130
w. Schulze and E.-D. Schulze
50% of this carbon gain had been assimilated by old and 50% by young
leaves (Rawson and Woodward 1976). In the wild type, the carbon gain is
either transported to the phloem or stored as starch at different rates,
depending on leaf age. Fourteen percent (Penning de Vries 1975) is used for
respiration in a ratio of 1: 6 for old versus young leaves (Thornley 1976).
Old leaves deliver assimilates into the root and support root growth and the
necessary fraction of root respiration, and they supply assimilates to sink
leaves. These assimilates, together with the assimilatory carbon gain of sink
leaves, suport growth. In the case of the wild type, the amount of carbon
supporting growth is 70% of the daily carbon gain.
Caspar et al. (1986) showed that dark respiration in leaves of the
starchless PGM mutant 69.19 nmol CO 2 g-l h- 1 compared with 11.5 nmol
CO 2 g-l h- 1 in the wild type, while root respiration was the same. This does
not greatly decrease the supply of assimilates from old to young leaves.
However, if starch formation is lacking, young leaves are expected to have
much greater respiration rates, which will consume a major fraction of the
carbon gain of the young leaves. In total, the carbon supply of young leaves
is decreased to 57% in PGM mutants, mainly because of the consumption of
carbon by respiration.
We think that the increased respiration which was observed by Caspar et
al. (1975) is due to an activation of the alternative pathway of cyanideresistant respiration, which regulates the level of Triose-P in the cytosol.
This may be important for releasing Pi in the case of the starchless mutant,
which otherwise may be bound in sugar phosphates, and affect metabolism.
We are aware that this is one possible interpretation, because Pi is also
regulated by other mechanisms, such as the activity of SPS. This mechanism, however, may not be effective if the sink leaf is not able to store
carbohydrates as starch.
6.6 Conclusions
Assimilatory starch promotes growth (1) because it allows a carbohydrate
supply over day and night for growth processes, which otherwise may
run out of carbohydrates at night, and (2) "protects" carbohydrates from
respiration, by keeping the cytosolic level of Triose-P low. The data show
that, independent of the daily starch turnover, accumulation of starch occurred at N limitation, and N accumulation occurred at carbohydrate
limitaion by low light. This indicates (3) a limited ability of plants to adjust
their partitioning pattern for resource acquisition and makes "emergency"
reactions, such as increased respiration, necessary to reduce soluble carbohydrate levels in the case of starchless mutants.
w. Schulze and E.-D. Schulze
50% of this carbon gain had been assimilated by old and 50% by young
leaves (Rawson and Woodward 1976). In the wild type, the carbon gain is
either transported to the phloem or stored as starch at different rates,
depending on leaf age. Fourteen percent (Penning de Vries 1975) is used for
respiration in a ratio of 1: 6 for old versus young leaves (Thornley 1976).
Old leaves deliver assimilates into the root and support root growth and the
necessary fraction of root respiration, and they supply assimilates to sink
leaves. These assimilates, together with the assimilatory carbon gain of sink
leaves, suport growth. In the case of the wild type, the amount of carbon
supporting growth is 70% of the daily carbon gain.
Caspar et al. (1986) showed that dark respiration in leaves of the
starchless PGM mutant 69.19 nmol CO 2 g-l h- 1 compared with 11.5 nmol
CO 2 g-l h- 1 in the wild type, while root respiration was the same. This does
not greatly decrease the supply of assimilates from old to young leaves.
However, if starch formation is lacking, young leaves are expected to have
much greater respiration rates, which will consume a major fraction of the
carbon gain of the young leaves. In total, the carbon supply of young leaves
is decreased to 57% in PGM mutants, mainly because of the consumption of
carbon by respiration.
We think that the increased respiration which was observed by Caspar et
al. (1975) is due to an activation of the alternative pathway of cyanideresistant respiration, which regulates the level of Triose-P in the cytosol.
This may be important for releasing Pi in the case of the starchless mutant,
which otherwise may be bound in sugar phosphates, and affect metabolism.
We are aware that this is one possible interpretation, because Pi is also
regulated by other mechanisms, such as the activity of SPS. This mechanism, however, may not be effective if the sink leaf is not able to store
carbohydrates as starch.
6.6 Conclusions
Assimilatory starch promotes growth (1) because it allows a carbohydrate
supply over day and night for growth processes, which otherwise may
run out of carbohydrates at night, and (2) "protects" carbohydrates from
respiration, by keeping the cytosolic level of Triose-P low. The data show
that, independent of the daily starch turnover, accumulation of starch occurred at N limitation, and N accumulation occurred at carbohydrate
limitaion by low light. This indicates (3) a limited ability of plants to adjust
their partitioning pattern for resource acquisition and makes "emergency"
reactions, such as increased respiration, necessary to reduce soluble carbohydrate levels in the case of starchless mutants.