Senescence Patterns of Several Physiological and Biochemical Parameters of Field Grown Flag Leaves of Triticum aestivum L. cv Kolibri

Senescence Patterns of Several Physiological and Biochemical Parameters of Field Grown Flag Leaves of Triticum aestivum L. cv Kolibri

ISABEL FLECK, ALBA FRANS! and DOLORS VIDAL

Departament de Fisiologia Vegetal, Facultat de Biologia, Universitat de Barcelona, Diagonal 637-647, 08028 Barcelona, Spain

Received July 23, 1985 . Accepted October 9, 1985

Summary Several physiological and biochemical parameters of flag leaves of wheat grown in experi

mental fields under Mediterranean climate were studied for two consecutive years from anthesis through senescence. The first events observed were the decline in RuBP-carboxylase activity and the soluble protein content. Approximately 20 days after anthesis a marked decline begun in chlorophyll content and specific activity of RuBP-Case on protein basis. At the same time, phosphorus content declined sharply, the specific leaf weight (SLW) achieved its maximum value and CO2 compensation points at 21 % at 2.5 % oxygen increased rapidly. We suggest that these events indicate the onset of an irreversible state of senescence. The role of phosphorus content of the leaves at the final stages of senescence is pointed out, as also the incidence of the environment on the magnitude of the parameters studied.

Key words: Wheat, senescence, RuBP-Case activity, phosphorus content, chlorophyll content, SL W, CO2 compensation points.

Introduction

The importance of the flag leaves of wheat during the grain filling period and its contribution to the final yield of the crops has been largely assessed (Patterson and Moss, 1979; Herzog, 1980; Stoddart and Thomas, 1982). This effect depends mainly on the longevity of the leaves (Spiertz, 1977), that is, the interval in which the leaves are physiologically active. As a consequence, the factors which could delay the initiation of senescence in the leaves may have a strong relation with productivity increase. The knowledge of the sequences and interrelationships of events accompanying senescence of the leaves may, therefore help to control it. The aim of this work was to study the pattern of several parameters of flag leaves during senescence of wheat grown in the field. We studied the variations in Ribulose-bisphosphate carboxylase (RuBP-Case) activity, soluble protein, chlorophyll, and total phosphorus content, specific leaf weight (SL W) and CO2 compensation concentration. RuBP-Case activity, chlorophyll content, and CO2 compensation concentration can be indicators of the photosynthetic capacity of the leaves, whereas phosphorus content and SL W can inform about their nutritional state. Much attention has been paid to the variations in

J. Plant Physiol. Vol. 123. pp. 327-338 (1986)

328 ISABEL FLECK, ALBA FRANSI and DOLORS VIDAL

activity and quantity of the carboxylating enzyme RuBP-Case during the development of flag leaves of wheat, both in a controlled environment (Thomas et al., 1978; Evans, 1983; Camp et al., 1982) and in the field (Wittenbach, 1978, 1979; Di Marco et al., 1979) due to its relation with photosynthetic assimilation. Likewise, the enzyme variations with plant aging have been related to other parameters such as chlorophyll content (Patterson and Moss, 1979; Evans, 1983) and proteolytic activity (Feller and Frismann, 1978). In this work, we studied the variations in phosphorus content due to its relation in inorganic form (Pi), with the export of assimilates (Heldt et al., 1977), photosynthesis (Usuda et al., 1984) and enzymes of the Benson-Calvin cycle (Machler et aI., 1984). We are aware of the complexity of interrelating the physiological processes occurring during the reproductive phase of the wheat plant. However, the trustworthyness of the tendencies observed was ratified by the fact that they were the same in two different years, and in spite of the obvious influences of the climate in each season, a pattern could be outlined in relation with the stage of development of the plant.

Materials and Methods

Plant material: Plants of spring wheat (Triticum aestivum cv. Kolibri) were cultivated in the experimental fields of the Faculty of Biology of Barcelona (Spain) during 1982 and 1983. We had 60 plots of 1.50 m x 1.50 m at our disposal. In the pre-anthesis period, wheat plants at the same phenological stage were marked to insure plant uniformity throughout the study. (Anthesis is considered when the anthers first emerge from the inflorescence of the culms). Sampling was set up in a completely randomized design, taking the plant material from the center of the plots to avoid effect of the edges. Samples of flag leaves were collected at 12 a.m. two or three times weekly beginning one week prior to anthesis and throughout senescence.

