Growth,allocation of N and carbohydrates,and stomatal conductance of greenhouse grown apple treated with prohexadione-Ca and gibberellins

Summary

Potted M.26 apple (Mahis domestica) liners were treated with the gibberellin biosynthesis inhibitor prohexadione-Ca (Apogee®) at 0 to 500 mg l”¹ as a foliar spray. Apogee inhibited stem elongation, leaf formation, total leaf area and shoot dry weight, while significantly increasing specific leaf weight, root dry weight and root: shoot ratio, regardless of rate. Foliar application of gibberellin A4₊₇ (GA4₊₇) at 200 mg l”¹ to Apogee-treated plants one day later reversed these effects, especially stem elongation, root dry-matter production and root: shoot ratio. Apogee increased N concentration in stems but not in leaves and roots. There was no effect on the pattern of N allocation amongst organs. GA4+7 increased leaf N concentration but decreased stem and root N concentrations compared with untreated controls, with N allocation shifting from roots to stem. Total nonstructural carbohydrates (TNC), expressed either on a concentration or content basis, increased in all parts of the Apogee-treated plants, due to increased levels of starch rather than soluble sugars, without altering allocation pattern. Conversely, GA4₊₇ reduced TNC levels (mainly starch levels) in all parts, with the pattern of allocation slightly shifted from roots to stem. The afternoon decline in stomatal conductance occurred earlier in the Apogee treated plants, measured 10 d after stem elongation had ceased. Starch buildup in the Apogee-treated plants appeared to be associated with this effect, suggesting an involvement of a feedback inhibition of photosynthesis in the Apogee-induced stomatal control.