Anatomy, Growth and Survival of a Long-hypocotyl Mutant of Cucumis sativus Deficient in Phytochrome B

Abstract

Plant dry matter (DM) partitioning, survival rates, stem anatomy, and stem water conductivity were investigated in wild-type (WT) and long-hypocotyl (lh) mutant seedlings of cucumber (Cucumis sativus) grown as isolated individuals under natural radiation. The lh mutant is severely deficient in phytochrome B. Wild-type seedlings accumulated more DM than lh seedlings over a 4-week growth period in the glasshouse. Leaf and root DM were higher in the WT but stem DM was higher in the lh mutant. Stem DM per unit length was larger in WT than in lh mutant seedlings, even when the two genotypes were compared at equal whole plant DM, which was achieved by growing the plants under different irradiance. In WT seedlings, the hypocotyl was shorter but thicker, with larger average cell diameter than the lh mutant. In hypocotyl transverse sections the area occupied by load-bearing tissues (xylem and phloem fibres) and the number and diameter of xylem vessels were larger in WT than lh seedlings. Survival rates of the lh mutant were normal in the glasshouse but very low outdoors due to hypocotyl fracture. The water conductivity of hypocotyl sections was higher in WT than lh seedlings, but no significant differences in water conductivity were observed when the root remained attached to the hypocotyl. These results suggest (a) that compared to the WT, tall and slender lh plants are more susceptible to mechanical stresses created by wind impact, and (b) that if the lh lesion affects the phyB gene only, phytochrome B plays a role in the elicitation of anatomical and morphological changes that specifically increase fitness in open environments.