Tabasco pepper transpiration by the heat dissipation probe ... - UFRB

16Marinho et al.Figure 7. Relationship between the average daily sapflow period (FS) for each plant with and without compensationof the natural temperature differences in thestem - NTD (A) and relationship between the lysimetermeasured and cumulative daily sap flow with and withoutNTD compensation (B)The mean values of sap flow estimated by Eq.(7), with and without compensation for NTD, arecorrelated in Figure 7A. There is a strong trendof underestimation in the method when NTD arenot compensated (average of 14.9%). Vellame etal. (2011) found underestimation trends of 13.15%for sap flow in adult mango trees, using the heatdissipation probe method.When thermal differences are correctedby subtracting the estimated natural thermaldifferences, the trend of underestimating theflow by the method decreases and the accuracy inestimating the sap flow increases, as one can seeby the increase of the determination coefficient(R 2 ) shown in Figure 7B.Delgado-Rojas (2003) and Delgado-Rojaset al. (2004) found, in coffee and lemon plants,respectively, that the mentioned gradient showshigh interference on the estimations.Girardi et al. (2010) noted that errorsinherent to the NTD and injuries in the stemtissues affected the accuracy of the sap flowmeasurements, with the heat dissipation probemethod of “Valência” orange tree (Citrussinensis (L.) Osbeck) grafted on “Cravo” lemon(Citrus limonia Osbeck). However, Delgado-Rojas et al. (2006), studying rubber trees,verified no interference of NTD on the sap flowestimations (Fs) due to the closure of plantcanopy, resulting in nearly 100% shading.According to these authors, it seems that whenplants have a low basal area (young plants) orlow population density, there may be a higherincidence of thermal load from the environmenton them, suffering more easily with theinterference of natural thermal gradient (NTD).Also, the smaller the plants the stronger theeffects, especially because of the reduced stemdiameter, thus more easily affected, both forthe energy transferred by advection and for theenergy transformed on that place.The xylem and supporting structures(conductive area) occupied an area of 70.9%,medulla, 9.5% and epidermis, cortex, fibers andphloem, 19.6% of the total stem area of Tabascopepper plant.The relation between the conductive sectionarea and the outer perimeter of the stem had a goodadjustment following a quadratic polynomial, Eq.(8).2 2AS = 0. 112P − 0. 819P+ 2. 765 R = 0.991in which,AS - area of the conductive section, cm 2P - outer perimeter of the stem, cmIn this study, using all the plants as destructivesamples, it was possible to establish the numberof sampled stems that allows a reliable estimationof the active xylem area.With a small sampling (around 10% or 3 stemsegments) the errors are smaller than 5%, whichallows studies of this nature with a reducednumber of destroyed plants (Figure 8).Figure 8. Percentage error in the relationship betweenmeasured and estimated conductive area (ASest/ASmed)versus percentage of stems sampled(8)The coefficients of variation (CV) for thedaily measurements of sap flow from the 29plants with and without correction to the sensorpositioning by Eq. (6) may be seen in Figure 9.It is noted that part of the variability of the dailymeasurements of sap flow between plants canbe explained by the positioning of the sensorin areas not conducting raw sap. For the entireperiod, the difference between CV values of theWater Resources and Irrigation Management, v.2, n.1, p.11-18, 2013.