B I O L O G I A P L A N T A R U M (PRAHA) 23 (1) : 58--67, 1981

Photosynthetic Capacity, Irradiance and Sequential Senescence of Sugar Beet Leaves

DANUBE H o D ~ . ~ o v ~ , I n s t i t u t e of E x p e r i m e n t a l Botany, Czechoslovak Academy of Sciences, Praha*

Abstract. I n field-grown sugar boot plants (Beta vulgaris L. ev. Dobroviek~ A), each of 66 successive leaves produced in the course of the vegetation period was different with respect~ to its photosynthetic capacity (Pc), lifo span, duration of leaf area expansion, a n d longevity a f t e r its m a x i m u m leaf area (Amaz) has developed. The proportionality between the seasonal changes in these characteristics was n o t the same if the sequential senescence of leaves was t a k e n into account. W i t h aging of individual leaves, Pc increased with the leaf area expansion h a v i n g a t t a i n e d t h e peak value between 75% to 100% of Am~x. The rate of ontogenetic changes in Pc of each leaf was specified b y the rate of its growth a n d development so t h a t even a t comparable ages the successive leaves constituted a series of different physiological units. The seasonal changes in q u a n t u m irradiance (PAR) were found to be responsible for differences in the g r o w t h characteristics between the successive leaves: Leaf expansion period w a s related with daily integrals of the incoming P A R (I0), while leaf longevity, after the Ama, h a d b e e n a t t a i n e d , was closely linked with P A R intercepted b y the canopy (I). Pc expressed per t h e total leaf area of the p l a n t was significantly correlated with 1, while Pc calculated per unit leaf area of the p l a n t was related to I0. Leaf potential to a d a p t Pc correspondingly to changes in P A R was greatest during leaf blade expansion; after the leaf h a d ceased to expand, changes in Pc were independent of differences in leaf irradiance. The results stress, a t least for field conditions, the inadmissibility of the extrapolation of a t t r i b u t e s from one leaf to the other ones sequentially seneseing on the plant.

Changes in photosynthetic activity of leaves may take place in response to the time-course of internal events such as leaf senescence. Leaf ontogeny affects the contribution of individual leaves to the total photosynthesis of a plant and foliage canopy (TREHARNE et al. 1968, WOLV.DGE and L~AFE 1976, ACOCK et al. 1978, HOD~OV~ 1979) and may result in dry matter production and yield by single crops (PATTERSON and Moss 1979, DE Vos 1979). Leaf senescence patterns are also found to be useful for the photosynthetic selection of new plant varieties (VIETOR et al. 1977, WILl, ELM and NELSON 1978). Detailed information on the extent to which leaf age may limit the photosynthetic capacity of individual leaves is, therefore, necessary wherever the intra- and/or interspecific differences in the rate of net photosynthesis have to be compared. Receive~ April 25, 1980; accepted Juns 18, 1980 * .4ddrees: Flemingovo n ~ n . 2, 160 00 P r a h a 6, Czeehoslovakia.

58

PHOTOSYNTHESIS, IRRADIANCE AND LEAF SENESCENCE

59

This study was intended to determine (1) the photosynthetic capacity and its changes with ontogeny of each individual leaf on a plant, (2) the potential differences in the photosynthetic activity of leaves sequentially produced by a plant during the whole growing period, (3) how these differences are related to the leaf growth characteristics, and (4) to what extent the lesf photosynthetic capacity is affected by changes in leaf irradiance which the leaves in the canopy have experienced during ontogeny and the vegetation season. MATERIAL AND METHODS Plant Material

Sugar beet plants (Beta vulgaris L. cv. Dobrovick~ A) used in experiment were grown in the field under normal agrotechnique (25 cm distance between individuals and 43 cm between N - S oriented rows) and regularly watered during the whole vegetation period. Leaf Age and Growth Characteristics

Measurements were performed on single leaves numbered according to their chronological age (beginning from the first true leaf). The appearance of leaves was recorded every 2 d. The leaf was estimated as having appeared when its blade 10 mm long was visible. The life span of leaves (expressed by a number of d of leaf positive C02 balance) was determined by taking into account (1) -- the duration of expansion period until the leaf has attained its maximum area (Amax), and (2) -- the time-interval from the attainment of Amax till the leaf senescence (i.e. till the leaf net photosynthetic rate has decreased to zero). In this way, the number of the active leaves (expanding and/or fully developed) and the number of those dying off at individual stages of plant development could be recorded. Leaf Area (A)

was measured by a gravimetric method using paper outline traces of individual leaf laminae. The leaf area index (1~) was calculated by summing up the leaf areas of individual leaves and plants per 1 m e of the ground (9 plants per 1 me). In each set of measurements, the leaf growth characteristics were determined on 27 plants, which number was sufficient to obtain results at 95 per cent confidence level. Photosynthetic Capacity of Leaves (Pc)

