Environmental controls on water use efficiency during severe drought in an Ozark Forest in Missouri,USA

To accurately predict ecosystem responses induced by climate warming at local‐to‐global scales, models are in need of more precise knowledge of response during periods of environmental stress such as drought. In this paper, we studied environmental control of canopy‐level water use efficiency (WUE) during drought at an eddy flux site in an oak‐hickory forest in central Missouri, USA. Two consecutive severe droughts in the summers of 2006 and 2007 afforded coverage of a broad range of environmental conditions. We stratified data to obtain subranges that minimized cross‐correlations among putative WUE‐controlling factors. Our results showed that WUE was subject to control by atmospheric saturation deficit (ASD), soil water potential (SWP) and the ratio of diffuse to total photosynthetically active radiation (I_f/I_t). Generally, WUE was found to scale with 1/(ASD)^0.5, consistent with predictions from stomatal optimization theory. In contrast, SWP and I_f/I_t were related to WUE in a linear fashion. ASD was better correlated with WUE than either of the other two factors. It was also observed that the relationship between WUE and any single controlling factor was subject to influence of the other two. One such example was an opposite response of WUE to SWP between low and high ASD values, suggesting a breakdown of stomatal optimality under severe environmental stresses and a shift from optimal stomatal regulation to nonstomatal regulation at leaf scale. We have demonstrated that different data handling (stratified vs. nonstratified) or selection (hourly vs. daily) could lead to different conclusions on the relationship between WUE and its controls. For this reason, we recommend modelers to be cautious when applying WUE‐response formulas at environmental conditions or at time scales different from those at which they are derived.     

Photosynthesis and Osmotic Adjustment of Two Sugarbeet Cultivars Grown under Saline Conditions1

The response of photosynthesis in two sugarbeet cultivars exposedto 180 mol m^–3 NaCl was investigated. An increase in photosynthesis was found in both cultivars duringthe initial 4 d after exposure to salinity. With longer timeperiods, CO₂ fixation in one cultivar, Monriac, was not affected,while under the same conditions, photosynthesis of the othercultivar, Kawemegapoly, was significantly reduced. Full osmoticadjustment was obtained in both cultivars, and the sensitivityof Kawemegapoly to salinity was not due to loss of turgor. Thepossible mechanisms responsible for this phenomenon are discussed.Rubisco activity responded to salinity as CO₂ fixation ratein both cultivars. Key words: Photosynthesis, osmotic adjustment, salinity     

Nitrate and Nitrite Reduction in Wheat Leaves as Affected by Different Types of Water Stress

The effect of different types of water stress on nitrate and nitrite reductases of wheat (Triticum vulgare L. cv. Mivhor) leaves was investigated. Water stress was applied either to leaf tissue by its incubation in mannitol or various salt solutions, or to intact plants by exposure of the root system to low temperatures or to salinity. Nitrite reductase was much less sensitive to water stress than nitrate reductase, and was not sensitive to salinity up to osmotic potentials of about — 13 bars. The decrease in nitrite reductase activity by water stress was attributed to a direct inhibition of the enzyme rather than to a repression of enzyme synthesis. This was based on the fast response of the enzyme after exposure of leaf tissue to reduced osmotic potential, on the lack of a continuous decrease in enzyme activity during a prolonged stress, and on the fact that light activation of reductase was unaffected by water stress. The inhibition of nitrate reductase under water stress was attributed to both a direct inhibition and a reduced rate in enzyme synthesis. This is concluded from the fact that a decrease in its activity was obtained already within 1 h after stress application and from the fact that light induction of the enzyme was inhibited by stress.     

Reassessment of the in vivo Assay for Nitrate Reductase in Leaves

The in vivo assay procedure for nitrate reductase and its dependence on the concentration of nitrate and other ions were examined. It was found that high ion concentrations led to an increased release of nitrite to the reaction media which could be interpreted as a stimulated nitrate reductase activity. This phenomenon is not an osmotic effect, since equivalent concentrations of mannitol did not lead to identical results. The effect of ions on the enhanced nitrite production was attributed to changes in cell membrane permeability rather than to a supply of substrate. This conclusion is based on several findings: (a) in in vitro assays, the rate of nitrite production was not affected by ion concentrations: (b) the stimulation of nitrite production was obtained by various ions and not only by nitrate; (c) pretreatment of alfalfa leaves with nitrate did not increase the NO₂^? release rate to the external solution; and (d) nitrate and nitrite export from leaf discs to the solution was stimulated even in discs which were enzymatically inactive. Calcium ions in the presence of KNO₃ inhibited the enhanced nitrite production, probably due to alteration of membrane stability. The effect of ions on the rate of nitrite production was reversible and the high rate of nitrite production was reduced to the control rate when discs were transferred to a solution of low concentration.     

The Role of Roots in Control of Bean Shoot Growth

Restriction of root growth by growing bean plants (Phaseolusvulgaris L.) in very small pots led to the development of dwarfplants. The leaves of those plants were smaller and their internodesshorter than those of control plants which were grown in largerpots and had developed a more extensive root system. A largequantity of starch—much more than in control plants —accumulated in the leaves and shoots of the dwarf plants. Increasingthe amount of minerals which was supplied to the roots, enhancedleaf growth of the control plants but failed to affect the dwarfones, in spite of the fact that in both cases the treatmentincreased the content of N, P and K in all the plant organs.The leaf water content was similar in both treatments, but theleaf water potential was higher in the dwarf plants. Exogenousapplication of gibberellic acid (GA₃) to the dwarf plants overcamethe reduction of stem growth completely, and that of the leavespartially. Application of the cytokinin, benzyladenine (BA)did not affect stem growth, but increased that of the primaryleaves. A combined supply of GA₂ + BA restored completely thegrowth of the stem and the primary leaves, and partially thatof the trifoliate leaves. It is concluded that a limited rootsystem restricts shoot growth through an hormonal system inwhich at least gibberellins and cytokinins are involved, andthat the dwarfing is not a consequence of mineral or assimilatedeficiency, or due to water stress. Phaseolus vulgaris L., leaf growth, stem growth, root restriction, gibberellic acid, benzyladenine, cytokinin     

Carbon Dioxide Fixation and Ribulose-1, 5-bisphosphate Carboxylase Activity in Intact Leaves of Sugar Beet Plants Exposed to Salinity and Water Stress

The influence of salinity in the growing media on ribulose-1,5-bisphosphate (RuBP) carboxylase and on CO₂ fixation by intactsugar beet (Beta vulgaris) leaves was investigated. RuBP carboxylase activity was mostly stimulated in young leavesafter exposure of plants for 1 week to 180 mM NaCl in the nutrientsolution. This stimulation was more effective at the higherNaHCO₂ concentrations in the reaction medium. Salinity also enhanced CO₂ fixation in intact leaves mostlyat rate-limiting light intensities. A 60 per cent stimulationin CO₂ fixation rate was obtained by salinity under 450 µEm^–2 s^–1. At quantum flux densities of 150 µEm^–2 s^–1 (400–700 nm) this stimulation was280 per cent. Under high light intensities no stimulation bysalinity was found. In contrast, water stress achieved by directleaf desiccation or by polyethylene glycol inhibited enzymeactivity up to fourfold at –1.2 MPa. Beta vulgaris, sugar beet, ribulose-1, 5-bisphosphate carboxylase, salt stress, water stress, carbon dixoide fixation, salinity