Photosynthetic responses to chilling in a chilling-tolerant and chilling-sensitive Miscanthus hybrid

Miscanthus is a C-4 perennial grass being developed for bioenergy production in temperate regions where chilling events are common. To evaluate chilling effects on Miscanthus, we assessed the processes controlling net CO2 assimilation rate (A) in Miscanthus x giganteus (M161) and a chilling-sensitive Miscanthus hybrid (M115) before and after a chilling treatment of 12/5 degrees C. The temperature response of A and maximum Rubisco activity in vitro were identical below 20 degrees C in chilled and unchilled M161, demon strafing Rubisco capacity limits or co-limits A at cooler temperatures. By contrast, A in M115 decreased at all measurement temperatures after growth at 12/5 degrees C. Rubisco activity in vitro declined in proportion to the reduction in A in chilled M115 plants, indicating Rubisco capacity is responsible in part for the decline in A. Pyruvate orthophosphate dikinase activities were also reduced by the chilling treatment when assayed at 28 degrees C, indicating this enzyme may also contribute to the reduction in A in M115. The maximum extractable activities of PEPCase and NADP-ME remained largely unchanged after chilling. The carboxylation efficiency of the C4 cycle was depressed in both genotypes to a similar extent after chilling. Phi(P):Phi(CO2) remained unchanged in both genotypes indicating the C-3 and C-4 cycles decline equivalently upon chilling.