(estimated as the rate of net photosynthesis under constant near-optimum conditions) was determined from dry mass increments in discs (0.8 cm diameter) cut out of leaf blades. The discs were placed in an annular chamber (~ETLiKand ~EST~K 1971) and exposed for 6 h to the quantum irradiance 1600 ~mol m -2 s -1 (photosynthetically active radiation, PAR, 400--700nm), continuous water saturation ~f leaf tissue, CO2 concentration 600 mg m -8, air flow rate 55• 10-31 s-l, and the temperature 25 ~ Prior to each measurement the leaves were cut off from the plants and placed with petiols immersed in water for 12 h into a dark box with a constant temperature o f 25 ~ and 95% relative humidity. Each set of measurements included 12

60

D. HOD/~OVA

discs in 6 replications for each leaf. Each sample contained discs cut out uniformly from the basal, central and apical parts of the leaf lamina. Pc per unit area of each successive leaf was determined separately and the photosynthetic capacity of the entire plant (~.e. per the total area of all constituent leaves, Pep, and per unit leaf area oi the plant, Pen) was calculated at successive stages of plant development. Fig. 2

Fig. 1

to~

dead

i:

i JUNE[JULY

IAU~USTI s ~

IC~T.

ORDEROF LEAF SEC~UISNCE

Fig. 1. The t o t a l n u m b e r of leaves produced and dead on the sugar beet plant and the n u m b e r of leaves actually present -- expanding a n d fully developed -- a t different times during the vegetation period. Fig. 2, The leaf appearance rate, d u r a t i o n of leaf area expansion a n d t h a t of the m a i n t e n a n c e of the m a x i m u m leaf area and t h e lifo span of the successive sugar beet leaves. Daily Totals of PAR [

incident (I0) and intercepted by the canopy [I) were measured on clear days during the whole vegetation period by means of selective integrators (KvBi~ and HL.~DEK1963) with photocells exposed horizontally 2 m above the canopy and close above the ground between and within the rows. RESULTS

AND DISCUSSION

The total number of leaves produced by a plant and the number of leaves actually present (expanding and fully developed) and of those dead at different times of the vegetation period are given in Fig. 1. During 172 d of the growing period analysed, the sugar beet plants produced on average 66 leaves. At the beginning of plant development the new leaves have appeared more rapidly than the old leaves died. Through the second half of the growing period after the foliage canopy has been fully completed (cf. Fig. 5), a nearly constant number of leaves (26) on the plant (with a constant ratio 0.54 between the expanding and fully developed ]eaves) was maintained. The successive leaves have not appeared at regular intervals and differed in their life span which ranged from 17 to 67 d (Fig. 2). Differences were also

P H O T O S Y N T H E S I S t IRRADIAlCCE AND L E A F SENESCENCE

61

found in the rate of leaf area development, which followed a saturation curve in each leaf. Up to leaf 15, the Amax was attained in 12 to 32 d and then the duration of this period has progressively decreased to 16 d in leaves which unfolded and matured at the end of the growing period. After having ~eached Amax, the leaves remained functional for about 6 to 39 d (with the

N

r.

20

Fig. 3. PhotosynthetiC capacity (Pc) of the successive sugar beet leaves (the order of leaf sequence is indicated b y the figures a t individual curves) as related to their life span, leaf area expansion (blaok area) a n d duration of the period after the leaf a t t a i n e d the m a x i m u m area (dotted area).

