The relationship between loss of ribulose-1,5-bisphosphate carboxylase (RuBPCase) and the decline in photosynthesis during the senescence of barley primary leaves was assessed. Loss of RuBPCase accounted for about 85% of the decrease in soluble protein. The senescence of leaves is usually associated with loss of soluble protein, predominantly RuBPCase2 (5,12,13,21, 25, 26).Protein degradation and remobilization provide an important source of N and S for other parts of the developing plant (4, 21). The relationship between the function of RuBPCase as a storage protein (7, 12) and its catalytic function has not been well defined. Loss of RuBPCase during leaf senescence is usually accompanied by a loss in in vitro RuBPCase activity and a decline in CER (5,12,13,21,22, 25, 26). It is generally assumed that RuBPCase breakdown results in an increase in mesophyll resistance and a decline in gross photosynthesis, a component of CER. However, the concentration of RuBPCase catalytic sites in leaves can be as high as 3 mm (G. Lorimer, personal communication). Thus, RuBPCase concentration may not always limit in vivo RuBPCase activity and photosynthesis. The decline in CER is probably due not only to a decrease in gross photosynthesis but also to an increase in photorespiration (22). In addition, the decrease in CER may not be entirely a result of an increase in mesophyll resistance, as previously reported (26) in stomatal resistance (22, 24). In fact, stomatal aperture has been recently suggested as one of the main controlling factors in senescence (17). To date, no comprehensive study has been reported of the relationship between RuBPCase degradation and alterations in gross photosynthesis (as opposed to CER) and stomatal and mesophyll resistance. In the present study, all of these parameters were measured and were related to changes in RuBPCase activity, soluble protein, protease activity, total free amino acids, Chl, and carbohydrates. These measurements were made on intact leaves of barley (Hordeum vulgare L.). Although detached leaves are commonly used to study senescence, recent reports (9, 19) have demonstrated that detached leaves do not necessarily senesce in the same manner as intact leaves.MATERIALS AND METHODS Plant Culture. One hundred barley (H. vulgare L., var. Numar) seeds were planted at a depth of 2 cm in plastic pots (13.5 cm in diameter x 15.0 cm tall) containing Vermiculite. Each pot received 800 ml of nutrient solution at planting. Additional nutrient solution (100 ml/day) and distilled H20 were supplied via cotton wicks extending from the bottom of each pot into a 1-liter glass jar. The nutrient solution contained, in mmol/l: Ca(NO3)2, 5; KNO3, 1; K2SO4, 2; MgSO4, 4; NH4H2PO4, 2; and in ,tmol/l: MnSO4, 18.3; H3BO3, 8.0; ZnSO4, 3.8; CuSO4, 1.5; (NH4)6Mo7024, 0.1; NaCl, 28.2; and Fe as Fe-ethylenediamine di-(O-hydroxyphenylacetate), 110.4. Li'ht intensity at pot height in the growth chamber was 550 ,uE m s-as provided by a mixture of incandescent and metal halide lamps. Air temperature during t...

The objective of this experiment was to elucidate the manner in which N metabolism is influenced by S nutrition. Maize (Zen mays L.) seedlings suppled with Hoagland solution minus s042-exhibited S deficiency symptoms 12 days after emergence. Prior to development of these symptoms, a declie in leaf blade nitrate reductase (NR, EC 1.6.6.1) activity was observed In S-deprived seedlins compared to normal seedlngs. Twelve days after emergence, in vitro NR activity was diminished 50% compared to normal seedings. Glutamine syathetase (EC 6.3 Higher plants generally accumulate N and S in amounts proportional to that incorporated into protein (4,7,15,21). However, when plants are S-deficient, protein synthesis is inhibited and nonprotein N (e.g. amides, N03 -N) is accumulated (4,7,15,21 The objective of the present experiment was to determine if S deprivation affects NR activity preferentially and prior to other alterations in N metabolism. Besides NR, the activities of glutamine synthetase (GS) and both NAD-and NADP-specific glutamate dehydrogenase (NAD-GDH, NADP-GDH) in maize seedling leaf blades were determined. Recent evidence (1 1) suggests that GS, rather than GDH, is the primary route of NH4' assimilation in higher plants. We also measured the effect of S deprivation on fresh wt and on the concentration of soluble protein, N03 -N, and Chl a and b.MATERIALS AND METHODS Culture. The experiment was conducted in a growth chamber maintained at 31/21 C day/night temperature. Daylength was 16 hr (lights on at 0400 hr) and photosynthetic photon flux density was 32.5 nE cm-2 sec-' at pot height. Seedlings of the maize hybrid, W64A x W182E, were germinated in plastic pots (four seedlings/pot) containing 1 liter of Vermiculite that had been thoroughly rinsed with distilled H20. Starting 2 days after emergence, 25 ml of Hoagland nutrient solution No. I (10) was added to each pot every other day. Each pot had holes in the bottom so that excess solution would drain into a plastic cup placed underneath. The nutrient solution contained 250 aM Fe3' as sodium ferric ethylenediamine di-(0-hydroxyphenylacetate) (Geigy Agricultural Chemicals, New York). S-deprived seedlings were obtained by replacing the s042-salts in the nutrient solution with Cl-salts. Two additional treatments were obtained by adding 5 ml of 0.5 M K2SO4 to a portion of the pots containing control and S-deprived seedlings 12 days after emergence.Seedlings were harvested 7, 9, 10, 11, 12, 14, and 16 days after emergence between 0830 and 1030 hr. Three pots from each of the treatments were harvested at each date. The four seedlings in each pot were excised at the second node and composited to form a single sample from which two plant fractions were obtained. The first consisted of culms, leaf sheaths, and unfurled leaves (hereafter termed stems). The second consisted of fully extended leaf blades and leaf material extending above the most recently formed collar (hereafter termed leaf blades). The leaf blade fraction was cut into 6-mm sections, weighed, and su...

