Negative shoot phototropism in a climber and an epiphyte

When a root-climber Ficus pumila and an epiphyte Peperomia quadrangularis were exposed to a light gradient, over 90% of the shoots grew away from light. This is the first demonstration of negative phototropism in these species. Whereas in climbers, negative phototropism of the shoots may help in remote location of support, its function in epiphytes remains uncertain.

Methods
This is an experimental test of the occurrence of negative phototropism in an adventitious-root climber Ficus pumila L. from S.E. Asia, and a neotropical epiphyte Peperomia quadrangularis A.Dietr. The plant material was obtained from cultivated stock. Rooted cuttings of Ficus pumila were planted in round pots 11 cm in diameter and 10 cm tall, and those of P. quadrangularis in square pots 6.5 cm wide and 8 cm tall. The test for phototropic growth involved exposing young plants to a gradient of light intensity from fluorescent sources and recording the growth direction of new shoots. 

In the experiment with F. pumila the method described in Wyka (2023, Tree Physiol 43:1874–1885) was used. Each young potted plant (n=33) was placed centrally in a larger (22 cm in diameter, 95 mm deep) pot and stabilized by filling the space in the outer pot with keramsite. The outer pot was inserted into a 30 cm tall tube made of glossy white photo paper. Half of the inner surface of the tube was lined with black landscape cloth, dividing the horizon of each plant into equal black and white surfaces . These setups were arranged on a table under Sylvania Pentron fluorescent lamps, with the black and white surfaces oriented in random directions. The plants were maintained under direct photosynthetic photon flux density PPFD=50.9 ± 7.2 µmol m-2 s-1 (mean ± st. dev.; n=20 measurements), measured with sensor held horizontally at the plant apex. The horizontal gradient of scattered light was measured by holding the sensor at the plant apex and orienting it sideways towards the center of the white and the black surface. The ratio of PPFD scaterred  from the white surface to that scattered from the black surface was on average 2.9 (n=20). Plants were checked weekly and considered to have reached the end point when the main shoot reached the tube. 

The experiment with Peperomia quadrangularis used a different set up. The light gradient was created by suspending a black cloth along the middle of a 1 m×1.4 m growing table and mounting Philips Master TLD fluorescent tubes above the edges of the table. Plants were arranged on each side of the cloth at 10 cm distance from the cloth (n=36 plants in total). The PPFD of light scattered from the black cloth at plant apex was 3.1-3.7% of the light arriving directly from the lamp (1.9 ± 0.3 µmol m-2 s-1 vs. 60.6 ± 11.6 µmol m-2 s-1, means ± st. dev., n=12 measurements in each direction). 

In both species, the plant response to light gradients was measured as the angle of deviation between the direction of shoot growth and the nearest point of the black surface.