The combined effects of soil moisture and irradiance on growth,biomass allocation,morphology and photosynthesis in <Emphasis Type="Italic">Amomum villosum</Emphasis>

Amomum villosum grown in the traditional way for economic purpose in Xishuangbanna, southwest China, causes some damages to the local tropical seasonal rain forests. It is important to evaluate the effects of irradiance and soil moisture on A. villosum in order to find out ways to maintain the local forest ecosystems on the one hand and to promote its economic cultivation with agroforestry practices on the other hand. Soil moisture was the main determinant of plant growth. Seedling height, leaf number and area, biomass, relative growth rate (RGR) and net assimilation rate decreased significantly with the decrease of soil moisture. There were no interactions between soil moisture and irradiance on all the plant traits evaluated in this study. The effects of soil moisture and irradiance were orthogonal; drought reduced RGR at a similar degree at all irradiance levels. The plant can acclimate to high irradiance combined with low soil moisture. Under this condition, A. villosum modified its biomass allocation in favor to roots rather than to leaves so that whole-plant level water balance could be well maintained. Furthermore, many small-sized slender leaves were formed to facilitate leaf thermal loss, its carotenoid content increased in favor for photoprotection, and contents of chlorophyll and light-harvesting complex of photosystem II decreased so that its irradiance interception was better balanced for plant adaptation. Our results indicated that soil moisture is a more important factor to concern than irradiance when planting A. villosum in agroforestry practices and that A. villosum can be grown in high light habitats such as secondary forests and artificial forests when soil moisture is adequate.