Effects of young cladodes on the gas exchange of basal cladodes of Opuntia ficus-indica (Cactaceae) under wet and dry conditions

Crassulacean acid metabolism (CAM) plants, such as the cultivated Opuntia ficus-indica, are resistant to drought, but their responses can be affected by development of new organs, such as young, developing cladodes. The effect of a variable number of daughter cladodes on net CO2 uptake, nighttime intercellular CO2 concentration, stomatal conductance, and relative water content for mother cladodes exposed to wet and dry conditions was evaluated. Daily net CO2 uptake by mother cladodes of O. ficus-indica was significantly reduced as the number of daughter cladodes increased under dry conditions but was less affected under wet conditions. Stress caused by daughter cladodes and dry soil affected the expression of CAM phases in mother cladodes, curtailing CO2 uptake in the late afternoon (phase IV) and reducing carbon gain during the night (phase I) and early morning (phase II), the reduction being most marked under dry conditions. Dry soil and daughter cladodes reduced the relative water content (RWC) of mother cladodes, decreased the net CO2 uptake during the day and at night, and increased the internal cellular CO2 concentration. Under wet conditions, the RWC of mother cladodes was only slightly affected by daughter cladodes; nevertheless, daughter cladodes affected the recovery of phases I, II, and IV after substantial rainfall interrupted drought, and they also affected the net CO2 uptake in phases II and IV; this is one of the first times that an ontogenic factor has been shown to affect net CO2 uptake under well-watered conditions. The increase in the number of daughter cladodes did not increase the photosynthetic capacity of mother cladodes under wet or dry conditions.