Sexual differences in photosynthetic activity, ultrastructure and phytoremediation potential of Populus cathayana exposed to lead and drought.

Lead (Pb) and drought frequently coexist in China's forests and seriously affect their biomass, net primary productivity and biodiversity, particularly among dioecious trees with different sex-related reproduction costs. Here, the effects of Pb, deposited into soil (Pbsoil) and leaves (Pb(leaf)), on the traits related directly or indirectly to photosynthetic activity were evaluated in the dioecious tree, Populus cathayana Rehd. In addition, we analysed the potential of P. cathayana males and females for phytoremediation based on the photosynthetic activity, cellular ultrastructure and phytoremediation-related parameters. The Pb level was 2.7 mmol Pb(NO3)2 kg(-1) dry soil in the Pb(soil) treatment and 1.8 mmol Pb(NO3)2 per plant in the Pb(leaf) treatment. In parallel experiments, two watering regimes, 100 and 50% of the field capacity, were applied. The stress duration was 2 months. Dry mass production, net photosynthetic rate increased in both sexes, particularly in females, when exposed to Pb(leaf) alone, Pb(soil) with drought and Pb(leaf) with drought. The study revealed that males exhibit greater plasticity in the photosynthetic capacity than females. Severe damage to cellular ultrastructure was also observed in the leaves of males and females exposed to Pb(leaf), but more strongly in females. However, the Pb(soil) treatment alone did not affect these traits as significantly compared with other treatments. Moreover, drought significantly increased the sensitivity to Pb stress in both sexes, but more so in females. In addition, changes in the photosynthetic capacity and cellular ultrastructure combined with the analysis of tolerance index, translocation factor, bioconcentration factor and Pb concentration showed that males and females could be employed for different purposes in phytoremediation: females are promising candidates for phytoextraction, whereas males are promising candidates for phytostabilization.