Pathophysiological changes during the symptom development of pine wilt disease are reconsidered from recent investigations. The symptom development is divided into two stages: the early and the advanced stages. In the early stage, small number of nematodes migrate in cortex, then in xylem of the stem, and induce denaturation and necrosis of parenchyma cells. These changes in parenchyma are regarded as defense reactions of pines which result in terpene synthesis in xylem cells and embolism in tracheids. Such changes in the early stage can be induced in both susceptible and resistant pine species by either virulent or avirulent isolates of pinewood nematode (Bursaphelenchus xylophilus), or by B. mucronatus. No change occur in physiological status of leaves, and nematode reproduction is suppressed during this stage. Pine trees can survive if symptom does not progress from this stage. The symptoms of the advanced stage usually occur only in susceptible pines infected by virulent nematode isolates. At the beginning of the advanced stage, enhanced ethylene production by stem which coincides with cambial destruction occurs, and results in embolism of the outermost xylem in the portion. The embolism causes decrease in leaf water potential and cessation of photosynthesis. After cessation of photosynthesis, symptoms develop drastically with a burst of nematode population. There seems to be some unknown mechanism which suppress nematode reproduction and invasion to the cambial zone. This mechanism is thought to be photosynthesis-dependent, so that in photosynthesis-decreased conditions, even avirulent nematodes can multiply and invade cambium to induce tree death. Water stress in hot and dry summer should accelerates symptom development from the early to the advanced stage through such decrease of photosynthesis-dependent "cambial resistance."

Reactions of N(4S) atoms with NO and H2 have been investigated using direct detection of N atoms by the atomic resonance absorption technique in a shock tube apparatus, where N(4S) is generated by photodecomposition of NO by 193 nm laser radiation behind reflected shock waves. The rate constant of the reaction, N+NO→N2+O (1) has been determined using pseudo first-order kinetic analysis to be k1=(1.3±0.3)×1013 (cm3 mol−1 s−1) over 1600–2300 K temperature range, which agrees very well with the estimation by Baulch et al. [Evaluated Kinetic Data for High Temperature Reactions (Butterworths, London, 1973), Vol. 2]. No (or very small) activation energy of this process was confirmed. Also, the rate constant of the reaction, N+H2→NH+H (2) has been decided by adding H2 to NO–Ar mixtures; it is k2=(2.8±0.2)×1014 exp(−Ea/RT) (cm3 mol−1 s−1), where Ea =33±7 kcal/mol. A quantum mechanical calculation performed in order to determine the mechanism of this reaction suggests that the reaction N(4S)+H2→NH+H proceeds via a direct abstraction of H atom from H2, and it gives calculated activation energy which is in good agreement with the present experiment.

In order to clarify the mechanism of pine wilt caused by the pinewood nematode (PWN), Bursaphelenchus xylophilus, nematode migration in tissues and disease symptoms in Pinus thunbergii seedlings were investigated. One-year-old seedlings were inoculated with different pathogenic isolates of PWN under two different temperatures. At an early stage of symptom development, a virulent isolate of PWN multiplied in both bark and xylem and was distributed in cortical resin canals, cortical tissue, and xylem resin canals at 30°C. Cell death and disease symptoms developed in both bark and xylem. The virulent isolate of PWN at 25°C and the avirulent isolate of PWN at 30°C were distributed mainly in cortical resin canals, but rarely in xylem resin canals and cortical tissue. Disease symptoms and cell death occurred in cortical resin canals and rarely occurred in other tissues. These results demonstrated that the virulent isolate of PWN at low temperature and avirulent nematodes could not easily migrate to xylem resin canals and cortical tissue. It was shown that cell death and early symptom development coincided with PWN migration and, therefore, PWN invasion induces cell death and early symptom development.

Alfalfa (Medicago sativa L.) is one of the most important forage crops and has high protein and highly digestible fibre contents. It can be cultivated in moderate salt‐alkaline soils and has been widely cultivated as an economic crop worldwide. We quantified the effects of salt (1:1 molar ratio of NaCl to Na2SO4, pH 7.01–7.05) and alkali (1:1 molar ratio of NaHCO3 to Na2CO3, pH 9.80–10.11) stresses on germination, growth, photosynthesis and ion accumulation in alfalfa. The results showed that both stresses significantly reduced germination and radicle elongation, indicating that alfalfa was relatively sensitive to both stresses during seed germination and early seedling growth stages. The relative growth rate, water content, chlorophyll content, intercellular CO2 concentration, stomatal conductance, net photosynthetic rate (PN) and transpiration rate decreased slightly with increasing salinity under salt stress, but were markedly reduced under alkali stress. Conversely, water use efficiency increased with increasing salinity under both stresses. The Na+ content increased and the K+ content decreased with increasing salinity under both stresses, indicating competitive inhibition between the absorption of Na+ and K+. Intracellular imbalance of Na+ and K+ caused by high pH of alkali stress might be one of the reasons for the visible decrease in PN. Both Ca2+ and Mg2+ contents decreased with increasing salinity under both stresses. Our study found that the deleterious effects of alkali stress were more severe than those of salt stress.

