The Regulation of Invertase Activity in the Latex of Hevea brasiliensis Muell. Arg: THE EFFECTS OF GROWTH REGULATORS, BARK WOUNDING, AND LATEX TAPPING

Treatments which increase latex yield, e. g. bark scraping,latex tapping, and bark application of 2, 4-D or 2-chloroethylphosphonicacid (CEPA) were found to enhance the activity of latex invertase.In previously untapped trees, both the introduction of tappingand the application of 2, 4-D brought about an increase in thelevel of invertase. In regularly tapped trees, the amount oflatex invertase is several times higher than in untapped treesand evidence was obtained that its activity is regulated bythe variation of latex pH. The pH of latex of the clone investigated(PR 107) was shown to vary between 6.3 and 7.1 whereas the activityof invertase, as assayed directly in the latex, has a sharpoptimum at pH 7.5 and falls rapidly with the shift of pH tothe acid side. There was no increase in the content of latexinvertase when trees adapted to regular tapping were treatedwith 2, 4-D. The effect of auxin on actual invertase activitywas essentially mediated through related increase of latex pH.The CEPA and bark scraping were also shown to increase latexpH in tapped trees. The treatment of the bark of tapped trees with CEPA increasedthe level of latex sucrose, as did auxins. Bark scraping alsohad a slight stimulatory effect. The K_m of latex invertase asa function of pH was found to change in the same way as V_max,being highest at pH optimum.     

Sucrose Synthetase in the Latex of Hevea brasiliensis Muell. Arg

Sucrose synthetase (EC 2.4.1.13 [EC] ) was found in the latex of therubber tree but the activity of sucrose phosphate synthetase(EC 2.4.1.14 [EC] ) was not detected. The enzyme was purified andsome properties have been investigated. Examination of the kineticsof sucrose synthesis revealed K_m of 0.56 mM for uridine diphosphoglucoseand 3.85 mM for fructose. Mg²⁺ and cyanide activated sucrosesynthesis but reduced the cleavage reaction. Increased pH hadthe same effect, the synthetic activity being higher than theactivity of sucrose breakdown within the physiological levelsof latex pH. In the latex of regularly tapped trees, the total enzyme activityin the direction of synthesis was about 10% or less of the totalinvertase activity at pH 7.0. Because of the strong limitationof invertase under natural conditions, the proportion of actualsynthetase activity is, however, much higher and evidence ispresented that in the latex of regularly tapped trees this activitysignificantly reduces carbohydrate breakdown. Some indications have been obtained that this involvement ofsucrose synthetase is weakened by application of Ethrel to thebark. A reduction of its synthetic activity, accompanied byan acceleration of sucrose utilization in latex cytoplasm andby an increase of latex yield, could be observed before thetreatment-induced rise of pH enhancing inver.