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We have studied the effects of MCP and low O2, applied singly and in combination, on apple fruit ripening at 1, 7, and 18 °C. The single application of 2 ppm MCP is more effective in delaying the onset of the C2H4 climacteric than is 1% O2. However, the combined application has a much larger effect than the single applications of either MCP or 1% O2. For instance, at 7 °C, the onset of the C2H4 climacteric occurs at 15, 50, and 90–95 days for the controls, 1% O2 and 2 ppm MCP, respectively, whereas the combined application of 2 ppm MCP and 1% O2 suppressed the initiation of the C2H4 climacteric for 200 days, the duration of the experiment. The retardation of the climacteric onset by the treatments is associated with the suppression of ACC-synthase (ACS1) and the putative receptor ERS1. The accumulation of their transcripts is critically dependent on the rate of C2H4 evolution. As expected, the combined application of MCP and 1% O2 completely suppressed the expression of both genes. Yet when the fruits were transferred to 18 °C in air, they ripened normally. A similar pattern of inhibition in response to the above treatments was also observed with a C2H4-dependent MAPK. The expression of ETR1, ETR2 and ACC-oxidase was not affected by the treatments. The nature of this strong effect of the combined application of MCP and low O2 is not clear. It should be pointed out that MCP does not inhibit the induction of hypoxic proteins such as ADH.
`Granny Smith' apples were stored at 1 °C, 5 °C and 20 °C, then treated at the preclimacteric stage with 2 ppm MCP, various O2 concentrations, and MCP + low O2. All the treatments greatly retarded the onset of the C2 H4 climacteric, and hence ripening. MCP + low O2 was much more effective than were the applications of MCP and low O2 singly. Even at 20 °C, 4.04 kPa O2 inhibited the rise in C2 H4 evolution for 145 d. Neither low O2 nor MCP inhibited the System 1 C2 H4 evolution. The suppression of the climacteric rise in C2 H4 evolution was accompanied by a strong inhibition of the accumulation of ERS1 C2 H4 receptor and ACS transcripts. On the other hand, ETR1 receptor was constitutively expressed. When climacteric fruits were treated with MCP, and with low O2 + MCP the rate of C2 H4 evolution decreased sharply. This occurred simultaneously with a decrease in ERS1 mRNA. Moreover, the decrease in ERS1 mRNA paralleled the decrease in C2 H4 evolution. The data thus indicate that the initiation and sustainment of the C2 H4 climacteric requires the presence of functional C2 H4 receptors. The expression of ETR2 and ERS2 is also under investigation.
Cover crop management in growing horticultural produce has attracted attention for reducing soil erosion and limiting the input of synthetic fertilizers and pesticides. Hairy vetch (Vicia villosa Roth.), one of the cover crops, exhibits desirable attributes such as high N fixing ability, biomass quality, adaptability to low temperatures, resistance to pests, and fitness in vegetable production, particularly in rotation with tomatoes. The interactions between the cover crop mulch and the tomato plant in the field plots result in delayed leaf senescence and increased disease tolerance. The mechanisms underlying these interactions are largely unknown. Limits in pursuing these studies year-round in the field—growing season and complexity and variability of the field environment—could be circumvented if the observed responses of tomato plants to hairy vetch mulch in the field could be reproduced under greenhouse conditions. We have tested tomato plants for two years in the greenhouse using soil residues brought from field plots where respective cover crops had been previously grown. Treatments were a) bare soil from a fallow, weed-free field plot, b) soil from a field plot that had been planted into a rye cover crop, and c) soil from a field plot that had been planted into a hairy vetch cover crop. Pots with soil from the rye or vetch field plots were further topped with rye or vetch residues, respectively, after transplanting the tomato plants. Additional N was applied to 50% of the plants in each treatment. In the greenhouse, cover crop residue-supplemented tomatoes exhibited high vigor, higher marketable yield and delayed senescence compared to those grown in bare soil. All treatments responded favorably to additional N from commercial fertilizers. Delayed leaf senescence correlated with the accumulation of rubisco large subunit and chitinase, two proteins central to photosynthesis and pathogenesis, respectively. This study shows that the responses of tomato plants to cover crops seen in the field can be mimicked under greenhouse conditions.