PHOTOSYNTHETIC GAS EXCHANGE AND WATER RELATIONS DURING DROUGHT IN 'SMOOTH CAYENNE' PINEAPPLE (ANANAS COMOSUS (L.) MERR.) GROWN UNDER AMBIENT AND ELEVATED CO₂ AND THREE DAY/NIGHT TEMPERATURES

The effect of water deficits on net CO₂ assimilation (A), titratable acidity, and water relations of pineapple, the most important crop species having Crassulacean acid metabolism (CAM), was studied under ambient (350 μmol mol^-1) and elevated (700 μmol mol^-1) CO₂ levels and day/night temperatures of 35/25, 30/25, and 30/20°C. After six months of acclimatization to the above treatments, water was withheld for 70 days. A substantial decrease was noticed after 10 days of drought, particularly at elevated CO₂, where stomatal conductance (g_s) initially was low but declined more rapidly. Plants grown at elevated CO₂ had higher water use efficiency (WUE) than plants grown at ambient CO₂, both when well-watered or subjected to drought. The rate of decline of g_s and A during drought also varied with the day/night temperature. For plants at 35/25 and 30/25°C, after 40 days of drought, there was an 80 to 90% decrease in the diurnal variation in titratable acids (TA) due to a decline in nocturnal CO₂ uptake. However, the diurnal variation in TA for plants at 30/20°C dropped by only about 65% at the end of 70 days of drought at both ambient and elevated CO₂. Diurnal variation in TA at the end of the drought period indicated that pineapple continued to re-fix substantial quantities of respiratory CO₂. There was a slower rate in the decrease of leaf water content and osmotic potential during drought for plants grown at elevated CO₂ than at ambient CO₂ resulting in higher predawn leaf water potential and turgor pressure at the end of the drought period. The elevated CO₂ levels experimentally engineered to occur would help pineapple, under prolonged drought, to maintain a better water status and higher WUE compared to plants of this species growing at current CO₂ levels. Increased temperature due to global warming would not alter this result.