EFFECT OF SHADE ON LlMONOID AND NITROGEN COMPOSITION IN SHOOTS OF Swietenia macrophlla.(KING)
The main factor limiting cultivation of mahogany, Swletenia macrophylla King(Meliaceae), in plantations is damage by shoot borers (Hypsloyla spp.). Shade hasbeen repeatedly cited as reducing shoot borer attack but the responsiblemechanisms have not been determined by experimentation. Shade may altereither secondary compounds such as limonoids or nitrogen concentrations or bothin the plants making them unsuitable for insect survival and development.Previous studies have not examined whether shade influences Iimonoid andnitrogen content of S.marcrophylla and thereby alter shoot-borer attack.Therefore, these studies were designed to identify the variation in incidence ofshoot borer attack under three levels of shade treatments in the field conditionsand the variation in plant chemistry of Ssmarcrophylla shoots grown under threedifferent artificial shade treatments. The hypotheses tested were under high lightavailability (i) incidence of shoot borer attack increases (ii) the concentration oflimonoids present in the shoots of S. macrophylla decreases ind(iii) total nitrogencontent of shoots increases.
This study was done from 1997-99 in Sri Lanka and UK. Attack by the shootborer was assessed 54 weeks after planting. The Limonoid content wasinvestigated by Thin Layer Chromatography (TLC) and the total nitrogen byKjeldhl method using freeze-dried ppowdered shoots. The incidence of shootborer attack was significantly higher under low shaade (dJ.=2, F=8.6, p=O.0003).The intensity of the green-blue fluerescence under UV seen in the TLC plates wasgreater using the extracts from the high shade treatment compared to that from thelow shade treatment. The total nitgrogent content was satistically lower in thehigh shade (1.l14± 0.22%, n=33) compared to that in full light (1.36± 0.22%;n=31). These results suggest that light environment may have a significant effecton the concentrations of limonoids and nitrogen in S.macrophylla which in turninfluences the shoot borer attack.
The study forms part of a larger project on 'Silvicultural prescription formahogany plantation establishment; Research grant funded by DFlD, UK isgreatfullyacknowledged.