Influence of drought stress on the gas exchange of Cupressus funebris Endl. seedlings of different ages in the limestone area of Chongqing,China

The influences of experimentally imposed drought stress on,photosyhthesis,photo-parameters and CO2-parameters of potted Cupressus funebris Endl.seedlings of two different ages in the limestone area of Chongqing,were studied using a Li-6400 portable photosynthesis system.Net photosynthetic rate(Pn),transpiration(E),and stomatal conductance(Gs)of the seedlings generally decreased under drought stress,but 1-year old seedlings decreased more than 5-year old seedlings.Intercellular concentration(Ci)of 1-year seedlings increased with aggravating drought stress while the stomatal limitation value(Ls)decreased.In contrast,Ci of 5-year seedlings decreased and Ls increased before they suffered serious drought stress,but they had the same trend as 1-year seedlings under serious stress.These results implied that the photosynthetic depression of 1-year seedlings was caused by non-stomatal limitation,while the depression of Pn in 5-year seedlings was predominantly due to stomatal limitation before suffering serious stress,but transferred to non-stomatal limitation after suffering serious stress.Water use efficiency(WUE)in 1-year seedlings decreased with the aggravation of drought stress,but 5-year seedlings showed the reversed trend,indicating that 5-year seedlings had greater drought resistance.Under drought stress apparent quantum yield(AQY),carboxylation efficiency(CE)and photosynthetic ability(A0)of 1-year seedlings decreased,while light compensation point(LCP),CO2 compensation point(Γ),light respiration rate(RP)and dark respiration rate(Rd)increased.For 5-year seedlings,AQY,LCP,RP and A0showed similar trends,but CE and Γ decreased at first,then increased at the end,while Rd followed the reversed pattern.This also indicates that 5-year seedlings could cope better with drought stress.