Gas exchange in litchi under controlled and field conditions

Gas exchange of 3-year-old potted ‘73-S-20’ litchi (Litchi chinensis Sonn.) plants were measured under controlled conditions. At 28.8 ± 0.3 °C, dark respiration (Rd) was estimated at 0.6 CO2 μmol m−2 s−1, quantum yield (Φ) was 0.024 mol CO2 mol−1, and light compensation point (PPFDcomp) was 24 μmol m−2 s−1 photosynthetic photo flux density (PPFD). Maximum net CO2 assimilation (ACO2ACO2) (6.5–8 μmol m−2 s−1), stomatal conductance (gs) (0.07–0.09 mol m−2 s−1) and transpiration (E) (0.7 mmol m−2 s−1) were recorded at PPFD >800 μmol m−2 s−1, leaf temperature (TL) between 27 and 32 °C, and vapor pressure deficit (VPD) <0.7 kPa. When VPD and TL simultaneously increased maximum ACO2ACO2 was obtained between 25 and 28 °C. CO2 compensation point (Ccomp) was recorded between 90 and 100 ppm. ACO2ACO2 saturated at external CO2 concentration >800 ppm. ACO2ACO2 responses to light, and VPD were mainly regulated by stomata. On the other hand, ACO2ACO2 response to internal CO2 was mainly through leaf photochemistry. At TL < 28 °C leaf photochemistry dominated ACO2ACO2 responses, while at TL > 28 °C stomatal regulated ACO2ACO2 responses. Effects of leaf age and location as well as cropping on gas exchange were also studied on 9-year-old in the field trees. Leaf ACO2ACO2, gs and E on the well-exposed, younger flushes and adjacent to fruit were greater than those on shaded older flushes and woods or those on de-fruiting shoots. Our findings indicated that potential depression of leaf photosynthetic efficiency in litchi might occur due to low temperature in bloom season and early fruit set, as well as due to hot and dry mid-day in early summer.