Possible role of growth regulators in adaptation to heat stress affecting partitioning of photosynthates in tomato plants

Abstract

Tomato plants of two cultivars: Roma - sensitive and Robin - tolerant to heat stress were grown in greenhouse up to the flowering stage and then under controlled environmen­tal conditions. The partitioning of recently fixed <sup>14</sup>CO<sub>2</sub> by mature tomato leaves was examined as a posteffect of 24-h heat stress (38/25°C day/night) with the interaction of growth regulators (GR) sprayed on the flowers with solution of β-naphthoxyacetic (NOA) and gibberellic (GA<sub>3</sub>) acid (denoted as NG), or Zeatin + NOA + GA<sub>3</sub> (denoted as ZNG). In both cuitivars GR strongly stimulated fruit growth and transport of <sup>14</sup>C-photosynthates to the clusters at the expense of vegetative organs. Heat stress decreased export of <sup>14</sup>C-phoiosynthates from the blades in plants not treated with GR, but even more in cv. Roma. In Roma plants not treated with GR (with very small fruitlets and fruits) the heat stress retarded <sup>14</sup>C-transport just in the petioles, diminishing the <sup>14</sup>C-supply to the fruits. Reduction of the current photosynthate supplied to the fruits seems to be causally connected with inhibition of the specific activity of acid invertase in that organ. Growth regulators reduced the negative effect of high temperature - they alleviated depression of <sup>14</sup>C-export from the blades and increased invertase activity. <sup>14</sup>C-photosynthate transport to the fruits, presumably owing to their higher sink strength, was less affected by heat stress. In Robin plants (which had bigger fruits during the experiment) high temperature depressed <sup>14</sup>C-fruit supply only in the NG-series, in contrast to enhacement of <sup>14</sup>C-Movement to that sink in the control and ZNG-series. In spite of these facts, after heat stress, the specific activity of acid invertase decreased in all the experimental series, but much less in the GR-treated series. Therefore, in the Robin cv. there was no relation between invertase activity and <sup>14</sup>C-mobilization by fruits, as was observed in Roma plants. The possible explanation of the different response of the two cultivars with contrasting sensitivity to heat stress; with special reference to the role of GR; diminishing injury of the plants by high temperature is discussed.