Name the structures labelled J to L.
Outline how this leaf anatomy adapts the plant for rapid carbon fixation at high temperatures.
Sorghum is a C4 plant, and Sorghum bicolor is an important food crop in dry tropical regions. The leaves of S. bicolor are coated with a wax layer made from a mixture of esters and free fatty acids, and this wax melts at $77$-$85\ ^\circ\text{C}$. Waxes from the leaves of non-tropical plants usually have lower melting points than this. For example, wax from the bayberry, Myrica sp., has a melting point of $45\ ^\circ\text{C}$. Suggest how the wax on sorghum leaves helps the plant to survive in dry, tropical regions.
An investigation examined how sorghum responds to a brief period at low temperature. Soybean plants, which are better adapted than sorghum for growth in subtropical and temperate climates, were used for comparison. Sorghum and soybean plants were maintained at $25\ ^\circ\text{C}$ for several weeks and then transferred to $10\ ^\circ\text{C}$ for three days. The temperature was then raised back to $25\ ^\circ\text{C}$ for seven days. Day length, light intensity and carbon dioxide concentration were kept constant throughout. The uptake of carbon dioxide, in mg $\text{CO}_2$ absorbed per gram of leaf dry mass, was measured: - at $25\ ^\circ\text{C}$ before cooling - on each of the three days at $10\ ^\circ\text{C}$ - for seven days at $25\ ^\circ\text{C}$ Table 4.1 shows the results.
Compare how carbon dioxide uptake changed in sorghum and soybean over the three days at $10\,^{\circ}\text{C}$.
During cooling, the ultrastructure of the sorghum chloroplasts altered. The thylakoid membranes moved closer together, removing the spaces between them. The grana became smaller and fewer in number. Explain how these alterations could account for the low rate of carbon dioxide uptake by sorghum even after it has been returned to a temperature of $25\,^{\circ}\text{C}$.