(Possible relationship to potassium deficiency)
by Joe Traynor
Putrescine is one of a number of nitrogen (N) compounds that are intermediate metabolic products as plants convert fertilizer nitrogen to more usable forms. Some N compounds are toxic to plants — cyanide (CN) is an example but is rarely a problem in commercial agriculture. Putrescine, whose precursor is arginine, can, under certain conditions, accumulate to levels that are toxic to plants – do a Search for putrescine toxicity and you will find hundreds of reports on the subject but none (to my knowledge) for almonds.
Putrescine toxicity is also associated with potassium deficiency (do a Search for putrescine potassium). Apparently potassium (K) is needed to convert putrescine to safer metabolites and if K is deficient, putrescine will accumulate to toxic levels – high putrescine levels in plant tissue can be as good an indicator of K deficiency as K levels alone (and can also indicate excess nitrogen). It should not be surprising that symptoms of putrescine toxicity are identical to those of K deficiency – insufficient K allows putrescine to build to toxic levels. This can lead to confusion if one does not recognize that putrescine toxicity and K deficiency can act independently of each other.
Can putrecscine accumulate to toxic levels when plant K levels are sufficient? Yes! – under certain conditions. In an apple study, normal leaves containing ample K were fed putrecscine and soon showed classic K deficiency symptoms. Detailed UC studies on grapes introduced the term False Potassium Deficiency (FK) for grape vines that showed classic K deficiency symptoms but had K levels in plant tissue that were slightly depressed but were considered sufficient. Leaves from these FK plants showed putrescine levels up to 30 times higher than normal levels. K levels in leaves bounced back and symptoms disappeared in a few weeks without any supplemental potassium.
We know that putrescine toxicity occurs in K-deficient plants but how does putrescine accumulate in plants with adequate K? The UC grape study showed that putrescine toxicity (or FK) is often seen when a cool spell follows a warm spell. There is likely an accumulation of putrescine when plants (and trees) are given nitrogen in excess of their needs and cool weather prevents or slows down the metabolic conversion of putrescine to less toxic N compounds. The UC study showed that plant analysis for total N or ammonium-N can be used to distinguish putrescine toxicity from K-deficiency – false potassium (FK) plants had very high N and ammonium-N levels.
In 2010, there were scattered reports of classic K deficiency symptoms on young, non-bearing almond trees and K-deficiency was diagnosed as the cause. This flies in the face of everything we know about K-deficiency – that crop load, not vegetative growth, drives the demand for K; this is especially true for almonds where a bumper crop removes 200+ pounds of K from the soil, but K taken up by vegetative growth is negligible
Growers can minimize putrescine toxicity on young, non-bearing trees by being careful not to over-fertilize with nitrogen. Adding potassium fertilizer to non-bearing trees is a dubious undertaking as this K can be tied up in the soil and less available when the trees really need it – when they start bearing. Adding a triple-mix (N-P-K) fertilizer to newly planted trees can backfire since the K in such fertilizers is almost always in the form of potassium chloride and chloride can retard the growth of new trees.
If you see K-deficiency symptoms on non-bearing trees – marginal and interveinal leaf scorch and upward curling of leaves — resist the temptation to apply potassium fertilizers unless leaf analysis for K shows definitively that what you are seeing is true K deficiency. Instead, cut back or temporarily eliminate N fertilization and it is likely that the problem will self-correct during warmer weather as the trees work the excess putrescine out of their systems. Save your K fertilizer dollars for when they are needed down the road—when your trees start cranking out those bumper crops.
Comparative Leaf Tissue Analysis of Potassium Deficiency and a Disorder Resembling Potassium Deficiency in Thompson Seedless Grapevines; Peter Christensen et al, Amer. J. Enol. Vitic., 41(1)77-83 (1990).
Elevated Putrescine Levels in Grapevine Leaves That Display Symptoms of Potassium Deficiency; Douglas Adams et al., Am. J. Enol. Vitic. 41(2)121-125. (1990).
Effects of potassium deficiency on nitrogen metabolism of fruit plants. G. Forshey and M. McKee. J. Amer. Soc. Hort Sci. 95:727-729 (1970).
This article appeared in the Feb.11, 2011 issue of Nut Grower and PCA (Pacific Nut Producer)