Cool-night temperature induces spike emergence and affects photosynthetic efficiency and metabolizable carbohydrate and organic acid pools in Phalaenopsis aphrodite

After being acclimated to constant warm (28 degrees C day/28 degrees C night) and cool-night temperature (28 degrees C day/20 degrees C night) regimes in growth chambers for 2 weeks, the two groups of mature Phalaenopsis aphrodite subsp. formosana plants both clearly exhibited a diurnal oscillation of stomatal conductance, net CO2 uptake rate, malate and starch levels, and the phosphoenolpyruvate carboxylase (EC 4.1.1.31) and NAD(+)-malic enzyme (EC 1.1.1.39) activities. Hence, P. aphrodite is an obligate crassulacean acid metabolism plant. Nevertheless, different night temperature greatly affected both the stomatal conductance and the contribution of ambient and respiratory CO2 to the nocturnal accumulation of malate. However, the amounts of nocturnal accumulated malate and daily deposited starch appeared to have no significant difference between the two groups. These results demonstrate that P. ahrodite is congruent with the characteristics of CAM plants having great flexibility and plasticity in response to changes in environmental conditions. In addition, the formation of reproductive stem, viz. spike, was noticeably inhibited by a constant warm temperature, but induced by a fluctuating warm day and cool night condition. The relationship between the metabolic pool variation and spike induction of Phalaenopsis is also discussed.