Decline of Activity and Quantity of Ribulose Bisphosphate Carboxylase/Oxygenase and Net Photosynthesis in Ozone-Treated Potato Foliage

Abstract

The effect of ozone (03) on ribulose bisphosphate carboxylase/ oxygenase (Rubisco) activity and quantity and net photosynthesis in greenhouse-grown Solanum tuberosum L. cv 'Norland' foliage was studied in relation to oxidant-induced premature senes-cence. Plants, 26 days old, were exposed to 0.06 to 0.08 micro-liters per liter 03 from 1000 to 1600 hours for 4 days in a controlled environment chamber. On day 5, plants were exposed to a 6-hour simulated inversion in which 03 peaked at 0.12 microliters per liter. Net photosynthesis declined in response to 03 but recovered to near control levels 3 days after the exposure ended. Rubisco activity and quantity in control potato foliage increased and then decreased during the 12-day interval of the study. In some experiments foliage studied was physiologically mature and Rub-isco activity had peaked when 03 exposure commenced. In those cases, 03 accelerated the decline in Rubisco activity. When less mature foliage was treated with 03, the leaves never achieved the maximal level of Rubisco activity observed in control foliage and also exhibited more rapid decline in initial and total activity. Percent activation of Rubisco (initial/total activity) was not affected significantly by treatment. Quantity of Rubisco decreased in concert with activity. The decrease in activities is most likely due to a decrease in available protein rather than a decrease in the percentage of Rubisco activated in vivo. The reduction in the quantity of Rubisco, an important foliar storage protein, could contribute to premature senescence associated with toxicity of this air pollutant. Ozone (03) is a major phytotoxic air pollutant responsible for crop loss in the United States (5). An early event observed in plants exposed to 03 is a reduction in net photosynthesis (18). Later in development, plants stressed by 03 often exhibit premature senescence (15). Both reduced photosynthetic rate and premature senescence may lead to reduced plant productivity. A key protein involved in both photosynthesis and, indirectly , in senescence is Rubisco3 (2). 3Abbreviations: Rubisco, ribulose bisphosphate carboxylase/oxy-genase; PPFD, photosynthetic photon flux density. 427 dual role, serving as the enzyme for net C02-fixation during photosynthesis (13) and as the major storage protein in the leaf(7). Changes in synthesis and degradation of Rubisco may be divided into three phases corresponding to leaf age (2, 1 1). The first phase is a period of rapid synthesis and almost undetectable degradation, coinciding with the period of leaf expansion. At full leaf expansion, rubisco quantity reaches a maximum and may constitute up to 70% of the total soluble leaf protein (12). The second phase is a steady state period in which synthesis and degradation rates are similar. The final phase is characterized by a period in which degradation is the main process. After full leaf expansion, less Rubisco is needed for photosynthesis, and the enzyme begins to degrade to export nitrogen and carbon to growing regions of the plant (7). Several studies have been conducted to investigate the effects of 03 on Rubisco. Nakamura and Saka (14) reported a 50 to 80% decline in activity of Rubisco in rice leaves after a 2-h exposure to 0.12 ,uL L' 03. Pell and Pearson (16) demonstrated a decrease in Rubisco quantity in alfalfa after a similar acute 03 exposure of 0.25 ,L L' for 2 h. The results of the latter study were especially intriguing, showing a 36 to 80% decrease in enzyme quantity even in those leaves exposed to 03 but without foliar symptoms. More recently, Lehnherr et al. (9) exposed field-grown wheat to 0.1 ,uL L' 03 for 8 h/ d for 2 months and reported a decrease in Rubisco activity in flag leaves. Evidence exists that 03 reduces net photosynthesis in many species (18) and changes in the status of Rubisco have been offered as a partial explanation. The implications of 03-induced decreases in Rubisco content could extend beyond the role ofthis enzyme in photosynthesis. As described above, Rubisco goes through a process of synthesis and degradation which characterizes, in part, the maturation and aging processes of the leaf. The 03-induced decrease in Rubisco could reflect an acceleration of this process and offer a partial explanation for 03-induced premature senescence. In the present study, we sought to more thoroughly evaluate the effect of 03 on the status of Rubsico throughout foliar matu-ration and senescence in potato. MATERIALS AND METHODS Plant Culture Solanum tuberosum L. cv 'Norland' plants were used for these experiments. This C3 species has been shown to exhibit