Apple seedlings (Malus domestica Borkh.) were grown in the greenhouse under a range of polyethylene glycol (PEG)-induced osmotic potential stresses up to −7.5 bars. Water use by seedlings (ml water consumed/dm2 leaf area) responded to ambient temperature fluctuations after 4 days in the solution. During the first 4 days after initiation of the stress, no response was obtained indicating that the seedlings were undergoing an “adaptation period.” Plants receiving higher osmotic stresses were less able to respond to ambient temperature fluctuations as measured by transpiration. Transpiration rate decreased as osmotic stress was increased. PEG-induced osmotic water stress and water stress in soil were compared, the latter by letting the soil mass dry out. Comparable transpiration rates from the 2 methods, when plotted, showed that PEG-induced osmotic stresses of −0.5 and −4.0 bar were equivalent to greenhouse potting soil at 75% of field capacity and approaching the wilting point, respectively. It was concluded that the PEG-induced water stress was similar to water stress in soil, thus PEG-induced stress can be used in experiments with apples to study various effects of water stress. Use of a freezing point depression osmometer in determining solution osmotic potentials of PEG-modified solutions is described.

Polyethylene glycol (PEG)-induced water stress in nutrient solutions decreased both water consumption and 45Ca uptake by apple seedlings (Malus domestica Borkh.) The decrease in water uptake was more severe than the decrease in 45Ca uptake. When 45Ca uptake was calculated on the basis of water consumption, it was found that 45Ca uptake was not dependent on water uptake although water was necessary for movement of 45Ca. In split-root experiments, PEG and 45Ca were either applied to the same half of the root or to separate halves. Calcium uptake decreased in plants subjected to water stress. The results indicated that the site of this decrease was at the root, not the aerial portion of the plant which, indirectly, may affect root function and thus 45Ca uptake. Split-root experiments also indicated that the unstressed half of the root cannot fully compensate for the stressed half of the root in either water or 45Ca uptake. Water use of plants with half of their root under a −5.0 bar water stress was decreased by 30%. Urea-nitrogen pretreatment did not modify the effect of osmotic stress on leaf Ca, Mg and K concentrations, water use or dry matter production during the period of applied water stress. All these parameters decreased with increasing solution osmotic stress.

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Proper plant nutrition that takes into consideration both the requirements of plants and ecological conditions is one of the most important precondition of successful plant production. An important element of the N-fertilization of wheat is that the optimum zone of nitrogen supply is significantly narrower than that of other plant species, therefore it can easily happen that we apply higher or lower nitrogen doses than the optimal one. A possible solution to this problem can be precision agriculture. Applying the methods of precision agriculture we can take into consideration the heterogeneity of fields. By applying precision methods either online or offline we can intervene faster than if we would rely only on regular soil and plant analysis procedures. The determination of the doses of nitrogen and the timing of application are influenced also environmental andeconomic aspects. The chlorophyll content of the leaves indicates the nitrogen status of plants, since there is a relationship between the nitrogen content and the amount of chlorophyll in the leaves. According to plant analysis results there was a strong and significant relationship between the values of the NDVI (Normalized Difference Vegetation Index)and the total nitrogen content measured in the leaves.

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