peak values in leaves 25 to 30) before they entered a period of senescence. The different lengths of the periods before and after the successive leaves had completed their expansion resulted in a faster or slower ontogenetic increase and decline of leaf Pc (cf. Fig. 3). Thus, in the leaves constituting a juvenile group (leaf 1 to 15), in which the period following the completion of Amax was shorter than that of leaf area expansion, the slower rate of leaf area development was "compensated" by an earlier attainment of leaf photosynthetic maturity characterized by the ontogenetic peak of Pc. Considerable diversity that the successive leaves show in their appearance rate, life span and the times taken to complete and maintain the final leaf area -- evidently due to both the cultivar used and the varying effects of environmental factors which the plants were subjected to - somewhat shifts the limits of variability in leaf growth characteristics established for sugar beet by FICK et al. (1975). In Fig. 3 the changes in Pc as related to the life span and leaf area development of the successive sugar beet leaves are given. -Pc of each leaf exhibited

62

D. HODA~OVA

a typical ontogenetic pattern having an initial increase with the leaf a r e a expansion up to the photosynthetic maturity of the leaf at which Pc w a s maximal, and then a decline with further leaf senescing. In leaf 5 to leaf 20, the peak values of Pc were recorded when the leaves have attained 75% to 86 % of Araax. In the other leaves the attainment of peak Pc coincided with, that of Amax.

i+

i

~

27

67

2O

ilql

S

10

15

'

20

l l l l l l l l | l f r i , t l , t ~ i l t r l + 1 +

25

)0

3S

40

4';

50

"

ORDER OF LEAF SEQUENCE

Fig. 4. Profiles of leaf photosynthetic capacity (Pc) for the sugar beet plant of different age.

With the increasing age of the plant, Pc of the newly produced leaves differed from that of the older ones: Thus, peak Pe rose steeply from leaf 5, reached the highest value 86 ng (dry m.) cm -2 s -1 in leaf 20 and then slowly declined with the advancing vegetation period approximately to one third of the value attained by leaf 20. The maximum Pc obtained in leaf 20 would be represented with 130 ng CO2 em -2 s -1, if Pc determined here from the dry mass increment was converted to Pc characterizing the net CO2 uptake. This value corresponds well to the highest values of the light-saturated net photosynthesis obtained from the gasometric measurements under the natural CO2 concentration, and reported for sugar beet e.g. by T~_~RXr and UL~IC~ (1973) and HoDX~ovX (1979). The ontogenetic inequality of the successive sugar beet leaves was the reason why the proportions between Pc of one leaf and those of its neighbours actually present on the plant were not constant at subsequent stages of its development (Fig. 4). Figs. 1, 2 and 3 demonstrate that the rhythms underlying the changes in Pc and growth characteristics during leaf ontogeny and with sequential senescence of successive sugar beet leaves were not quite the same. Many contradictory data exist concerning the extent to which each rhythm has a regular or irregular character. Constant as well as varying seasonal trends e.g. in the leaf appearance rate (RoBsoz, 1967, 1973, Ds,~mTT et al. 1979), the duration of leaf expansion (LITTLETO~ et al. 1979), the length of leaf developmental time (HAcKETT and RAWSON 1974, DEI~I~ETTet al. 1979), the ontogenetic synchronisation of the leaf peak Pc with the Amax (WADA 1968, RAWSO~ and HACKI~TT 1974) have been evidenced in both field and controlled conditions. Even if the relative importance of single internal and external factors which govern these processes is not yet clear, leaf irradianee

63

P H O T O S Y N T H E S I S , I R R A D I A N C E AND L E A F SENESCENCE

TaJsI~ 1 The correlation coefficients between the period of leaf area expansion (E) and that of the maintenanee of the m a x i m u m leaf area (M) in the successive sugar beet leaves, the seasonal changes in the photosynthetic capacity per u n i t leaf area (Peu) and the total leaf area (Pop) of the p l a n t and the daily integrals of the incoming (I0) a n d intercepted (I) P A R , respectively I0

I [ • 108 J m - a d -x]

E

0.7958++

-- 0.0788

[d] M

0.3817

0.6036+

0.8593 ++

0.3677

0.3087

0.9180 ++

[d] Pcu [ng (dry m.} cm-2 s-ll Pep [ng (dry m.) em -2 s -1] Level of significance: +0.95; ++0.99.

