Fronds of Ulva lactuca and Padina tetrastromatica were collected from the upper, middle, and lower littoral regions of the intertidal belt at Veraval (India), and their nitrate reductase activity (NRA) was estimated at different levels of desiccation by exposing them to air under laboratory conditions. Fronds of both species from the upper littoral region showed more NRA than their counter parts from the lower littoral region. There was a steep rise in NRA during the initial period of desiccation in all the fronds, followed by a decay in the activity of the enzyme, thereafter. In fronds of Ulva lactuca from the upper and middle littoral regions decay of the enzyme (NR) started when their moisture content feil below 25%, whereas in the fronds from the lower littoral region, it started to fall when their moisture content was äs high äs 82%. It is inferred from these results that plants inhabiting the upper limits of the intertidal belt, which are subjected to longer periods of desiccation than those in the lower region, have more NRA äs an adaptation to longer periods of desiccation. However, such responses could be species-specific and hence, generalizations may not be valid.
IntroducrionExposure to aerial conditions during emergence causes dehydration and consequent damage to the tissues and is a major factor affecting the intertidal seaweeds, especially under the high Isolation on tropical shores. The ability of some intertidal seaweeds to withstand desiccation has been well studied in the past, and Biebl (1952) found a relationship between vertical distribution and drought tolerance of some species. Johnson et al. (1974) observed increased rates of photosynthesis in algae from the upper and middle littoral regions when exposed to air (1.6 to 6.6 times more than under submerged conditions), while the trend in species from the lower littoral was the reverse. Schonbeck and Norton (1978, 1979 a, 1979b, through a series of experiments, have shown that tolerance to desiccation and ability to resume photosynthesis and growth on resubmergence, was greater in species from the upper shore and that partial, rather than complete dehydration, was more injurious to Fucoids from upper shore. According to them, mechanical dehydration causes a disruption in cell membranes making the fronds incapable of rehydration on re-submersion, whereas tissue water loss affects the biochemical reactions causing a metabolic imbalance, which becomes severe under high temperatures and partial dehydrated conditions. That drought hardening changes with seasons, has also been reported by Schonbeck and Norton (l 979 a). However, Dromgoole (1980) did not see any clear relationship between the rate of dehydration of different species under Standard conditions and their bathymetric position along the intertidal belt. Thus, resistance to desiccation äs a causative factor in determining the vertical distribution of intertidal seaweeds is still controversial though the majority of workers support such a possibility. In an attempt to augment such studies...
