Boron Deficiency and Chilling Injury Interactions in Tobacco Transplants Grown in the Float System

Abstract

OVERSTREET, LAURA FLINT. Boron Deficiency and Chilling Injury Interactions in Tobacco Transplants Grown in the Float System. (Under the direction of James W. Rideout.)Decades of agricultural research have failed to determine the precise mechanisms of infliction caused by the conditions of boron deficiency and chilling injury. Both conditions affect the quality and marketability of tobacco transplants grown in the float system. Interestingly, boron deficiency and chilling injury produce strikingly similar symptoms in young tobacco transplants; so similar, in fact, that they are often confused for one another. This has lead to severe boron toxicity when growers treated chilling injury as boron deficiency by applying boron to non-deficient float beds. The observation of nearly identical symptoms suggests that boron deficiency and chilling injury have interdependent effects on cell physiology and/or metabolism. Because little research has been conducted on tobacco transplants in the float system, two studies were conducted to determine general parameters for the boron deficiency threshold and effect of non-optimal temperatures and large day/night temperature differentials in this system. The boron deficiency study established that the deficiency threshold for transplants growing at 26/22º C is 10-20 µg B g -1 dry matter. These tissue levels occurred with solution concentrations of 0.19-1.9 µM B. The chilling injury study determined that root and shoot growth of flue-cured cultivars is near maximum at a constant 26/26º C temperature regime. Burley cultivars display a wider range of temperature tolerance, but in general constant day/night temperatures seem to provide the greatest shoot tissue accumulation.A reduction in night temperature resulted in decreased shoot growth in all cultivars. The chilling injury study also examined the effect of boron deficient conditions at each temperature treatment. In general, boron uptake declined at sub-optimal temperature regimes when supplied at concentrations sufficient for near-optimal temperatures. Shoot growth of flue-cured varieties at transplant stage was near maximal at a constant optimal day/night temperature regime (26/26º C) and adequate B concentrations. Sub-optimal temperatures may alter the boron deficiency threshold such that it decreases with decreasing temperatures or with stressful temperature differentials. This may be summarized in the following way: Temperature is the immediate limiting factor in tobacco transplant growth in the float system under conditions of sub-optimal temperatures and low B concentration, and B deficiency is an additional potential limiting factor.