Ratio of Nutrient and Minerals to Defensive Compounds Indicative of Plant Quality and Tolerance to Herbivory in Pear Trees

ABSTRACT

The phenolic ester 3-0-trans-p-coumaroyltormentic acid and the volatile compounds (E,E)-α-farnesene and methyl salicylate have been identified previously in pear tree cultivars ‘Bartlett,’ ‘NY10355,’ and ‘Conference.’ Here, using the same dried extract of leaf samples, the contents of total protein, chlorophyll, and the minerals calcium (Ca), magnesium (Mg), phosphorus (P), potassium (K), sodium (Na), carbon (C), and nitrogen (N) were determined. Differences between pear cultivars were found with regard to their absolute content in Ca, K, chlorophyll, and total protein, and to carbon/nitrogen values in uninfested leaves compared with leaves infested by Cacopsylla pyricola (Foester) and C. pyri L. (Homoptera, Psyllidae) on partially and completely infested trees. The ratio of nutrients and minerals to the constitutive and Cacopsylla-induced phenolic and two volatiles reflects a diminution of the leaf quality as food for Cacopsylla nymphs in infested leaves of all cultivars. In uninfested leaves, the magnitude of this ratio in partially infested trees of cv. ‘Conference’ with local response, together with Ca ion content and chlorophyll loss, point to classification of this cultivar as tolerant to herbivory.

Keywords:

• pear
• Cacopsylla
• defensive compound
• systemic
• local
• nutrient
• mineral
• ratio
• quality
• tolerance

ACKNOWLEDGMENTS

We thank Jan Bruin for his comments on this manuscript; Anette Johansson for total protein analysis; P. Bregman for C and N analysis; P. van Egmond for chlorophyll analysis; Leo Hoitinga for Ca, Mg, K, P, and Na determination and also discussions; and Ludek Tikovski for help with grafting the scions of pears. Dr. P. Scutareanu is grateful to Maurice Sabelis for giving him the opportunity to work in his department, while both authors thank B. Bartlett (National Clonal Germplasm Repository, Corvallis, Oregon, USA) for providing the grafts of pear varieties ‘Bartlett’ and ‘NY10355.’