Photosynthetic Light Utilization Efficiency, Water Relations and Leaf Growth of C3 and CAM Tropical Orchids under Natural Conditions

Shawn Tay,Tim Wing Yam,Jie He

doi:10.4236/ajps.2015.618290

Shawn Tay, Tim Wing Yam + Show 1 more

Open Access

https://doi.org/10.4236/ajps.2015.618290

Copy DOI

Abstract

Native orchid species of Singapore in their natural conditions experience stress from high irradiance, high temperatures and periods of extended low rainfall, which impact orchid plant physiology and lead to reduced growth and productivity. In this study, it was found that there was a reduction in photochemical efficiency of photosystem II (PSII) in 6 native orchid species under high light (HL) and Bulbophyllum membranaceum under low light (LL). There was chronic photoinhibition in these 6 orchid species over a period of 3 months after transplanting onto the tree trunks without watering and fertilization, especially in Coelogynes mayeriana and Bulbophyllum membranaceum under both HL and LL. This chronic photoinhibition caused by sustained period of water deficit in their natural conditions was later reversed by natural re-watering conditions from higher rainfall. These results indicate that water deficit has a greater impact on photosynthetic light utilization efficiency than excess light. The present study also showed that after natural rewatering, relative water content (RWC) of leaves and pseudobulbs generally increased. During the natural re-watering, total leaf area also gradually increased and reached maximum expansion after 7 weeks under both HL and LL, with some exceptions due to leaf abscission or decline in total leaf area, possibly a strategy for water conservation.

Highlights

Singapore has a tropical rainforest climate with no distinctive seasons, temperatures ranging from 22 ̊C to 35 ̊C, high relative humidity averages ranging from 60% to 100%, and annual rainfall about 2300 mm, with November to January being the wettest months
Diurnal Photosynthetic Photon Flux Density (PPFD) was much higher for the plants grown under high light (HL) than low light (LL) on a sunny day at 1000 h, 1200 h and 1400 h (Figure 1(a) and Figure 1(b)) and the diurnal Fv/Fm ratio at these timings were correspondingly lower in HL than LL (Figure 1(c) and Figure 1(d)), with the biggest difference in B. vaginatum and B. membranaceum
Our present study reports a reduction in the efficiency of open photosystem II (PSII) reaction centres of all the 6 orchid species under exposure to HL and C3 orchid B. membranaceum under exposure to LL, towards midday (Figure 1), indicating photoinhibition

Summary

Introduction

Singapore has a tropical rainforest climate with no distinctive seasons, temperatures ranging from 22 ̊C to 35 ̊C, high relative humidity averages ranging from 60% to 100%, and annual rainfall about 2300 mm, with November to January being the wettest months. Considerable efforts have been made in the past decade to re-introduce native orchid species back into Singapore as part of conservation [1]-[3] Many of these re-introduced species are epiphytes adapted to relatively dry habitats, native to low-elevation forests, and are dependent upon their microclimate. Crucial to these epiphytic orchids is the requirement for water economy where high evapotranspiration combined with scarce water supply challenges plant water balance [4]. Epiphytic orchids grow well under optimal environmental conditions and propagate naturally When they experience stress from high irradiance, high temperatures and periods of extended low rainfall, these factors significantly impact their physiology and lead to reduced growth and productivity

Objectives

Methods

Results

Discussion

Conclusion