Calcium deficiency in potato (Solanum tuberosum ssp tuberosum) leaves and its effects on the pectic composition of the apoplastic fluid

Potato plants (Solanum tuberosum ssp, tuberosum cv, Adelheld), multiplied i n vitro, mere cultivated in growth chambers on nutrient solution at calcium regimes of 1000, 90, 60 or 30 mu M Ca. An absolute Ca deficiency, particul arly at the Iom Ca-supply levels of 30 and 60 mu M Ca, manifested itself in itially in tbe form of marginal necrosis in younger, but not in the younges t, leaves of the potato plants. Further symptoms mere rolling of the leaf l amina, browning of veins and roots, and finally necrosis also of the younge st leaves. Only in an advanced stage of Ca deficiency, the meristem of the shoots died, Ca-deficiency symptoms could be expected at a Ca content in th e leaves of less than 5 mg Ca (g dry weight)(-1) However, there was no clos e negative correlation between the extent of leaf damage and the total Ca c ontent of the leaves. In order to obtain information about tbe Ca concentra tion in the apoplast fluid of the leaves, apoplastic mashing fluid mas extr acted by an infiltration-centrifugation technique. A low Ca supply reduced the Ca concentration both in the apoplast fluid of the leaves and in the ce ll walls. Up to 60% more diffusible pectin fragments mere then found in the apoplast of younger leaves, as compared to the control supplied with an op timum Ca level of 1000 mu M The amount of diffusible pectins accounted for 1-2% of the total pectin content of younger potato leaves. The size of the existing pectin fragments varied depending on the Ca supply, Compared with an optimum Ca supply of 1000 mu M; fewer monomers and up to 7 times more di ffusible pectin fragments with a degree of polymerization 9-20 were present at the low Ca-supply level (30 mu M). In addition, polygalacturonase activ ity in tissue homogenates increased remarkably with Ca deficiency. Thus it appears that one major effect of Ca deficiency was a stimulation of the act ivity of polygalacturonase, which could control the breakdown of pectic pol ysaccharides in the cell wall. Whether the release of potentially biologica lly active pectic fragments in cell malls might be involved in the occurren ce of Ca-deficiency symptoms is discussed.