The effect of rootstock on tree size, root distribution and leaf mineral content of ‘Orlando’ tangelos on 11 rootstocks was studied. Pronounced differences in depth of rooting, weight of feeder roots and tree height were detected. Depth of rooting was correlated to tree height, (r = .58, 1970; r = .83, 1971) i.e., the tallest trees had the deepest root systems. Feeder root wt and tree height were not related. The level of leaf N, K, Ca and Mg but not P was related to rootstock, suggesting a differential absorption of mineral nutrients by rootstock. The level of N appeared to be influenced by root distribution. Trees with deep extensive root systems or with a large number of feeder roots near the surface had high leaf N. Leaf K was significantly correlated with depth of rooting, (r = .96, 1970; r = .84, 1971). The results suggested the maximum performance of all rootstocks was not attained under the uniform cultural conditions of this experiment and therefore the need to examine each rootstock under conditions optimum for it.

Two distinct periods of fruit drop, summer (mid-June to mid-August) and summer-fall (late August to late October), of navel sweet orange [Citrus sinensis (L.) Osbeck] were observed during the period between the end of the June drop (mid-June) and legal maturity (late October) in the 1978-79 and 1979-80 seasons. Summer drop reduced fruit number in 1979 by as much as 16.5% and was mostly caused by a yellowing and subsequent decay of the secondary fruit. A separation zone was found between the central axis and the secondary fruit of those with yellow navels. Summer-fall drop was responsible for an additional 14.5% reduction in fruit number based on catching frame counts. It was caused primarily by stylar-end decay, dieback of branches, splitting and brown rot (Phytophthora spp.). Causes of stylar-end decay, branch dieback and splitting were not identified. Incidence of stylar-end decay and splitting was greatest on fruit with a large (> 12 mm) stylar-end aperture (navel) of irregular shape, and adjacent thinner than usual peel. Stylar-end decay was associated with protrusions of rind-like tissue from the secondary fruit into segments of the primary fruit.

The pattern of soil water extraction by ‘Orlando’ tangelo (Citrus paradisi Macf. × C. reticulata Blanco) trees on rough lemon (C. jambhiri Lush.), sour orange (C. aurantium L.), sweet orange (C. sinensis (L.) Osbeck), and ‘Rusk’ citrange (Poncirus trifoliata Raf. × C. sinensis) rootstocks was studied using the neutron moderation method. Root distribution data from a previous study of the same trees were used to compare to water extraction data and to calculate “apparent root efficiencies.” Soil water loss was significantly correlated to feeder root dry weight. The general pattern of moisture extraction showed that the largest water losses initially occurred at the surface with increasing contributions from deeper roots as water from the surface layers was depleted. Roots at each depth and from each rootstock were not equally efficient. The adaptability of trees on the 4 rootstocks to the soil of the experiment site appeared to be related to their respective root distributions and efficiencies.

No abstract

After refinement, a fluorometric technique was found adequate for quantitatively determining certain gibberellins in the flowers and young fruits of navel sweet oranges. Using this method, significant changes in gibberellins were shown to occur, both in tissue concentrations and total amounts per fruit, in samples collected during the bloom and early period of fruit growth. Two relationships, a correlation between gibberellin concentration and rate of fruit growth, and an effect of total gibberellins per fruit on cumulative fruit growth, were found. These data indicated a cause and effect relationship between endogenous gibberellins and the early stages of fruit growth of the navel orange.