Drought tolerance in dipterocarp species improved through interspecific hybridization in a tropical rainforest

Abstract

Gene transfer through interspecific hybridization can enhance functions such as growth performance, although its role in the ecophysiological functions of tropical tree species remains unclear. Hybrid seedlings of genus Shorea (Dipterocarpaceae), which dominates the canopy layer, are found in fragmented tropical rainforests in Southeast Asia. To elucidate changes in dipterocarp environmental adaptability due to hybridization, we studied the ecophysiological traits of dipterocarp seedlings including leaf morphology, photosynthesis, and drought tolerance in two parent species (Shorea curtisii and Shorea leprosula) and their hybrids, including F1 and backcross hybrids. Then, we induced drought stress to investigate drought tolerance in these seedlings. Photosynthetic ability was higher in S. leprosula than in S. curtisii, with the F1 and backcross hybrids showing intermediate characteristics. By contrast, drought tolerance, as indicated by stem hydraulic safety, cuticle thickness, and leaf mass per area, was higher in S. curtisii, lower in S. leprosula, and intermediate in their hybrids. The leaf turgor loss point decreased in S. curtisii due to osmotic adjustment under drought conditions, which led to lower predawn leaf water potential and higher water absorption ability from drier soil. By contrast, these changes were smaller and drought tolerance was weaker in S. leprosula. The hybrids including backcrosses had higher drought tolerance than S. leprosula, suggesting that gene transfer through introgression may facilitate the acquisition of drought tolerance. Although increased drought stress due to climate change and forest degradation are likely to limit seedling regeneration in S. leprosula, drought-tolerant genes obtained through hybridization with S. curtisii may contribute to its survival.