Low growth temperature inhibition of photosynthesis in cotyledons of jack pine seedlings (Pinus banksiana) is due to impaired chloroplast development

• Cotyledons of jack pine seedlings (Pinus banksiana Lamb.) grown from seeds were expanded at low temperature (5degreesC), and total Chl content per unit area of cotyledons in these seedlings was only 57% of that observed for cotyledons on 20degreesC-grown controls. Chl a/b ratio of 5degreesC-grown jack pine was about 20% lower (2.3 +/- 0.1) than 20degreesC controls (2.8 +/- 0.3). Separation of Chl-protein complexes and SDS-PAGE indicated a significant reduction in the major Chl a containing complex of PSI (CP1) and PSII (CPa) relative to LHCII1 in 5degreesC compared to 20degreesC-grown seedlings. In addition, LHCII1/LHCII3 ratio increased from 3.8 in control (20degreesC) to 5.5 in 5degreesC-grown cotyledons. Ultrastructurally, 5degreesC-grown cotyledons had chloroplasts with swollen thylakoids as well as etiochloroplasts with distinct prolamellar bodies. Based on CO2-saturated O-2 evolution and in vivo Chl a fluorescence, cotyledons of 5degreesC jack pine exhibited an apparent photosynthetic efficiency that was 40% lower than 20degreesC controls. Seedlings grown at 5degreesC were photoinhibited more rapidly at 5degreesC and 1200 mumol.m(-2).s(-1) than controls grown at 20degreesC, although the final extent of photoinhibition was similar. Exposure to high light at 5degreesC stimulated the xanthophyll cycle in cotyledons of both controls and 5degreesC-grown seedlings. In contrast to winter cereals, we conclude that growth of jack pine at 5degreesC impairs normal chloroplast biogenesis, which leads to an inhibition of photosynthetic efficiency.

Nyckelord

chloroplast

growth

temperature

photosynthesis

photoinhibition

Pinus banksiana Lamb.

ultrastructure

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