Photosynthesis and related processes of two mangrove species: rhizophora mucronata and ceriops tagal at Gazi Bay, Kenya

Abstract

Measurements were made of the gas exchange properties, water use efficiency and water relations of two mangrove species of the Rhizophoraceae family in the dry and wet seasons of 1992 at Gazi Bay, Kenya. The rates of photosynthesis differed significantly (P < 0.001) between the two species, with Rhizophora mucronata consistently having higher CO2 assimilation rates than Ceriops tagal. Maximum rates of photosynthesis recorded (C02 uptake per unit area of one leaf) in C. tagal in the dry season (February- March) ranged from 0.l3-5.93 umol m-2 s-1 and from 0.l3-6.l2 umol m-2 s-1 in R. mucronata with an average of 1.25 and 1.53 umol m-2 s-1 respectively. During the wet season (May-August 1992) the saplings of both species had higher rates of net photosynthesis and greater stomatal conductances than their adult mangrove counterparts. Maximum rates of light saturated photosynthesis were 0.26-13.37 umol m-2 s-1 in R. mucronata saplings and 0.34-8.70 umol m-2 s-1 in C. tagal saplings with an average of 5.82 and 3.97 umol m-2 s-1 respectively. By contrast, maximum rates of photosynthesis in the wet season in the adult mangrove trees ranged from 0.18 - 12.2 umol m-2 s-1 in R. mucronata and from 0.l5-11.5 umol m-2 s-1 in C. tagal with an average of 4,04 and 3.20 umol m-2 s-1 respectively. Similarly, maximum stomatal conductances amongst the saplings in the wet season of 1992 were about 50% greater than those of the adult trees. The diurnal course in the rates of photosynthesis was such that CO2 assimilation was high in the morning (between 8 and 10 hours) decreasing thereafter before rising slightly in the evening from about 15 hours. The rates of photosynthesis and stomatal conductances did not show any significant (P > 0.05) variation in relation to leaf position in the canopy. However, the bottom leaves of Ceriops tagal had higher CO2 assimilation rates than the 'sun' leaves located at the top of the canopy in the dry season. Proportional changes in the rates of stomatal conductances and CO2 assimilation were closely and positively correlated in both species and were highly significant (P<O.OOI). As photosynthesis was measured on leaves exposed to a wide range of light conditions, it was possible to derive approximate photosynthetic light response curves for leaves of R. mucronata and C. tagal for the uppermost level in the canopy. Photosynthesis in leaves of both species was light saturated at photon flux densities of about 800 umol m-2 s- I and above. In some cases there was indication that photosynthetic rates may be depressed by photon flux densities above 1900 umol m-2 s- I, but the trends were not sufficiently clear to be certain. Internal CO2 concentration responded positively to stomatal behaviour in both species, being on average about 82 ubar in the dry season and 136 ubar in the wet season when stomatal conductances increased from about 0.030 to 0.44 mol m? S-l in the dry and wet season respectively. Leaf water potential for both species ranged between -2.1 to -2.4 MPa in the morning and thereafter decreased as solar radiation increased, reaching a maximum of -4.6 MPa at about noon. There were significant differences between the water potentials of shoots at the top of the canopy and those collected from the bottom of the canopy, with water potential becoming more negative with increasing leaf height above the ground. There was a close inverse relationship between water potential and measured gas parameters such as CO2 assimilation rate and stomatal conductance. Furthermore, there was evidence that even during the late wet season (August), the mangrove trees were operating for at least part of the day at turgor potentials that are close to zero. However, this phenomenon did not induce stomatal closure and photosynthesis continued albeit at low values.