Gas exchange characteristics and chlorophyll-a fluorescence kinetics of two morphotypes of Centella asiatica under open and shaded conditions

The differences in leaf Ψw, pigment levels and photosynthetic activity of two contrasting leaf morphotypes (PT1 and PT2) of Mandukaparni (Centella asiatica L.) grown in open (sun) and under artificial green shade nets of 25% and 67% shade levels were investigated. Open and shade plants of PT1 and PT2 showed similar diurnal fluctuations in Ψw. The Chl a, Chl b, carotenoid content and Chl: carotenoid ratio in plants under 67% shade were 1.53, 1.63, 1.10 and 0.87 times of open plants in PT1 whereas PT2 had 1.86, 1.96, 1.37 and 0.69 times the respective value under shade. PT1 recorded much lower net photosynthetic rate (P_n) compared to PT2 and the peak P_n in PT1 was 66.7% and 57.1% of PT2 in open and 25% shade, respectively. Fluctuation in stomatal conductance (g_s) during the day was more acute in PT2 compared to PT1 and g_s closely coupled with P_n at different light intensity in PT2 compared to PT1. Both the plant types recorded higher transpiration (E) in 25% shade. Chlorophyll fluorescence values were significantly altered under low light levels although primary photochemistry was least affected by high light. Efficiency of photosystem II (ΦPS II) increased by 4.3 and 3.6 times of open under 67% shade in PT1 and PT2, respectively. The photochemical quenching (qP) increased substantially under 67% shade with 48% and 42% increase compared to open in PT1 and PT2 respectively, whereas non photochemical quenching (qN) increased by 22% and 30% in open compared to 25% and 67% shade. Increase in shade resulted in reduction in electron transport rate (ETR) value in both plant types. Leaf size increased 3.38 and 1.54 times in PT2 and PT1 under 67% shade. Specific leaf area (SLA) also increased by 1.31 and 1.42 times under 25% and 67% shade respectively, in PT1. Results revealed that PT2 was less sensitive to high light and exhibited higher plasticity to varying light intensity.