RuBP-Case activity and protein content: RuBP-Case activity was assayed using samples of one gram of fresh tissue taken from the material obtained by cutting leaves into small pieces. Four replicates of five leaves each were assayed per day and the leaves were always collected at 12 a.m. The extraction was performed as described by Di Marco et al., 1979 and the enzymatic activity of the extract was measured by spectrophotometric end-point titration of formed DPGA in a 60 sec assay at 25 °C (Di Marco and Tricoli, 1983). Protein content of the extract was determined by the method of Lowry et al., 1951.

Chlorophyll content: Chlorophyll content was measured spectrophotometrically using the method of Arnon, 1949.

Phosphorus content: Phosphorus analysis was performed by the vanado-molybdat procedure after an acid digestion. Two replicates of ten leaves each were assayed per day.

CO2 compensation points, (r): The CO2 compensation concentration of the leaves was at 21 % and 2.5 % oxygen using a closed gas exchange system connected to an infrared gas analyser (Fleck and Di Marco, 1984).

Climatological data: Net irradiation corresponding to the period of growth of the flag leaves in 1982 and 1983 was measured using a Kipp and Zonen integrating solarimeter. Air temperature in the same periods was recorded with resistance thermometers (Leeds and Northrup).

Results

RuBP-Case activity showed a similar pattern in both seasons (Fig. 1 A, B) with higher activity in 1982. The maximum activity was observed both years at the stage


'c

30 e ~~ d-e <5

20 E !-~ 0

U a. CD ,

10 a:

I i i i

'c i:;-30 E 1 ~

~~ 100-1; <j, !~

Ef 1 E i-

~ i ,;

20 8oi2

3 i~ I~

~ 1:15 i.g

ci 60 jUl CD i-a , ,;si

10 a: I i

40-i I

t

1

o 10 DAYS AFTER ANTHESIS

1

I

r

MAY 1983

o DAYS AFTER ANTHESIS

Senescence in field grown wheat 329

A

I

I 1"1 ~!

B -'t80

~I ~I 51 :;! f-60 u

1 ~!

f-4O I

! "J i ~~~.-... I

.......... ""1 ~20

I 30

Fig. 1: RuBP-carboxylase activity (--), chlorophyll content (- - - - -) and total soluble protein (. -' _.) in flag leaves of wheat senescing in the field. A) 1982 season, B) 1983 season. Each value is the mean of four replicates of five leaves each. Vertical bars represent standard errors.

of anthesis, declining thereafter throughout the grain filling period. This decline was faster in 1983 (47% decline two weeks after anthesis in 1982 in comparison with 65 % in 1983). Observing the climatological data of temperature and irradiance of both seasons (Fig. 5: A, B) a relation with the enzyme activity was found. Daily increases in irradiation together with high temperatures accounted for peaks of RuBPCase activity, modifying at some points the decline pattern initiated at anthesis. Nevertheless, the environmental control of RuBP-Case activity seems not to be determinant in the period of anthesis, or immediately after it.

Soluble protein content was determined only in 1983 (Fig. 1 B). The highest



ci. '" E

MAY 1982

o 10 DAYS AFTER ANTHESIS

~ 1 .,;

'" a.

E

!-·····-f-····· __ ···i···/·········-I:····_··

DAYS AFTER ANTHESIS

Fig. 2: Total phosphorus content (--) and Specific Leaf Weight ( ..... ) of flag wheat leaves senescing in the field. A) 1982 season, B) 1983 season. Each value is the mean of two replicates of ten leaves each for phosphorus analysis. The Specific Leaf Weight values are the mean of ten replicates. Vertical bars represent standard errors.

content occurred at anthesis, declining thereafter. In the latest stages of senescence, the decline of RuBP-Case activity was faster than the loss of protein (83 % and 54 % respectively 20 days after anthesis). Soluble protein content did not change until the end of the experiment, whereas the RuBP-Case activity dropped to a barely detectable level.

Changes in chlorophyll (a+b) content are shown in Fig. 1: A, B. Due to the variations of water content in the leaf tissue, it seemed more appropiate to express chlorophyll on dry mass or leaf area basis. During the first weeks of the grain filling period,


90

60

70

60

~

50 ~

40 C

30

20

10

90

80

60 :i.

