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 availability to roots, as reflected in specific transpiration rate, stomatal conductance, and specific growth rate of very young leaflets of 'Kardinal' rose ( Rosa × hybrida L.), grafted on Rosa canina L. 'Natal Brier'. Plants were grown 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 hydraulic conductivities were calculated. Irrigation pulses were actuated according to predetermined media moisture tensions. Transpiration rate of plants was measured gravimetrically using load cells. Specific transpiration rate (STR) was calculated from these data and leaf area. STR and stomatal conductance were also determined using a steady-state porometer. Specific growth 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 in UC mix suggest this medium has insufficient free air space for proper root activity within this range. Above 2.3 kPa, unsaturated hydraulic conductivity 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 mix-grown plants. This study demonstrated the crucial role of reaching sufficient air-filled porosity in the medium shortly after irrigation. It also suggests that hydraulic conductivity is a more representative measure of water availability than tension. Transpiration (T) is an essential process in plant life. Optimiza- tion of dry matter production can be realized when actual T is close to potential T due to the dual role of stomata in controlling leaf temperature through water vapor exit and CO2 entry. The role of transpirational cooling in maintaining an optimal tissue temperature range is more critical in greenhouse-grown crops than in field-grown crops. Daytime air temperature within a green- house is generally higher than ambient. Moreover, radiative and convective heat losses from plant surfaces are smaller due to the presence of the greenhouse cover. Thus, the well being of a greenhouse-grown plant is more dependent on uninterrupted tran- spiration than its field-grown counterpart. Since water storage capacity of a rose plant ( Rosa × hybrida) is relatively small, a balance must be maintained between water flux into and out of the plant, so as to prevent water stress. Water flux from the root zone through the plant, to the air is determined by hydraulic conductance (C) of the soil-plant-air continuum. This variable is determined by a number of parameters including root zone hydraulic conductivity (K). In this report we