Environmental and Experimental Botany 44 2000 151 – 164
CO
2
enrichment enhances flag leaf senescence in barley due to greater grain nitrogen sink capacity
A. Fangmeier
a,
, B. Chrost
b
, P. Ho¨gy
a
, K. Krupinska
c
a
Institut fu¨r Pflanzeno¨kologie der Justus-Liebig-Uni6ersita¨t, Heinrich-Buff-Ring
26
-
32
IFZ , D-
35392
Gießen, Germany
b
Botanisches Institut der Uni6ersita¨t zu Ko¨ln, Gyrhofstraße
15
, D-
50923
Ko¨ln, Germany
c
Botanisches Institut und Botanischer Garten der Christian-Albrechts-Uni6ersita¨t, Olshausenstraße
40
, D-
24098
Kiel, Germany Received 21 March 2000; received in revised form 4 July 2000; accepted 4 July 2000
Abstract
Senescence is a highly regulated process which is under genetic control. In monocarpic plants, the onset of fruit development is the most important factor initiating the senescence process. During senescence, a large fraction of
plant nutrients is reallocated away from vegetative tissues into generative tissues. Senescence may therefore be regarded as a highly effective salvage mechanism to save nutrients for the offspring. CO
2
enrichment, besides increasing growth and yield of C
3
plants, has often been shown to accelerate leaf senescence. C
3
plants grown under elevated CO
2
experience alterations in their nutrient relations. In particular their tissue nitrogen concentrations are always lower after exposure to elevated CO
2
. We used a monocarpic C
3
crop — spring barley Hordeum 6ulgare cv. Alexis — grown in open-top field chambers to test the effects of CO
2
enrichment on growth and yield, on nitrogen acquisition and redistribution, and on the senescence process in flag leaves, at two applications of nitrogen fertilizer.
CO
2
enrichment 650 vs. 366 mmol mol
− 1
caused an increase both in biomass and in grain yield by 38 average of the two fertilizer applications which was due to increased tillering. Total nitrogen uptake of the crops was not
affected by CO
2
treatment but responded solely to the N supply. Nitrogen concentrations in grains and straw were significantly lower − 33 and − 24 in plants grown at elevated CO
2
. Phenological development was not altered by CO
2
until anthesis. However, progress of flag leaf senescence as assessed by chlorophyll content, protein content and content of large and small subunit of RubisCO and of cytochrome b559 was enhanced under elevated CO
2
concentrations by 4 days. We postulate that CO
2
enhanced flag leaf senescence in barley crops by increasing the nitrogen sink capacity of the grains. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Chlorophyll; Grain-filling; Hordeum 6ulgare; Open-top field chambers; Proteins; Redistribution; Yield www.elsevier.comlocateenvexpbot
1. Introduction
Leaf senescence is a complex and highly coordi- nated developmental process under genetic con-
trol Smart, 1994; Noode´n et al., 1997 which precedes cell death Gan and Amasino, 1997;
Corresponding author. Tel.: + 49-641-9935315; fax: + 49- 641-9935309.
E-mail address
:
andreas.fangmeierbot2.bio.uni-giessen.de A. Fangmeier.
S0098-847200 - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 9 8 - 8 4 7 2 0 0 0 0 0 6 7 - 8
Chrost et al., 1999. Several attempts have been made to distinguish between different stages dur-
ing the leaf senescence process, and somewhat different classifications and numbers of stages have
been proposed. It has been generally assumed that regulatory genes are involved in the initiation of
senescence. Later on, other senescence-associated genes SAG’s are expressed which are encoding
proteins involved in breakdown of macromolecules and, thus, in mobilization of nutrients, such as
RNAses, proteinases, and lipases Gan and Amasino, 1997; Thompson et al., 1998. Another
class of SAG’s expressed at later stages appears to be related to protective or stress response functions
Bleecker, 1998. Up to now, more than 30 SAG’s have been identified, cloned and characterized
Biswal and Biswal, 1999. Recently, it has also been shown that leaf senescence — at least at late
stages — is accompanied by programmed cell death Yen and Yang, 1998.
One of the most obvious events occurring during early senescence at the cellular level is the transfor-
mation of chloroplasts into gerontoplasts Smart, 1994; Noode´n et al., 1997. The thylakoids are
disrupted quite early whereas the plastid envelope remains intact until final stages of senescence
Thompson et al., 1998; Chrost et al., 1999. The disassembly of the photosynthetic apparatus pro-
ceeds in a highly regulated manner. In field-grown barley Hordeum 6ulgare, flag leaf senescence was
first detectable by reduced photosynthetic capacity which was accompanied by decreasing D1 protein
8 days after anthesis. About 6 days later, pigment content, photosystem II efficiency, cy-
tochrome f and the large subunit of ribulose-1,5- bis-phosphate carboxylaseoxygenase RubisCO
started to decline, whereas the small subunit of RubisCO remained high until 22 days after anthesis
Humbeck et al., 1996.
