and predicted the involvement of secondary regeneration cycles such as photorespiration [32,36,37]. Al-
though the first function of the leaf as the major site of C fixation is fairly well understood today, the sec-
ond function of the leaf as a source of reduced C for developing sinks is not well understood at the whole
plant level [4,5,101]. Terms such as source strength and sink demand define concepts affecting C parti-
tioning and allocation in plant tissue [101]. However, a limited number of methodologies provide quan-
titative data regarding fundamental C fluxes and exchanges such as the immediate export rate from source
leaves. The mass (^13 C) and radioactive (^11 C,^14 C) isotopes of C are valuable probes for quantifying as-
similate movements within the plant. The potential exists to use the mass isotopes such as^13 C and^15 N
more extensively; however, to date, user-friendly noninvasive techniques for mature plants have not been
devised. Although the radioisotope^11 C emits particles of sufficient energy to be used to study phloem
transport in a noninvasive manner, its use has been restricted. The more stable form of C,^14 C, has limi-
tations as a noninvasive probe. However, it remains a powerful tool in studying export during photosyn-
thesis. The steady-state labeling methodology outlined in this chapter, which depends on measurements
of immediate export being made when the transport sugar pools are in isotopic equilibrium with the^14 CO 2
being assimilated, provides estimates of mass transfer rates of C through export during photosynthesis.
As pointed out in this chapter, plant productivity in natural photosynthetic variants (C 3 , C 3 -C 4 inter-
mediate, and C 4 types) depends on many factors including the ability of the leaves to export C [4,5,30,40].
During the last 25 years, researchers have attempted to increase productivity of plants through genetic en-
gineering by altering the primary metabolic steps involved in the reduction of CO 2. It has been proposed
that modifications of properties of key photosynthetic enzymes such as Rubisco [25,102,103] or the trans-
fer of C 4 genes into C 3 species will alter leaf photorespiratory and photosynthetic capacity [104]. Furbank
and Taylor [102] noted that there are many challenges in the area of photosynthesis to use the large bulk
of data on the enzymes of the pathway and their regulation. Both fixation and export are functions of the
leaf. Much more data are needed at the whole plant level. Integration of photosynthesis and export pro-
cesses and not merely enzymes of C metabolism are required to understand how specific site mutations
affect diurnal patterns of C partitioning. With the integration of techniques such as the use of metabolic
engineering coupled with traditional biochemical and physiological approaches, we may develop the
means to improve photosynthetic performance, assimilate partitioning, and growth in higher plants.
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