Root shoot interactions in passionfruit (Passiflora sp.) under the influence of changing root volumes and soil temperatures

Passionfruit are grown in the tropics and subtropics where mean monthly soil temperatures at 15 cm range from about 10-degrees to 30-degrees-C. The choice of rootstock can also influence production with most industries exploiting either the purple (Passiflora edulis f. edulis) or golden passionfruit (P edulis f. flavicarpa). We examined the relationship between shoot and root growth in purple x golden hybrid E-23 grafted onto golden passionfruit seedlings. Growth was manipulated by varying the volume of the soil available to the roots or temperature of the root zone. Shoot and root growth increased as root zone volume increased from 0.3, 1.4, 4, 12 to 24 1. Shoot weight (W(S)) was correlated with root weight (W(R)):W(S) = 12.697 + 5.272 W(R) + 0.195 W(R)2 (r2 = 91%, P <0.001), with the plants allocating a smaller proportion of dry matter to the roots as root weight increased. Differences in shoot growth with pot volume were not due to changes in water or nutrient status. In the temperature experiment, the two critical root zone temperatures at 90% of maximum growth were about 20-degrees and 35-degrees-C for vine extension, leaf area, node and leaf production, and 20-degrees and 30-degrees-C for flower production. Leaf and stem dry weight were optimal between about 18-degrees and 34-degrees-C, while maximum root growth occurred at 38-degrees-C. There was a weak relationship between shoot (W(S)) and root dry weight (W(R)): W(S) = -19.346 + 24.500 W(R)-1.046 W(R)2 (r2 = 53%, P <0.001). Apparently, variations in shoot growth at different soil temperatures cannot be explained solely by differences in root growth. Reduced growth at 10-degrees-C was associated with lower chlorophyll concentration, stomatal conductance and net CO2 assimilation, but not lower leaf water potential. The concentration of most nutrients were lower at 10-degrees-C than at higher temperatures, but none was outside the range which would be expected to restrict growth. There appears to be a co-ordination of shoot and root growth as the soil volume available for root growth increases, whereas root temperature affects the roots and tops differently. The results of the pot volume experiment demonstrate the importance of rootstock vigour in passionfruit breeding. Productivity would be affected in cool subtropical areas with soil 30-degrees-C.