Many abnormal embryosacs (43% of the total observed) were recorded in ears from corn plants that had been drought stressed twice over a period of 10 days during embryosac formation (treatment T3). Also, hermaphrodite spikelets were commonly observed. Yery few grains were produced by these plants. In ears of plants drought stressed once over a period of five days during embryosac formation (treatment T2), 15% of the observed embryosacs were abnorraal compared with only 2.5% in ears of the unstressed control plants. The grain yield of T2 plants was not significantly reduced. Both T2 and T3 treatments greatly delayed silking; in the former there was a lag of 13 days between anthesis and silk emergence, and 16 days in T3 as compared to 3 days in the controls. Plants were hand‐pollinated to prevent sterility caused by the lack of pollen during silking, so that the large reduction in grain yield by treatment T3 was due mostly to embryosac abortion.
The surmise that femaleness of corn plants was reduced by drought stress because of a reduction in auxin levels was discussed.
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SUMMARYThe response of the roots of maize seedlings to physical impedance was studied in glass chambers that enabled roots to be grown either unimpeded or in loosely packed small glass spheres (ballotini) and flushed with humidified air or other gas mixtures. Impedance decreased root length when measured after approxmiately 40 h by 30-60 "" and increased root diameter 10 cm behind the tip hy a similar percentage. Fresh weight was reduced by up to 28-40%, while ethylene evolution from roots was increased 2-to 2-S-fold. Supplying ethylene (5-13 //I 1 ') in the air flow to unimpeded roots simulated closely the effects of impedance.The volatile inhibitor of ethylene action, 2,5-norbornadiene (approx. 900//I 1 '), overcame the effects of ethylene (5 4//I 1 ') completely, but the growth of impeded roots was unchanged by norbornadiene. Smiilarly, aminoethoxyvinylglycinc (O-5-O-75 mmol m •') inhibited endogenous ethylene formation, but did not modify the growth responses of roots to impedance, even though ethylene production was suppressed to rates well below those of unimpeded roots. The concentration of ahscisic acid was not affected by impedance.It is concluded that despite a widespread view to the contrary, ethylene is not the cause of the morphological response of roots to physical impedance. Abscisic acid is also unlikely to be involved.
Inflorescence leaves improve fruit set on sweet orange trees. We sought an explanation for this effect in terms of carbon demand by developing fruit versus potential supply from adjacent leaves. Our assessment was based upon measurements of fruit growth, leaf photosynthesis and 14C distribution patterns in plants grown under controlled conditions.
Leafy inflorescences had sufficient foliar surface (1.24 dm2) and photosynthetic capacity (CO2 10.1 mg · dm‐2· h‐1) to support early development of fruits on the same shoot, and to make a substantial contribution towards subsequent growth. 14C‐assimilates derived from new leaves were distributed towards adjacent fruit which showed strong competition for labelled substrate.
By contrast, fruit borne on leafless inflorescences had to obtain all their assimilates from older leaves whose photosynthetic capacity (CO2 3.5–4.6 mg · dm‐2· h‐1) and individual area (0.2 dm2) were generally insufficient to wholly sustain fruit growth, so that a large number of old‐leaves were needed; these fruit would be more susceptible to competition from other sinks.
The influence of magnesium nutrition on fruit quality of sweet orange was investigated in sand culture, 'and the relationships between Mg and fruit quality were explored in the field.In the glasshouse, fruit produced over 3 seasons by cuttings, cv. 'Washington Navel' grown witk high and low Mg nutrient solutions were compared With controls (5.29, 0.37, and 1.62 milliequivalents Mg++/L). High Mg resulted in fruit with a much greater juice acid content than the control, other effects on fruit quality attributes were .small and mostly non-significant. High Mg depressed uptake of Ca, but not K indicating a Ca-Mg antagonism. Phosphorus uptake was depressed by both high and low Mg compared to the controls. Leaf concentrations of Mg were: 0.595, 0.178, and 0.392 per cent (means over 3 seasons) in the high, low Mg and controIs.In the field, fruit from 'Washington Navel' trees on Rough Lemon rootstock had acid levels that correlated well with leaf K, also with (K + Mg) : Ca ratio.Leaf levels of Mg were below 0.4 per cent. 'Late Valencia' on Poncirus tri]oliata rootstock produced fruit With higher acid levels which correlated with leaf Mg (mostly > 0.4 per cent), also with K + Mg: Ca ratio. Therefore, Mg was a significant predictor of juice acidity when Mg levels were high.
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