Effect of High Temperature On Gene Expression, Volatile Biosynthesis and Postharvest Quality of Banana Fruit During Ripening and Senescence

Banana (Musa AAA group) is one of the most consumed fruit in the world due to its eating quality and nutritional value.  Banana fruit quality is dependent on color, flavour (aroma and taste), sweetness and nutritional composition.  It has been reported that chlorophyll breakdown and color formation in banana is inhibited by ripening temperatures above 24°C.  At this temperature thylakoid membranes are retained resulting in reduced chlorophyll degradation, but faster senescence.  In this study, green (immature), untreated banana fruit were obtained from a local whole-sale market and half of the fruit were subjected to ethylene treatment at 10µl/L for 24 hours.  After ethylene treatment, both treated and untreated fruit were stored at 20 or 30°C for 7 days. During this period, fruit were sampled after 0, 1, 4 and 7 days of storage and evaluated for color, chlorophyll fluorescence, volatile production and expression of genes related to volatile biosynthesis.  Storage at 30°C reduced yellow color development in the peel, decreased chlorophyll fluorescence (Fv/Fm), but increased Fo, indicating possible heat stress of the fruit.  A total of 22 volatile compounds were identified using SPME/GC/MS.  Both ethylene treatment and high temperature enhanced volatile production.  Principle component analysis (PCA) revealed a correlation in the increase of Fo and ethyl acetate with the stress of high temperature.  Using RT-PCR, expression of genes related to volatile biosynthesis including hydroperoxides lyase (HPL), alcohol acetyl transferase (AAT), lipoxygenase (LOX), alcohol dehydrogenases (ADH, short and medium chains) and pyruvate decarbolxylase (PDC) were investigated in both peel and flesh tissue.  Among the tested genes, AAT, HPL and ADH (medium chain) in peel and flesh tissue increased significantly during ripening and in response to ethylene and storage at 30°C.  ADH (short chain) was induced by ethylene only in the peel.  LOX decreased during ripening and storage at 30°C in the peel, but increased in the flesh tissue of ethylene-treated fruit.  No significant change in gene expression of PDC was found.  This study demonstrates that both ethylene and high temperature influence volatile biosynthesis in banana fruit at the transcriptional level and confirms findings that high temperature accelerates banana fruit ripening.