Physiological Changes in Portulacaria afra (L.) Jacq. during a Summer Drought and Rewatering

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Abstract
The changes of titratable acidity, enzyme activity, water status, and pigment composition were studied in Portulacaria afra (L.) Jacq. during a normal summer drought and rewatering. Two groups of plants were grown outside under a clear plastic canopy with water stress initiated at 2-week intervals in May 1986. Drought resulted in a linear decrease of fresh weight for 80 days and there was no further fresh weight change for the next 65 days. Nocturnal CO2 uptake remained measurable for 83 days. Cessation of exogenous CO2 uptake corresponded to the point where the pressure potential (Ψp) became zero. Ribulose-1, 5-bisphosphate (RuBP) and phosphoenolpyruvate carboxylase were reduced to 50% of this activity by the end of the drought period. Phosphoenolpyruvate carboxykinase activity was undetectable after 120 to 140 days of drought. Chlorophyll (Chl) levels decreased with a preferential loss of Chl a over Chl b. Carotenoid content was relatively constant over the course of the drought period. After 145 days of drought, plants responded to rewatering within 24 hours; Ψp became positive and daytime CO2 uptake resumed after 24 hours. After 3 days, RuBP carboxylase activity reached control levels. Activity of the CAM pathway recovered after 5 days, as noted by increased diurnal acid fluctuations. Phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase activity fully recovered within 6 days. Chl levels were greater than control levels within 5 days. Chl a/b ratios took 27 days to return to control levels. The results indicated that P. afra can withstand a normal summer drought by utilizing the CAM and CAM-idling pathway for 130 to 140 days. The plants respond rapidly to rewatering because of the conservation of enzyme activity and the quick recovery of Ψp.