The Evolution of a Weed

Abstract

Saccharum spontaneum L. is an innocuous grass species of the Old World, but one of its ecotypes behaves as a pernicious weed in Central India. The species is believed to have evolved in the ecological mosaic of the sub-Himalayan valleys into a wide range of polyploids (2n = 40 to 128), from where its higher polyploids may have migrated into South-east Asia and Africa while the lower ones spread over the Indian sub-continent. The species shows much morphological diversity, and a remarkable plasticity and adaptation to environment. Its ecotypes propagate themselves variously, and many clones possess rhizomes for tiding over long dry seasons. The weed ecotype is one of the latter.

When a collection of S. spontaneum clones was grown in an experimental garden, most of the clones from the Himalayan river basins and the larger deltas showed no tendency to rhizome formation, while almost all occurring in the Central Indian plateau were highly rhizomic; the lower peninsular clones showed various degrees of predisposition.

These differences were seen to be associated with the presence or absence of soil droughts and long dry seasons in the original habitat. Apparently, natural selection for the rhizome character has taken place largely on the basis of this factor of the environment.

Genes for rhizomes may have arisen in the sub-Himalayan area and emerged into the Gangetic valley via areas felled indiscriminately and with inefficient agricultural follow-up. They may have accumulated under a high-selection pressure in the Central Indian plateau area and given rise to the pernicious weed ecotype here.

S. spontaneum does not yield to ordinary weeding; mechanised deep ploughing produced good results, but later the infestation returned. The weed may yield to irrigation, as this would dislocate it ecologically. Irrigation would boost its hot weather growth, which could be blade-harrowed away. A summer crop of rice or sunn hemp (for green manure) would keep the weed down until it is too late for it to grow. Pilot studies would show whether in this way the rhizomes could be starved out. Under the altered agro-ecological conditions, re-infestation may not succeed.