No abstract

Four-year-old `Redblush' grapefruit (Citrus paradisi Macf.) trees on either the relatively fast-growing rootstock `Volkamer' lemon (VL) (C. volkameriana Ten. & Pasq.) or on the slower-growing rootstock sour orange (SO) (C. aurantium L.) were transplanted into 7.9-m3 drainage lysimeter tanks filled with native Candler sand, irrigated similarly, and fertilized at three N rates during 2.5 years. After 6 months, effects of N application rate and rootstock on tree growth, evapotranspiration, fruit yield, N uptake, and leaching were measured during the following 2 years. When trees were 5 years old, low, medium, and high N application rates averaged about 79,180, or 543 g N/tree per year and about 126,455, or 868 g N/tree during the following year. Recommended rates average about 558 g N/tree per year. A lysimeter tank with no tree and additional trees growing outside lysimeters received the medium N treatment. Nitrogen concentration in the drainage water increased with N rate and exceeded 10 mg·liter-1 for trees receiving the high rates and also for the no tree tank. Leachate N concentration and total N recovered was greater from trees on SO than from those on VL. Average N uptake efficiency of medium N rate trees on VL was 6870 of the applied N and 61 % for trees on SO. Nitrogen uptake efficiency decreased with increased N application rates. Trees outside lysimeters had lower leaf N and fruit yield than lysimeter trees. Overall, canopy volume and leaf N concentration increased with N rate, but there was no effect of N rate on fibrous root dry weight. Fruit yield of trees on SO was not affected by N rate but higher N resulted in greater yield for trees on VL. Rootstock had no effect on leaf N concentration, but trees on VI. developed larger canopies, had greater fibrous root dry weight, used more water, and yielded more fruit than trees on SO. Based on growth, fruit yield and N leaching losses, currently recommended N rates were appropriate for trees on the more vigorous VL rootstock but were 22% to 69 % too high for trees on SO.

SUMMARYRootstock seedlings of Carrizo citrange {Poncirus trifoliata (L.) Raf. x C Sinensis (L.) Osheck) and sour orange (Citrus aurontium L.) were grown in a sandy soil low in phosphorus (P) and either inoculated with Glomus intraradices Schenck & Smith (vesicular-arbuscular mycorrhizal; VAM) or fertilized with soluble P (non-mycorrhizal; NM). Five-month-old VAM and NM seedlings of each rootstock were comparable in size, P sufficiency, and relative growth rate whether they were kept well-watered or subjected to two drought-stress cycles of short duration. Under well-watered conditions, whole plant transpiration, leaf water status, and root hydraulic conductivity were similar for VAM and NM plants of each rootstock. During drought-stress and recovery periods, VAM plants also had very comparable whole-plant transpiration rates and leaf water potentials to NM plants, but mycorrhiza reduced root hydraulic conductivity of Carrizo citrange and sour orange 66 and 49%, respectively. These data do not support the hypothesis that mycorrhiza significantly enhance water relations of citrus under the droughtstress conditions studied.

Growth rates of seasonal leaf flushes of 'Valencia' orange [Citrus sinensis (L.) Osbeck] were measured and water relations characteristics of young (new) and over-wintered (old) citrus leaves were compared. New flush leaves had lower specific leaf weights and lower midday leaf water potentials than comparably exposed old leaves. Spring and summer flush new leaves had higher osmotic potentials than old leaves. These differences became non-significant as the new leaves matured. During summer conditions, water. stressed new leaves reached zero turgor and stomatal conductance also began to decrease in them at higher leaf water potentials than in old leaves. Old leaves were capable of maintaining open stomata at lower leaf water potentials. Opened flowers and new flush leaves lost more water, on a dry weight basis, than flower buds, fruit or mature leaves. The results illustrate differences in leaf water potential and stomatal conductance which can be attributed to the maintenance of leaf turgor by decreases in leaf osmotic potentials as leaves mature. These changes in citrus leaf water relations are especially important since water stress resulting from high water loss rates of new tissues could reduce flmvering and fruit set.