RELATIONSHIP BETWEEN CHLORIDE AND NITRATE AND ITS EFFECT ON GROWTH AND MINERAL-COMPOSITION OF AVOCADO AND CITRUS PLANTS

Two rootstocks of avocado (Persea americana Mill.), the salt-tolerant 'Degania-113' and the salt-sensitive 'Smith', and two rootstocks of ci trus, the salt-tolerant 'Cleopatra' mandarin (Citrus reshni Hort. ex T an) and the salt-sensitive 'Troyer' citrange (Poncirus trifoliata x Ci trus sinensis [L.] Osbeck), were grown in a sandy soil and irrigated d aily with nutrient solutions containing various chloride concentration s. Increasing the concentration of chloride resulted in elevated chlor ide levels in all avocado plant parts, and toxic symptoms that were mo re pronounced in the 'Smith' than in the 'Degania-113' avocado rootsto ck. When leaves of both rootstocks had accumulated similar chloride le vels and showed scorching damage, the leaves of 'Degania-113' abscised , while those of 'Smith' did not. High chloride reduced the total dry matter yield of the root more than that of the shoot, decreasing the ' 'root: shoot'' dry weight ratio in both rootstocks. Addition of nitrat e to the irrigation water reduced chloride accumulation in the plant a nd alleviated its adverse effects. Accumulation of nitrogen in the pla nt exceeded that of chloride in all cases. High nitrate reduced phosph orus levels in the plant and caused chlorosis in young leaves. High ch loride partially reversed this effect. Citrus rootstocks responded sim ilarly, but leaves of the chloride-sensitive 'Troyer' accumulated more chloride than 'Cleopatra'. High chloride resulted in scorching damage in the leaves and branches of 'Troyer', but not in 'Cleopatra'. Nitra te reduced chloride accumulation and toxic symptoms as well as boron l evels in the leaves. The results of this study suggests that water con taining high chloride levels may be used to irrigate avocado and citru s orchards, provided that nitrate is supplied continuously at a molar concentration equivalent to half that of chloride. Our results also su ggest that a nitrate supplement applied to citrus will reduce the unde sirable uptake of boron. The difference in mode of resistance to chlor ide between avocado and citrus rootstocks is discussed.