Seasonal Differences in Diurnal Patterns of Metabolites and Enzyme Activities in Pitaya (Hylocereus undatus) Grown in a Temperate Zone

Abstract

Pitaya is a crassulacean acid metabolism (CAM) plant that is normally grown in subtropical regions, but it would be useful to cultivate this crop in temperate regions. In this study, we measured concentrations of organic acids and carbohydrates, and activities of the enzymes phosphoenolpyruvate carboxylase (PEPC), malic enzyme (ME), and malate dehydrogenase (MDH) in pitaya grown in a temperate zone, and compared the diurnal changes in these components between winter and summer. In summer, the diurnal changes in malate, citrate, and starch in pitaya were typical of starch-using CAM plants. Activities of PEPC and ME also showed typical CAM-type diurnal patterns in summer. In particular, changes in ME activity were closely associated with changes in malate content. In winter, changes in malate content showed a typical CAM pattern, although the amount accumulated was only half of that accumulated in summer, however, the citrate content in winter remained at an almost constant low level throughout the day. PEPC and ME activities were almost constant through the day in winter, however, PEPC activity in winter was similar to its minimum level in summer, whereas ME activity in winter was similar to its maximum level in summer. MDH activity was higher in summer than in winter, but there was no distinct diurnal pattern observed in either summer or winter. These results suggest that pitaya shows normal photosynthesis and metabolism in summer. In winter, however, malate accumulation is restricted, result in decreased concentrations of downstream products and metabolites. This may be because of decreased PEPC activity and increased ME activity during the night in winter. Our results suggest that temperature is not the only factor that affects CAM in this plant, because the summer temperature in this study was similar to the winter temperature in our previous study, which was carried out in a subtropical region (Ishigaki Island). Furthermore, diurnal profiles of metabolites and enzyme activities in both regions were similar in summer and winter; therefore, daylength may also be an important environmental factor.