is one of the most meaningful. In controlled conditions, leaf growth and senescence patterns are explicitly related to PAR which the plants are exposed to (TAYLOR et al. 1968, MILFORD and LENTEN 1976, BUNCE et al. 1977, CLOVGH et al. 1979, CHABOTet al. 1979, JURIK et al. 1979). The reason why such relationship has not been always confirmed in plants grown in the field (DE~N~TT et al. 1979, LITTLETONet al. 1979) lies not only in a great variability of PAR in natural conditions but also,in the analytical approach applied: Generally, the single leaf growth and developmental characteristics have been linked with the incident radiation only. In fact, radiation is effective not only through maeroclimate but also through canopy microclimate and so it may be assumed that differences between the successive leaves are brought about by the growth of the entire canopy: The leaf appearance rate, the growth rates of leaves, their ultimate size, the photosynthetic activity, the senescence pattern, etc. may be modified in response to changes in leaf irradiance within the canopy, which are induced by the increase in plant size, changes in leaf area density, inclination and height of insertion of leaves in the canopy, etc. In this experiment, the time duration of lamina expansion in the sugar beet leaves and the duration of their activity after the completion of expansion till senescence were estimated with respect to the seasonal changes in PAR, both incident and intercepted by the canopy (Table 1). Leaf expansion period was found to be related with I0 while leaf longevity after the leaf attained its A m~x was closely linked with I. These relations correspond well with the ontogenetic changes of leaf insertion and irradiance in the canopy. When expanding, the leaf is situated in the central part of the leaf rosette and being practically unshaded it is fully exposed to PAR incident upon the canopy. When the leaf grows older, it descends to the bottom of the canopy and its life-processes are realized under the lowered PAR which the upper foliage strata allow to pass through. The correlation coefficients found would probably be higher if some "inaccuracies" accompanying the calculation were eliminated: Thus, the measurements of PAR were only taken on clear days, only the average daily integrals of PAR for each particular period were considered and the recorded I represents a

64

D. HODJd~OVA

s u m m a r y value irrespective of leaf insertion and changes in P A R with the d e p t h of the canopy. As to the sequential aging of the successive sugar beet leaves, it is shown in Fig. 1 t h a t the number of the expanding and fully developed leaves on the plant was nearly constant during the second half of the vegetation

Io

.

I I

$.~

@.

6@

go

s

@

0

J!

~'ig. 5. The life spans of the successive sugar beet leaves, the photosynthetic capacity expressed per unit leaf area (Pea) and per total leaf area (Pep) of the plant, the leaf area index (L), and the daily integrals of the incoming (I0) and intercepted (1) P A R related to the time of the vegetation period.

period. This documents a strict sequential synchronisation of the initiation, appearance, growth and developmental processes of each leaf and of its successors during this time. Because only small changes in the P A R extinction coefficient were found during this period (HOD~OV~, 1972), it may be assumed t h a t this synchronisation was under the control of relatively stabilized light microclimate when about 90 ~ to 95 ~h of the incoming P A R was intercepted by the canopy (of. Fig. 5). As is shown in Table I, the photosynthetic capacity calculated per total leaf area of the plant (Pep) was found to correlate significantly with the seasonal course of I while t h a t expressed per unit leaf area of the plant (Peu) was related with I0. These relationships are illustrated more clearly in Fig. 5 in which the seasonal changes in Pep, Pen, I0 and I are related to the relevant changes in L and the successive leaves actually present on the plant. Thus, under the seasonal maximum of I0 and I in July -- when the canopy has attained the ceiling L ~ 4.6 (i.e. 0.5 m 2 leaf area per one plant) -- the leaves of sugar beet plants showed the highest Pe (cf. Fig. 3 and Fig. 4) and the largest A. Owing to those values the plants have reached the seasonal maximum of Pcu and Pep. Because the photosynthetic potential of an entire plant is a function of complex relations between the Pc and the size of constituent leaves, its seasonal changes reflect the specifity of changes in each characteristic. It follows from the figure that Peu ranged between 61% (at the beginning of the vegetation period) and 36~ (at its end) of the seasonal maximum, while at the same time, the total leaf area of the plant was reduced up to 29 ~ and 6 5 ~ , respectively. This provides evidence

P H O T O S Y N T H E S I S , IRRADIAhYCE AND L E A F S E N E S C E N C E

65

that leaf growth characteristics were more subjected to change during the first half of the vegetation period than Pen. In order to determine the photosynthetic adaptation potential of the aging sugar beet leaf in relation to its light environment in the canopy, we have compared Pc of leaf 30 normally exposed (i.e. the leaf whose position

w Lv 2O

0

Fig. 6. The ontogenetie changes in photosynthetic capacity (Pc), leaf area (A) and dry matter (W) of leaf 30 exposed in normal and vertical positions in the sugar beet canopy. Vertical bars are 95 per cent confidence limits for the mean.

