Characteristics of photosynthesis and conductance of potato canopies and the effects of cultivars and transient drought

Gas-exchange characteristics of potato (Solanum tuberosum L.) canopies were determined with crop enclosures under condition of optimal water supply, during soil water depletion, and during recovery from drought-stress. The plants were grown under a rainshelter in large containers with 2-m² surface area. Control plants received water throughout the growth period. In Expt. 1, transient drought was imposed by interrupting water supply completely for 22 days. During the 33-day period of drought treatment in Expt. 2, the stressed plants received half the amount of water that was given to the control plants (irrigations 2–3 times per week).

During soil water depletion, stomatal conductance of the canopy was reduced earlier and more than photosynthesis. This resulted in a lower internal carbon dioxide concentration in the leaves of stressed plants and a higher ratio between photosynthesis and stomatal conductance. The value of that ratio increased as soil water potential dropped.

Water-use efficiency ( ) increased during drought stress. The cultivar Bintje showed a larger ratio between photosynthesis and stomatal conductance of the canopy and a higher than cv. Saturna, both for optimal water supply and during drought stress.

Photosynthesis and conductance of the canopy started to recover immediately upon rewatering, even when relative values had declined to 20–30% during the preceding stress period. After release from stress, the leaves in the top layer of the canopy had a higher stomatal conductance in treated plants than in control plants.     

Genetic variation for nitrate content between cultivars of endive (Cichorium endiviae L.)

Summary A high nitrate content is a negative quality trait in vegetables. Endive (Cichorium endiviae L.) contributes considerably to human nitrate intake in Northwestern Europe. A collection of 125 commercially available cultivars of endive were grown on nutrient solution and evaluated for genetic variation between cultivars for the nitrate concentration in expressed sap. In two subsequent experiments 23 cultivars, representing the range of nitrate levels found in the first experiment, were grown both in soil and on nutrient solution. There were significant differences between cultivars in nitrate accumulation. Cultivar Vicor consistently had the lowest nitrate content. Averaged over all experiments the nitrate concentration in expressed sap of this cultivar was 0.7 g·l^-1 (19%) lower than that of two recommended cultivars. The results indicate that there may be some possibilities to reduce the nitrate content of endive by cultivar choice or breeding. However, the chances of obtaining a substantial reduction of the nitrate content of endive seem less than in some other crops, such as lettuce. The nitrate levels of the cultivars showed interactions with experiments and growth medium. There was no correlation between the cultivar means for the concentrations of nitrate and chloride, or between the concentration of nitrate and the osmolarity of the plant sap.     

Inheritance of partial resistance to Myzus persicae in lettuce

Summary The genetics of partial resistance of lettuce to Myzus persicae was studied using F₁ and F₂ generations of two crosses between a susceptible and partially resistant accession (Norden x Batacer and Liba x Norden) and three crosses in which both parents were partially resistant (Batavia la Brillante x Batacer, Batacer x Liba and CGN4741 x Batacer). Partial resistance to M. persicae inherited quantitatively, without important dominance effects. Only in the cross Batacer x Liba were significant departures of the F₁ and F₂ from the midparent found, which were probably caused by epistatic effects. Reciprocal F₁s had similar resistance levels, indicating the absence of cytoplasmic or other maternal effects. Estimates of broad-sense heritability ranged from 0.34 to 0.61. The results indicated that lines with an improved resistance level can be obtained from crosses between partially resistant accessions, preferably by line selection or the application of indirect marker aided selection.Abbreviations PR partial resistance, partially resistant - S susceptibility, susceptible     

The influence of cyst nematodes and drought on potato growth

In two experiments in the Wageningen Rhizolab with potato cv. Mentor planted in soils with or without potato cyst nematodes (Globodera pallida, W) the number of roots per cm² was observed two weekly by video camera, in horizontally placed minirhizotrons at depths varying from 5 to 100 cm. In both experiments initial root growth was more rapid in the top soil of the uninfected soil. In the first experiment under optimal water supply, root formation continued longer in the top 30 cm of infested soil leading to twice as many roots at the end of the growing season as in uninfested soil. In the subsoil from 30 to 100 cm, however, root formation was strongly reduced by cyst nematodes leading to an uneven distribution of roots throughout the profile. In the second experiment potato cyst nematodes only increased rooting in the top soil with reduced irrigation. Potato cyst nematodes did not affect the water use efficiency of the crop whereas reduced irrigation increased water use efficiency by about 22%. Without potato cyst nematodes the soil profile was depleted of mineral nitrogen until a depth of 1 m whereas with high initial population densities no nitrogen was taken up in the subsoil between 30 and 100 cm. The spational heterogenity of roots and nitrogen in the soil is an important mechanism of damage. This finding may lead to improved cultural practices and breeding for tolerance.