Photoinhibition and downregulation

Young leaves, even those uppermost in the canopy, did not suffer chronic photoinhibition at peak irradiance, and any decrease in photochemical efficiency was transient, and reversed as the irradiance declined again (Murchie et al 1999). Examination of kinetics of recovery from potentially photoinhibitory conditions indicated that the dark-adapted F/Fm was suppressed over a lag period, but these decreases were relatively small and diminished in line with the change in irradiance.

In older leaves, when a significant proportion adopted a more horizontal orientation, photoinhibition was stronger, and did not recover so readily as in younger erect leaves when the irradiance declined. Again, however, the decline in Fv/Fm did not persist, and recovery was complete overnight. It is hard to argue that, in an irrigated crop, in a tropical environment as found at IRRI, photoinhibition represents a major source of photosynthetic loss.

Equally, however, under these conditions, acclimation to high light reached its maximum in terms of the irradiance-dependent increase in Pmax and xanthophyll cycle pool size (see Table 1), and could be considered just at the threshold of significant light stress. Therefore, in more extreme conditions, where light intensities are higher and sustained for longer periods, and where water management is perhaps less comprehensive than at IRRI, photoinhibition could become a problem. Certainly, the fact that rice under favorable conditions is so close to light stress indicates that many other crops in more marginal habitats probably suffer considerable photosynthetic losses through photoinhibition.

Table 1. Effect of growth irradiance on photosynthetic characteristics of field-grown rice. New plant type rice (IR65600-42-5-2) plants were grown in the field and exposed to differential shading for 4 wk. The irradiance values are the approximate mean peak irradiances at midday. Pmax, Chi content, and xanthophyll cycle contents were determined for upper leaves as described by Murchie et al (1999). Rubisco content was determined following polyacrylamide gel electrophoresis of soluble protein, and quantified by densitometry of the Coomassie stained gel, relative to Rubisco standards. Note the near saturation of Pm„ at 800, but the large increase in Rubisco at 2,000. Increased photoprotection when acclimation of photosynthesis is saturated at 1,000 is evident from the decrease in Chi and elevation of xanthophyll cycle pool size.

Irradiance Pma( Chi content Rubisco Xanth cycle

(Hmol m~2 s"1) (nmol C02 rrf2 s_1) (mg nV2) (g irf2) (% total carotenoid)

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