Excess ammonium in foliar tissue: a possible cause of interveinal chlorosis in strawberry (Fragaria x ananassa Duch. cv. Nyoho)

Substrate-grown strawberries (Fragaria x ananassa Duch. cv. Nyoho) in excessively fertigated peat bags often suffer interveinal chlorosis (leaf yellowing) in their immature leaves shortly after planting. Full recovery was observed in such plants following a 4 - 7 d restraint in the supply of nutrients. Hence, the cause of this phenomenon could reasonably be attributed to excess NH4-N accumulation in plant tissues. As has been previously shown, NH4-N accumulation in plant tissue can be induced by inhibition of glutamine-synthetase (GS). Thus, a GS inhibitor (glufosinate-ammonium) was applied at various dosages to peat bag-grown 'Nyoho' plants, foliar NH4-N concentrations were determined and yellowing symptoms were observed. After 7 d of treatment, foliar NH4-N concentrations increased dramatically, 1 - 2 d prior to the onset of severe yellowing. Subsequently, the relationship between nitrogen (N)-source and leaf yellowing was investigated. NH4-fed plants initially had higher NH4-N concentrations in their immature leaves than NO3-fed plants, and later suffered from interveinal chlorosis. Potted plants dipped in the relevant nutrient solutions exhibited seven-fold higher NH4-N concentrations in their immature leaves than plants that were manually supplied with 50 ml of the relevant nutrient solutions twice a day. In this study, we also investigated whether the combined effect of a N-source under various environmental conditions (e.g., light intensity and air temperature) affected NH4-N accumulation in plant tissues, as has been suggested previously. We observed that, in 'Nyoho' plants, elevation of foliar NH4-N concentrations and the appearance of yellowing symptoms began earlier and was more severe under conditions of higher solar radiation and air temperature. The absence of interveinal chlorosis in plants that exhibited low NH4-N concentrations, regardless of treatment, led to the conclusion that high leaf NH4-N concentrations and excess accumulation of NH4-N may play an important role in the leaf yellowing phenomenon.