BIOENERGETICS, RESPIRATION COST AND WATER RELATIONS OF DEVELOPING AVOCADO FRUIT

Avocado fruit of cultivars with different susceptibility to physiologi cal fruit disorders were examined in situ for respiration, stomatal co nductance, transpiration and xylem water potential. Therein, attached fruit of cv. Fuerte (susceptible to mesocarp discolouration) were comp ared with Hass (susceptible to small fruit) over the 3-month period fr om anthesis until the fruit reached 60 g fresh matter: 1. Avocado frui t respired more CO2 in the dark than in the light due to fruit photosy nthesis. Respiration rates decreased with fruit ontogeny from 14.5 to 1.5 mu mol CO2 m(-2) s(-1) and increased from 1 to 4.5 mu mol CO2 frui t(-1) h(-1), reflecting the higher energy requirement of the avocado r elative to other fruits and of cv. Hass relative to Fuerte fruit. 2. F ruit respiration accumulated to 4.8 g (Fuerte) or 5.1 g (Hass) CO2 (= 63 or 66 kJoule), with growth respiration (Fuerte) of 0.324 kg CO2 (7. 35 MJ) kg(-1) dry matter twice as large as found in fruits of other sp ecies and maintenance respiration of 1.98 kg CO2 or 26 MJ kg(-1) dry m atter. 3. Stomatal conductance decreased from 0.17 mmol m(-2) s(-1) at pre-anthesis to 0.01 mmol m(-2) s(-1) at 60 g fruit weight. 4. Fruit of cv. Hass transpired more water than those of Fuerte, i.e. 0.2 to 1 mmol H2O m(-2) s(-1) or 0.2 to 2 mmol H2O fruit(-1)day(-1) 5. A 60-g f ruit accumulated 51 mL water and transpired 155 mt or 171 mL H2O, resu lting in a water consumption of 206 mt (Fuerte) or 222 mt (Hass) water with a fruit water use efficiency (FWUE), i.e. mol CO2 respired per m ol water transpired, of 9.5 x 10(-3). 6. Midday fruit water potentials ranged between -0.4 to -0.9 MPa with values obtained for cv. Fuerte f ruit exceeding those of the respective leaf. These results present the first data on energy requirement, maintenance and growth respiration, transpiration, water use and water potential of developing avocado fr uit, which are discussed with respect to physiological fruit disorders .