The role of N and S remobilization in ear development of maize (Zea mays L.) has not been adequately assessed. The objective of this greenhouse experiment was to determine the effect of N deprivation during grain‐filling on N and S remobilization and the relationship between N and S accumulation. Maize plants were grown in nutrient solution containing 15N‐nitrate and 35S‐sulfate. After silking, the plants were placed in unlabelled solutions, with or without nitrate‐N. The amount of labelled and unlabelled N and S in several plant fractions was determined at silking and 1, 2, 3, 5, and 7 weeks after silking. The remobilization of labelled N and S was not markedly affected by N deprivation. Unlabelled N (when supplied) and unlabelled S accumulated in all plant fractions, but did not affect the concentration of N or S in the ear. Nitrogen deprivation resulted in only a 19% decrease in ear N content. Labelled N constituted 100% of the total ear N in plants deprived of N as compared to 84% for control plants. Labelled S comprised 73% of the total ear S in N‐derived plants as compared to 56% in plants supplied with N. The ratio of reduced N to reduced S (NR/SR) in the ear was lower in N‐deprived plants. The NR/SR, ratio in the leaves was not affected by N deprivation, and declined during grain‐filling. Apparently, the remobilization of N and S contributes more to ear development than does the absorption of N and S after silking.

The activities of glutamate dehydrogenase (GDH), glutamine synthetase (GS), and nitrate reductase (NR) and the levels of soluble protein and NO‐3 were assayed in soybean (Glycine max [L.] Merr.) leaves over a 48‐h period with the initial 24 h under a light‐dark cycle (LD 16:8) followed by 24 h of continuous light (LL). Plants had been entrained for 30 days under the LD regime. Maize (Zea mays) leaves (10 days old) under a LD 15:9 cycle were assayed only for NR and nitrite reductase (NiR). Data were subjected to frequency analysis by the least squares method to determine probabilities for cosine function periods (τ's) between 10 and 30 h. NR activities for both soybean and Zea leaves had 24 h τ's with P values < 0.05 indicating circadian periodicity. GDH in soybeans had a 24‐h rhythm under LD conditions which lengthened under LL conditions. The 24‐h rhythm of GDH displayed maximal activity toward the end of the dark period of the LD cycle whereas the highest activity of NR was early in the light period. Total soluble protein displayed a rhythm with a best fitting τ of greater than 24 h under both LD and LL. GDH, GS, NR, NO3, and soluble protein in soybeans and NiR in Zea, all displayed that were ultradian (10–18 h), indicating that a τ of about one half a circadian periodicity may be a common characteristic of the enzymes of primary nitrogen metabolism in higher plants. These data also demonstrate that although both NR and GDH are circadian in their activity, the 24‐h rhythm may be greatly influenced by ultradian oscillations in activity.

Maize (Zea mays L.) has a noted propensity for accumulating large amounts of nitrate‐N (NO3‐N). However, the availability of such NO3‐N for assimilation during periods of restricted N supply has not been assessed. In addition, the availability of sulfate‐S (SO4‐S) affects the accumulation and assimilation of NO3‐N. Little is known about the effect of NO3‐N supply on SO4‐S accumulation. Such information is vital to efforts to improve the efficiency of nutrient utilization by crops. The objective of this greenhouse experiment was to determine the effect of N deprivation during grain‐filling on the accumulation of dry matter, NO3‐N, and SO4‐S. Maize plants were grown hydroponically in a modified Hoagland's solution. At silking the nutrient solution was replaced with fresh solution, with or without NO3‐N. Plants were harvested and separated into seven fractions at silking and 1, 2, 3, 5, and 7 weeks after silking. Plants that were supplied NO3‐N during grain‐filling contained almost twice (1.8×) as much N at physiological maturity (7 weeks after silking) as did N‐deprived plants. Ear dry weight was not affected by N deprivation. The concentration of NO3‐N in all fractions declined during grain‐filling; however, N deprivation enhanced this decline in the roots, lower stem, and upper stem. Lack of N did not affect NO3‐N concentration in the lower leaves, upper leaves, or ear. The concentration of SO4‐S in the roots nearly doubled between silking and physiological maturity when plants were deprived of NO3‐N, but did not change when N was supplied. The concentration of SO4‐S in all other parts was not markedly affected by N deprivation. Apparently, maize plants can compensate for a restricted N supply during grain‐filling by utilizing NO3‐N stored in the roots and stem.