during the later stages of inifectioni. The lcvels of chlorophyll. N.I)HPH-dlial)horase and aldolase Also (leclille(l. h'l'ere xwas no detectaile (lifferenice in the respiration of inlfecte andi noninifected leaves ulnitil 48 hoturs after inlocuilatioln. After this tilmie, the infected leaves shoxved a higher respiration, the n7aximum differenlce occtirrinig about 14-lholors after iniocuilationl. The resl)iratorv incirease was nlot acconmpalie(l bx significallnt changes in the levels of enzymies of glvcolvsis anid( the tricarboxvlic acid cycle \vith the excel)tion of walate dehvdrogenase vhiclh -vas lower in inifected leaves. In conitrast, the activities of glutcose-6-P dlehldrogenase anid 6-P-gluconate (lehvdroge-nase shox\vedl chalnl-es similar to that observed for respirationi.Tlhe respiration anid the activities of glIcose-6-P dehvdrogeniase and( 6-0P-gluconlate delhvdrog-enase did niot ilncrease in inifected leaves of etiolate(l plants, even

Partial photochemical activities and concentrations of electron carriers were measured relative to chlorophyll in barley (Hordeum vulgare L.) thylakoids, isolated from primary leaves during ontogeny and senescence. Thylakoids from mature leaves generated somewhat higher quantum efficiencies than thylakoids from premature or senescing leaves; this phenomenon did not appear to be caused by any deficiency of water-splitting enzyme. Under conditions of saturating light, the noncycic electron flux from water to the reducing side of photosystem I increased during leaf ontogeny, peaked at maturity, and declined during senescence. However, electron fluxes appeared to be Umited at different steps before and after leaf maturity. Before leaf maturity, the rate-limiting step was located prior to the reoxidation of plastohydroquinone. After leaf maturity, the decline in noncyclic electron flux correlated with a decrease in the concentration of cytochromesf and b6. This correlation, together with a consideration of mechanisms of entry and exit of electrons in 3-(3,4-dichlorophenyl)-1,1-dimethylurea-treated thylakoids, suggests that the cytochrome f/b6-containing complex, and not plastocyanin or P700, is the site of entry of electrons from the reduced forms of 2,6-dichlorophenolindophenol and diaminodurene. It is therefore proposed that in senescing leaves the cytochrome f/bs-containing complex limited electron transport by constraining the rate of reduction of cytochromefby plastohydroquinone.It has been well established that maximum rates of net photosynthesis, as measured by 02 evolution or CO2 assimilation, are achieved at or just before leaf maturity, followed by an extended period of decline (7,24,26). Previous research on changes in chloroplast electron transport during foliar ontogeny and senescence (for a review, see Sestak [25]), have shown that rates of chloroplast electron transport in isolated thylakoids, expressed on a Chl basis, generally follow similar patterns of rise and decline to net photosynthesis in whole leaves.Most methods for measuring rates of electron transport in vitro rely on artificial electron donors, acceptors and inhibitors. Recently, the sites of action of many of these compounds have been clarified (13,(29)(30)(31) Stromal enzymes appeared to be absent from thylakoid preparations, as evidenced by the absence of bands corresponding to the mol wt of both large and small subunits of ribulose bisP carboxylase-oxygenase when thylakoid preparations were subjected to SDS-polyacrylamide gel electrophoresis (6). Estimations of Chl a and b (28) were compared with total Chl (5), to show that degradation products were not significant (8).Photochemical Activities. Relationships between electron donors, carriers, and inhibitors used during this investigation are shown in Figure 1.Photochemical Activities Measured Spectrophotometrically. The light-driven reduction of ferricyanide or DCPIP in both reactions a and b (below) was followed by an Aminco-DW2a

