Beta gauging leaf water status: influence of changing leaf characteristics.

Since the intvroduction (3) o,f the technique of beta gauging as a method for continuous and nondestructive mon!itoriing the water status of leaves, severcal reipo,rts (1-4) have appeared verifying the technique and its appl,ication in physio;logical research. Impliciit in this applicatiioni o?f beta gauging is the assutmption that, except for diurinal variations in water content, the physical characteristics of the leaf at the time it is removed for calibralbio,n are not sign1ifican-tly differenit fronm those that prevai,l while leiaf vater content is being mon,i(tored. In other wordis, the fuincitional reliati,on between beta-ray absorption by the leaf and some measutre of leaf water stress estiabl'iished wihleni the leaf is calibrated s'houlld not be dcifferent from thait whiich prevailed while the leaf was attached to the plant. If the atitached leaf is monitioreTl for onily severall hours there is liittle probability that eirrors in prediction wou,ld arise fr,o!m c'hianges in leaf comiposition, and it is reasonable to ass,ume that variatiolns in beta-ray attenualtion are soilely the result of vari,ations in mass of water per uniit area of leaf surface. At the time of its proposal, the belta gauge technique was envisioned as one thatt woulld enable monitoring oif leaf-water status for a period of at least 24 hours or perhiaps for several days. In thle course of calibrating, that is, determining tihe regression cutrve relating relative water contenit, R, (5) to count rate for a large nutmiber of soybean leaflets we observed that under conditions of constaant source-gaulge geometry, the curves folr a group oif youtng, developing leaflets were significantly differeniit from those fo,r older, inore mature leaflets. Youtng leaflets have characteristically less dry matter and less water per unit area at saturation contenit (100 % R) and shrink more with lo,ss of water thfan matuire, ful,ly devel,oped leaflets. We hypothesized thait because there is a rapid, progres).aive change in the physical characteristics of youing developing leaflets they shoutld not be monitored fo!r mo,re than a few hours befoire removing the leaflet for calibration. But mature leaflets with either constant or onily slowily changing physical characteristics would provide sites for