Millennium Post

Timed to perfection

But this long-held belief had not been put to test, till recently. A study has utilised the difference in timings of the arrival of the southwest monsoon and the northeast monsoon in India to test whether plants fruit in sync with the rains.

Comparing a common set of plants found in regions which receive rainfall at different times of the year, the study demonstrates that peak fruiting occurs just before the rains. This ensures that germination takes place during the wet season. Thirty-three common, wind-dispersed, native species were chosen for the study, including 5 shrubs, 11 climbers and 17 trees found in the Udupi-Mangalore region (which experiences the southwest monsoon) and in the Chennai-Pondicherry region (which experiences the northeast monsoon). The researchers examined whether the peak fruiting season of the plants were in congruence with the onset of monsoon in their respective regions. They found that plants in the southwest monsoon path fruit in the month of April, ahead of the beginning of the wet season in June, while the same species, when they experience the northeast monsoon, fruit in the month of August, before the wet season in October.

Wind currents generally precede the wet season and some species are strongly dependent on the wind to disperse their fruits before the monsoon. Millions of years of evolution has hard-wired these plants to alter their biological schedules, else the rain would wet the dispersal structures and impair dispersal. ‘Think of silk cotton trees and what would happen to the seeds bearing the silk when they are wet,’ says Uma Shaanker, co-author of the study. On the other hand, if monsoons did not come after fruiting is complete, it could be catastrophic for the plant population. ‘In the common jack-fruit tree, for example, seeds have a viability period of only two weeks. Thus if the rain is late, the?seeds will die,’ explains Shaanker.

What happens with fluctuations in the arrival of the monsoon as a consequence of climate change? ‘There can be a certain degree of phenotypic plasticity in plants that allows them to adjust to these fluctuations,’ says Shaanker. The researchers have found that the effect of monsoon arrival on fruiting time is stronger in trees than in shrubs and climbing plants. Thus trees would be more threatened by climate change or rainfall variation than shrubs and climbing plants. ‘Our study shows that important medicinal plants like the Aristolochia indica (a gastric-stimulant), Holarrhena pubescens (used in treating dysentery and helminthic disorders), Hemisdesmus indicus (a diuretic) could be affected in the future by variation in monsoon arrival,’?says Aravind Madhyastha, lead-author of the study. ‘The mechanism underlying fruiting patterns could be genetic and/or environmental. The authors recommend further experimental studies to understand such mechanisms and predict how plants might respond to future climate change,’ says Deepak Barua, assistant professor, Indian Institute of Science Education and Research, Pune.

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