50 L

40 o

30

20

10

MAY


--21% 0,

------ 2.5 % 0,

1

o

DAYS AFTER ANTHESIS

-- 21 '/, 0,

-------- 2.5% 0,

1 o

o

• .. ---_ ... --...

DAYS AFTER ANTHESIS

o o

30

o

o

30

I I

I

B

A

o o

Fig. 3: CO2 Compensation Concentration at 21 % O 2 (--) and 2.5 % O2 (- - - - -) of detached flag leaves of wheat during senescence. A) 1982 season, B) 1983 season. The curves fit the equations: y = 0.12 x2 -28 x + 1733; r = 0.94; P<0.05 in 1982 and y = 0.05 x2

- 11.18 x + 729; r = 0.92; P<0.05 in 1983.

the total chlorophyll content per unit area increased, reaching a maximum about 20 days after anthesis in both years. After that time, a rapid loss of chlorophyll occurred, being faster in 1982. The maximum chlorophyll content per unit area was higher in 1982 (77 Jtg . cm - 2) than in 1983 (60 Jtg . cm - 2). During 1982, the total chlorophyll content per unit area of the flag leaf was compared with the second leaf (leaf immediately below the flag leaf). Also this leaf showed, during the post-anthesis period, an increase of chlorophyll content although lower in respect to the flag leaves. The rapid decline in chlorophyll initiated a few days before that observed in the flag leaf (Data

J Plant Physiol. Vol. 123. pp. 327-338 (1986)

332 ISABEL FLECK, AlBA FRANS! and DOLORS VIDAL

50

40

30

20

10

0

--1982 .. a -- .. --.. - 1983 o

c5' u

~ 1 o U

o o o

o

o DAYS AFTER

b~ --1982

is' 3 u

1·

•••••••••• 1983

......... ! ............ 0 0 _ ... _____ ! ______ ~ ___________________ ---u_ .... -

o o o o

o 0

o

•

30

B

O~---~---~----_r----~-----J)

AFTHER ANTHESIS

A

Fig. 4: A) 02-independent component (r 0) of the CO2 compensation concentration in flag wheat leaves in 1982 (--) and 1983 (- - - - -). The curves fit the equations: y = 0.012' eG.OS7X; r = 0.90; P<0.05 in 1982 and y = 0.001' eG.066x; r = 0.94; P <0.05 in 1983. B) Ordependent component (-y) of the CO2 compensation concentration in flag wheat leaves in 1982 (--) and 1983 (- - - - -). The curves fit the equations: y = 0.001 r-0.94 x + 60.18; r = 0.72; P<0.05 in 1982 and y = 0.001 x2-0.22 x + 15.83; r = 0.51; P<0.05 in 1983.

not shown}. Interestingly, in both seasons, the maximum values of chlorophyll content were obtained during the periods in which temperatures were the highest, with mean daily temperatures over 25 0c. Once chlorophyll decline began, the influence of the environmental factors were no longer noticeable.

Total phosphorus content. In 1982, phosphorus content on dry weight basis increased after anthesis and reached the highest values (4.2 mg' g.d.w. -I) about fifteen

J Plant Physiol. Vol. 123. pp. 327-338 (1986)

10

30 i , E -; ~

§ 20 0

~ a:

10

10 DAYS AFTER ANTHE~S

1

'0 10 DAYS AFTER ANTHESIS


A

-_1982

--------- 1983

30

B

/

--1982

---------- 1983

20 30

Fig. 5: A) Maximum and Minimum daily temperatures during the development of flag wheat leaves in 1982 (--) and 1983 (- - - - -). - B) Irradiance during the development of flag wheat leaves in 1982 (--) and 1983 (- - - - -).

days later. Thirty days after anthesis, the total phosphorus content declined to a value ot 2.2mg·g_d.w.- 1 (Fig. 2 A). During the spring of 1983, phosphorus content increased also from anthesis, but reached the maximum (3.5 mg -g.d.w. -I) in advance in comparison with the previous year. Thirty days after anthesis the P-content in 1983 was 1.3 mg' g.d.w. -\ (Fig. 2 B). The losses in P-content from the maximum until 30 days after anthesis were 0.32 mg P.ieaf- 1 in 1982 and 0.30 mg P.leaf-l in 1983, corresponding to 59 % and 68 % of the highest value.

