TEMPERATURE-GROWTH RELATIONS OF THE ROOTS AND HYPOCOTYLS OF COTTON SEEDLINGS

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Abstract
Cotton seedlings were grown for 7-14 days in darkness at tepm. from 18[degree] to 39[degree] C. Germination was >90% at all tepms. except 39[degree]. Protrusion of the radicles occurred most promptly at 33[degree], and less and less promptly at 36[degree], 27[degree], 39[degree], 30[degree], 24[degree], 21[degree], and 18[degree]. The opt. tepm. for the elongation of primary roots for 3 days and for the attainment of a length of about 80 mm. was 33[degree]-36[degree], after which it shifted to 27[degree]. The opt. for hypocotyls was 33[degree] for 4-5 days and for a length of 160 mm., after which it shifted to 36[degree]. The tepm. minimum for some growth of either primary roots or hypocotyls was below 18[degree], and the max. above 39[degree], but the latter tepm. induced heat injury after the 2d day. The secondary roots emerged earliest at 33[degree]-36[degree], but after 7 days of culture their total length was greatest at 30[degree], and they were most numerous at 27[degree]. The optimal for early emergence of the cotyledons from the testas was 27[degree]-30[degree]. After 7 days of culture, only about 40% of the seedlings at 33[degree] had freed their cotyledons from the testas; only 20% of those at 36[degree]; and none of those at 39[degree]. At all temps., in the 7-day period, ash content increased and dry wt. and organic matter decreased, the loss being greatest at 36[degree]; but, with respect to both the 7-day elongation of hypocotyls and increase in water content at the tepms. from 24[degree] to 36[degree], organic matter was most efficiently consumed at 30[degree] and 33[degree]; and the greatest increase in wt. occurred at 30[degree].