2 2oo ~' ~ 100 I t0

I 20

I I 40 .50 LEAF AGE [ d ] 30

~

-

0

80.

during ontogeny naturally lowered in the canopy and which was thus subjected to continuously decreasing irradiance) and of the leaf fixed in a constant near-vertical position (at 85 ~ by means of a mechanical rest (i.e. the leaf was protected from direct shading by its neighbours) -- Fig. 6. Significant differences in Pc between the two leaves were found only at the beginning of their expansion: Pc of the treated leaf was higher by 43 % than that of the control lear. Later, both leaves responded in a similar way, i.e. the changes in leaf Pc were independent of differences in irradiance. The results confirm the conclusions of JURIK et al. (1979) who found that the leaf potential to adapt Pc to P A R is greatest in blade expansion and decreases as expansion is completed. The tested leaves had a similar patterns of the leaf area development, reached the same size but differed with respect to ontogenetic changes in leaf mass per unit surface area: After having attained Amax, the treated leaf had less dry, matter than the control leaf. The fact that the treated leaf responded to P A R by changing dry mass late in ontogenesis makes it difficult to explain its photosynthetic adaptability at early s~ages of development by changes in leaf thickness as it was proposed by e.g. BuNcE et al. (1977), CLouoH et al. (1979) and JURIK et al. (1979). Tile results presented here stress the importance of partial changes within the sugar beet leaf system which consists at every moment of a group of leaves at diverse stages of ontogeny -- some expanding, some mature, some senescing and dying away. However, eve~a with comparable ages, the .successive leaves constitute a series of different physiological units. This accentuates the necessity to locate each leaf in the time scale of plant development and thus prevent the life history of a particular leaf from being replaced by that of any incidental leaf unfolding formerly or later.

6~

D. HOD~OV~

Many differences in growth processes and Pe which occur at the level of individual sugar beet leaves have the adaptive character and may be explained by the effects not only of the incoming but also intereeptec~ PAR. REFERENCES

