(5,12,13,14) and Perilla (10) leaves. Fraction I protein as usually isolated can probably be considered to be crude RuDPCase' since it often contains several other enzymatic activities in addition to that of RuDPCase (15,18,36). We recently reported that intact barley seedlings placed in extended darkness lost soluble protein, almost all of which was accounted for by the enzyme RuDPCase (9,25). The results of these investigations are not surprising, since this enzyme is considered to be the major storage protein of many plant leaves (18,26). Since RuDPCase is also principally responsible for net fixation of CO2 in plants, its fate during senescence is important. This study is a further extension to show the effects of some chemicals and light on the loss of RuDPCase and to correlate the concurrent development of proteases and esterases during senescence. Evidence is also presented for the turnover of RuDPCase under these conditions. The process of senescence has been studied in intact plants, detached leaves, and leaf discs in both light and dark (31, 37). In general, senescence is speeded by detaching the leaves and/or imposing darkness. Perhaps the most striking changes in senescence are losses of Chl, nucleic acids, and protein (39), accompanied by a general increase in proteolytic activity as measured in vitro. Cytokinins (32,33,37) or light (7)

Nitrate and N02-transport by roots of 8-day-old uninduced and induced intact barley (Hordeum vulgare L. var CM 72) seedlings were compared to kinetic patterns, reciprocal inhibition of the transport systems, and the effect of the inhibitor, p-hydroxymercuribenzoate. Net uptake of N03-and N02-was measured by following the depletion of the ions from the uptake solutions. The roots of uninduced seedlings possessed a low concentration, saturable, low Km. possibly a constitutive uptake system, and a linear system for both N03-and N02-. The low Km system followed Michaelis-Menten kinetics and approached saturation between 40 and 100 micromolar, whereas the linear system was detected between 100 and 500 micromolar. In roots of induced seedlings, rates for both N03-and N02-uptake followed Michaelis-Menten kinetics and approached saturation at about 200 micromolar. In induced roots, two kinetically identifiable transport systems were resolved for each anion. At the lower substrate concentrations, less than 10 micromolar, the apparent low Kms of N03-and N02-uptake were 7 and 9 micromolar, respectively, and were similar to those of the low Km system in uninduced roots. At substrate concentrations between 10 and 200 micromolar, the apparent high Km values of N03-uptake ranged from 34 to 36 micromolar and of N02-uptake ranged from 41 to 49 micromolar. A linear system was also found in induced seedlings at concentrations above 500 micromolar. Double reciprocal plots indicated that N03-and N02-inhibited the uptake of each other competitively in both uninduced and induced seedlings; however, Ki values showed that N03-was a more effective inhibitor than N02-. Nitrate and N02-transport by both the low and high Km systems were greatly inhibited by phydroxymercuribenzoate, whereas the linear system was only slightly inhibited.

ABSTIRACIThe effect of NaCl and Na2SO salinity on NO3-assimilaon in yonng barley (Horden m vlgare L var Numar) seed was studied. The induction of the NO-tranporter was affected very little the major effect of the salts was on its activity. Both a-and SO42-salts severely inhibited uptake of NO-. When compared on the basis of osmolality of the uptake solutions, a-salts were more inhibitory (15-30%) than S042-salts. At equal concentrions, SO42-salts inhibited NOj-uptake 30 to 40% more tha did a-salts. The absolute concentrabons of each io seemed more important as inhibitors of NOi-uptake than did the osmolality of the uptake solutios. Both KI and Na salts inhibited NO3-uptake similarlr, hence, the process seemed more sensitive to anionic linity than to cationic salinity.Unlike NO3-uptake, NO3-reduction was not affected by salinity in short-term studies (12 honrs). The rte of reduction of endogeos NOiin leaves of seelings gron on NaCl for 8 days decrased only 25%. Nitrte redutase activity in the salt-treated laves also decrased 20% but its activity, determined either in vitro or by the 'amerobic' in vivo assay, was always greater than the actual in sits rate of NOQ reducton.When salts were added to the assay medium, the in vitro enzymic activity was severely inhibited; whereas the anaerobic in vivo nitrate reductase activity was affected only slightly. These results indicate that in sits nitrate reductase activity is protected from salt injury. The susceptibility to injury of the NO-transporter, rather than that of the NO3-reduction system, may be a critical factor to plant survival during salt stress.The assimilation of NO3-, the predominant form of N available in an aerobic environment, is critical if plants are to adapt, grow, and reproduce in saline conditions. Not only is NO3 assimilation required for growth and development, but some of its metabolites accumulate during stress (1 1, 16, 30). It is well known that both proline (1 1, 16, 30) and betaine (10) accumulate during stress. Proline apparently originates from recently formed glutamate (5). Methylated quaternary ammonium compounds and possibly some amino acids accumulating in stressed plants could serve as osmotica for osmoregulation (10,16,30 Stargrass. In contrast, NaCI slinization had little effect on N uptake in winter barley but impaired its incorporation into the protein fraction (13). In leafdiscs ofNicotiana rustica, salt stress reduced both the uptake of L-leucine and its incorporation into proteins (3).The reported effects of salinity on N assimilation are controversial, because no studies were done that measured all of the processes of NO3-assimilation simultaneously. Measuring only uptake or internal reduced N does not yield a balance sheet needed to determine which processes are affected.This report describes the effects of salinity on the processes of NO3-assimilation. MATERAS AND METHODSSeedling Growth. Two varieties of barley (Hordeum vulgare L.), Numar and Arivat, were grown. Numar is a salt-tolerant and Arivat is a salt-sensitive variet...

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summary Although information has rapidly developed concerning the intracellular localization of plant proteins, relatively few reports concern the intracellular location of endo‐ and exo‐proteolytic activities. Relatively few proteases have been purified, characterized, and associated with a specific cellular location. With the exception of the processing proteases involved in transport of proteins across membranes, little progress has yet been made concerning determination of in vivo products of specific proteases. Information on the turnover of individual proteins and the assessment of rate‐limiting steps in pathways as proteins are turned over is steadily appearing. Since chloroplasts are the major site of both protein synthesis and, during senescence, degradation, it was important to show unambiguously that chloroplasts can degrade their own constituents. Another important contribution was to obtain evidence that the chloroplasts contain proteases capable of degrading their constituents. This work has been more tenuous because of the low activities found and the possibility of contamination by vacuolar enzymes during the isolation of organelles. The possible targeting of cytoplasmic proteins for degradation by facilitating their transport into vacuoles is a field which hopefully will develop more rapidly in the future. Information on targeting of proteins for degradation via the ubiquitin (Ub) degradation pathway is developing rapidly. Future research must determine how much unity exists across the different eukaryotic systems. At present, it has important implications for protein turnover in plants, since apparently Ub is involved in the degradation of phytochrome. Little information has been developed regarding what triggers increased proteolysis with the onset of senescence, although it appears to involve protein synthesis. Thus far, the evidence indicates that the complement of proteases prior to senescence is sufficient to carry out the observed protein degradation. This field of study has great practical implications, e.g. maintaining photosynthesis during seed‐fill in order to obtain greater crop yields. The current use of ‘stay green’ variants in the populations of several crop plants to produce increased yields shows the potential for future development. The near future should see exciting discoveries in these areas of research that will have far reaching effects on the construction of transgenic plants for future research accomplishments and agricultural use.