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We compared the structural, biochemical, and physiological characteristics involved in photorespiration of intergeneric hybrids differing in genome constitution (DtDtR, DtDtRR, and DtRR) between the C(3)-C(4) intermediate species Diplotaxis tenuifolia (DtDt) and the C(3) species radish (Raphanus sativus; RR). The bundle sheath (BS) cells in D. tenuifolia included many centripetally located chloroplasts and mitochondria, but those of radish had only a few chloroplasts and mitochondria. In the hybrids, the numbers of chloroplasts and mitochondria, the ratio of centripetally located organelles to total organelles, and the mitochondrial size in the BS cells increased with an increase in the constitution ratio of the Dt:R genome. The P-protein of glycine decarboxylase (GDC) was confined to the BS mitochondria in D. tenuifolia, whereas in radish, it accumulated more densely in the mesophyll than in the BS mitochondria. In the hybrids, more intense accumulation of GDC in the BS relative to the mesophyll mitochondria occurred with an increase in the Dt:R ratio. These structural and biochemical features in the hybrids were reflected in the gas exchange characteristics of leaves, such as the CO(2) compensation point. Our data indicate that the leaf structure, the intercellular pattern of GDC expression, and the gas exchange characteristics of C(3)-C(4) intermediate photosynthesis are inherited in the hybrids depending on the constitution ratio of the parent genomes. Our findings also demonstrate that the apparent reduced photorespiration in C(3)-C(4) intermediate plants is mainly due to the structural differentiation of mitochondria and chloroplasts in the BS cells combined with the BS-dominant expression of GDC.

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ABSTRACISoybean (Glycine max [L.] Merr. cv Braxton) plants were grown in sandy soil with only natural rainfall (N) or with supplemental irrigation (I). Water-stressed plants grew more extensive root systems, whereas irrigated plants developed larger shoots and smaller root systems. Maximum stomatal apertures were observed at the beginning of each photoperiod. Partial stomatal closure occurred each afternoon, but stomata of I plants remained open longer than those of N plants. Significant reductions in net carbon fixation rate generally accompanied decreases in stomatal aperture, which coincided with periods of high temperature, low relative humidity, maximum solar radiation, and water stress. Leaf water potential decreased from morning to afternoon, with a greater decrease observed for N plants. Midafternoon stomatal closure did not occur in N plants with very large root systems following a heavy rain which saturated the soil profile. With smaller root systems and greater evaporative demand from larger shoots, the I plants continued to show midafternoon stress following the heavy rain. The large root systems of the N plants absorbed sufficient water to meet shoot evaporative demand for several days following the rain. Root soil system resistance apparently contributed to the aftemoon water stress in the I plants.-11 bars may affect photosynthesis through stomatal closure. Thus, leaf water status, particularly turgor, can affect photosynthetic carbon fixation rate.For maximum photosynthetic productivity, leaves must maintain turgor by obtaining sufficient water from the xylem to replace evaporative loss (9). The xylem, in turn, obtains water by root absorption from soil reserves. If the water supply is not adequate, leaves will initiate stomatal closure, thus limiting photosynthesis. Ishihara et al. (14) noted that leaf water stress induced stomatal closure even in rice plants growing in flooded paddies. The time of stomatal closure coincided with periods of high temperature and maximum solar radiation, conditions which would otherwise facilitate maximum photosynthetic rates (13). Because the rice roots grew in standing water, any reduction in leaf water potential under conditions of high evaporative demand implies that water movement from soil to and through roots and the xylem transport system was too slow.Maintenance of an active root system requires continued growth into new regions of soil to replace older roots which become suberized or die. Thus, continued growth of both shoot and root systems must be coordinated and regulated as the plant adapts to its environment during the growing season (22). The regulatory mechanisms are poorly understood at present (5); this study was initiated to determine effects ofdiurnal and long term water stress on soybean shoot and root growth.Inasmuch as photosynthetic rate is a function of temperature and PPFD3, the highest CO2 fixation rates should be measured during midafternoon when light intensity and temperature reach their daily maximum, provided all other factors are...

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