The effect of potassium deficiency on chemical, biochemical and physical factors commonly associated with blackspot development in potato tubers

Blackspot bruise is a physiological disorder of potato tubers resulting fro m mechanical damage to tissues during handling. This study examined the eff ect of potassium nutrition on biochemical and physical changes normally ass ociated with blackspot bruise in potato tubers. Four potato (Solanum tubero sum L.) genotypes (TXA 763-5 [very resistant], Ranger Russet [very suscepti ble], Russet Burbank [susceptible], and Lemhi Russet [very susceptible]) di ffering in susceptibility to blackspot bruise were grown at three potassium nutrition levels (100%, 10% and 0% of the potassium in full strength Hoagl and's solution). Tubers from the three treatments were tested for their che mical, biochemical and physical characteristics commonly related to blacksp ot bruise susceptibility. The potential to develop malanin pigments was mea sured as total oxidative potential (optical density, OD475) of clarified tu ber homogenates. Tuber tissue extracts were assayed for polyphenol oxidase (PPO) activity over time. with catechol, measured as increase in OD410 Spec ific gravity of tubers was determined with a brine solution series. Physica l property measurements were made by impacting cores of tuber tissue in a s pecially designed instrumented pendulum. Shock wave speed, Young's modulus, failure stress, and failure strain were all measured dynamically during im pact. Tubers produced under both the 10% and 0% potassium regimens had almo st 2x the free tyrosine of control tubers. The 0% potassium tubers had sign ificantly higher PPO activity than the control. Both of these factors likel y contributed to the significantly higher total oxidative potential that wa s observed in tubers from both the 10% and 0% potassium treatments. Tubers from both the 0% and 10% potassium treatments had a significantly lower You ng's modulus (a measure of stiffness), significantly lower failure stress a nd the shock wave of impact propagated through both of them significantly s lower. Although the cultivars were significantly different in their suscept ibility to black pigment development, they all responded similarly to the t reatment with regards to the variables measured. The TXA-763-5 clone was mu ch more susceptible to developing black pigments in these tests than expect ed. The other three cultivars performed as they do in commerce.