Photosynthesis of Ulva sp. I. Effects of desiccation when exposed to air

The photosynthetic performance of intertidal Mediterranean Ulva sp. during emergence has been investigated and compared with certain field desiccation levels of this alga. Net photosynthetic rates were constant from the onset of desiccation up to a water loss of 20%; at higher desiccation levels rates decreased. Positive net photosynthesis was maintained until a relative water content of ≈ 35%. Comparing these results with maximal natural dehydration rates in the summer, it may be predicted that net photosynthesis in the morning is positive for ≈90 min after exposure, while the corresponding time during midday is 30 min only. Ulva growing in the lower mid-littoral zone, was not exposed enough time to cause a reduction in photosynthetic rates. At higher levels plants did desiccate to a point where photosynthetic rates would be adversely affected, but net rates were still positive during the whole day. Plants growing at the highest level were dehydrated to such an extent as not to allow for positive net photosynthesis for many hours of the day. No apparent barrier for water loss from Ulva fronds was found. The ability, however, to perform positive net photosynthesis when mildly desiccated may be a feature permitting this plant to grow in the intertidal zone.