10) Bock, H.; Goebel, I.; Zdenek, H.; Liedle, S.; Oberbammer, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 187. (11) (a) Leonard, N. J.; Coll, J. C.; Wang, A. H.-J.; Missavage, R. J.; Paul, I. C . J. Am. Chem. S a . 1971,93,4628. (b) Wan& A. H.-J.; Missavage, R. J.; Byrn, S. R.; Paul, I. C. (17) Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J. B.; Schlegel, H. B.; Raghavachari, K.; Robb, M. A.; Binkley, J. S.; Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.; Pople, J. A.; Gaussian 90, Rev. I; Gaussian,Reactions of the ethynyl (C2H) radical with C2H2, H2, and D2 were studied over the temperature range 298-438 K by time-resolved mass spectrometry. The rate of the reaction with C2H2 was followed by measuring the appearance rate of C4H2. The rates of the reactions with H2 and D2 were deduced by measuring the dependence of C4H2 production (arising from the reaction of C2H with the C2H2 precursor) on the partial pressure of added H2 or D2. The rate constants for the reaction C2H + C2H2 -C I H~ + H were also measured following reflected shock waves by monitoring H atom resonant absorption at 121.6 nm. In both experiments, the C2H radical was generated by ArF (193-nm) laser photolysis of C2H2. A rate constant of (1.5 f 0.3) X cm3 molecule-' s-l was obtained for reaction 1 without any temperature dependence at T = 298-2177 K. The results for the reactions C2H + H2 -C2H2 + H and C2H + D2 -C2HD + D could be represented by the Arrhenius expressions, k2 = (1.8 f 1.0) X lo-" exp(-(1090 f 299)/T) and k3 = (1.4 f 0.8) X lo-" exp(-(1377 f 301)/T) cm3 molecule-' s-', over the range of T = 298-438 K. The classical barrier height for reactions 2 and 3 was estimated to be 2 kcal/mol on the basis of conventional transition-state theory. The isotope effects on reactions 2 and 3 calculated with Wigner tunneling correction were in good agreement with the present results.

Pairing tests with isolates of Armillaria in culture were used to identify species of Armillaria and their distribution throughout Japan. The existence of 10 intersterile groups of Armillaria was determined by pairing haploid single spore isolates obtained from 20 basidiocarps from a wide geographic distribution in Japan. Two to four haploid tester isolates from each intersterile group were then paired to biological and morphological species with haploid tester isolates from Europe and North America identified in previous studies. Japanese haploid tester isolates were then paired with 190 haploid or diploid isolates and compatibility reactions were used to identify these to species. Of the 10 intersterile groups identified in Japan, 7 were authenticated as A. gallica, A. nabsnona, A. ostoyae, A. cepistipes, A. mellea, A. sinapina, or A. tabescens. Three of the groups were not compatible with any of the tester species. The distribution and host relationships of the Japanese biological species are also discussed.

No abstract

Anatomical and cytochemical changes in the current-year stem cuttings of Japanese black pine (Pinus thunbergii) were investigated in the early stage of infection by a virulent isolate and an avirulent isolate of pine-wood nematode (Bursaphelenchus xylophilus), and an avirulent isolate of another nematode species, Bursaphelenchus mucronatus. Accumulation of lignin- and suberin-like substances around the resin canals in the cortex was shown as a new symptom of the infection by these isolates. Experiments with girdled cuttings demonstrated that more nematodes inhabit and move in the bark than in the xylem and pith at the early stage of infection by the virulent isolate. Death of pine cells occurred first in the epithelial cells of resin canals in the cortex after inoculation with the virulent isolate and then in the cortex and periderm, pith and xylem, and finally the cambium. In branches of 5-year-old seedlings inoculated with avirulent and B. mucronatus isolates, wound periderm was formed surrounding resin canals in the cortex, and cortical cells surrounding the wound periderm were alive. Evidence indicates that nematodes first enter resin canals in the cortex and then invade the surrounding cortical tissue, and that the ability of the virulent nematodes to move into the cortical tissue may be greater than that of the other isolates, accounting for differences in virulence. Key words: Bursaphelenchus xylophilus, Bursaphelenchus mucronatus, virulence, periderm, pine wilt disease, pine-wood nematode.