Photosynthesis-irradiance response curves and leaf nitrogen contents were measured weekly by destructive sampling over the life cycles of leaves 10, 15, 20 and 25 of sunflower plants (cv. Prosol 35) grown in large pots in the open under optimum conditions of temperature and high irradiance. Individual leaf responses were adequately described by a hyperbola of three parameters, viz. Pmax, the rate of photosynthesis in saturating irradiance; R, the rate of dark respiration adjusted for temperature (30�C); and ε, the apparent quantum efficiency of photosynthesis at low irradiance. Pmax (range 0-40 μmol CO2 m-2 s-1) and R (0-4 μmol CO2 m-2 s-1) were non-linearly related to nitrogen content per unit leaf area (NL) (range 0.3-2.9 g N m-2) across all leaf positions and for all leaf ages. ε (mean value 0.050 mol mol-1, s.e. 0.001) was independent of NL. The equations for net photosynthesis derived from pot studies were shown to explain (r2 =0.80) leaf photosynthesis in a crop of the same cultivar over a wide range of NL and irradiance.

Bondada et al. (1995) studied T. urticae damage to cotton grown in the field and found alterations to the sto-Spider mites are important pests of cotton (Gossypium hirsutum matal apparatus and internal damage to the mesophyll L.), capable of dramatically affecting growth, yield, and fiber quality. This study investigated the physiological response of cotton leaves to cells, which resulted in declining photosynthesis in paralfeeding damage by the two-spotted spider mite (Tetranychus urticae A.A. Reddall and L.J. Wilson, CSIRO Plant Industry and Australian Cotton CRC, Locked Bag 59, Narrabri NSW 2390, Australia; V.O.

Bondada et al. (1995) studied T. urticae damage to cotton grown in the field and found alterations to the sto-Spider mites are important pests of cotton (Gossypium hirsutum matal apparatus and internal damage to the mesophyll L.), capable of dramatically affecting growth, yield, and fiber quality. This study investigated the physiological response of cotton leaves to cells, which resulted in declining photosynthesis in paralfeeding damage by the two-spotted spider mite (Tetranychus urticae lel with declining stomatal conductance and transpira-Koch) in two experiments in the field over two seasons. Mite colonies tion. However, studies of the relationship between the initially established and developed in the basal areas of leaves, where density of mites on leaves and effects on the photosynthe leaf blade joins the petiole. These infestations caused rapid and thetic rate of cotton leaves in the field are limited. Many severe reductions in photosynthetic rate, stomatal conductance, tranother studies on the impact of spider mites on host plants spiration, transpiration efficiency (TE), and chlorophyll content. In such as peach [Prunus persicae (L.) Batsch], almond basal areas, a peak of 68 adult female mites per leaf caused photosyn-(Prunus dulcis Mill.), apple (Malus domestic Borkh), thesis to decline to zero, while undamaged leaves averaged 33 mol tomato (Lycopersicon esculentum Mill.), strawberry (Fra-CO 2 m Ϫ2 s Ϫ1 . Differences in plant responses to mites occurred between garia ananassa Duch.), and peppermint (Mentha piperita seasons despite similar infestation levels, possibly related to later timing of infestation and harder leaves in the second season. Compen-L.) have recorded both reduced leaf photosynthesis and sation for mite damage was not apparent at the leaf level, since photo-transpiration rates (Hall and Ferree, 1975; Poskuta et synthesis was reduced on undamaged portions of damaged leaves. al., 1975; DeAngelis et al., 1983; Youngman et al., 1986; The sequence of mite damage events on the undamaged leaf portions Youngman and Barnes, 1986; Hare and Youngman, was determined to be: first, reduction of stomatal conductance; second, 1987; Royalty and Perring, 1989; Mobley and Marini, reduction of transpiration; third, reduction of photosynthetic rate; and 1990; Nihoul et al., 1992). Reductions in photosynthesis finally, reduction of transpiration efficiency. At the leaf level, the have been shown to result from decreased stomatal openoverall effect of mite damage on photosynthesis was greater thaning and increased mesophyll resistance (Welter, 1989). expected because undamaged areas surrounding those visibly dam-However, the order in which these changes occur and aged were also affected.how this varies with different mite densities and intensity of damage has not been clarified previously.A further limitation of the above studies was that A.A. Reddall and L.J. Wilson, CSIRO Plant Industry and Australian Cotton CRC, Locked Bag 59, Narrabri NSW 2390, Australia; V.O.

We assessed the effects of timing and intensity of spider mite (Tetranychus urticae) infestation on the oil yield of cotton (Gossypium hirsutum L.) crops. Artificially infested crops of cv. Deltapine 90 (normal leaf) and Siokra 1-4 (okra leaf) were compared in 2 field experiments. Actual timing of infestation ranged from 59 to 127 days after sowing and intensity of infestations ranged from 64 mites per leaf in the most severe treatment to almost nil in controls. Mites significantly reduced oil yield in both experiments; yield reductions were greater in the normal-leaf than in the okra-leaf type. Oil yield losses declined with the delay in the onset of infestation. Under low and moderate levels of infestation, oil yield losses were associated with reductions in seed production. Under more severe infestations lower seed oil content also contributed to the reduction in oil yield.

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