Assay conditions in vivo for determining nitrate reductase activity (NRA) in shoot and root tissues of winter barley (Hordeum vulgare L. cv. Igri) were studied. The composition, pH and temperature of the incubation medium, and the size of leaf slice were varied. Maximum leaf NRA was obtained with 5 mm transverse slices incubated at 32°C in a medium containing 0 . 1~ phosphate buffer, pH 7.0, 0.02M KN03, 30 ml litre-' propan-1-01 and 0.1 ml litre-' Triton X-100. The optimum asssay conditions for roots were similar to those for leaves, except that the pH was 7.5. There was negligible nitrite reductase activity in leaves incubated in the dark.

A B S T R A C TWinter barley (Hordeum vulgare L cv lgri) plants were grown for 5 weeks in a controlled environment room using a recirculating nutrient solution culture system at a root and shoot temperature of 10°C.When solution NO,-N concentration was varied between 0 and 6 4 m g litre-', shoot endogenous and induced nitrate reductase activity ( N R A , and N R A i respectively) increased asymptotically f o m zero in plants grown with 0 mg NO,-N litre-' to a maximum activity in solutions containing 3 16 mg N 0 3 -N litre-'. Shoot Nitrogen Response Index ( N R I = N R A J N R A , )showed an opposite trend decreasing fiom a maximum of 2.6 at 2 mg NO,-N litre-' to a minimum approaching unity when 3 3 2 mg N litre-' was supplied.In a separate experiment, nitrogen form (NO,-N, NH,-N or a 1:l mixture) had a marked influence on shoot N R A . Enzyme activity was negligible in tissue fiom plants absorbing NH,-N, highest in NO,-fed plants and intermediate in those supplied with both N forms. N R l , however, approached unity in all treatments. The results provide evidence to suggest that N R l may be used as an indicator of the N status of barley plants grown with varied N nutrition.

A B S T R A C TIn three separate experiments, the efects of rooting medium pH (PH 4-0,5*3 and 6.5), root temperature (5, 10 and I S O C ) and shoot temperature (6 and 17°C) METHODS Culture systemWinter barley plants (Hordeurn uulgare L cv Igri) were prepared and grown in a controlled environment using flowing nutrient solutions described elsewhere (Hall rt a1 1989). (1) Three root temperatures, 5f1, 10f1 and 15f 1"C, were maintained by (2) The pH of each of the three nutrient reservoirs was kept within the range 6-3-6.8Experiment 3: Influence of shoot temperature on the growth, NRA and N status of winter barleyThe same controlled environment room used to conduct Experiments 1 and 2 was partitioned into two sections. Conditions were identical to those described in the previous section except for the following:(1) Two air temperatures were imposed, namely 6 + 2 and 17f3"C. 437In Experiment 2, harvesting of shoots and roots of all treatments took place 21 days after imposition of the temperature treatments. Plants subjected to 5 and 10°C root temperature had reached the 3rd and 4th leaf stages respectively, whilst those receiving the 15°C treatment were slightly more developed having reached the 4th leaf stage on day 18. In Experiment 3, plants grown at 17°C air temperature had matured to the 5th leaf stage and those at 6°C had not quite reached the 4th leaf stage after 21 days.The fresh weight yield of shoots of all plants in Experiments 1, 2 and 3 were recorded and fresh samples of shoots and roots were assayed for in-vivo endogenous NRA (NRA,) and induced NRA (NRA,) (Sym 1984). The remainder of each sample was dried, finely milled and analysed for reduced N using a semi-micro Kjeldahl technique. Nitrate was determined in cold water extracts by the phenol disulphonic acid method. Total N was expressed as the sum of reduced N +NO,-N. RESULTS AND DISCUSSION Experiment 1 Yield, total N and NO,-N concentrations of dry matterAlthough prolonged exposure to low external pH can damage roots and impair their ability to absorb nutrients (Bassioni 1971), this is dependent on duration of exposure. There is no evidence from these results to suggest that NO,-N absorption was adversely affected at pH 4.0 since a comparatively faster rate of N03-N uptake ( Table 1) and significantly (Pc0.05) greater shoot yield ( Table 1) was recorded at pH 4.0 compared with pH 6.5. This is because NO,-N uptake proceeds at a rate inversely proportional to pH (Rao and Rains 1976). Under acid conditions there is less competition between NO; and OH-for absorption (Bassioni 1971) and/or less suppression of the NO, transport system by OH-(Rao and Rains 1976).There was little difference in the quantity of reduced N ( Table 1) assimilated in the shoots and roots of plants grown at pH 4.0, 5.3 and 6.5. However, nitrate contents of both shoots and roots were significantly ( P < 005) greater in plants grown at pH 4.0compared with pH 6.5. Whereas the rate of NO,-N absorption was the principal limiting factor to growth at pH 6.5, its rate of reduction m...