Impact of seasonal variations on photosynthetic behavior of subtropical Krishna mangroves, East coast of India

Abstract

Gilakaladindi and Malakayalanka are two mangrove growing estuaries of Krishna District located at latitudes 16° 8ˈ56.1624ˈˈ, 15°58ˈ34.5354 and longitudes 81°9ˈ46.029ˈˈ, 81°5ˈ53.3616ˈˈ respectively.  Photosynthetic performance in terms of photosynthetic rate (µmoles m-2s-1 CO2 fixed), stomatal conductance (m mol CO2 m-2s-1), transpiration (mmol m-2s-1), internal CO2 concentration (µmol mol-1) and water use efficiency was recorded at 1600 Quanta (µmol quanta m-2s-1) on eight mangroves such as Lumnitzera racemosa, Ceriopsis tagil, Avicennia marina, Rhizophora apiculata, Avicennia officinalis, Bruguiera cylindrica, Xylocarpus granatum and Exocaria agallocha growing at Gilakaladindi and Malakayalanka sites was investigated during mid-summer with a mean temperature of around 38±1. The pH and electrical conductivity values at Gilakaladindi and Malakayalanka estuaries were reported as 8.1, 8.3 and 14.98, 15.44 respectively.  At these high pH and Electrical conductivity conditions, mangroves growing at Gilakaladindi such as Avicennia marina  and Rhizophora apiculata showed highest photosynthetic rate (8.63 µmoles m-2s-1 CO2, 7.85 µmoles m-2s-1 CO2) along with high stomatal conductance (0.13 m mol CO2 m-2s-1, 0.11 m mol CO2 m-2s-1), transpiration rate (2.98 mmol m-2s-1, 2.46 mmol m-2s-1), internal CO2 concentration (249.92 µmol mol-1, 258.86 µmol mol-1 and 235.97 µmol mol-1, 231.21 µmol mol-1) and water use efficiency (2.89 µmol quanta m-2s-1, 2.88 µmol quanta m-2s-1 and 3.19 µmol quanta m-2s-1, 2.83 µmol quanta m-2s-1).  Mangrove species Xylocarpus granatum, Rhizophora apiculata and Ceriopsis tagil even though showing high water use efficiency (3.23 µmol quanta m-2s-1, 3.19 µmol quanta m-2s-1 and 3.08 µmol quanta m-2s-1) but unable to perform more photosynthetic rate and this is may be due to low stomatal conductance (0.08 m mol CO2 m-2s-1, 0.11 m mol CO2 m-2s-1 and 0.08 m mol CO2 m-2s-1) and high transpiration (2.08, 2.46 and 1.99) respectively.  In mangroves growing at Malakayalanka i.e. and Rhizophora apiculata reported high photosynthetic rate (8.31 µmoles m-2s-1 CO2), stomatal conductance (0.13 m mol CO2 m-2s-1) and transpiration rate (3.00 mmol m-2s-1) coupled with high internal CO2 concentration (235.97 µmol mol-1) in turn results in high water use efficiency (2.88 µmol quanta m-2s-1).  In both of these ecosystems the minimum photosynthetic performance was observed in Lumnitzera racemosa and Bruguiera cylindrica with a photosynthetic rate of 3.53 µmoles m-2s-1 CO2, 3.53 µmoles m-2s-1 CO2, stomatal conductance 0.03 m mol CO2 m-2s-1, 0.03 m mol CO2 m-2s-1 and transpiration rate 1.13, 1.08 respectively.  The decrease in photosynthetic performance of these species may be because of reduced internal CO2 concentration in Gilakaladindi (204.61 µmol mol-1, 206.10 µmol mol-1) and Malakayalanka (199.07 µmol mol-1, 173.82 µmol mol-1) coupled with low stomatal conductance (0.05 m mol CO2 m-2s-1, 0.06 m mol CO2 m-2s-1 and 0.03 m mol CO2 m-2s-1).