Chap 7a n 7b

A farmer removes 50% of developing apples from a tree. The remaining apples grow larger because:

• Photosynthesis rates in leaves double to compensate.

• Reduced sink size increases resource allocation per remaining fruit.

• Xylem transport becomes more efficient.

• The symplastic unloading pathway is activated.

During drought, a plant switches from apoplastic to symplastic phloem loading. Why?

• To conserve ATP by avoiding active transport.

• Plasmodesmata allow faster water movement.

• P-proteins block sieve plates under stress.

• Oligosaccharides like raffinose reduce water loss.

In a plant with high sugar export demand, which companion cell type would dominate?

Radiolabeled carbon shows sucrose moving from leaves to roots but not to immature leaves. This is because:

• Immature leaves lack plasmodesmata.

• Immature leaves are sinks, not sources.

• Roots actively block sucrose transport.

• P-proteins degrade sucrose in transit.

A pathogen blocks sieve plate pores. How does the plant respond short-term?

• Produces callose to seal pores permanently.

• P-proteins form plugs to limit sap loss.

• Increases symplastic unloading in sinks.

• Activates xylem to bypass phloem.