Explain how rice plants are adapted for growth when their roots are kept below the water surface.
Some cultivated rice varieties can produce long internodes when they are submerged in water, which keeps the leaves and flowers above the water surface (an internode is a section of stem between leaves). These varieties are called deepwater rice. The snorkel genes $SK1$ and $SK2$, believed to cause this response, were identified in a deepwater rice variety, C9285. A non-deepwater variety, T65, lacked these genes. When rice plants are submerged, they release the gaseous plant hormone ethene. Because this has very low solubility in water, it accumulates in the aerenchyma tissue in the rice stems. Fig. 4.1 shows the concentration of ethene in the aerenchyma of T65 and C9285 when the plants are submerged in water for $18\ \text{hours}$.
With reference to Fig. 4.1, describe the effect of submergence in water on ethene production in rice.
With reference to Fig. 4.2, compare the effect of ethene on internode elongation in C9285 and T65.
The snorkel genes were shown to be expressed when the plant was exposed to ethene. Expression of these genes leads to a higher production of gibberellin, GA. Fig. 4.3 shows the effect of submergence on GA production in C9285 and in T65. With reference to Fig. 4.3, and your knowledge of the functions of GA, suggest an explanation for the differences in the effects of ethene in C9285 and T65 shown in Fig. 4.2.
Cultivated rice has been developed from the wild rice species Oryza rufipogon and Oryza nivara. O. rufipogon shows a strong deepwater elongation response, whereas O. nivara shows only a slight elongation response. Another species, Oryza glumaepatula, shows a strong elongation response. O. rufipogon has both the snorkel genes, $SK1$ and $SK2$. O. nivara has $SK1$, but an addition (insertion) mutation has created a stop triplet within $SK2$. O. glumaepatula has $SK2$, but not $SK1$. Describe what this information indicates about the relative importance of the genes $SK1$ and $SK2$ in the deepwater elongation response.
Explain how an addition mutation could create a stop triplet.
Deepwater rice is the principal food crop in many regions that are flooded during the rainy season. Many deepwater rice varieties yield less than non-deepwater varieties. Suggest how a deepwater rice variety with high yield could be produced, using artificial selection.