J. Plant P/rysiol. Vol. 123. pp. 327-338 (1986)

334 ISABEL FLECK, ALBA FRANS! and DOLORS VIDAL

Specific Leaf Weight (SLW). SLW on dry weight basis increased slowly ten days after anthesis coinciding with the grain filling and reached a maximum of 1.0g.d.w. ·dm-2 in 1982 and O.7g.d.w. ·dm- 2 in 1983 (Fig. 2A, B). Mean flag leaves area in 1982 was 16.7±0.5cm-2

, whereas in 1983 was 20.8±0.8cm- 2• A certain influence of the environmental parameters (temperature and irradiance) could be noticed on the variations of the SL W pattern. In fact, specially in 1982, high temperature and irradiance coincided with increases of the Specific Leaf Weight.

CO2 Compensation Concentration (r). CO2 compensation concentration at 21 % O 2 showed, in both seasons, high values during pre-anthesis and maintenance of the values around 40 - 45 1'1 . 1- I during the grain filling period until the third week after anthesis. Afterwards the values rose rapidly (Fig. 3: A, B). Applying the equation of Forester et al., 1966, the oxygen independent (ro) and dependent ('Y) components of the CO2 compensation concentration could be calculated, observing an increase of both parameters with senescence (Fig. 4: A, B).

Table 1: Climatological data during the growth period of flag leaves of Triticum aestivum L. cv Kolibri in the spring of 1982 and 1983.

1982 1983

May June May June Net Radiation (MJ· m -2. day-I) 16.33 19.30 16.30 18.25 Mean daily temperature (0C) 18.91 21.93 16.11 20.64 Mean daily maximum temperature (0C) 21.85 25.18 19.58 24.13 Mean daily minimum temperature (0C) 15.96 18.68 12.63 17.14

Environmental parameters. In 1982, the mean maximum temperatures from the pre-anthesis period to 10 days after anthesis were over 20°C, whereas in 1983 they were slightly lower. In the middle of the grain filling period temperatures rose over 25 °C in both years. In 1982 these values were maintained, whereas in 1983 the temperature decreased to around 20°C and remained milder compared to 1982 until the end of the filling period. (Table 1, Fig. 5 A). Solar irradiation during May in 1982 and 1983 (corresponding to the pre-anthesis and the beginning of the anthesis period) were practically the same in both seasons, whereas in June it was higher in 1982 (Table 1, Fig. 5B).

Discussion

From our results, we observe that during the two periods studied, the sequence of events and interrelationships accompanying senescence of field grown flag leaves of wheat were similar. RuBP-Case activity showed an increase during the pre-anthesis period, reached a maximum at anthesis and then declined throughout the grain filling period. A similar decline has already been described in flag leaves of wheat by others (Thomas et al., 1978; Di Marco et aI., 1979). Highest activities during the grain filling



period have also been reported {Wittenbach, 1979}. The difficulty in following correctly the phenological state of the leaves could lead to a certain variation of the patterns. In this work, we tried to avoid this problem by marking the plants as described in materials and methods. RuBP-Case is not only a key enzyme in CO2 fixation but also, due to its quantity amounting to 50 - 60 % of the soluble protein in wheat leaves (Wittenbach, 1979; Makino et al., 1983) and 20% of total leaf nitrogen (Evans and Seemann, 1984), an important storage material. The products of breakdown of this protein can be rapidly translocated to the growing parts, e.g. the grains. The increase of activity that we observed during the pre-anthesis period needed to provide assimilates to the developing ear, is followed by a decrease of activity which should reflect a decrease in the RuBP-Case protein concentration (Evans, 1983). During senescence, RuBP-Case constituted the major proportion of soluble protein loss (85 %) {Friedrich and Huffaker, 1980}. Its loss after anthesis is interpreted as utilization by the spike at the moment in which its sink strength intensifies. In fact, a more or less pronounced decline in photosynthesis depending on the variety and condition of the cultivar and environment follows the maximum photosynthetic rates which occur at anthesis (Patterson and Moss, 1979).

