activities, if any, revealed limitations to nitrate assimilation by substrate availability. In soybean, substrate was completely nonlimiting in roots but was severely limiting in leaves. In cotton, substrate had a slight effect which did not vary between roots and leaves. Soybean accumulated a greater fraction of the plants total nitrate in the roots, and translocated a much smaller fraction to the leaves, than did cotton. The results imply that the ability of soybean roots to transport nitrate into the xylem was restricted, compared to cotton roots. Such a postulated restriction can help to explain differences between the two species in their distribution of nitrate reductase activity between roots and leaves.

Sucrose Synthase Localization during Initiation of Seed Development and Trichome Differentiation in Cotton Ovules

Sucrose synthase in cotton (Gossypium hirsutum L.) ovules was immunolocalized to clarify the relationship between this enzyme and (a) sucrose import/utilization during initiation of seed development, (b) trichome differentiation, and (c) cell-wall biosynthesis in these rapidly elongating “fibers.” Analyses focused on the period immediately before and after trichome initiation (at pollination). Internal tissues most heavily immunolabeled were the developing nucellus, adjacent integument (inner surface), and the vascular region. Little sucrose synthase was associated with the outermost epidermis on the day preceding pollination. However, 1 d later, immunolabel appeared specifically in those epidermal cells at the earliest visible phase of trichome differentiation. The day following pollination, these cells had elongated 3- to 5-fold and showed a further enhancement of sucrose synthase immunolabel. Levels of sucrose synthase mRNA also increased during this period, regardless of whether pollination per se had occurred. Timing of onset for the cell-specific localization of sucrose synthase in young seeds and trichome initials indicates a close association between this enzyme and sucrose import at a cellular level, as well as a potentially integral role in cell-wall biosynthesis.

The apoplastic pool of abscisic acid in cotton leaves in relation to stomatal closure

Suboptimal nitrogen nutrition, leaf aging, and prior exposure to water stress all increased stomatal closure in excised cotton (Gossypium hirsutum L.) leaves supplied abscisic acid (ABA) through the transpiration stream. The effects of water stress and N stress were partially reversed by simultaneous application of kinetin (N6-furfurylaminopurine) with the ABA, but the effect of leaf aging was not. These enhanced responses to ABA could have resulted either from altered rates of ABA release from symplast to apoplast, or from some “post-release” effect involving ABA transport to, or detection by, the guard cells. Excised leaves were preloaded with [14C]ABA and subjected to overpressures in a pressure chamber to isolate apoplastic solutes in the exudate. Small quantities of 14C were released into the exudate, with the amount increasing greatly with increasing pressure. Over the range of pressures from 1 to 2.5 MPa, ABA in the exudate contained about 70% of the total 14C, and a compound co-chromatographing with phaseic acid contained over half of the remainder. At a low balancing pressure (1 MPa), release of 14C into the exudate was increased by N stress, prior water stress, and leaf aging. Kinetin did not affect 14C release in leaves of any age, N status, or water status. Distribution of ABA between pools can account in part for the effects of water stress, N stress, and leaf age on stomatal behavior, but in the cases of water stress and N stress there are additional kinetinreversible effects, presumably at the guard cells.

Seed Development in Cotton: Feasibility of a Hormonal Role for Abscisic Acid in Controlling Vivipary

In maturing cotton (Gossypium hirsutum L.) fruits, embryos acquire the capacity to germinate in vitro about 16 days before fruit maturity and dehiscence. Vivipary is believed to be prevented by abscisic acid (ABA) originating in the seed coat and diffusing to the embryo (the Ihle-Dure hypothesis). Although endogenous ABA levels are much greater in embryos than in seed coats during the period of germinability, in «donor-receiver» experiments movement of (14)C-ABA is strongly polar in favor of the embryo. Compartmental efflux analysis showed that embryos contain 90% of their ABA in a vacuole-like compartment and an insignificant amount in a cytoplasm-like compartment. In contrast, seed coats have only 60% of their ABA in the «vacuole» and a much greater fraction than embryos in the «cytoplasm». As a result, efflux across the plasma membranes of seed coat cells is much faster than from embryo cells. Increasing external pH strongly inhibits ABA uptake by isolated seed coats and embryos, indicating a role of pH gradients in its partitioning (i.e. ABA tends to be transferred from acidic to alkaline compartments). Aqueous extracts of seed coats are much more acidic than those of embryos. This difference, presumably originating in the «vacuoles», can account for the different intracellular distributions of ABA in the two tissues and therefore can account for the polarity of ABA diffusion between tissues. The results implicate intracellular pH gradients in the control of ABA movement between seed coat and embryo. Demonstration of the feasibility of inward ABA movement, despite apparently unfavorable diffusion gradients, provides direct support for the Ihle-Dure hypothesis.

