Potassium Deficiency-induced Changes in Stomatal Behavior, Leaf Water Potentials, and Root System Permeability in Beta vulgaris L.

Abstract

Studies of the water relations of potassium deficient sugar-beet plants (Beta vulgaris L.) revealed two factors for stomatal closure. One component of stomatal closure was reversible by floating leaf discs on distilled water to relieve the water deficit in the leaves; the other component was reversible in the light by floating the leaf discs on KCI solution for 1 hour or more. Potassium-activated stomatal opening in the light was observed when the guard cells were surrounded by their normal environment of epidermal and mesophyli cells, just as observed by previous workers for epidermal strips. Leaf water potentials, like stomatal apertures, appear to be strongly related to leaf potassium concentration. Potassium-deficient plants have a greatly decreased root permeability to water, and the implications of this effect on stomatal aperture and leaf water potential are discussed. In contrast, petiole per-meability to water is unaffected by potassium treatment. A number of authors (5-7, 9, 13, 15) have reported recently on the role of potassium in stomatal opening. Most studies have been made using excised epidermal strips (5-7, 9) and this technique has permitted rapid advances in the study of stomatal behavior (18). From a purely mechanical point of view, guard cell activity is considered to be due to an increase in turgor relative to the adjacent epidermal cells (17) with which guard cells may exchange water deficits in very short time intervals as Raschke (14) has recently reported. Such relationships between the guard cells and the epidermal cells are generally destroyed when epidermal strips are used as the test material. The mesophyll is removed and almost all epidermal cells are ruptured during separation (5). In the present studies, the response of guard cells was observed in the intact leaf and this technique also confers the advantage of allowing the use of a porometer to estimate stomatal apertures. Measurements of the leaf water potential profile and the root system permeability are also included in this report as they contribute to the understanding of the stomatal responses. MATERIALS AND METHODS Sugarbeet plants (Beta vulgaris L. MS NB1 X NB4) were grown in plant growth chambers in a complete nutrient culture solution (8) until they were well established (about 5 weeks after germination). The photoperiod was 16 hr, and the light intensity was 43,000 lux, supplied by fluorescent lamps supplemented with incandescent tungsten lamps. Some plants were then harvested, and half the remainder were transferred to solutions similar to the complete nutrient solution but lacking in potassium. Measurements and further harvests of plants were made at intervals up to 15 days after potassium cutoff. Night temperature was 20 C throughout, whereas day temperature, initially 25 C, was increased step-wise to 29 C to maintain the evaporative demand on the growing plants. Leaf viscous resistance was measured with an Alvim-type porometer (1), similar to the one described by Bierhuizen et al. (3). Leaf discs (diameter 1.5 cm) were floated on water or KCI solution in Petri dishes at 25 C under lights and on removal , gently dried in a standard manner with absorbent paper tissue before being placed in the porometer. The light source and intensity were the same as in the plant growth chambers. Leaves are numbered from the oldest leaf. Root system permeability was measured by placing the de-capitated root systems in a pressure chamber containing the nutrient solution. The solution was aerated by a bleed tube, and a pressure of 2 bars was applied to the chamber. Fluxes were measured at the cut stump which was exposed to the atmosphere. Petiole permeability was measured likewise in a similar chamber with a 7.5 cm length of petiole. The leaf water potential was determined by using the Shardakov technique as described recently by Knipling (11). Further experimental details have been published elsewhere (8). RESULTS In Figure 1, three sets of leaf viscous resistance measurements are presented for different leaves. Resistance is represented by the length of the bars, and, as there is an inverse relationship between resistance and stomatal aperture, a high resistance value is interpreted as a small stomatal aperture and vice versa. Each set of data shows that for intact plants the resistance to the movement of air through the leaf under pressure was much higher where the potassium status was low. Stomatal apertures of low K leaves were substantially increased when full turgor was restored. This is demonstrated as a decrease in leaf viscous resistance when leaf discs were floated on distilled water for 4 hr (2). Whereas this treatment 105