Night temperature and source-sink effects on overall growth, cell number and cell size in bell pepper ovaries.

BACKGROUND AND AIMS: Ovary swelling, and resultant fruit malformation, in bell pepper flowers is favoured by low night temperature or a high source-sink ratio. However, the interaction between night temperature and source-sink ratio on ovary swelling and the contribution of cell size and cell number to ovary swelling are unknown. The present research examined the interactive effects of night temperature and source-sink ratio on ovary size, cell number and cell size at anthesis in bell pepper flowers. METHODS: Bell pepper plants were grown in growth chambers at night temperatures of either 20 °C (HNT) or 12 °C (LNT). Within each temperature treatment, plants bore either 0 (non-fruiting) or two developing fruits per plant. Ovary fresh weight, cell size and cell number were measured. KEY RESULTS: Ovary fresh weights in non-fruiting plants grown at LNT were the largest, while fresh weights were smallest in plants grown at HNT with fruits. In general, mesocarp cell size in ovaries was largest in non-fruiting plants grown at either LNT or HNT and smallest in fruiting plants at HNT. Mesocarp cell number was greater in non-fruiting plants under LNT than in the rest of the night temperature/fruiting treatments. These responses were more marked in ovaries sampled after 18 d of treatment compared with those sampled after 40 d of treatment. CONCLUSIONS: Ovary fresh weight of flowers at anthesis increased 65 % in non-fruiting plants grown under LNT compared with fruiting plants grown under HNT. This increase was due primarily to increases in mesocarp cell number and size. These results indicate that the combined effects of LNT and high source-sink ratio on ovary swelling are additive. Furthermore, the combined effects of LNT and low source-sink ratio or HNT and high source-sink ratio can partially overcome the detrimental effects of LNT and high source-sink ratio.

Gravity control of growth form in Brassica rapa and Arabidopsis thaliana (Brassicaceae): Consequences for secondary metabolism.

How gravity influences the growth form and flavor components of plants is of interest to the space program because plants could be used for food and life support during prolonged missions away from the planet, where that constant feature of Earth's environment does not prevail. We used plant growth hardware from prior experiments on the space shuttle to grow Brassica rapa and Arabidopsis thaliana plants during 16-d or 11-d hypergravity treatments on large-diameter centrifuge rotors. Both species showed radical changes in growth form, becoming more prostrate with increasing g-loads (2-g and 4-g). In Brassica, height decreased and stems thickened in a linear relationship with increasing g-load. Glucosinolates, secondary compounds that contribute flavor to Brassica, decreased by 140% over the range of micro to 4-g, while the structural secondary compound, lignin, remained constant at ∼15% (w/w) cell wall dry mass. Stem thickening at 4-g was associated with substantial increases in cell size (47%, 226%, and 33% for pith, cortex, and vascular tissue), rather than any change in cell number. The results, which demonstrate the profound effect of gravity on plant growth form and secondary metabolism, are discussed in the context of similar thigmostresses such as touch and wind.