Rubisco limits photosynthetic CO 2 fixation because of its low catalytic turnover rate (k cat ) and competing oxygenase reaction. Previous attempts to improve the catalytic efficiency of Rubisco by genetic engineering have gained little progress. Here we demonstrate that the introduction of the small subunit (RbcS) of high k cat Rubisco from the C 4 plant sorghum (Sorghum bicolor) significantly enhances k cat of Rubisco in transgenic rice (Oryza sativa). Three independent transgenic lines expressed sorghum RbcS at a high level, accounting for 30%, 44%, and 79% of the total RbcS. Rubisco was likely present as a chimera of sorghum and rice RbcS, and showed 1.32-to 1.50-fold higher k cat than in nontransgenic rice. Rubisco from transgenic lines showed a higher K m for CO 2 and slightly lower specificity for CO 2 than nontransgenic controls. These results suggest that Rubisco in rice transformed with sorghum RbcS partially acquires the catalytic properties of sorghum Rubisco. Rubisco content in transgenic lines was significantly increased over wild-type levels but Rubisco activation was slightly decreased. The expression of sorghum RbcS did not affect CO 2 assimilation rates under a range of CO 2 partial pressures. The J max /V cmax ratio was significantly lower in transgenic line compared to the nontransgenic plants. These observations suggest that the capacity of electron transport is not sufficient to support the increased Rubisco capacity in transgenic rice. Although the photosynthetic rate was not enhanced, the strategy presented here opens the way to engineering Rubisco for improvement of photosynthesis and productivity in the future.

The effects of overexpression of Rubisco activase on photosynthesis were studied in transgenic rice expressing barley or maize Rubisco activase. Immunoblot and SDS-PAGE analyses showed that transgenic lines from both gene constructs expressed the foreign Rubisco activase at high levels. The activation state of Rubisco in transgenic lines was slightly higher than that in non-transgenic plants (NT). In addition, light activation of Rubisco was significantly more rapid in transgenic lines compared with NT. These findings indicate that the overexpression of Rubisco activase can enhance Rubisco activation. However, despite enhanced activation of Rubisco in these transgenic plants, the CO(2) assimilation rate at ambient CO(2) conditions was decreased. This decrease in CO(2) assimilation rate was observed in both young developing and mature leaves independent of nitrogen nutrition. The contents of nitrogen and Chl did not differ significantly between transformants and NT; however, Rubisco content was substantially decreased in transgenic lines. There was no evidence for reduced transcription of RbcS or RbcL in these transgenic lines; in fact, transcript levels were marginally increased compared with NT. These results indicate that the overexpression of Rubisco activase leads to a decrease in Rubisco content, possibly due to post-transcriptional mechanisms.

In a 3-year study, seasonal and daily soil water fluctuations in a California blue oak woodland were investigated by measuring soil water potential (Ψ) at hourly intervals. Soil water potential remained relatively high well into the annual summer drought, with values above -0.5 MPa until June even in a dry year. As drought progressed, Ψ (at 25, 50, 75, and 100 cm depth) decreased to less than -3 MPa, providing evidence for continued blue oak root activity throughout the summer. We observed diurnal Ψ fluctuations (gradual increase at night and rapid decrease during daytime) characteristic of hydraulic lift, a process by which plant roots redistribute water from wet to dry soil layers. These diurnal fluctuations were observed at all four soil depths and began to appear when Ψ reached approximately -0.3 MPa. When Ψ reached approximately -3 MPa, fluctuations became "offset" from those typical of hydraulic lift. These offset fluctuations (apparent at low water potentials when temperature fluctuations were large) closely followed diurnal fluctuations in soil temperature. We propose that these offset patterns resulted from a combination of hydraulic lift cessation and an over-correction for temperature in the model used to calculate Ψ from raw sensor data. The appearance and disappearance of hydraulic lift fluctuations seemed to depend on Ψ. While soil temperatures and dates at which hydraulic lift appeared (and disappeared) were significantly different between wet and dry years, Ψ values associated with hydraulic lift appearance were not significantly different. Hydraulic lift occurred too late in summer to benefit annual forage grasses. However, water released by blue oak trees at night could slow the rate of soil water depletion and extend blue oaks' growing season.

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