Correlations between extractable leaf chlorophyll (Chl) concentration and portable, nondestructive leaf “greenness” meter readings imply that such estimates can be used as surrogate measurements of leaf nitrogen (N) status. However, few studies have actually found a direct relationship between Chl meter readings and leaf N. We evaluated the utility of two handheld transmittance-based Chl content meters (SPAD-502, Minolta Corp. and CCM-200, Opti-Sciences) and one reflectance-based meter (Observer, Spectrum Technologies), in estimating Chl and N concentrations in intact leaves of several citrus cultivars. Total Chl determined analytically, correlated well with nondestructive Chl meter readings (r2: 0.72 to 0.97; P < 0.0001), but regression models differed among cultivars using the same meter and also among meters for a given cultivar. The relationships were generally more linear and stronger at low Chl concentrations (<0.5 mmol·m-2) than at higher Chl concentrations, reflecting increased variability in Chl meter readings with increasing leaf Chl. Significant relationships between Chl meter readings and measured leaf N concentrations were also found in all the cultivars tested (r2: 0.23 to 0.69; P < 0.01), but the data were more variable than those for Chl. Field-grown leaves were significantly thicker and had higher Chl meter readings than greenhouse-grown leaves of similar Chl or N concentrations. The results suggest that nondestructive Chl content meters can overestimate Chl and N in thicker leaves and/or leaves with high Chl concentrations. A single prediction equation derived from a wide range of Chl or N concentrations could be applicable across the range of citrus cultivars when grown in the same environment. Potential limitations associated with leaf thickness as influenced by environmental factors may necessitate the development of more specific calibration equations.

ently maintain or very quickly recover P and water uptake capacity. This behaviour is consistent with an The effects of prolonged exposure to dry surface soil overall rooting strategy where essentially no surface on the capacity of roots to take up water and phosroots are shed following prolonged exposure to dry phorus were examined in mycorrhizal sour orange soil. (Citrus aurantium L.) seedlings grown in pots with upper and lower portions separated hydraulically. In Key words: Drought, split-root, 32P, citrus, uptake kinetics, the first experiment, upper portions of the pots were phosphorus uptake, water uptake, sour orange, Citrus either irrigated every 2-3 d, droughted for 14 d, aurantium. droughted for 43 d, or droughted for 42 d followed by 8 d re-irrigation. Lower portions of the pots were irrig-

Because the timing and amount of N fertilizer applications are important in producing high yield in cotton (Gossypium hirsutum L.), determining changes in plant N status is a valuable management tool. This study evaluated the Spectrum Technologies CM-1000 chlorophyll meter as compared with the GER model 1500 spectroradiometer for remote estimation of leaf chlorophyll concentration (Chl) under N-deficit conditions. In both instruments, measurements of leaf reflectance (R) at 700 and 840 nm (± 5.0 nm) were used to compute R 840 /R 700 , a measure associated with general plant stress. Plants were grown outdoors in large pots using half-strength nutrient solution (control) via a drip-irrigation system until some three-row plots received a restricted supply of N. Treatments comprised control N from emergence to maturity, 20% of control N at first floral bud (square) onward, and 0 and 20% of control N at first flower onward. Following 27 July 2000, treatment rankings for Chl and N were control > 20% N first flower > 0%N first flower ≥ 20% N first square. On most sampling dates, significant linear correlation was obtained between Chl and R 840 /R 700 across N treatments, particularly in August (r =0.55-0.93), when stress-induced changes in Chl were 171

Despite the frequent occurrence of hot, dry surface soil, little is known about the fate of roots in this soil layer during and following periods of surface drought. Phosphorus uptake kinetics were examined in surface roots of citrus seedlings exposed to different periods of drought. Exposure to dry soil for up to 43 days had no effect on phosphate uptake of excised roots measured at solution concentrations of 50, 750, and 1500 μm phosphate. Effects of surface drought on fine root behavior of seedlings and bearing trees were examined in the field using a split-pot arrangement. At the end of 5 weeks of drought, only about 1% of the roots in the top pot of either the seedling or tree had died. By 8 to 9 weeks of drought, about 26% of the fine laterals of the adult citrus had died, but only 6% had died in the seedling. After 15 weeks of drought, 33% of the mature tree fine roots had died but only 8% had died in the seedling. Root-soil respiration per gram root at this time was at least twice as high in the seedling as in the mature tree. Fine roots of citrus are remarkably tolerant to dry surface soils of about 5 weeks duration, both in terms of root survival and resumption of physiological activity after rewetting.