(a) Striga is a parasitic weed that attacks cereal crops such as sorghum in parts of Africa. (i) Explain what is meant by a parasitic weed. [2] (ii) Describ...

(a)(i) A parasitic weed is a plant that attaches itself physically to a host crop plant [1] and obtains water, mineral nutrients and sometimes organic compounds directly from the host's vascular system, rather than from the soil through its own roots. [1] This makes it fundamentally different from ordinary weeds, which compete for the same soil resources but grow independently.

(a)(ii) Two ways Striga reduces sorghum yield (any two): [2]

1. Striga penetrates the roots of the sorghum plant using specialised structures called haustoria, diverting water and dissolved nutrients away from the crop and into the parasite. [1]
2. This causes severe wilting, stunting and chlorosis (yellowing) of the sorghum plant, dramatically reducing its ability to photosynthesise and fill grain, leading to lower yields or complete crop failure in heavy infestations. [1]

(b) Three cultural methods to reduce Striga, each with explanation: [6]

1. Crop rotation with non-host crops (e.g. groundnuts, cowpeas, cotton): Striga seeds require a chemical signal (strigolactone) released by the roots of a host cereal to trigger germination. Growing a non-host crop means no germination signal is produced, so Striga seeds remain dormant in the soil and the seed bank declines over time as seeds lose viability. [2]
2. Hand-pulling Striga plants before they flower and set seed: each Striga plant can produce tens of thousands of tiny seeds that remain viable in the soil for over 10 years. Removing and destroying plants before seed-set prevents replenishment of the soil seed bank, gradually reducing the infestation over successive seasons. [2]
3. Improving soil fertility with manure or fertiliser: well-nourished, vigorous sorghum plants can tolerate a degree of Striga parasitism without severe yield loss. Higher soil nitrogen levels have also been shown to reduce the amount of strigolactone exuded by crop roots, which may reduce Striga germination. [2]

Another acceptable method: adjusting the sowing date to allow Striga seeds to undergo suicidal germination (germinating in response to soil moisture but dying without a host before the crop is planted).

(c) A trap crop such as cotton releases root chemicals (strigolactones) that are similar enough to those produced by sorghum to stimulate Striga seeds in the soil to germinate. [1] However, once the Striga seedlings attempt to attach to the cotton roots, they cannot successfully parasitise the non-host crop and die. This depletes the Striga seed bank in the soil before sorghum is planted in the following season, reducing the number of parasites that will attack the crop. [1]

(d) Breeding Striga-resistant sorghum varieties is the most sustainable long-term solution because (any three): [3]

1. Resistant varieties either do not stimulate Striga germination (producing low levels of strigolactones) or prevent successful attachment of the parasite to the crop roots, so the parasite never establishes. [1]
2. The resistance is built into the seed itself, so the farmer does not need to purchase herbicides or hire extra labour for Striga control each season, reducing recurring costs. [1]
3. It is environmentally sustainable because no chemicals are applied to the soil, avoiding contamination of water sources and harm to beneficial soil organisms. The protection is inherited by the seed and works every season without additional inputs. [1]