During the last years, agent technologies have witnessed an steady, if not fast, increase in popularity. Even so, its adoption, specially in commercial products has been sluggish and scant. Probably the main hurdle to a wider adoption of agent technology are the security issues involved, and the lack of sound and reliable security infrastructures in such environments. This is specially relevant in mobile agent systems, where software agents have the ability to move between several execution platforms over a network. The security concerns that mobility brings to the picture, which are not present in more traditional distributed systems, have to be carefully taken into account. Among the security problems of mobile agents, an outstanding one is resource access control and authorisation management. Practical mobile agent systems demand lightweight, flexible, and scalable solutions for access control in order to cope with the highly heterogeneous nature of their clients. In this thesis, we tackle the access control and authorisation management problem in mobile agent systems. We start with an analysis of the problem, the current state of the art in the field, and the most relevant available technologies for access control and authorisation management. We also consider how other distributed technologies are dealing with the access control problem. Then, we propose concrete solutions for the problem, taking into account the particularities and requirements of mobile agent systems. The proposal considers role based access control and the delegation of authorisations as the backbone of the infrastructure. We also provide solutions for specific requirements such as subscription like services and consider other related aspects such as the usability and applicability of such systems.

In Partial Fulfillment of the Requirements

The Absorption of Inorganic Ions by Chlorella pyrenoidosa.

A greenhouse study was
conducted to explore the effect of various rates of potassium sulfate (K2SO4)
nanoparticles on alfalfa (Medicago
sativa L.)
growth and physiological response under salt stress. One salt-tolerant genotype
(Mesa-Sirsa) and one salt-sensitive genotype (Bulldog 505) were selected based
on germination under salt and were planted in pots containing 2 kg of sand. The
two genotypes were subjected to 0 and 6 dS·m-1 salt
levels using CaCl2·2H2O: NaCl (2:1) mixed with
Hoagland solution. Three K2SO4 nanoparticle treatments
consisting of, 1/4, 1/8, and 1/10 of the potassium (K) level in full strength
Hoagland solution (235 mg·L-1) were
applied. Adding K2SO4 nanoparticles at the 1/8 level
resulted in the highest shoot dry weight, relative yield, root length and root
dry weight in both genotypes. The different rates of K2SO4 nanoparticles affected significantly Na/K ratio and the concentrations of
Calcium (Ca), Phosphorus (P), Copper (Cu), Manganese (Mn), and Zinc (Zn) in
plant tissue. The application of K2SO4 nanoparticles at
the 1/8 rate enhanced the plant’s physiological response to salt stress by
reducing electrolyte leakage, increasing catalase and proline content, and
increasing antioxidant enzymes, activity. These results suggest that the
application of K nanoparticles may have better efficiency than conventional K
fertilizers in providing adequate plant nutrition and overcoming the negative
effects of salt stress in alfalfa.

/pdf/response-of-alfalfa-under-salt-stress-to-the-application-of-3y481rmu22.pdf

Response of Alfalfa under Salt Stress to the Application of Potassium Sulfate Nanoparticles

Increasing concentrations of K (20, 200, 2000 mum) in the nutrient solution depressed Ca content and concentration in barley plants growing in nutrient solutions of low Ca concentrations (250 and 2500 mum). Increasing K from 20 to 200 mum depressed Ca absorption more than increasing K from 200 to 2000 mum K.The strong depression of Ca absorption by low concentrations of K must involve a different process from that studied by other workers at high concentrations of K. Since the depression in net absorption of Ca was as great at 250 as at 2500 mum Ca the results fail to support previous suggestions that a specific mechanism for Ca absorption operates at low Ca concentrations. It is suggested that, at the low concentrations of K and Ca likely to be found at the root surface in many soil solutions, the above mentioned effect of K in inhibiting Ca absorption may be important in the Ca nutrition of plants.

Interactions Between Potassium and Calcium in Their Absorption by Intact Barley Plants. I. Effects of Potassium on Calcium Absorption

/pdf/some-effects-of-boron-supply-on-the-chemical-composition-of-3qq3jl29bk.pdf

Some effects of boron supply on the chemical composition of tomato leaflets.

The nutritive values of food crops and the factors which may modify them are subjects of increasing interest. In recent years vitamin and mineral constituents of plants have been used to evaluate some aspects of nutritive value, and the importance of a variable supply of minerals from plant sources in animal nutrition has been discussed by Maynard (15). During the summers of 1941 and 1942, extensive experiments have been conducted at this laboratory on the effects of varying the relative proportions of macro-nutrients1 supplied to tomato plants on the vitamin content of tomato fruits (6, 11). Although variations in the mineral constituents of the tomato fruit are of relatively minor nutritional importance (10), studies relative to mineral composition were made to aid in an understanding of the physiological processes involved in mineral absorption by plants. In soil studies of the mineral nutrition of plants, Jenny and Ayers (12) have clearly distinguished two aspects of the problem : 1. Soil colloidal phenomena as they are influenced by fertilization and as they in turn influence the composition of soil solutions ; and 2, the absorption and accumulation of the nutrients by plants from the soil solution. For convenience, they have termed the second aspect "the biological accumulation phase by root hair cells." More exactly this aspect might include the localized retention of ions by the living system. Since the cytoplasmic constituents of any organism are changing continuously, chemical analysis of any organ at any given interval can only be empirical. Investigations with artificial culture solutions of known composition have been profitable in studying those empirical relationships involved in ionic absorption by plants. Such factors of nutrient composition as the presence of essential ions, the relative concentration of the constituent ions, the osmotic concentration of the solution, the reaction of the medium, and the total quantity of the nutrient available to the plant can be varied over wide ranges under carefully controlled conditions. It is the purpose of this paper to present data concerned primarily with the mineral composition of tomato plants as it is influenced by the composition of the nutrient medium with special attention given to ionic interaction. In many investigations of the effect of changes in nutrient supply on plant composition, nutrient solutions of fixed composition were utilized to ? The term macro-nutrients as used in this paper refers to the six nutrients (Ca, K, Mg, N03, S04, and P04) in contrast to the micro-nutrients (Cu, Mn, B, Zn, Mo, and Fe). The distinction between these two groups of nutrients is based upon the fact that the former group is needed by the plants in much greater amounts than the latter group.

C. B. Lyon

Papers

Ionic absorption by tomato plants as correlated with variations in the composition of the nutrient medium.