During dissassembly of the chloroplasts, 70 of the total nitrogen contained in the cells is
remobilized and allocated to sink tissues with high nutrient demand Smart, 1994; Gan and Amasino,
1997. In monocarpic plants, developing seeds or fruits are the most important sink for nutrients
allocated from senescing leaves. In experiments with wheat, we found nitrogen concentrations in
flag leaves to decrease by 78 from anthesis to maturity, and phosphorous concentrations to de-
crease by 82. Correspondingly, at maturity the wheat grains contained 85 of total shoot nitrogen
and 94 of total shoot phosphorus, respectively Fangmeier et al., 1997.
In monocarpic species the onset of fruit develop- ment is the most important factor initiating the
senescence process Noode´n et al., 1997. In addi- tion the onset and progression of senescence is
influenced by many external factors such as day- length or insufficient supply of resources light,
water, nutrients Smart, 1994; Kleber-Janke and Krupinska, 1997. The molecular mechanism of
senescence initiation in relation to fruit develop- ment is not yet clear. Nutrient demand from the
developing reproductive tissues might trigger the senescence process in leaves Kelly and Davies,
1988. Other authors, however, postulate the exis- tence of a ‘death hormone’ initiating the senescence
process Noode´n and Leopold, 1978; Wilson, 1997. Whatever the signal transduction pathway
is, senescence at least in monocarpic species may be regarded as a highly effective nutrient salvage
mechanism for plants to transfer the nutrients to their offspring. It is likely that the nutrient salvage
function of senescence is the primary reason for the evolution of such a complicated, multi-factorial
process, and that the end of the life-cycle of the organs concerned is just an inevitable by-product
Bleecker, 1998.
Nutrient relations in plants are affected by the concentration of atmospheric CO
2
which repre- sents the most important plant nutrient in the
biosphere 45 of plant dry matter is carbon. Due to human activities disturbing the global
carbon cycle, atmospheric CO
2
concentrations have been increasing from 280 mmol mol
− 1
in pre-industrial times to more than 360 mmol mol
− 1
today, and a further increase to at least 450 or, more likely, 700 mmol mol
− 1
by the end of the next century appears inevitable Anonymous, 1995.
Atmospheric CO
2
enrichment may be regarded as a global fertilization of the biosphere with the most
abundant plant nutrient. Elevated atmospheric CO
2
concentrations do not only promote growth and biomass of plants most effectively in C
3
species, Poorter et al., 1996 but have also conse- quences for the demand for other nutrients
by vegetation. Under CO
2
enrichment, CN
ratios in tissues of C
3
-plants have been shown to increase considerably Conroy, 1992; Cotrufo
et al., 1998. This is not caused by a simple ‘dilution’ due to higher carbohydrate concentra-
tions, but is rather due to a decreased demand for nitrogen in green tissues. Under CO
2
enrichment, optimization of resources within the photosyn-
thetic apparatus may occur Webber et al., 1994 since ribulose-bisphosphate- RuBP and phos-
phate- P
i
regeneration rather than carboxylation by RubisCO will limit the rate of CO
2
assimila- tion Harley and Sharkey, 1991. Decreased con-
tent of RubisCO Moore et al., 1999 which comprises up to 60 of soluble leaf protein Ja-
cob et al., 1995 will reduce total nitrogen demand of green tissues. Additionally, the depression of
the photorespiratory pathway approximetaly half at doubled CO
2
concentrations, Sharkey, 1988 will also decrease the leaf nitrogen demand be-
cause of smaller contents of enzymes of the glyco- late pathway Webber et al., 1994; Fangmeier and
Ja¨ger, 1998.
In previous studies with cereal crops, we could demonstrate that nitrogen uptake by the crops
was not affected by CO
2
enrichment, but was dependent on nitrogen supply Fangmeier et al.,
1997. At the same time, grain yield was signifi- cantly increased under CO
2
enrichment. We also observed a faster progress of senescence, and an
earlier remobilisation of proteins, in flag leaves of wheat crops under CO
2
enrichment Vermehren et al., 1998. Similar observations have been made
by Sicher and Bunce 1998. We assume that enhanced flag leaf senescence
during grain filling in cereals under CO
2
enrich- ment may be triggered by the different effects of
elevated [CO
2
] on grain production which is thought to be increased on the one hand, and on
the acquisition and storage of nitrogen in vegeta- tive pools used during grain filling which are
thought to be not affected, on the other hand. By this means, CO
2
enrichment might accelerate senescence via increased grain nutrient sink capac-
ity. We also speculate that the effect of elevated CO
2
on flag leaf senescence will not be mitigated by additional nitrogen fertilization as additional
N will lead to increased biomass and yield rather than to higher nitrogen pools that would be avail-
able per unit grain yield. To test these hypotheses, we used open-top field
chambers to expose spring barley H. 6ulgare cv. Alexis crops to CO
2
enrichment at two nitrogen supplies and assessed the effects on growth and
yield, on nitrogen acquisition and redistribution by the crops, and on the progress of senescence in
barley flag leaves.
2. Materials and methods