Maize plants (Zea mays L. hybrid INRA 508) were placed under controlled conditions of light and CO2 partial pressure. The K+, Cl− and P contents were then determined by X‐ray microanalysis in the bulbous end of guard cells and in the center of subsidiary cells. The results were interpreted in connection with the stomatal conductance at the time of sampling. In normal air, the K+ and Cl− contents in guard cells only rose from a light threshold of about 300 μmol m−2 s−1 at which stomata were already largely open. At 600 μmol m−2 s−1, the K+ and Cl− levels in guard cells attained values that were 3‐ and 8‐fold greater, respectively, than the values observed in darkness. The K+ and Cl− contents in the subsidiary cells remained quite constant irrespective of the light conditions. CO2‐free air in darkness induced a significant K+ influx towards guard and subsidiary cells. Under light and in CO2‐free air, the K+ and Cl− contents dramatically increased in the guard cells, but slightly decreased in the subsidiary cells. Thus, when subjected to strong light in CO2‐free air, the K+ and Cl− contents in the subsidiary cells were approximately equal to those measured in normal air conditions. In the guard cells, stomatal opening was associated with a marked shift of the Cl−/K+ ratio – from 0.3 for closed stomata to ca 1 for fully open stomata. This could imply a slow change in the nature of the principal counterion accompanying K+ during stomatal opening. The content of P in guard cells appeared, in contrast to that of K+ and Cl−, to be practically independent of stomatal aperture.
Water vapor exchange between sunflowers (Helianthus annuus L.) and the atmosphere was studied in darkness and in light using THO as a tracer. In the presence of tritiated water vapor, the exchange of THO measured as tritiated water in the leaves was slower in darkness than under illumination. This is explained by the variation of stomatal conductance. The authors show that a single curve is capable of summarizing the tritium exchange under any environmental condition, provided adequate units are used; this curve characterizes the behaviour of sunflowers with regard to tritiated water exchange. Using a numerical simulation model, we showed that the tritiated water vapor exchange is well represented by Fick's equation and affects only 80% of the total leaf water (lamina plus veins).
In normal air, illumination with a low level of blue or red light (40 μmol m−2 s−1) did not induce stomatal opening in maize plantlets. In CO2‐free air, 40 μmol m−2 s−1 of blue or red light promoted an enhancement in stomatal opening. At the same quantum flux, blue light was more efficient than red light and stomatal closure occurred more rapidly with a significantly shorter lag phase after blue light. Anoxia inhibited light‐dependent stomatal opening, even under 320 μmol m−2 s−1 illumination. However, after 60 min of illumination with 40 μmol m−2 s−1 of blue light in anoxia, transient stomatal opening was observed when the plant was returned to darkness and normal air. This transient stomatal opening was weaker after pretreatment with red light. We conclude that a blue‐light‐dependent process induced under anoxia leads to stomatal opening provided oxygen is present. Possible mechanisms associated with blue‐light‐effect and the nature of the oxygen‐consuming processes are discussed.
The oxygen requirement for stomatal opening in maize plants (Zea mays L. hybrid INRA 508) was studied at different CO2 concentrations and light intensities. In the absence of CO2, stomatal opening always required O2, but this requirement decreased with increasing light intensity. In darkness, the lowest O2 partial pressure needed to obtain a weak stomatal movement was about 50 Pa. This value was lowered to ca 10 Pa in light (320 μmol m−2 s−1). On the other hand. in the absence of O2, CO2enabled stomatal opening to occur in the light, presumably due to the evolved photosynthetic O2. Thus, CO2, which generally reduced stomatal aperture, could induce stomatal movement in anoxia and light. The effect of CO2 on stomatal opening was closely dependent on O2 concentration and light intensity. Stomatal aperture appeared CO2‐independent at an O2 partial pressure which was dependent on light intensity and was about 25 Pa at 320 umol m−2 s−1. The presence of a plasmalemma oxidase, in addition to mitochondrial oxidase, might explain the differences in the O2 requirement at various light intensities. The possible involvement of such a system in relation to the effect of CO2 is discussed.
In corn (Zea mays L.) root and shoot studies particular attention has been paid to environmental and soil influence on seedling root growth. Experimentation in this field has been limited by the difficulty in investigating root elongation, since no nondestructive in situ method was available other than glass‐sided boxes. The purpose of this study was to show that the neutron radiography is capable of supplying in situ root images permitting qualitative and quantitative interpretation so that comparative root and shoot growth may now be analyzed in a new way. The experiments consisted of shoot and root growth studies up to 21 days for two corn seeds sown at depths of 1 and 6 cm. The plants were grown in a growth chamber in a 2‐cm‐thick aluminum box containing coarse sand maintained at 8% volumetric water content. Daily neutron radiographs were made by the direct method using the 3.2° diverging thermal neutron beam of the MIRENE reactor at Valduc, France. The images obtained allowed a qualitative interpretation not only for seminal root but also for laterals. Root development was not harmed visibly by irradiation and repeated environmental changes due to neutron radiography experiments. The seminal root elongation rate was analysed vs. the plant age. Quantitative data show that the root growth was greater during the day than at night. Root and shoot lengths were related in a linear manner between 7 and 15 days after sowing.
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