Changes in soluble protein content are compared with those of RuBP-Case activity in Fig. 1 B. We can observe that the specific activity of the enzyme expressed on soluble protein content is not always constant during senescence. During the first week after anthesis the soluble protein content is almost constant, whereas the RuBPCase activity drops rapidly. A period of constant specific activity follows until 20 days after anthesis. Afterwards, the levels of soluble protein remain fairly constant, whereas enzyme activity declines at a faster rate, indicating a decline in specific activity of the enzyme. No changes in specific activity of RuBP-Case on protein basis during senescence have been observed by Wittenbach, 1979 and Makino et aI., 1983, among others. However, the results of Wittenbach in detached leaves of wheat senescing in the dark indicate that when the so-called «irreversible senescence» begins, the values of specific activity decline, probably due to a more rapid loss of active sites than immunochemical recognition sites. The decline in specific activity of the enzyme 20 days after anthesis in our results occurs at the same time at which most parameters show strong variations in their patterns, suggesting that at this moment the irreversible state of senescence is achieved.

The decline in chlorophyll content begins effectively 20 days after anthesis, that is, 20 days after the initial loss in RuBP-Case activity. This is in agreement with the results obtained by Friedrich and Huffaker, 1980, using barley leaves, who suggested that the mechanism responsible for chlorophyll loss was not the same as that for RuBP-Case; chlorophyll may be temporarily protected from degradation because of its association with the thylakoid membrane of the chloroplast. A similar result has been reported by Makino et aI., 1983, in rice leaves. They found that during leaf senescence RuBP-carboxylase activity decreased at a much faster rate than chlorophyll content and that the loss of chlorophyll was not correlated with that of photosyn-



thesis. It is interesting to note that chlorophyll content increased during the first week after anthesis in both years. This is coincident with an increase of irradiance and temperature which occurred during the same periods. This increase in chlorophyll content at the middle of the grain filling period can be a mechanism of the leaf to increase the efficiency of photosynthesis by increasing light absorption as suggested by Terry, 1980 and Lin and Ehleringer, 1982, during a period in which the assimilation rate is declining.

The CO2 compensation concentration observed in both seasons followed the ontogenic changes reported by Ticha and Catsky, 1981. The values increased at both 21 % and 2.5 % oxygen at the time of the initiation of the irreversible state of senescence, around 20 days after anthesis. In 1982 the increase in CO2 compensation concentration (r) was faster, affected probably by the higher temperature and irradiance of this period. r depends not only on the kinetic characteristics of the enzyme RuBP-Case/Oxygenase but on the contribution of the dark respiration, Rd. (Azc6nBieto et al., 1981). Increases in r with age may be explained by increases in the relation RdlVc max (Dark Respiration/maximal RuBP-Case activity) leading to increases in the Orindependent (r 0) and dependent ('Y) part of r, as observed in a previous study on detached barley leaves (Fleck and Di Marco, 1984).

Phosphorus concentration expressed on dry weight (Fig. 2: A, B) and on leaf area basis (data not shown) follows trends similar to those of Specific Leaf Weight until two weeks after anthesis. After that, the remobilization of assimilates and phosphate appeared to be asynchronous since P content declined 20 days after anthesis when the Specific Leaf Weight was still increasing. The maximum Specific Leaf Weight indicated a high level of assimilation and coincided approximately 20 days after anthesis, with the maximum accumulation of carbohydrates in the leaves of wheat (Evans et aI., 1980). This fact may indicate that at this stage phosphorus transport is independent from assimilate translocation, possibly through a way different from phloem, when senescence reaches the internodal parts of the stem, as suggested by Martin, 1982. The inorganic phosphorus, Pi, generally reflects the variations of Pstatus in vegetative organs (Chapin and Bieleski, 1982). It has an important role in assimilate partitioning in isolated chloroplasts (Heldt et al., 1977): low Pi concentration in the medium leads to a decrease in assimilate export and an increase in starch synthesis. Likewise, in phosphorus deficient leaves, starch accumulation has been observed (Sawada et al., 1982; Ariovich and Cresswell, 1983). We therefore suggest that the ontogenic decrease of total P concentration reported here affects the metabolic Pi levels leading to carbohydrate accumulation and then to the simultaneous increase in Specific Leaf Weight. It is known that stromal Pi affects the A TP / AD P ratio (Herold, 1980). This, in turn, can affect the regeneration capacity of the specific substrate of RuBP-Case, RuBP (Robinson and Walker, 1981). However, the decline in total phosphorus content and then in Pi content is much less than the decline in RuBPCase activity observed in this work, and obviously also of its active sites. Since in healthy, not senescent leaves RuBP is considered not limiting (Perchorowicz et al.,



1981) it is unlikely that the decline in phosphorus content in late senescence could limit the regeneration capacity of RuBP and then the carboxylation capacity of the leaves «in vivo».