Acocx, B., CHARLES-EDWARDS, D. A., FITTER, D. ~., HAND, D. VlT., LUDWIO, L. J., W~mRE~ W~sol~, J., WXTHERS, A. C." The contribution of leaves from different levels within a t o m a t o crop to canopy n e t photosynthesis: An experimental examination of two canopy models. -J. exp. Bot. 29 9 815--827, 1978. BuNc~., J. A., PATTEI~SON, D. T., P~.ET, M. M.: Light acclimation during and after leaf expansion in soybean. -- P l a n t Physiol. 60 : 255--258, 1977. CI~BOT, B. F., Jul~Ix, T. W., CHA~OT, J. F.: Influence of instantaneous a n d integrated light-flux density on leaf a n a t o m y a n d photosynthesis. -- Amer. J. Bot. 66 : 940--945, 1979. (~LOUGH, J. M., ALBERTE, R. S., TERRI, J. A. : Photosynthetic adaptation of Solanu~n dulcamara L. to sun a n d shade environments II. Physiological characterization of phenotypic response to environment. -- Plant. Physiol. 64 : 25--30, 1979. DENNE~r, M. D., ELSTON, J., M~I~OI~D, J. R.: The effect of t e m p e r a t u r e on the growth of individual leaves of ViciaJaba L. in the field. -- Ann. Bot. 43 : 197--208, 1979. FIcK, G. W., LooMIs, R. S., W~LIAMS, W. A.: Sugar beet. -- I n : E v ~ s , L. T. (ed.): Crop Physiology. Some Case Histories. Pp. 259--295, Cambridge Univ. Press, Cambridge 1975. HACK~.~, C., RAWSON, H. M.: A n exploration of the carbon economy of t h e tobacco plant, l I . P a t t e r n s of leaf growth a n d dry m a t t e r partitioning. -- Aust. J. P l a n t Physiol. | : 271--281, 1974. HoDA~ov~, D.: Structure a n d development of sugar beet canopy I. Leaf area-leaf angle relal~ions. -- P h o t o s y n t h e t i c a 6 : 401--409, 1972. H o D . ~ o v i , D.: Sugar beet canopy photosynthesis as limited b y leaf age a n d irradiance. Esti. m a t i o n b y models. -- P h o t o s y n t h e t i c a 13 : 376--385, 1979. JURIK, T. W., CHA~OT, J. F., CHABOT, B. P.: Ontogeny of photosynthetic performance in Fragar/~ virginiana under changing light regimes. -- P l a n t Physiol. 63 : 542--547, 1979. K~rsiN, 8., HL/[DEK, L.: A n integrating recorder for photosynthetically active radiant energy with improved resolution. -- P l a n t Cell Physiol. 4 : 153-- 168, 1963. Lrl~rr-mTON, E. J., DENNETT, M. D., ELSTON, J., MONTEITH, J. L.: The growth a n d development of cowpeas (Vigna unguiculata) u n d e r tropical field conditions. 1. Leaf area. -- J. agr. Sci. 93 : 291--307, 1979. MJJ~ORD, G. F. J~., LENTON, J. R.: Effect of photoperiod on growth of sugar beet. -- Ann. Bot. 40 : 1309--1315, 1976. PATTERSON, T. G., Moss, D. N.: Senescence in field-grown wheat. -- Crop Sci. 19 : 635--640, 1979. RAWSON, H. M., HAOKETT, C.: A n exploration of the carbon economy of the tobacco plant. I I I . Gas exchange of leaves in relation to position on the stem, ontogeny a n d nitrogen content. -Aust. J. P l a n t Physiol, I : 551--560, 1974. ROBSON, M. J. : A comparison of British a n d N o r t h African varieties of tall fescue, (Irestuca arundi. nacea). I. Leaf growth during winter a n d the effects on it of t e m p e r a t u r e a n d day le~ng~h. -- J. appl. Eeoh 4 : 475--484, 1967. RUBSON, M. J.: The growth a n d development of simnlated swards of perennial ryegrass I. Leaf growth a n d dry weight change as related to the ceiling yield of a seedling sward. -- Ann, Bot. 37 : 487--500, 1973. ~ E ~ i x , I., ~ESTXK, Z.: Use of leaf tissue samples in ventilated ehamhers for long t e r m measurem e n t s of photosynthesis. -- I n : ~ . S T ~ , Z., ~ATS~r~2, J., J ~ w s , P. G. (ed.): P l a n t Pho~osynthetic Production. Manual o f Mebhods. Pp. 316--342. Dr. W. J u n k N. ~ . -- Publ~., The T~YLOR,. T. H., COOPER, J. P . , T ~ m ~ E , K. J.: Growth response of orchardgrass (Dac$y//s ~lo~nera$a L.) to different light a n d t e m p e r a t u r e environments. I. Leaf development a n d senescence, -- Crop Sci. 8 : 437--440, 1968. T ~ a v , N., ULRiCh, .%.: Effects of phosphorus deficiency on the photosynthesis a n d recpiv&bion of lea~os of mlgar beet. -- P l a n t Physiol. ~1 : 43--47, 1973. ~ , K. J.,~ Cool,~R, J . P., T/~x~o~, T. H.: Growth response of orehardgrass {~Dar ~/omsraaa L.) to different light a n d t e m p e r a t u r e environments. H . Leaf age a n d photo~~jmthetio activity. -- Crop Sci. 8 : 441--445, 1968.

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VIETO~, D. M., A~rrA~A~'AGA~, R. P., MwsoP.Av~, R..B.: Photosynthetic selection of Zea maye L. I. P l a n t age a n d leaf position effeote a n d a relationship between leaf a n d oanopy rates. --- Crop Sci. 17 : 567--573, 1977. Vos DE, N. M.: Cultivar differences in p l a n t a n d crop photosynthesis. -- I n : Crop Physiology a n d Cereal Breeding. Proc. E u e a r p i a Workshop, Wageningen, The Netherlands, 14--16 N o v e m b e r 1978. Pp. 71--74. Wageningen 1979. WADA, Y.: Changes of photosynthetic a n d respiratory activities a n d of chlorophyll cohtent in growing leaves of some tobacco varieties. -- Bot. Mag. (Tokyo) 81 : 25--32, 1968. WrLH~LM, W. W., NELSOX, C. J. : Leaf growth, leaf aging, a n d photosynthetic rate of tall fescue genotypes. -- Crop Sol. 18 : 769--772, 1978. WOLEDGE, J., LEAFE, E. L.: Single leaf a n d canopy photosynthesis in a ryegrass sward. -- Ann. B o t . 4 0 : 773--783, 1976.