Optimization of transpiration and potential growth rates of 'Kardinal' rose with respect to root-zone physical properties

Physical characteristics of two media were studied concerning water availab ility to roots, as reflected in specific transpiration rate, stomatal condu ctance, and specific growth rate of very young leaflets of 'Kardinal' rose (Rosa x hybrida L.), grafted on Rosa canina L.'Natal Brier'. Plants were gr own in UC mix [42% composted fir bark, 33% peat, and 25% sand (by volume)] or in coconut coir. Water release curves of the media were developed and hy draulic conductivities were calculated. Irrigation pulses were actuated acc ording to predetermined media moisture tensions. Transpiration rate of plan ts was measured gravimetrically using load cells. Specific transpiration ra te (STR) was calculated from these data and leaf area. STR and stomatal con ductance were also determined using a steady-state porometer. Specific grow th rate (R-SG) of young leaflets was calculated from the difference between metabolic heat rate and respiration rate, which served as an indicator for growth potential. Low STR values found at tensions between 0 and 1.5 kPa i n UC mix suggest this medium has insufficient free air space for proper roo t activity within this range. Above 2.3 kPa, unsaturated hydraulic conducti vity of UC mix was lower than that of coir, possibly lowering STR values of UC mix-grown plants. As a result of these two factors, STR of plants grown in coir was 20% to 30% higher than that of plants grown in UC mix. STR of coir-grown plants started to decline only at tensions around 4.5 kPa. Yield (number of flowers produced) by coir-grown plants was 19% higher than UC m ix-grown plants. This study demonstrated the crucial role of reaching suffi cient air-filled porosity in the medium shortly after irrigation. It also s uggests that hydraulic conductivity is a more representative measure of wat er availability than tension.