Acknowledgements

We wish to thank Drs. A. Caballero and G. Di Marco for helpful discussions and M. Berbel for environmental data.

References

ARIOVICH, D. and C. F. CRESSWELL: The effect of nitrogen and phosphorus in starch accumulation and net photosynthesis in two variants of Panicum maximum J acq. Plant Cell. Environ. 6,657-664 (1983).

ARNON, D. 1.: Copper enzymes in isolated chloroplasts. Polyphenol oxidase in Beta vulgaris. Plant Physiol. 24, 1-45 (1949).

AZCON-BIETO, J., G. D. FARQUHAR, and A. CABALLERO: Effects of temperature, oxygen concentration, leaf age and seasonal variations of CO2 compensation points of Lolium perenne L. Planta 152, 497-504 (1981).

CAMP, P. J., S. C. HUBER, J. J. BURRE, and D. E. MORELAND: Biochemical changes that occur during senescence of wheat leaves. 1. Basis of the reduction of photosynthesis. Plant Physiol. 70, 1641-1646 (1982).

CHAPIN, F. S. III and R. L. BIELESKl: Mild phosphorus stress in barley and related low-phosphorus-adapted barleygrass: phosphorus fractions and phosphate absorption in relation to growth. Physiol. Plant. 54, 309-317 (1982).

DI MARCO, G., S. GREGO, and D. TRICOLl: RuBP-carboxylase/oxygenase in field grown wheat. J. Exp. Bot. 30,851-861 (1979).

DI MARCO, G. and D. TRICOLl: RuBP-carboxylase determination by enzymic estimation of D-3-PGA formed. Photosynth. Res. 4, 145-149 (1983).

EVANS, L. T., 1. F. WARDLAW, and R. A. FISCHER: Wheat. Crop Physiology. Evans L. T. ed. Cambridge Univ. Press. 2nd. ed. London: 101-149 (1980).

EVANS, J. R.: Nitrogen and photosynthesis in the flag of wheat (Triticum aestivum L.). Plant Physiol. 72, 297 -302 (1983).

EVANS, J. R. and J. R. SEEMANN: Differences between wheat genotypes in specific activity of ribulose-l,5-bisphosphate carboxylase and the relationship to photosynthesis. Plant Physiol. 74,759-765 (1984).

FELLER, U. and K. ERISMANN: Veranderungen des Gaswechsels und der Aktivitaten proteolytischer Enzyme wahrend der Seneszenz von Weizenblattern (Triticum aestivum L.). Z. Pflanzenphysiol., 90, 235-244 (1978).

FLECK, 1. and G. DI MARCO: Ribulose-1,5-bisphosphate carboxylase activity, net photosynthetic rate and CO2 compensation concentration in detached barley leaves senescing in the dark. Photosynthetica 18, 248-254 (1984).

FORRESTER, M. L., G. KROTKOV, and C. D. NELSON: Effect of oxygen on photosynthesis, photorespiration and respiration in detached leaves. I. Soybean. Plant Physiol. 41, 422-427 (1966).

FRIEDRICH, J. W. and R. C. HUFFAKER: Photosynthesis, leaf resistances and ribulose-1,5-bisphosphate carboxylase degradation in senescing barley leaves. Plant Physiol. 65, 1103 -1107 (1980).

HELDT, H. W., C. J. CHON, D. MARONDE, A. HEROLD, Z. S. STANKOVIC, D. A. WALKER, A. KRAMlNER, M. R. KIRK, and U. HEBER: Role of orthophosphate and other factors in the regulation of starch formation in leaves and isolated chloroplasts. Plant Physiol. 59, 1146 -1155 (1977).



