The roles of photosynthetic pseudobulb of epiphytic C3 oncidium in responses to short- and long-term drought and rewatering

Abstract

The pseudobulb of epiphytic orchids has been shown to be an integral organ in the survival and growth of orchids. These 'false bulbs' are known to function as storage organs for water, carbohydrates and minerals accumulation and photosynthesis of green pseudobulbs (GPSB) can contribute positively to carbon balance. In our study, epiphytic C3 Oncidium Golden Wish was subjected to mild and severe DS (MDS and SDS) and rewatering (RW) by shading the GPSB to investigate the roles of photosynthetic GPSB in drought response and recovery to alter the source/sink ratio in relation to the main photosynthetic sources of green leaves (GL).<br><br>There were significant decreases in water content of GPSB as well as GL after SDS unlike after MDS. After 3 weeks of RW, the water content and RWC both fully recovered. The FW and length of new shoot was significantly lower in MDS and the lowest on SDS plant treatments. Interestingly, results were lower for FS-GPSB than FI-GPSB for both MDS and SDS groups. Data for Chl fluorescence parameters such as Fv/Fm ratio, ΔF/Fm’, qP and qN used to describe the photosynthetic state of the plant showed that SDS had a profound effect on the plant as compared to MDS. When exposed to MDS, midday Fv/Fm ratio of GL was not affected but in SDS plants, this was apparent and despite 3 weeks of RW, did not fully recover. The ETR results coincides well with data of Fv/Fm ratio ratio, qP and qN of GL and GPSB after MDS and SDS whereby values of SDS+FS-GPSB plants were the lowest after SDS and following 3 weeks of RW. SDS resulted in significant decreased in total Chl content of GL with greater decrease in FS-GPSB plants that was not observed in GL of MDS plants. Both MDS and SDS resulted in decreases of total Chl content of GPSB with greater decreases in FS-GPSB plants. No significant differences in total carotenoids content in both GL and GPSB among the different treatments after MDS and RW were observed. Less GL SS and IS was observed in FS-GPSB treatments as the photosynthetic role of GPSB had been removed, making less carbohydrate readily available as compared to FI-GPSB treatments during DS. The impact was more discernible after SDS. Our findings support that GPSB is more susceptible to DS when shaded. It takes a longer period of RW for GPSB to fully regain photosynthetic capacity, highlighting the crucial role of GPSB photosynthesis in drought tolerance for Oncidium Golden Wish plants.