Accumulation and turnover of abscisic acid in osmotically stressed cotton leaf tissue in relation to temperature

Abstract Leaf discs of cotton ( Gossypium hirsutum L. cv. Deltapine 70) were osmotically stressed by floating them on solutions of polyethylene glycol 8000. The tissue produced copious amounts of abscisic acid (ABA) when stressed. Accumulation of ABA depended strongly upon temperature during the incubation, displaying a maximum at 20°C. At 35°C, the amount of ABA accumulated after 24 h was 45–80% less than at 20°C. Temperature did not affect leakage of ABA into the medium. Turnover rate of [ 14 C]ABA was more than 3 times greater at 35°C than at 20°C. This rapd turnover at 35°C could account for the decreased ABA accumulation. Three 14 C-containing metabolites of ABA were extracted from the tissue. At 20°C, two of these accumulated and retained substantial 14 C over 16 h. At 35°C, though, the 14 C in one of these compounds was almost completely lost during the last 8 h of the incubation. Although the metabolites are not identified, the results show some specific effects of temperature on ABA metabolism. The strong effect of temperature on ABA accumulation may contribute to patterns of ABA-dependent processes (such as stomatal closure) during water stress.

Water Relations in Controlled Environments and the Field

Water availability is often a primary limitation to biomass production. With the aim of explaining and predicting water use in the field, plant physiologists frequently study the fundamental biology of water uptake, transport and evaporation in controlled environments (glasshouses, growth chambers). However, despite the accumulation of knowledge, an understanding of the fundamental biological principles operating in controlled environments rarely leads to improved predictions of plant water relations in the field. This paper discusses some of the factors contributing to these apparently irreconcilable differences. Of the many possible sources of error in controlled environment experiments, a few predominate. Common problems are: (1) rooting restrictions resulting from using plants grown in pots; (2) the use of plants at an inappropriate developmental stage, usually very early in their life cycle and; (3) environmental conditions which fail to simulate the field environment adequately, and are critical because they fundamentally alter plant water status or responses to water status. In many cases, resistance to water stress in the field varies with developmental changes in the water transport capacity of the root system as the plant matures. To study such effects in controlled environments requires mature plants, grown in large pots that do not alter normal root development, and an evaporative demand sufficient to reveal hydraulic limitations imposed by the root system. Studies in controlled environments can never completely match those in the field, but the gap between the two can be narrowed. The sources of error in controlled environments are often easily addressed. Focussing on such critical factors will remove obstacles to interpreting results, improve relevance to the field, and speed the application of basic principles to predicting performance under field conditions.

Interaction of Nitrogen and Water Deficits on Stomatal Behavior in Cotton

Stomata of normal and mildly N-deficient cotton plants (Gossypium hirsutum L.) reacted very differently to water stress. In N-deficient plants, stomata closed at water potentials up to 10 bars higher than in normal plants. Data presented here show: 1) stomata of both normal and N-deficient plants responded little to intercellular CO2 concentrations (Ci) before the imposition of drought; 2) during drought, stomata became sensitive to C-i, closing at elevated concentrations; 3) stomata of N-deficient plants began to respond to C-i at a higher water potential than those of normal plants; 4) sensitivity to Ci was also induced by application of abscisic acid (ABA). The results strongly imply that in N-deficient plants, ABA was synthesized at higher water potentials than in normal plants.