Stigmatic RNase in calamondin (Citrus reticulata var. austera x Fortunella sp.)

In calamondin. which is self‐compatible, ribonuclease (RNase) activity was found in the stigmatic diffusate. Tissue print experiments using calamondin styles demonstrated that most of the RNase is localized in the stigma. Stigmatic RNase activity was monitored at different developmental stages of the flower and was found to peak at anthesis. An SDS‐PAGE‐zymogram indicated the molecular mass of this RNase to be 24 kDa. In vitro germination of calamondin pollen showed a higher percentage of germination in the presence of diffusate from one stigma than in the control. However, diffusates from 3.5 and 7 stigmata per 100 μl aliquots of growth medium, exhibiting 15. 25 and 35 units ml⁻¹ RNase activity, respectively, had successively stronger inhibition effects on the percentage of germination. Diffusate from 7 stigmata inhibited pollen tube elongation, as well as pollen germination. Both the 24‐kDa Stigmatic RNase and RNase TI significantly inhibited pollen germination and pollen tube elongation. In pollen tubes treated with either the 24‐kDa stigmatic RNase or RNase TI, considerable deposition of callose was observed, as compared to the control which had only a thin callosic cell wall.