OBSERVATIONS OF THE EFFECT OF NITROGEN AND POTASSIUM ON THE FRUITING OF THE TOMATO

Abstract

Summary The effect of nitrogen and potassium on the fruiting of the tomato under glass is studied by observations on plants grown on completely manured plots and plots from which nitrogen and potassium respectively have been omitted from the scheme of manuring over a period of years. Nitrogen starvation reduces the number of blossom‐buds formed, the percentage of buds that open and the mean numbers of fruits per flower‐truss. Potassium starvation does not affect the number of blossom‐buds formed or the percentage of buds that open but reduces the mean numbers of fruits per flower‐truss. An outstanding feature of nitrogen starvation is retardation of the rate of development of successive flower‐trusses. With potassium starvation the rate of development is accelerated. Growth, estimated from increase in height of plants trimmed to a single axis by continuous removal of incipient axillary shoots, is retarded by both nitrogen and potassium starvation. The “maturation period” (between opening of the blossom and ripening of the fruit) is lengthened by potassium starvation, shortened by moderate nitrogen starvation and lengthened by severe nitrogen starvation. From the present results, in conjunction with data published previously, it is concluded that the “maturation period” is shortened by a high carbohydrate‐nitrogen balance and lengthened by a low carbohydrate‐nitrogen balance. In mid‐season all plants suffer a check to growth and development demonstrated by (a) retardation of the rate of differentiation of leaves and flower‐trusses, (b) cessation of growth in height, and (c) the occurrence of a high proportion of fruit with arrested development. This “mid‐seasonal check” is accentuated by nitrogen and potassium starvation, especially the former, corresponds with the period of maximal weight of developing fruit, and is attributed to competition for nutrient supply between the fruits and vegetative parts of the plants, leading to apparent antagonism between these processes. The fruiting and foliage symptoms of the potassium‐starved plants are attributed to the following effects: (a) The potassium in starved plants is undergoing a continuous cycle, being translocated from older parts of the plant to the growing point, incorporated in young leaves and flower trusses and subsequently re‐translocated as these organs are approaching maturity.(b) The level of potassium supply regulates nitrogen metabolism, a moderately low potassium supply being associated with high protein level and a greater degree of seventy of potassium starvation with low protein level. The results are discussed in relation to the problems of overbearing and potassium manuring. The observed effects of potassium deficiency on fruiting (acceleration of development of flower trusses, failure of pollination and prolongation of the “maturation period” of the fruit) are those also associated with carbohydrate deficiency relative to nitrogen supply.