Overview: Perspectives in Ecophysiological Research of Photosynthesis
557
cycle. It appears that this coordination is keyed to the carbohydrate status,
and that intermediates of carbohydrate metabolism such as sucrose and
fructose-1, 6-bisphosphate have special regulatory capacity (Stitt 1993a).
The problem of regulating respiration and sugar metabolism leads immediately to the question of how growth is linked to photosynthesis. The
correlation between both processes is poor (Peirera, Chap. 8). Growth
varies much more than photosynthesis. Only when nitrogen is limiting does
a correlation between growth and photosynthesis emerge (Fichtner et al.,
Chap. 7). Obviously, the partitioning of carbon and nitrogen between root
and shoot has a long-lasting effect on whole-plant carbon relations and on
photosynthesis, which overrides in feedback the actual effect of carbon gain.
Storage of carbohydrates, such as starch, and of nitrate are intermediate
accumulatory processes, which essentially exclude each other. Maximum
growth rate occurs when both these accumulatory processes reach a minimum
(Stitt and Schulze 1993; Fichtner et aI., Chap. 7).
Thus far, ecological models fail to explain partitioning on a mechanistic
basis. Molecular tools involving use of "antisense" mutants of carboxylase,
invertase, etc. are beginning to lift "the lid of this black box" (Stitt 1993a,b).
It appears that sucrose has a regulatory function at the genome level and
determines, along with the fructoses, partitioning between root and shoot
(Stitt 1993b). When nitrate and carbohydrate mutants were investigated at
different nutrition levels, a linear relationship of shoot and root growth
emerges when plotted against free sucrose (W. Schulze et al. 1993).
Monitoring photosynthetic integrity, such as by leaf fluorescence characteristics, provides early indications and assessment of plant stress at extreme
temperatures (Larcher, Chap. 13). Membranes are sensitive sites of temperature injury and photosynthetic function is easily impaired when membranes associated with the photosynthetic apparatus are damaged. Thus,
much can be learned about the progress of temperature acclimation, avoidance or tolerance of temperature extremes and critical threshold temperatures by measuring leaf fluorescence.
26.4 Balancing Photosynthesis and Transpiration
Since the early days of ecophysiology, it has been recognized that leaves
must make a compromise in regulating stomatal aperture to allow carbon
gain without undue water loss. Both fluxes are directly coupled by the
diffusive path through the stomatal pore. Therefore, much effort has gone
into understanding stomatal function (Losch and Schulze, Chap. 9). Direct
observations of stomatal movement (Kappen et aI., Chap. 11) and theoretical considerations (Cowan, Chap. 10) suggest that feedback responses
through plant internal water status are more important than previously
557
cycle. It appears that this coordination is keyed to the carbohydrate status,
and that intermediates of carbohydrate metabolism such as sucrose and
fructose-1, 6-bisphosphate have special regulatory capacity (Stitt 1993a).
The problem of regulating respiration and sugar metabolism leads immediately to the question of how growth is linked to photosynthesis. The
correlation between both processes is poor (Peirera, Chap. 8). Growth
varies much more than photosynthesis. Only when nitrogen is limiting does
a correlation between growth and photosynthesis emerge (Fichtner et al.,
Chap. 7). Obviously, the partitioning of carbon and nitrogen between root
and shoot has a long-lasting effect on whole-plant carbon relations and on
photosynthesis, which overrides in feedback the actual effect of carbon gain.
Storage of carbohydrates, such as starch, and of nitrate are intermediate
accumulatory processes, which essentially exclude each other. Maximum
growth rate occurs when both these accumulatory processes reach a minimum
(Stitt and Schulze 1993; Fichtner et aI., Chap. 7).
Thus far, ecological models fail to explain partitioning on a mechanistic
basis. Molecular tools involving use of "antisense" mutants of carboxylase,
invertase, etc. are beginning to lift "the lid of this black box" (Stitt 1993a,b).
It appears that sucrose has a regulatory function at the genome level and
determines, along with the fructoses, partitioning between root and shoot
(Stitt 1993b). When nitrate and carbohydrate mutants were investigated at
different nutrition levels, a linear relationship of shoot and root growth
emerges when plotted against free sucrose (W. Schulze et al. 1993).
Monitoring photosynthetic integrity, such as by leaf fluorescence characteristics, provides early indications and assessment of plant stress at extreme
temperatures (Larcher, Chap. 13). Membranes are sensitive sites of temperature injury and photosynthetic function is easily impaired when membranes associated with the photosynthetic apparatus are damaged. Thus,
much can be learned about the progress of temperature acclimation, avoidance or tolerance of temperature extremes and critical threshold temperatures by measuring leaf fluorescence.
26.4 Balancing Photosynthesis and Transpiration
Since the early days of ecophysiology, it has been recognized that leaves
must make a compromise in regulating stomatal aperture to allow carbon
gain without undue water loss. Both fluxes are directly coupled by the
diffusive path through the stomatal pore. Therefore, much effort has gone
into understanding stomatal function (Losch and Schulze, Chap. 9). Direct
observations of stomatal movement (Kappen et aI., Chap. 11) and theoretical considerations (Cowan, Chap. 10) suggest that feedback responses
through plant internal water status are more important than previously