Summary 1. This review is concerned with the obligate parasitism of the downy mildew, powdery mildew and rust fungi. 2. Mechanisms which determine resistance and susceptibility are incompletely understood. The three possibilities so far advanced, namely, hypersensitivity, phyto‐alexins and nutrition are all inadequate to explain specificity. It is postulated that compounds carrying specificity are released by the invading pathogens and the interaction of the compounds with specific sites in the host cell determines whether a successful relationship is established. This hypothesis is based on the observation that host and pathogen protoplasts do not come in direct contact with each other and cells removed from the invading pathogen are affected. It should be possible to investigate this question now that some rust fungi have been cultured axenically. 3. Invasion of host tissue by these pathogens leads to alteration in host metabolism. Respiration rates and the activity of the pentose phosphate pathway are enhanced. Possible mechanisms for the reoxidation of NADPH by molecular oxygen are discussed. In the later stages of infection, photosynthetic activity is decreased and this decrease is coupled to the enhanced activity of the pentose phosphate pathway through NADPH availability. It is still not clear whether photosynthetic activity is increased or decreased during the early stages of infection. Nutrients from other parts of the plant accumulate at sites of infection, resulting in retarded growth of the host. Possible roles of hormones in nutrient mobilization and ‘green island’ formation are discussed. Changes in intermediary metabolism can be correlated with changes in amounts of nucleic acids in infected host cells. Chloroplast polysomes and ribosomes decrease during the early stages of infection but cytoplasmic ribosomes increase during patho‐genesis. This increase more than compensates for the decrease in chloroplast ribosomes resulting in an overall increase in cellular RNA. 4. Some members of the Pucciniaceae and one member of the family Melamp‐soraceae have now been cultured axenically. Nutritional studies indicate that these fungi are relatively non‐specific with respect to their carbohydrate and amino‐acid nitrogen sources but do have a specific requirement for either cysteine, methionine or homocysteine. Essential metabolites such as cysteine and cysteine‐containing peptides move from hyphae into the medium and these may be important in amino‐acid transport. Two stages in the development of axenic rust cultures are still imperfectly understood ‐ the transition from germ tube to saprophytic mycelium and the fate of the colonies once formed. Further knowledge of the factors which control these stages may well suggest approaches to the axenic culture of other groups of obligately parasitic fungi.

No abstract

Scorn. 1975. Macroinfauna of northern New Eneland marine sand. 11. Amphi+-dominated intettidd communities. Can. J. h l . 53: 42-51.Tbi: is a 5.year study of macroinfaunal communities of moderately exposed to sheltered intert~dal sands OF New Hampshire nnd southwestern Mame. Habitats also differed in slope, width, predominant sand grain size, and carbon-nitrogen concentrations. Thirty-one species were recorded. Abundance and dry we~ght biomass at four intensively st~tdied habltats averaged 50M/rnz and 2.1 glmz. respect~vely, durlng 1971. The most exposed habitat yielded rwice thls prksence de six esptces d'amphipdes. la plupan der Hausroriidac. don1 la longivitt est relativernent courte. 1-8 variation dans I'ahondance et la biomasse des amphipodes err plus g m d e d'un habitat i I'autre que d'une saison i I'autre au sein du msme habitat. Leq espkces de folychete:. sont plus nombreuses dans les hahifats prorbg.5~ et les moltusques bivalves sont tmrtis aux zones abritees. Ln comrnunautk typique deq sables exposis moder6ment, dans le Maine, au sud de la baie de Penohscot, esr caractkrisie par un proupe recurrent de neuf espkces (cinq arnphipdes). La stabilire de la cornmunautk depend, en partie. des habitudes reproauctrices des arnphtpodcc dominant<. tn discussion pone sur la zonation, les rapports avec In zone suh-tidale, Its mouvemenrs saisonnlers. enfin, les modes de dominance des espkces d'amphi-W e s . Onttablir Cgalement certainescomparaisons struc~urales avec des carnrnuna~~tes boniales europeennes ainbi quc d'autres cornmunaues habatant les sables de la zone ~ntertidaie de la cbte Atlantique.[Traduil par le jo~~rnall