HEROLD, A.: Regulation of photosynthesis by sink activity - the missing link. New Phytol. 86, 131-144 (1980).

HERZOG, H.: Source and sink relations during the grain filling period in six winter wheat cv. Z. Acker- und Pflanzenbau 149, 472-487 (1980).

LIN, Z. F. and J. EHLERINGER: Effects of leaf age on photosynthesis and water use efficiency of Papaya. Photosynthetica 16, 514-519 (1982).

LOWRY, O. H., N. J. ROSEBROUGH, A. LEWIS-FARR, and R. J. RANDALL: Protein measurement with the Folin phenol reagent. J. BioI. Chern. 193, 265-275 (1951).

MAcHLER, F., H. SCHNYDER, and J. NOSBERGER: Influence of inorganic phosphate on photosynthesis of wheat chloroplasts. I. Photosynthesis and assimilate export at 5°C and 25°C. J. Exp. Bot. 35,481-487 (1984).

MAKINO, A., T. MAE, and K. OHIRA: Photosynthesis and ribulose 1,5-bisphosphate carboxylase in rice leaves. Changes in photosynthesis and enzymes involved in carbon assimilation from leaf development through senescence. Plant Physiol. 73, 1002-1007 (1983).

MARTIN, P.: Stem xylem as a possible pathway for mineral retranslocation from senescing leaves to the ear in wheat. Aust. J. Plant Physiol. 9, 197-207 (1982).

PATTERSON, T. G. and D. N. Moss: Senescence in fieldgrown wheat. Crop ScielJ.ce 19, 635-640 (1979).

PERCHOROWICZ, J. T., D. A. RAYNES, and R. G. JENSEN: Light limitation of photosynthesis and activation of ribulose bisphosphate carboxylase in wheat seedlings. Proc. Nat!. Acad. Sci. USA 78, 2985-2989 (1981).

ROBINSON, S. P. and D. A. WALKER: Photosynthetic carbon reduction cycle. In: The Biochemistry of Plants. A Comprehensive Treatise. vol. 8. Photosynthesis. M. D. HATCH, N. K. BOARDMAN (eds.) pp. 193-236. Academic Press, New York, 1981.

SAWADA, S., T. IGARASHI, and S. MrrACHI: Effects of nutritional levels of phosphate on photosynthesis and growth studied with single, rooted leaf of dwarf bean. Plant & Cell Physiol. 23, 27 -33 (1982).

SPIERTZ, J. H. J.: The influence of temperature and light intensity on grain growth in relation to carbohydrate and nitrogen economy of the wheat plant. Neth. J. Agric. Sci. 25, 182-197 (1977).

STODDART, J. L. and H. THOMAS: Leaf Senescence. In: Nucleic Acids and Proteins in Plants. I. Encyclopedia of Plant Physiology. New Series vol. 14 A. pp. 592-626, Springer Verlag, N.Y., 1982.

TERRY, N.: Limiting factors in photosynthesis. I. Use of iron stress to control photochemical capacity «in vivo». Plant Physiol. 65,114-120 (1980).

THOMAS, S. M., N. P. HALL, and M. J. MERRET: Ribulose 1,5-bisphosphate carboxylase-oxygenase activity and photorespiration during the ageing of flag leaves of wheat. J. Exp. Bot. 29, 1161-1168 (1978).

TICHA, I. and J. CATSKY: Photosynthetic characteristics during ontogenesis of leaves. 5. Carbon dioxide compensation concentration. Photosynthetica 15, 401-428 (1981).

USUDA, H., M. S. B. Ku, and G. E. EDWARDS: Activation of NADP-malate dehydrogenase, pyruvate Pi dikinase and fructose 1,6-bisphosphatase in relation to photosynthetic rate in maize. Plant Physiol. 76,238-243 (1984).

WIITENBACH, V. A.: Breakdown of ribulose bisphosphate carboxylase and change in proteolytic activity during dark-induced senescence of wheat seedlings. Plant Physiol. 62, 604-608 (1978).

- Ribulose bisphosphate carboxylase and proteolytic activity in wheat leaves from anthesis through senescence. Plant Physiol. 64, 884-887 (1979).


Documents

Senescence Patterns of Several Physiological and Biochemical Parameters of Field Grown Flag Leaves of Triticum aestivum L. cv Kolibri