Mittäglicher Stomataschluß bei Aprikose (Prunus armeniaca) und Wein (Vitis vinifera) im Freiland trotz guter Bodenwasser-Versorgung

Abstract

The fact that plants respond to dry and hot environmental oonditions with midday-depression of gas exchange has been known since the early 1920s. At first the increase in stomatal resistance at midday was explained as a response to internal water status of the leaves after high transpirational water loss during the morning where soil water supply was limiting. However, recent investigations with apricot trees cultivated in the Negev desert demonstrated that stomata respond to ambient air humidity which induces stomatal closure at midday independent of bulk leaf water conditions. Although these measurements were carried out with irrigated trees, the possibility that some water stress occurred could not be fully excluded. In the present investigations, therefore, stomatal behavior was examined in plants with definitely unlimited water supply in the root zone. Apricot ( Prunus armeniaca cv. “Ungarische Beste”) and grapevine plants ( Vitis vinifera cvs. “Silvaner" and “Riesling”) were placed in containers and grown under field conditions (Würzburg, Botanical Garden). Soil was kept at field capacity by frequent irrigation. Transpiration and CO 2 -exchange were recorded for attached leaves on intact plants enclosed in temperature and humidity conditioned cuvettes that tracked natural environmental conditions. Stomatal resistance was calculated from transpiration rate and water vapor concentration difference between leaves and ambient air. The measuring period from June to September 1976 was exceptionally warm and dry. In spite of good soil-water supply, apricot stomatal diffusion resistance increased during midday under hot and dry atmospheric conditions. The closing resulted in depression of net photosynthesis and often also of transpiration (Fig. 3). Twenty-seven diurnal courses exhibiting stomatal closure were recorded during the growing period under conditions of high saturation deficits between leaf and air (Fig. 4). Grapevine responded in a similar manner (Fig. 6). However, in this plant diurnal courses with midday-depression of gas exchange were restricted to the second part of the summer period (Fig. 4). In spring and early summer, grapevine exhibited low stomatal resistance at midday under the same environmental conditions to which apricot responded with strongly depressed gas exchange. Branches of apricot were detached under water. The cut branches were kept in water and the foliage portion remained enclosed in the measurement cuvette. Net photosynthesis decreased during the days after detachment when compared to an intact control branch of the same plant. However, in spite of unrestricted water availability, the detached twig exhibited midday-depression of CO 2 -uptake of the same order of magnitude as its attached counterpart (Fig. 7). The diurnal course of gas exchange and stomatal resistance of both species was modified when they were subjected to soil water stress during a clear weather period (Fig. 8, 9). Compared to well-watered plants, the increase of stomatal resistance at midday in non-irrigated plants was strongly enhanced. Under severe stress conditions, net photosynthesis was positive only during short periods in the morning, and CO 2 -exchange remained at or below the compensation point for the rest of the day. Plants recovered fully after re-irrigation. The results observed at high soil water potentials and with detached branches indicate that stomata of both species studied were controlled by external air environmental conditions and most importantly by the evaporative demand of the ambient air. Sensitivity of stomata in this respect was different in apricot and grapevine and changed for the latter during the course of the year. Stomatal aperture is seen also to depend on internal plant water status. This hydraulic feedback control, however, became effective only under conditions of strong soil water stress. From an ecological point of view, midday-depression of stomatal conductance appears unnecessary and even disadvantageous when soil water supply is not restricted. On the other hand, atmospheric dryness is usually correlated with the danger of water shortage in the habitats of these plants. Effective use of water would then confer an advantage. According to the theoretical considerations of C owan and F arquhar , feedforward control of stomatal resistance, i.e. their direct response to humidity, is a mechanism for optimization of water-use efficiency. Low stomatal sensitivity of grapevine in spring and early summer and increasing sensitivity later in the year may possibly be an adaptation to environmental conditions where water shortage occurs mainly during the second part of the growing period. The observation, that stomata respond to atmospheric dryness even in well-watered plants, may have agronomic implications. Midday-closure is disadvantageous if photosynthetic productivity is reduced in spite of good soil water supply. In such cases One should try to avoid middaydepression of gas exchange by increasing air humidity. Sprinkler irrigation might therefore be more successful than gravity irrigation.