SUMMARYYield stability in cassava requires genotypes that produce well under the variable moisture conditions encountered during the growth cycle. Plant characteristics related to yield stability were studied in two cassava clones subjected to 105 days of water stress in a field drainage lysimeter. Stress conditions commenced 117 days after planting, and the plants were allowed to recover at the end of the stress period for the rest of the growth cycle. Water stress restricted the growth of leaves and stems, but root yields were increased or remained unaffected. Leaf water potential varied little with stress, but gas exchange rates were about 75% those of the control throughout the stress period in both cultivars. Under stress, the plants partially closed their stomata and extracted deep soil moisture slowly. A high yield in both wet and stressed environments was associated with high mean LAI, better leaf retention, and greater partitioning of shoot biomass into leaf formation.
Measurements of CO2 and H2O exchange rate and the calculated leaf conductance of attached leaves were conducted over a range of leaf-to-air vapour pressure difference (VPD) (1.5 to 5.5 kPa) to compare the response of the parasitic mistletoe, Phthirusa pyrifolia, with that of its host, the mandarin orange, Citrus reticulata. Seedlings of the host infected with the parasite were grown in well-watered and adequately fertilized large pots outdoors at the CIAT headquarters, Palmira, Colombia, South America. Observations of leaf anatomy of the parasite and nutrient analysis of young tissues of both the parasite and host were made. The photosynthetic rate of the host decreased linearly with increased VPD, whereas the parasite showed a constant rate. This trend coincided with similar responses in leaf conductance. Due to the insensitivity of the parasite stomata, the transpiration rate increased linearly with VPD as compared with an initial increase and then a decrease in the host transpiration rate. The higher photosynthetic rate and the closure of stomata of the host resulted in high water use efficiency as compared with that of the parasite. The parasite accumulated in its leaves more N, P, K and less Ca and Mg than the host. The significance of the host-parasite differential response to air humidity is discussed in relation to mechanism underlying stomatal sensitivity and in the context of host-parasite association.
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